Use of Reamers
Tools for Putting Quality into Quality
What to Ream – The primary question for the tool engineer is, “What are the limitations of reaming?” It may be answered by asking a few more:
What is the material? What is its hardness? What are the tolerances? Which should be bored? Which should be ground? Which should be honed? Which should be lapped?
The conditions under which reamers are used and the results which are desired vary so widely that it is impossible to set up any guide to cover each possible combination. A few of the variables which affect reamer cutting action are:
- Use of guiding bushings
- Condition of machine
- Rigidity of set-up
- Rake of cutting edges
- Reliefs on reamer
- Amount of stock to be removed
- Finish Required
- Tolerance of hole
However, there are certain general principles which should be kept in mind.
Material to Ream
If the material is free cutting, any lightly constructed tool is adequate. If the material is hard, tough, stringy, or of a work-hardening nature, careful attention is needed to choose a solid reamer or a rugged type of inserted-blade design. No deflection can be allowed. Clearance, chamfer relief, and rake angles must be specified to suit the work to be done. If the Brinell reading is high, the heat treatment of the reamer must be correct when high-speed steel is used; and if the reading is above Brinell 415, it will be advisable to go to carbide tipping. Work hardening is often experienced in certain types of stainless steel and can be avoided only by helical fluting, narrow lands, a good secondary clearance, and the omission of the usual concentric margin. Should a concentric margin be demanded, it ought to be kept to mere hairline width. Floating holders are almost a “must” on stainless, as they hold work hardening to a minimum. Carbide- tipped reamers improve the finish and last longer in stainless than high-speed.
Amount to Ream
The usual practice is to ream from 0.004 to 0.012 inches on diameter, i.e. 0.002 to 0.006 inches on a side, in finishing operations. Seldom ream over 0.012 inches. As drill sizes vary in increments of 1/64 in., it will be found that theoretically 1/64 in. has been left for reaming, but actually it is only 0.012 in., as most drills cut oversize by 0.002 to 0.004 in. In the smaller sizes, especially those below ½ in., it will be advisable to ream only 0.006 or 0.008 in. in diameter. This is relatively easy to arrange, as drill sizes come in smaller increments within this range.
Maintenance and Salvage
Any cheap tool may seem economical in first cost, but the question is, “Which is the cheapest in the overall cost?” Taper reamers, of course, can be reground many times before size is lost. With a solid straight reamer the only salvage is to regrind it to smaller size – a condition not always convenient – or to convert it into some other type of tool.
Reamers should be sharpened as soon as they become dull enough to prevent proper cutting. It usually is necessary to sharpen only the cutting edge or the chamfer, but if it is necessary to regrind the diameter, the correct circular-land width must be maintained, as well as the back taper.
Floating fixtures vs. Floating holders
When a reaming fixture is light in weight, it is practical to use a rigid drive and allow it to “float” on the reaming table. If the fixture is heavy, it is more advisable to use a floating holder drive to permit self-alignment. There are many excellent commercial models available.
In screw machines and turret lathes, the reamer will always follow the bored or drilled hole; and if a rigid holder were used, a tapered or “bell-mouth” hole would result. Therefore, the proper style of floating holder must be specified. It must be “full” floating and allow the tool to adjust itself in directions freely and easily.
Various types of floating drivers are in use, each possessing individual merits. Some types permit angular relation between the center lines of the driving spindle and the reamer. Others permit side movement of the axis of the reamer with that of the driving spindle, the two being parallel. Still others combine these two features.
There are two principles that must be embodied in satisfactory floating reamer drivers. First are some of the angular positions that the reamer must be permitted to assume. Secondly, is what might be called parallel floating. Here the reamer axis may move away from the spindle axis while the two are in parallel.
The ideal floating driver must be able to do both of these. In other words, it must be able to adjust itself to misalignment of both types and in all positions throughout its circle of rotation. The amount of permissible angular or parallel float should be adjusted to cover the greatest amount of misalignment that is likely to occur on any given job.
Correcting Inaccurate Holes
Occasionally the designer is confronted with the task of supplying a reamer for straightening holes that have run off from the true center line owing to irregularities in the subject piece, uneven conditions in the metal, or plain carelessness in the shop. When the errors are not too great, the job can be saved by substituting an end-cutting reamer for one with the conventional chamfer. An end-cutting function can be given to any reamer by grinding the end off square with the axis as shown at A (Fig 15-5), removing the chamfer entirely. In effect, the tool cuts like an end mill. However, the desired results will not be obtained unless it is well guided and suitably supported – details that the designer should check.
Reamer Driving and Holding Methods
The most usual means for holding and driving reamers is the three-jaw chuck. Another is the straight-sleeve and setscrew method. Taper shanks are sometimes flattened on one side and used in what is known as a “use-‘em-up” sleeve or socket. Reamers with adapters for quick-change chucks are used on production set-ups.
