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?
By subdividing the problem into these several groups, the answer is more or less automatic.
Material to ream
With the modern materials from which reamers can be made, including the cobalt high-speed steels and sintered carbide tipping, it is a reasonably safe statement that practically all metals can be reamed economically and accurately including heat-treated steel up to 415 Brinell hardness.
The decision, therefore, as to which materials to ream rests more on the problem of which process of finishing is the more economical under the existing circumstances.
Reamers are mass-production tools
Present-day precision in high-speed manufacturing is very largely dependent upon the highly developed types of the plain everyday reamer. Tiny single- and two-fluted stubs are used in automatics at tremendous speeds. Sturdy, adjustable, carbide-tipped models are employed in massive single-purpose machines to turn out work on a tonnage basis. Line reaming of the main bearings in automobile and truck crankcases is going on hourly, and many of the precision tools with which the modern plant operates are made with jig boring reamers cutting to tolerances of .0002 inches. Reaming fits into almost any type of manufacture. The question is usually one of economic procedure: How does it work into the sequence of operations?
The “adjustable” is usually of the inserted-blade type adapted to the finest grade of precision work and differs from the “expansion” type. The latter, though adjustable within a narrow range, is not always dependable for accurate work, relying as it does upon an expansion bolt to spring the cutting edges. The taper pin reamer is differentiated from other taper reamers in that it serves a very particular purpose and is widely used. Rose reamers have wide lips and deep grooves at the entering end, making them especially adaptable to rough reaming operations and reasonably heavy cuts. They have no radial relief and will ream remarkably straight, although the hole is not so smooth as with other reamers. Each of the titles of the other tools seems to be self-explanatory. There are a number of reamer manufacturers who have developed their own varieties of specialty reamers, many of which are remarkable and serve their particular fields admirably. These can be procured in excellent standardized sets, in assemblies of cutters and bars, and in specially designed arrangements for a broad assortment of mass-production tooling. It would be well to be acquainted with most of the catalogues of such tools.
Threaded-end hand reamers are not included in the listing, having become practically obsolete. They are not made as a standard item but are occasionally found useful. The threads are generally 16-or 18-pitch with sharp V-crests on the first or entering series of threads. There are six or eight threads in all, the point diameter being from 0.008 inches less that the nominal diameter for ½ inch reamers to 0.020 inches less than nominal on large reamers.
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.
Unless unavoidable all straight reamers should have helical flutes and all tapered ones should have spiral flutes. The only exceptions should be roughing reamers for extremely coarse or heavy work in straight reaming and steep tapers requiring stepped tools. The angle of helix need not be heavy to accomplish its purpose, and it may be either left or right hand on straight reamers but must always be left hand on taper reamers. There is more about this later. Helical fluted reamers are mandatory for use in holes having keyways, splines, oil passages, or surfaces interrupted by a considerable number of cross holes.
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. Taper boring is preferable to taper reaming. Carbide-tipped reamers improve the finish and last much 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.
Tolerance and Quality
These two go hand-in-hand, as hole size includes roundness and absence of taper both of which are elements of quality. The reamer must be of correct size to meet the required tolerance; the cutting edges must be adequately supported to maintain size consistently; and the body must be solid enough to preclude deflection.
Maintenance and Salvage
Any cheap tool may seem economical in first cost, but the question is, “Which is the cheapest in the overall cost?” In designing a special reamer the engineer has to consider maintenance costs. These usually indicate the adoption of inserted-blade types, as solid reamers wear quickly and have little salvage value – at least for straight hole work. Taper reamers, of course, can be reground many times before size is lost, but on straight holes the adjustable type is the more economical. 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.
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.
Scarcely enough can be said regarding the advantages and applications of line reaming. From the diagram, it will be seen how the pilots in the detain C can be utilized to support the reamer in guide bushings. This type of tool is used in countless ways and is invaluable when precision and true alignment are necessary. It may be of either the solid or the shell style, as long as 4 or 5 ft. and occasionally even longer. For long setups, the best practice is to use adjustable shell reamers with straight holes to fit the reaming arbor. On short setups solid reamers are used for the smaller sizes, but the adjustable type ought to be specified on all jobs where size will permit.
Straight-line reaming calls for special reamers; and as long as the designer sticks to the correct specifications for cutting-edge details coupled with good common sense regarding the arbor and hole diameters in the bodies of his shell reamers, he will have little or no difficulty. It is in the jobs which seem to be simple that we most often run into difficulty, and that is usually due to false ideas on economy. If the workpiece sketched in Fig. 15-15 is taken as an example, it may be assumed that a refutted twist drill with a square end would be sufficient to counterbore the A diameter and that a common stock reamer would do for the B diameter, provided, of course, each is properly guided in jig bushings. In fact, the idea has been pointed out previously in the book that the refitting of regular tools to suit special purposes in an excellent form of economy.
This idea, however, has its limitations and does not apply in a job like this because a twist drill has too little body to support it in a bushing. The better way would be to design a special end-cutting mill as at D. This will have full cylindrical bearing in the jig bushing and hold to true alignment. If followed by a special reamer E, the line reaming operation will be done correctly, which would not be the case if a stock reamer were used. Stock reamers usually have recesses between the body and the shank, as indicated by dotted lines. In this project the recess would be in the bushing when the reamer started in the cut at F, leaving it in supported though the gap C.
