Reamer Applications
Reaming Applications
The primary question for a 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, ground, or lapped?
The conditions under which reamers are used and the results that are desired vary widely. Some variables which affect reamer cutting action are:
- Speeds and Feeds
- Use of guiding bushings
- Material
- Condition of the machine
- Rigidity of set-up
- Rake of cutting edges
- Reliefs on reamer
- Amount of stock to be removed
- Finish required
- Tolerance of the hole
However, there are general principles to keep in mind.
Maintenance and Salvage
Any inexpensive tool may seem economical at first in terms of initial cost, but one should ask which is the most inexpensive in overall cost. Taper reamers can be reground many times before its size is lost.
With solid straight reamers, the only way to salvage is to regrind it to smaller size (a condition not always convenient) or to convert it into some other type of tool.
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.
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.
Uneven Motions
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.
Methods of 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.
Using Reamers In Lathes and Hand/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.
Reamer Guiding
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.
Reaming
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.
Chatter
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.
Reamer binds
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.
PROBLEMS
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