When reamers must guide themselves into previously made holes, they require floating holders to maintain alignment and prevent tapered, out-of-round, and bellmouth holes. There are several types of floating holders, some of which permit an angular float; others permit a parallel float; still others permit both features.
It is almost universally accepted as good practice to design reamers of the solid, shell, or inserted-blade types with broken or irregular flute spacing. The purpose is to eliminate the liability to vibrate, or “chatter.” This is accomplished by a “broken” indexing when the flutes are milled before heat treatment and when sharpening in finishing and in service. Broken or uneven spacing is carefully planned to occur according to prescribed rules set down by the individual manufacturer to meet his own requirements. Starting with one flute at “zero” as basic, the second flute may be minus a definite number of degrees from even spacing, the third will be plus a similar number, the next minus a different number, the following will be plus still another number of degrees, and so on until “zero” is reached again.
Product Design as Related to Reaming
Better reaming usually results when the product is designed to facilitate that operation. Where possible, provision should be made for the reamer to pass through the work piece. This avoids the necessity for reaming a blind hole; when this is unavoidable, the depth of cut should be controlled to prevent bottoming and probably damaging the reamer. A reamer should enter a hole at right angles to the work surface to permit all teeth to engage for a good start, because reaming at an angle makes it to turn out good work. Plain reaming should not be depended on to align a series of holes and center on them on a common axis, because it merely trues the diameter of the individual hole. Concentricity and alignment require line reaming, which requires that the holes be of equal or progressively smaller diameters to permit entrance and withdrawal of the reamers. Provision must be made for guiding the reamer bar or arbor at both ends.
Reaming in Aluminum
The usual line of reamers may be used in aluminum, but helical fluting is best for straight holes and spiral for tapered. Duplex taper reamers give splendid performances on precision work in this material. Extra care is needed to see that sufficient stock is left in the holes to give the reamers work to do. Otherwise burnishing will occur.
The Use of Bushings
The use of bushings as guides for reamers is of great help. The ideal job would employ a fixed jig and bushing with a rigid spindle and a minimum of overhang. It is particularly important when using bushings that the spindle be accurately aligned with the bushing in order to prevent the reamer from hitting the top of the hardened bushing. In radial drill presses or in table presses equipped with sliding jigs, great care must be used. Sometimes bronze or fibre caps are placed over the bushings to act as a lead on device, but even this will not protect the reamer unless the tool is properly centered in the bushing.
The same problem arises if work is reamed on a table press without a bushing. The use of a short entering taped on the front of the reamer, or in some cases a shirt pilot section, may be of help in leading the reamer into the hole.
However, the practice of using a reamer as a locating device and forcing it to drag the work over into alignment is not good, and is likely to result in excessive wear or chipped edges of the reamer.
When holes are to be located at exact distances from some point or some other hole, the only sure method is to do the reaming in jigs or fixtures. In such jigs or fixtures the work is located and held securely, and the reamer is guided in bushings set in exact relation to locating points in the work.
For this type of reaming the ideal arrangement is to guide the reamer on both sides of the work, especially if the hole is comparatively long. A special piloted reamer is required for this purpose. Guide bushings should fit snug to the pilots, but should not be so right they will seize and bind. Pilots should be grooved throughout their length. These grooves serve the double purpose of permitting the cutting fluid to lubricate the pilots and to scavenge any chips that tend to wedge between the pilots and bushings.
If the holes to be reamed are short, the reamer may be guided at the entering side of the hole only. The guide bushing then may be made to fit the outside diameter of the reamer flute.
For either of these types of applications a rigid drive is satisfactory, because any misalignment of the machine spindle and the work tends to correct itself by means of the guiding bushings. By a rigid drive, we mean one where the reamer shank is help directly and rigidly in the machine spindle.
If, on the other hand, the reamer is to guide itself into a previously made hole, a rigid drive is no longer satisfactory, because any existing misalignment of the machine spindle with the work will result in reamed holes that are
bell-mouthed, tapered, or out of round. A floating reamer driver must then be used.
CONTROL OF THE CAUSES OF TOOL WEAR AND FAILURE
Good tool design is accomplished with consideration of many inseparably related factors: the comparison, hardness, condition, and shape of the work piece material; the rate and volume of the specified production; the type, motions, power, and speed of the machine tool to be used; the toolholders and workholders available or be designed; the specified accuracy and surface of the finished work piece, and many other factors, common or specific to the particular operation.
Set-up rigidity is vital to the maintaining dimensional accuracy of the cut surface, since the tool shifts into or out of the cut with the accumulation of static deflections and take-up of loose fits. Rigidity also maintains surface-finish quality, avoiding the marks made by elastic vibration and free play of loose fits and backlash. In the control of vibration, rigidity of the part and cutting tool can make the difference between success and failure of the machining operation.
Increasing mass reduces vibration amplitude and resonant frequency, while dampening reduces amplitude by dissipating vibratory energy as frictional heat. Since each part of the cutting system (i.e. the machine, the fixture, the tool and the work piece) can affect the mode and amount of vibration most should be made oversize and broadly supported. This provides design latitude for those members having more severe costs and space limitations. Tools should be rigid for fast, efficient cuts.
