Actual size of the #102 endmill

[TDA (John Torrez)]

Quick preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.

@jwr

I’ve been doing some test cuts with the #102 endmill trying to check the dimensional accuracy of the Nomad and was surprised to find that my cuts were quite a bit off (by 0.05mm-0.07mm).

I then did some cuts using a 2mm endmill and these were spot on, down to what I can actually measure (0.01mm).

So I took my calipers to the #102 and it turned out that it isn’t really a 1/8" endmill (3.175mm), it’s closer to 3.12mm.

I’m having trouble wrapping my head around how that is possible and how nobody notices? Is that just the one endmill that I have? What does your #102 measure?

@Moded1952

I realized I’d never actually looked at the tolerances:

• DIXI Polytool specifies ISO e8 (-0.020/-0.038mm)
• Harvey tool specifies +0.00/-0.04mm
• Precise Bits specifies +0.00/-0.05mm

This is for a number of reasons. First, is that we are dealing with centerless ground blanks to make tooling from. These start out smaller than the actual shank size (about 0.0002" / 0.005mm). Then you have to add margin drop, grind flutes, an edge, and possibly add reliefs. So even if you had perfect blanks, grinding machines, and wheels the tool would be smaller after you add the minimum steps to turn it into a cutting tool.

What that functionally means is that there is a looser tolerance on any tool that has a cutting diameter the same as the shank. In many cases this is also why you see a +0,-X spec on those tools and a ± on all the tools less than the shank size.

There is a way to solve this problem by starting with a larger blank then post grinding the shank to the correct diameter. However, this adds a LOT of cost and can make the tool less accurate as the shank you were using to grind the tool is being modified and can lead to tool runout.

@Moded1952

You can’t accurately measure the cutting diameter of an endmill with calipers. Calipers can be used to measure the diameter of cylindrical objects but the geometry of an endmill is too complex.

In machine shops, they generally use an optical comparator for a task like this.

For us, the best way to measure it is to do test cuts (e.g. mill a square pillar) with high radial engagement but very very low axial engagement and see how far off it is. High radial and low axial in order to reduce deflection.

@Luke

I would add that measuring a end mill with callipers won’t give you an accurate measurement. As I type this @Moded1952 just chipped in with some good info.

@jwr

I do realize that it’s difficult to measure and endmill with calipers: it took me a while (and many tries) to get a result. You can get quite close by rotating the endmill while it’s in the calipers and trying to find the size that stops it from rotating.

Not only is it not possible to get the actual cutting diameter this way (margin drop, relief, etc) but very easy to damage the tool without realizing it. One of the big issues is that to get the tool to not rub you grind away material behind the edge. This means that the only way to measure it with calipers “accurately” is to measure on the edge… the weakest part of the tool. You WILL damage the tool measuring it this way. Might not be enough to see or make a difference in the next few cuts but it will make that part of the edge wear faster as it is now fatter from dulling or chipping it.

Hope that helps to explain some of the issues. Let me know if there’s something I can help with.