HSM May/June 1988 - The Surface grinder (Part4)

[carrdo]

A basic question as to why one should have as sharp cutting tools as is possible to produce, specifically end mills in this case.

I pride myself on being able to do precision machine work in a home shop environment.

With a new commercially produced brand name end mill with the equipment I have, I can produce work that is square, parallel, flat and straight to +- 0.0005" over 3-4 inches having an excellent surface finish but with an end mill that has been used and still relatively sharp, I can only manage 0.002" - 0.003" over the same distance with a less than fine surface finish as the end mill will no longer make those very fine finishing cuts due to the slight dulling of the cutting edge which causes the cutter to start to ride over the surface of the work rather than cleanly shearing it.


Back to the topic at hand. What is end relief on an end mill? See the attached photo of an almost new 6 flute larger non center cutting end mill which has the end relief built in at the time of manufacture by the manufacturer. The end relief is the circular depression seen in the center of the end mill. It is the reason this type of end mill is called a non center cutting end mill as the cutting lips on the end mill do not extend all of the way to the center of the cutter. 99 % of the work I (we) do can use this type of end mill as it will mill a perfectly flat and straight surface. Center relief also makes it easier to sharpen the cutting lips on a surface grinder with the standard commercially available end mill sharpening fixtures and a straight grinding wheel when the cutter gets dull. But a some point, if we continue to sharpen the end of of the end mill, this center relief will disappear - then what?

What a non center cutting end mill cannot do is plunge end mill unless we first driil a hole which is larger than the circular end relief built into the cutter (use this type of end mill to spot face for example or to counterbore. Actually, a dedicated and piloted spot facing or counterboring tool will do the job much better than an end mill but often the budget does not allow for this).

Center cutting end mills have the cutting lips which extend all of the way to the center of the end mill and have no end relief. See the photo of a brand name nearly new two flute center cutting end mill. When new, they will plunge mill effectively. However, they are much more difficult to sharpen as one has to grind each cutting lip exactly to the center of the cutter and to be effective the cutting lips at the center have to meet as a fine line (for a two flute cutter) or as a point (with three or more flutes). Next to impossible to do in a home shop without having a dedicated tool and cutter grinder in good condition and having a ton of accessories.

Next, we will delve into the cutting tooth geometry on an end mill which is really important.
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