The strength of each member can be considered separately and related to the magnitude and application of the forces it will transmit. It should clearly be sufficient to prevent breakage or deformation beyond the elastic limit when the operation is performed correctly. The designer must also consider overloads and damage that may be encountered, providing abundant strength wherever economically possible. In particular, generous size and material specification should give good working life to areas subject to abrasive wear and work hardening under impact loads. But chatter, packed chips, or binding due to set-up misalignment can multiply normal operating forces many times. Tool failure, mechanical malfunctions, and operating errors threaten destructive casualties.
One sure way to overload a cutting tooth is to block the path of the chip flowing across its face so that the chip is re-cut. Chip breakers are frequently employed to curl an otherwise stringy chip so that it will break in the form of a figure nine and fall away. It is difficult to remove work materials like soft steel or copper alloys and titanium, whose chips tend to weld onto the tool face. Tool design should provide space for chip flow and means of disposal, which may well be the solution to many problems of tool chipping.
Another sure way to overload a cutting tooth is to increase the feed rate drastically beyond its structural or chip-disposal capacity. Machine structural deflection accomplishes this is the example of a drill breaking as it breaks through the work. As the heavy thrust of the chisel edge is relieved, structural members spring back toward their unstressed shape, and the drill lips plunge into the work for an oversize bite. Feed mechanisms may employ air or hydraulic fluid whose compression is elastic; or gearing and a leadscrew nut fit may introduce backlash. Machine way motion becomes jumpy at slow speeds (“slip-stick” motion), even when heavy lubrication. A milling cutter at slow feed may actually rub until pressure builds up. It then may dig into the work and surge ahead. Adding to the difficulty, the sudden change in cutting torque adds to the pounding caused be teeth entering the cut.
Torsional vibration and backlash tend to develop in a rotary drive train. Should cutter rotation become so erratic that it momentarily stops, carbide teeth will generally break at once by being bumped into the work. With some teeth gone, the entire cutter may fail progressively as each successive tooth is unable to carry the extra load left by the preceding damaged teeth.
SPECIFIC METHODS OF REAMING HAND REAMING
When the work piece is reasonable rigid, hand reaming may be performed by rotating the reamer by means of a double end tap wrench applied to the driving square. This type of wrench, which permits a balanced drive, should always be used in preference to a single-end wrench. The use of single-end wrenches makes it almost impossible to apply torque without distributing the alignment of the reamer and the hole.
The reamer should be rotated slowly and evenly, allowing it to align itself with the hole to be reamed. Wrenches should be large enough to permit a steady torque which will help to control vibration and chatter. The feed should be steady and large compared to the feed used in machine reamers. Feeds up to one quarter of the reamer diameter per revolution are not at all unusual.
In cases where the work piece is small enough to be handled with ease, it is often advisable to place the reamer vertically in a vise and rotate the work down over the reamer by hand. If the work is quite light it may not have enough mass to dampen vibrations. In such cases a holding device for the work may be employed which will add weight to the part to be reamed. This holding device should have two opposite handles on it, large enough in diameter to permit a stead controlled torque.
In cases where there is a large quantity of light parts to be reamed, the reamer is often mounted horizontally in a reaming machine. Most reaming machines are essentially chucks mounted on the output shaft of a motor-driven gear reducer. This drives the reamer at the necessary slow speed. The work piece is fed slowly and steadily over the reamer.
In all hand reaming done by any of the methods described, with solid, expansion, or adjustable reamers, the reamer should never be rotated backwards to remove it from the hole. For this results in premature dulling of the reamer. If possible, it should be passed on through the hole and removed from the far side without stopping the rotation. If this is not possible, it should be withdrawn without stopping the forward rotation.
THE USE OF REAMERS IN LATHES AND HAND OR SCREW MACHINES
When reaming is done in any of the machines listed above, certain problems arise because the reamer is stationary and the work revolves. This is the opposite of the condition normally encountered in drill-machining press reaming.
Theoretically, in this type of reaming the axis of the reamer coincides with the axis of machine spindle, and no difficulty should be encountered. However, in actual practice, this condition seldom exists. It probably would be wise to assume that it never exists and to expect the presence of misalignment on jobs of this nature.
The misalignment is of two distinct types:
The first is a case where the axis of the reamer is parallel to the axis of the spindle, but has been lowered slightly.
In the second, the shank of the reamer is in line with the spindle, but the reamer has been clamped at a slight angle.
In actual practice both of these errors are often present at the same time in a single set-up. Such misalignment can be caused by a combination of several things:
1. Worn ways on the machine
2. Worn or dirty holes in tool holder
3. Worn or dirty sleeves
4. Improper leveling of machine
5. Improper location or adjustment of tool slides
6. Errors in indexing mechanism
The use of a conventional reamer held rigidly in such a machine quite often results in poor finish and oversize holes, particularly at the start of the hole. Such holes are usually called “bell-mouthed.”
