 | New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
| New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
Basic Cutter Styles and Their Application - ( «Back )
( Which Cutter to Choose?) - screen [ 1 | 2 | 3 ]
Cost and Use Considerations
When form-relieved cutters are ordered for the first time, a form tool charge is made. This tool is required to cut the shape in the cutter blank during the back-off operation. For this primary reason and other minor factors, form-relieved cutters are generally more expensive than profile-ground cutters of the same size.Because of the rather complicated methods of backing off form-relieved cutters, the cutting geometry is somewhat restricted. This condition frequently demands lower speeds and feeds than could be applied to profile-ground cutters on similar cuts.
The majority of form-relieved cutters are made with unground form. This means that the shape is backed off with a special form tool while the HSS is still in the annealed condition (Fig. 58). After the shape is finished, the cutter is hardened and only the sides of the cutter and the hole are ground.
Since HSS is a fairly stable material, the amount of heat treatment distortion can be estimated within close limits, and the hardened cutter can be finished within a few thousandths or less of the desired shape. If greater accuracy is necessary, the form-relieved cutters may be form-ground for closer tolerances.
Form-relieved cutters are generally not recommended for straight sided cuts because cutting clearance or relief is in a radial direction and there is no axial or side clearance. Lack of clearance results in galling of the cutter and workpiece. Frequently the workpiece may be inclined so that the straight sided condition is avoided (Fig. 60). When this can be done, the application of a form-relieved cutter becomes feasible. An effort should be made so that the angular cutting surface is at least 10 degrees from the perpendicular to provide satisfactory cutting action with a form-relieved cutter.
Side Clearance Possible
When a straight sided condition cannot be avoided and a form-relieved cutter is still desirable, it is possible to manufacture the cutter with form-relieved side clearance. In this case the complete form of the cutter is shifted sideways while the form is being backed off and cutting clearance is generated on the side of the form (Fig. 61 ).If this condition exists on both sides of the cutter, side clearance can be applied from both directions. When this is done, however, the tooth thickness of the cutter will become narrower as the cutter is sharpened and therefore reduces one of the primary advantages of a form-relieved cutter. The reduced tooth thickness may sometimes be overcome by making the cutters in interlocking halves and then running as a set. As the cutters are sharpened, the two halves may be shimmed apart to obtain the original cutting dimension.
Side clearance on form-relieved cutters is common practice, but generally does not produce cutting conditions equal to the side cutting action of a profile-ground cutter. Due to this fact, side cutting form-relieved cutters are not recommended to cut materials which work harden or produce 'stringy' chips.
When To Use Which
Although form-relieved cutters have some limitations with regard to tool geometry, cutting qualities, and lower feeds, they definitely have their place, and cutter designers should make use of them when intricate forms are to be produced.While there are no hard and fast rules for deciding when to use profile-ground or form-relieved cutters, some generalizations can be made. To assist tooling engineers, a list of applications and conditions for proper selection of cutters follows above. Where both styles are designated, the selection should be made with regard to conditions shown at the bottom of this table or special requirements of the workpiece. If a particular job causes continued difficulty, another style of cutter may be a better choice with a small change in the part or fixture. The main idea is to understand the basic cutting action and design the cutter as simply as possible.