Tool engineers are expected to convert stock tools to special work, but this is an example of the type of work on which it cannot be done. It is mandatory that when quality of product demands precision tooling, it shall be supplied regardless of first cost. In the end, this proves to be true economy. The principal fact to remember is that in line reaming the tool must be fully supported at all times. To cover this requirement calls for careful study as to lengths and distances, arrangements of pilots, and position of bushings.
Oil feed reamers
All kinds of automatics and nearly all chucking, screw, and single-purpose machines call for a generous oil supply. This is frequently provided by using the type of reamer that has an oil hole through the solid metal of the body and shank, called an “oil feed” reamer. These require suitable oil connections in the machine, and such connections must be considered by the designer when drawing the tool. Except for the need of extra back taper (about twice the normal amount) and the use of a floating holder, no other special attention is demanded. Either high-speed or carbide tipping is satisfactory.
These may be distinguished from lining reamers in that the former are generally used in fixtures by means of hardening and ground bushings whereas the latter may be used directly within the work without fixtures or bushings. Also, lining reamers do not always have pilots in advance of the first cutting portion.
The diameters of pilots should always be made as large as practical to give rigidity. As the type is special, it may be either solid or shell type, the same economies prevailing as for other types.
Although many designers specify pilots in advance of the cut, it is better to have pilot shanks instead. Reamers properly designed with pilot shanks are fully as rugged, cheaper to buy and cheaper to use, and much cleaner to operate. Furthermore, if wear strips are included in the design, the whole setup is better. Wear strips may be of special bronze or tool steel inserts, carbide, or cast alloys such as Stellite, any of which are replaceable when worn or may be shimmed up for regrinding. Extension sleeves are sometimes used instead of pilot shanks and serve the same purpose. Bu utilizing extension sleeves for a pilot shank the designer can accomplish his purpose and still use a standard reamer.
Step, multiple-diameter, and similar reamers are included in this group, which is both broad and varied. Basically a “step” reamer would be as shown in Fig 15-17. This is valuable when reaming holes that have an interrupted surface due to cross holes, ports, or recesses. In such work the chips are apt to rest crossways on the edges of the interruptions and case scoring. By breaking up the operation as a step reamer does the chips are finer and wash away easily with the cutting fluid, saving much spoilage.
Step reamers are made with two or more reaming diameters, each of which cuts its individual portion and assures true alignment. The details of design are the same as for single reamers, the only extra care necessary being to arrange the relative positions according to work requirements.
In providing reamers for tapered holes, the designer is always faced with a tough problem. For gradual tapers, ¾ in. per foot or less, production jobs are usually handled by reaming directly from a straight drilled, bored, or punched hole, and this places a severe cutting operation on the reamers. Steeper tapers are rough-machined prior to reaming, but the chips accumulate rapidly and the end thrust is high. Therefore, any kind of taper requires careful attention to all design details, and at times it is necessary to use a lot of ingenuity before all the needs are met.
Some jobs require inspection with bluing gages to show 75 percent bearing or better, no small task. Therefore, the reamers must be very accurately made – often they are ground to fit a tapered ring gage – and they must be maintained with equal accuracy. The tool sharpener must be supplied with gages that are fully as exact as those used by the inspector.
Taper reamers are almost invariably cleared to the cutting edge. A concentric margin – even only a hairline – is unsatisfactory in a tool of this kind. Left-hand spirals flutes, varying from 5 to 45 degrees angle from the axis, are used because straight flutes cause serious chatter. Hand reamers may have comparatively low spiral angles. Machine reamers require fewer flutes and steeper spiral than hand reamers.
Duplex spiral taper reamers are particularly well adapted to precision finishing; and whenever the extra cost is justified, the designer will find a ready solution to some of his problems through their use. For this reason some of the highlights are reviewed here. They have flutes arranged at two or more different angles of spiral and will ream with greater ease and less liability to chatter than any other type of taper reamer. They are highly efficient on both roughing and finishing operations. If used in holes that have been bored reasonably close to size and taper, they will finish without the need of a roughing reamer.
One of the most perplexing jobs is the reaming of long taper pin holes, due to the small diameter and small taper (1/4 in. per foot). Straight-fluted reamers are practically useless except on comparatively short holes – not over four times the diameter at the small end. Ordinarily spiral reamers work reasonably well in free-machining materials. Where the work is tough, “helical” reamers, similar to Fig. 15-22, are most satisfactory. For general use the regular stock items are the best to choose. For special applications it may be necessary to procure special reamers with pilot shanks and operate through a guide bushing. Frequently a series of two or more reamers are needed for a single hole having precision tolerances, even though guided by long bushings. In such instances it is wise to consult the manufacturer for specification details.
There is a considerable difference if opinion as to the relative merits of right-hand helix angles for straight reamers. If you have never tried he right-hand helix, the best suggestion is to choose your job well and do so. The old idea that the hand or direction must be opposite that of the cut,
i.e. left-hand helix for right-hand cutting, is erroneous. Production reaming with right-hand helix fluting is being done in straight holes with increasing volume and an improvement in quality. The main problem is to be sure that there is not too much stock to be removed from the hole, that the alignment between work and tool is correct, that there is no lost motion (back lash) in the tool spindle, and that the work is firmly held.
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. When properly set up as explained in the preceding paragraph, a
right-hand helix is ideal for straight holes, and remarkably smooth holes will result. 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 designer should keep all these details in mind when planning for reaming in aluminum.
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.