In the case of a reamer held at an angle to the spindle of the machine, as this reamer is fed into the work, the lower flute will bear harder and harder against the wall of the reamed hole. The results in a scraping action by this flute which causes an oversize hole. The pressure also quite often causes a building up on this flute. When the pressure gets too high, this built-up portion breaks loose, causing gouges and tears in the reamed surface.
There are several means available to eliminate these torn, oversize holes. The first is to make corrections on the machine and tool holders. It is comparatively simple to make some of these corrections, such as cleaning tool holders, replacing worn sleeves or bushings, and relocating some times of tool holders. Others, such as correcting mis-indexing on a turret lathe or a multi-spindle automatic, may require a great deal of time.
The second method is to use floating holders for reamers. There are many kinds and designs of floating holders. Some correct for angular error only. Some correct for a parallel misalignment only, while some correct for both. Besides varying in function and design, these different floating holders also vary in their inherent rigidity. The problem in such holders is to allow movement in certain directions while restricting it in others. Naturally, this is not accomplished in the same manner in all designs and the rigidity of different types of holders, whether floating or not, varies considerably.
The presence of slight errors in the machine does not necessarily mean that floating holders must be used. It must be remembered that the looseness in the machine plus the flexibility in the shank of the reamer provide some float. Also, on some types of machines, it is comparatively simple to insert black bushings in the tool holder and rebore them from the headstock. On other machines, adjustable rather than floating holders may be used.
This oversize condition is not always cured by the use of floating holders alone. In the case of a turret which has worn low, an attempt to correct this condition is made through the use of a simple pin drive float.
Such a float allows the reamer to tilt slightly, but does not permit the shank of the reamer to move up or down or sideways. It will be noted that as the reamer feeds into the work the pressure will build up on the lower flute as before, still causing oversize and torn holes.
In order to overcome this condition it is sometimes necessary to grind back taper on the reamer. While the use of back taper on reamers will often eliminate these rough, oversize reamed holes, it must be remembered that this back taper reduces the reamer life, particularly on close tolerance holes. This special back taper varies between .001” and .005” per inch of flute. Tapers such as these are far in excess of the very slight taper which has been found best for stock reamers designed to do the average reaming job. The figure of .005”, in particular, seems excessive, and in the interest of economy the back taper should be kept as low as possible without interfering with the required hole size and finish.
REAMING OF TAPERED HOLES
The reaming of tapered holes has always presented a problem in reamer design and operation because of the very nature of the work to be performed. Usually one starts out with a drilled or bored hole of uniform diameter throughout. The diameter of this hole is slightly smaller than the finished diameter of the small end of the tapered hole to be produced.
The operation of taper reaming now consists of greatly enlarging one end of this hole, with this enlargement gradually decreasing towards the other end. This means that the Taper Reamer, instead of being a finishing tool, in reality becomes a tool for heavy stock removal, and further, that this tool at the finish of the operation is engaged in the cut throughout its length.
Because of the large amount of stock to be removed and the length of the cut, taper reamers are subjected to much greater torsional strains than the ordinary straight reamer that cuts on the end only. The tendency, therefore, is to chatter, with consequent poor finish or actual destruction of the reamer. The greater the taper, the worse these conditions will become.
Taper Reamers must, therefore, be constructed as sturdily as possible. Cutting edges must be adequately backed up, and flutes must be sufficiently large, with well rounded bottoms.
For reaming tapered holes with larger tapers, better results are obtained by first rough reaming the taper, followed by a finish taper ream to merely size and smooth the hole. In general, roughing reamers are made with fewer number of flutes than the finish reamers, and are of different construction as to hand of helix and degree of spiral. Quite often it is also necessary to use chip breakers ground in the cutting edges of the roughing reamer, usually in the form of a coarse pitch square thread. These may be particularly necessary when the tapered hole is quite long and the taper relatively small. As an added precaution, the number of flutes in a finishing reamer should never be an even multiple of the number of flues in the roughing reamer nor the same number of flutes.
For machine reaming, it has been found that a reamer having few flutes, and with cutting edges at a large spiral angle with the axis, is by far the best. Flutes may vary in number from two to five in the ordinary range of reamer sizes.
The spiral angle should be about 45 degrees and of a hand opposite to the direction of rotation. A greater spiral angle than this does not tend to improve the quality of the cut, but does materially increase the amount of end pressure required to feel the reamer into the work.
It should be realized, however, that reamers with a high left-hand spiral and right-hand cut tend to crowd the chips ahead of themselves. For this reason this design is not usually satisfactory for blind holes. For such holes, roughing reamers are often made of flat construction with little or no spiral angle.
Occasionally when the job conditions are such that a great deal of difficulty is experienced with chatter, or the holes are definitely out of round even though not chattered, it is necessary to employ a flute construction not generally used. This particular reamer construction embodies the use of an uneven or even number of flutes coupled with irregular flute spacing of the type that no two cutting edges are diametrically opposite each other. The reamers thereby are very difficult to measure for size and taper and their use, therefore, is to be avoided unless absolutely necessary.
