Choosing The Best Reamer
Factors that should influence the choice of reamers for a given job can be enumerated as:
- Material to be reamed
- Diameter of hole
- Amount of stock to be removed
- Accuracy and finish desired
- First cost
- Maintenance costs
- 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.
Thus, shell-type reamers can be used in comparatively small diameters for cutting free machining materials, but for hard or tough materials solid-type reamers are to be recommended even for holes of fairly large size.
No definite range of sizes can be given here, because of the widely varying conditions that surround different jobs. It should be kept in mind, however, that when holes are to be finished by reaming to very close tolerances, minimum deflection is permissible in the reamer itself.
Theoretically, a shell-reamer arbor should fully support the reamer so that it will not deflect, but in practice this is not always the case, because of the wear of the arbor or the presence of burrs, dirt, or other foreign materials. On production jobs where more or less unskilled operators are used, these factors must be taken into account.
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.
It is obvious, as diameters of reamers increase, the first cost of solid-type, or even of Shell-Type Reamers with integral cutting edges, will eventually become prohibitive. The Inserted Blade and Adjustable Type of reamer then is available. However, in changing to the latter type, the matter of maintenance costs to provide continuous rigidity and accuracy must be closely studied. Experience has shown that, unless reconditioning is carefully and competently done, the first and replacement cost advantages of these reamers are lost.
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.
The choice between a straight fluted and a helical fluted reamer is largely a personal one. Theoretically, a helical reamer is less likely to chatter than is a straight-fluted tool, but this is not always borne out in practice, because of the multiplicity of other factors involved.
In general, right-hand helical reamers cut slightly more freely than straight-fluted or left-hand helical tools, and due to 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.