It should be remembered that high-spiral Taper Reamers cannot be used on hand operations because of the end pressure required, but for machine operations this is by far outweighed by smoothness of operation, better quality of work, and longer reamer life. For hand operation, Taper Reamers with straight flutes, or with about ten degrees of spiral opposite in direction to that of the cut, are recommended.
End Mill cuts a variety of materials
The following points require consideration when using the suggested Weldon cutting speeds:
Work piece material class and hardness must be known before selecting cutting speed.
Hard materials necessitate low cutting speeds.
Highly alloyed materials containing nickel, chromium, cobalt, molybdenum, tungsten, and such, necessitate low cutting speeds even though they are not considered hard.
If experience with work material is lacking, it is best to start at the lowest cutting speed shown for that material and gradually increase speed until best results are obtained.
The range of cutting speed for each class of material is quite broad to allow for variables in each individual situation that could be compensated for by more or less cutting speed.
Longer tool life is obtained by operating in the lower cutting speeds with a generous feedrate per tooth when the diameter and length of end mill and set-up in general will permit.
If the selection of spindle speeds on the milling machine is so limited that it is impossible to run the end mill within the suggested range, then use whatever speed is available below the lowest in the range to prevent damage to the end mill.
When chips and end mill become discolored from heat, it is an indication of excessive cutting speed, which will result in premature damage to the tool.
When cutting edges dull rapidly without tool or chip discoloration, it is an indication of a highly abrasive work piece or high resistance to chip separation, and cutting speed must be reduced.
Maximum end mill life cannot be realized if erratic hand-feeding methods are used that cause a variation in tooth load and which invariably leads to tooth corner chipping or breakage at tool shank. Controlled power feed is recommended.
Climb cutting where and when possible will permit cutting speeds in the higher ranges and will also extend end mill life.
Higher production rates are possible with multiple-flute end mills since feed per tooth is multiplied by the number of teeth enabling a proportional increase in the feedrate.
OPERATING DATA FOR MACHINE REAMERS
*The 1/32” stock removal allowance for reaming the Brass, Bronze, and Aluminum groups may be reduced to 1/64” drilled holes rather than cores holes. The Cast Iron group do not include the alloyed Cast Irons, as these require 10% to 20% slower speeds.
The most efficient cutting speed for machine reaming depends on the type of material being reamed, the amount of stock to be removed, the tool material being used, the finish required, and the rigidity of the setup. A good starting point, when machine reaming, is to use one-third to one-half of the cutting speed used for drilling the same materials. Table H-5 may be used as a guide.
Where conditions permit the use of carbide reamers, the speeds may often be increased over those recommended for HSS (high-speed steel) reamers. The limiting factor is usually an absence of rigidity in the setup. Any chatter, which is often caused by too high a speed, is likely to chip the cutting edges of a carbide reamer. Always select a speed that is slow enough to eliminate chatter. Close tolerances and fine finishes often require the use of considerably lower speeds than those recommended in Table H-5. The very high surface speeds which have been found necessary for turning operations with carbide tools present some difficulties if applied to reamers. In lathe work, the cutting tool may usually be made of any desired cross-section and held in a large rigid tool-post with minimum overhang. In reaming, however, hole size limits cross-section of the tool, and usually considerable overhang is required. Both result in a loss of the rigidity so necessary with very high surface speeds if chatter is to be avoided.
As already emphasized, reamers do not work well when they chatter. Carbide Tipped Reamers in particular cannot stand even a momentary chatter at the start of a hole, as such a vibration is likely to chip the cutting edges.
Consequently, the primary consideration in selecting a speed is to STAY LOW ENOUGH TO ELIMINATE ALL CHATTER.
TABLE H-5 Reaming Speeds
Feeds in reaming are usually two to three times greater than those used for drilling. The amount of feed may vary with different materials, but a good starting point would be between .0015 and .004 inches per revolution. Too low a feed may “glaze” the hole, which has the result of work hardening the material, causing occasional chatter and excessive wear on the reamer. Too high a feed tends to reduce the accuracy of the hole and the quality of the surface finish.
Generally, it is best to use as high a feed as possible to produce the required finish and accuracy.
When a drill press that has only a hand feed is used to ream a hole, the feed rate should be estimated just as it would be for drilling. About twice the feed rate should be used for reaming as would be used for drilling in the same setup when hand feeding. In reaming with carbide tipped reamers, too low a feed may result in glazing, excessive wear, and in some cases, chatter. Feeds should always be high enough to apply sufficient chip loads on the cutting edges. The amount of feed may vary with the material, but a good starting point would be between .0015” and .003” per flute per revolution. This may be increased considerably on some types of work and the upper limit is usually determined by the finish required.
Carbide Reamers work best when well supported with a minimum of overhang. Any lack of rigidity in the set-up may result in a momentary vibration which may cause flaking and chipping of the carbide tips
Recommended Stock Removal
.003 to .005
Over 1/16 to 1/8 Incl .004 to .008
Over 1/8 to 1/4 Incl Over 1/4 to 3/8 Incl Over 3/8 to 1/2 Incl Over 1/2 to 3/4 Incl
.006 to .012
.008 to .014
.010 to .015
.012 to .018
Rockwell C 50 or harder Rockwell C 30 to 50
Feed in inches per revolution
.002 to .004
.004 to .008
Cast Iron & Malleable Iron .005 to .010
.05 to .012
Speed in surface feet per minute
Steel (All Types)
Rockwell C 60 or harder 8-20
Rockwell C 50 to 60 15-30
Rockwell C 40 to 50 20-40
Rockwell C 30 to 40 60-90
Under Rockwell C 30 50-85
Cast Iron and Malleable Iron
Aluminum, Brass, Bronze, Copper, Fibre, Plastic, Hard Rubber, etc 90-175
Steels harder than Rockwell C 50 Light Oil
Steels softer than Rockwell C 50 Light Oil for Good finishes or Soluble Oil and Water
Cast Iron and Malleable Iron Soluble Oil and Water
Non Ferrous Materials Soluble Oil and Water
The stock removal allowance should be sufficient to assure a good cutting action of the reamer. Too small a stock allowance results in burnishing (a slipping or polishing action), or wedges the reamer in the hole causing excessive wear or breakage of the reamer. The condition of the hole before reaming also has an influence on the reaming allowance since a rough hole will need a greater amount of stock removed than an equal size hole with a fairly smooth finish. Removal of too much stock by remaining often causes oversize and rough holes. Oversize holes, walls of holes roughened with grooves at or beyond the finished diameter, bellmouthed holes, or out-of-round holes are common causes of reamer failure. In improperly prepared holes the reamer has a tendency to wedge in the hole rather than machine it. The result is severe reamer wear and possible breakage. See Table H-6 for commonly used stock allowance for reaming. When materials that work harden readily are reamed, it is especially important to have adequate material for reaming. For hand reaming, stock allowances are much smaller, partly because of the difficulty in forcing the reamer through greater stock. A common allowance is .001” to .003”.
TABLE H-6 Stock Allowance For Machine Reaming
Technical Reaming Hints
Stock to be Removed – For the same reason, insufficient stock for reaming may result in a burnishing rather than a cutting action. It is difficult to generalize on this phase as it is tied in closely with the type of material, feed, finish required, depth of hole, and chip capacity of the reamer. For machine reaming, .010” on a ¼” hole, .015” on a ½” hole, up to .025” on a 1 ½” hole, seems a good s starting point. For hand reaming, stoke allowances are much smaller, partly because of the d difficulty in forcing the reamer through greater stock. A common allowance is .001” to .003”.
Alignment – In the ideal reaming job, the spindle, reamer, bushing, and hole to be machined are all in perfect alignment. Any variation from this tends to increase reamer wear and detracts from the accuracy of the hole. Tapered, oversize, or bell-mouthed holes should call for a check of alignment. Sometimes the bad effects of misalignment can be reduced though the use of floating or adjustable holders. Quite often if the user will grind a slight back taper on the reamer it will also be of help in overcoming the effects of misalignment.
Chatter – The presence of chatter while reaming has a very bad effect on reamer life and on the finish in the hole. Chatter may be the result of one of several causes, some of which are listed:
1) Excessive speed
2) Too much clearance on reamer
3) Lack of rigidity in jig or machine
4) Insecure holding of work
5) Excessive overhang of reamer or spindle
6) Excessive looseness in floating holder
7) Too light a feed
Carbide tipped reamers especially cannot tolerate even a momentary chatter at the start of a hole, as such a vibration is likely to chip the cutting edges. The remedy is to eliminate uneven motion and loose fits. Corrections may be made by reducing the speed, increasing the feed, putting a chamfer on the hole before reaming, using a reamer with a pilot or reducing the clearance angle on the cutting edge of the reamer.
Coolant – In reaming, the emphasis is usually on finish and a coolant is normally chosen for this purpose rather than for cooling. Quite often this means a change from that recommended for drilling.
Reamer Regrinding – In obtaining maximum economy from reamers, the same principles apply as in the case of most other cutting tools. One of these principles is not to allow a tool to become too dull. It is best practice to regind the chamfer on a reamer long before it exhibits excessive wear or refuses to cut. This sharpening is usually restricted to the starting taper or chamfer. It can be done on almost any tool and cutter grinder. Care must be taken so that each flute is ground exactly even, or the tool is likely to cut oversize. Sharpening the chamfer on a reamer by hand is not recommended because it is practically impossible to keep the cutting edges even.
Reamers are often required to produce holes that are in parallel alignment, at exact distances from location points on the work, and free from the bellmouth at either end. The jigs and fixtures needed for such accurate options must hold the work securely and incorporate bushings to guide the reamers properly. For long holes it is preferable to guide the reamer at both ends; for short holes usually the reamer is guided at one end, the entering side of the hole.
When the misalignment between the reamer and the hole to be machined amounts to about 30 percent or more of the stock to be removed, there is often wear on the reamer in the form of a thread. The lead is equal to the feed, and depth is equal to the misalignment. The thread appears to be more readily produced when reaming tough and abrasive materials. When reaming other materials, only excessive marginal wear may be noticed.
Consistently close tolerance reaming cannot be expected unless the following conditions are provided:
1. A center drill to completely spot a new center which will materially help in obtaining a true hole in the drilling operation.
2. The included angle of the center drill must be less than that of the drill to follow. This centers the drill up and materially helps to cut a concentric hole.
3. Care should be exercised to assure the equal length and identical angles of the drill’s cutting edges. A reamer cannot follow a hole that runs out and be expected to cut to size.
4. Spiral fluted reamers with narrow lands usually provide the best finish.
5. Reamers should be mounted in a floating holder to most consistently cut to size.
6. The length and angle of all cutting edges of a reamer should be identical if a reamer is expected to cut to close tolerances.
REAMING CHECK CHART TROUBLE CORRECTION
Cutting edges Burn
Check spindle speed. It may be too fast. Check cutting fluid as it must also be
good coolant. You may be using too rich a mixture and need paraffin to thin out, which is helpful in carrying off heat.
Cutting edges wear badly or dull rapidly
Check cutting fluid as it may be too rich in sulphur base oil and needs to be thinned out. Sulphur is abrasive and if your mixture is heavy, it will wear away cutting edge rapidly. Reamer should not be rotated backward to remove from hole. Either pass reamer through hole or withdraw without stopping forward rotation.
Hole cuts eccentric
Check chamfer. It must be concentric with all flutes. A poor start means a poor job. Check alignment of work with tool. Misalignment may be due to poor work-holding fixtures. Fine chips and incorrect setting will also cause this trouble. Try using a “floating holder.”
If you know reamer is sharp and correctly ground and your cutting fluids are satisfactory, reduce spindle speed.
Check the lands, if using a straight fluted reamer. They may be too wide and are rubbing, which causes chatter. Also sometimes caused by dull reamer, or drilled hole too large which does not let reamer get a good bite. There is less tendency to chatter with spiral fluted reamers. Check rigidity of tool holder; try small chamfer at start of hole.
Work glazes or burnishes
This occurs most when reaming 18-8 types. Reamer is not biting in deep enough to get good cut. Acts like letting a drill dwell and work-hardens surface of steel. Deeper bite will usually correct this fault.
Tool marks in finished reamed hole
Reamer was ground with too coarse a wheel. Use a finer grit, free-cutting grinding wheel; being careful not to burn edges of a reamer. It is characteristic for tools to leave the pattern of the grinding wheel on the part.
Be sure clearance and rake angles fall within certain limits. If so, they will not bind. Wide lands or insufficient back-off angle can also cause binding.
Nick in flutes
This comes from careless handling and storage when not in use. Handle then as carefully as the reamer manufacturer does when he ships them back to you. Store in individual boxes or racks with separations. Remember, the cutting edge is always vulnerable. Reamers should be well covered with oil when not in use, as a small rust spot on the cutting edge will start a pit or nick.
Selection Of Reamers
Factors that should influence the selection of reamers for a given job can be enumerated as:
1. Material to be reamed
2. Diameter of hole
3. Amount of stock to be removed
4. Accuracy and finish desired
5. First cost
6. Maintenance costs
7. Salvage value
The material to be reamed has much to do with selecting the best type of reamer. It is evident that if the material is free cutting, reamers of fairly light construction can be used to produce holes of satisfactory quality. But, if the material is hard, tough, or stringy in texture, adequate provisions must be made to meet these conditions. The power required to drive reamers through given materials must be compensated for in their design, so that no undue deflection will take place.
The amount of stock to be removed has a direct influence on the necessary driving force and in turn on the strength and rigidity the reamers must possess. While the driving force is not necessarily proportional to the amount of stock to be removed (as the frictional load is practically constant), the above statement is generally true.
While it is possible to produce reamed holes having a good finish, but poor accuracy, it may be said that, in general, accuracy and finish go together. Accuracy, in this case, takes into account tolerances on diameter, roundness, straightness, and absence of bell-mouth at ends of holes. To meet all of these conditions, it is necessary to use reamers with proper and adequate support for cutting edges. Solid Reamers should be selected for holes up to sizes for which the reamer is large enough in diameter to accommodate an arbor hole and yet leave sufficient wall thickness to support fully the cutting edges against deflection under load.
The salvage value of Solid Type Reamers will depend largely on the products that are manufactured and on the variety of sizes and holes. In some shops it is possible to regrind reamers that are worn undersize for a smaller size hole, where they can again be used with full wear life. This can only be done if the difference in sizes comes within the practical regrinding range.
In general, right-hand helical reamers cut slightly more freely than straight-fluted or left-hand helical tools, and because of their chip clearing ability have a slight advantage in the reaming of blind holes. However, in screw machine work, if the blind holes are not too deep, left-hand helical tools work satisfactorily.
For all applications it is essential that reamers be kept sharp.
Causes Of Breakage Or Excessive Wear Of Reamers
1. Dirt or burrs in spindle or socket in which reamer is held
2. Misalignment of two or more parts of the set-up. This condition can cause a bell-mouthed hole plus excessive wear of reamer margins.
3. Lack of chip space in set-up or flutes of the reamer
4. Too fast or too slow speeds
5. Too much or too little feed
6. Wrong type of coolant
7. No lubricant between guide bushing and reamer
8. Lack of lubricant
9. Bottoming in blind holes
10. Lack of sufficient stock to ream
11. Too much stock to ream
12. Entering work too fast
13. Badly drilled holes – too rough, tapered, or bell-mouthed. Bell-mouthed holes may cause the reamer to wedge rather than cut.
14. Faulty sharpening, which may consist of: improperly ground relief on cutting edges; grinding cracks from too fast and heavy grinding; unbalanced sharpening of cutting edges, causing one or two flutes of the reamer to take the entire cutting load; saw-tooth cutting edges from too coarse a grind; and incorrect end-cutting angle.
15. Poor handling of reamer
16. Oversize or undersize bushings
17. Chattering of reamer
18. Lack of rigidity in machine or work holder
19. Improperly designed reamer for the job
Other Reaming Problems
Oversized holes can be caused by inadequate work piece support, worn guide bushings, worn or loose spindle bearings, or bent reamer shanks. When reamers gradually start cutting larger holes, it may be because of the work material galling or forming a built-up edge on reamer cutting surfaces (Figure H-128). Mild steel and some aluminum alloys are particularly troublesome in this area. Changing to a different coolant may help. Reamers with highly polished flutes, margins, and relief angles or reamers that have special surface treatment may also improve the cutting action.
Bellmouthed holes are caused by misalignment of the reamer with the hole. The use of accurate bushings or pilots may correct bellmouth, but in many cases the only solution is the use of floating holders. A floating holder will allow movement in some directions while restricting it in others. A poor finish can be improved by decreasing the feed, but this will also increase the wear and shorten the life of the reamer. A worn reamer will never leave a good surface finish as it will score or groove the finish and often produce a tapered hole.
Too fast a feed will cause a reamer to break. Too large a stock allowance for finish reaming will produce a large volume of chips with heat buildup, and will result in a poor hole finish. Too small a stock allowance will cause the reamer teeth to rub as they cut, and not cut freely, which will produce a poor finish and cause rapid reamer wear. Cutting fluid applied in insufficient quantity may also cause rough surface finishes when reaming.
In reaming, the emphasis is usually on finish, and a coolant is chosen for this rather than for cooling. This may mean a change from the drilling coolant, but in general this list will be satisfactory.
Aluminum and its alloys: Soluble oil, kerosene and lard oil compounds, light non-viscous neutral oil, kerosene and soluble oil mixtures
Brass: Dry, soluble oil, kerosene and lard oil compounds, light non-viscous neutral oil Copper: Soluble oil, winterained lard oil, oleic acid compounds
Cast Iron: Dry or with a jet of compressed air for a cooling medium Malleable Iron: Soluble oil, non-viscous neutral oil
Monel Metal: Soluble oil, sulfurized mineral oil
Steel, ordinary: Soluble oil, sulfurized oil, high-EP mineral oil
Steel, very hard and refractory: Soluble oil, sulfurized oil, turpentine Steel, stainless: Soluble oil, sulfurized mineral oil
Wrought Iron: Soluble oil, sulfurized oil, mineral-animal oil compound
Cast iron is reamed dry, frequently with an air blast for cooling and blowing away the chips, but nearly all other materials require a suitable coolant. A water-soluble oil is satisfactory for most steels, malleable iron, brass, bronze, and aluminum. For fine finishes and close size control, a sulfurized and chlorinated oil-based cutting fluid is preferable. For heat-treated steels, a harder stainless steels, and the more difficult to machine of the nonferrous alloys, a chlorinated and sulfurized cutting oil may be required. These are oil-based cutting fluids and are consequently more expensive than the water-based soluble oils, but the usage is sometimes necessitated by the characteristics of the metal being reamed. Detailed information on cutting fluids and an applications chart may be found in Chapter 2.
To ream a hole to a high degree of surface finish, a cutting fluid is needed. A good cutting fluid will cool the work piece and tool and will also act as a lubricant between the chip and the tool to reduce friction and hear buildup. Cutting fluids should be applied in sufficient volume to flush the chips away. Table H-7 lists some cutting fluids used for reaming different materials.
TABLE H-7 Cutting Fluids Used for Reaming