roughing end mill nominal vs actual diameter

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petertha

H-M Supporter - Gold Member
H-M Supporter Gold Member
I discovered something as I was making a rather intricate part the other day. Thankfully I did not botch it, but it was skinny close. Turns out a 1/4" (=0.250") diameter fine tooth roughing end mill (Asian) measures 0.255-0.256" in actual diameter. I was doing work on a rotary table where I was offsetting by a radius using DRO. I gave myself what thought was a healthy allowance to clean up to final dimension with a regular end mill after the rougher. Only to discover the regular end mill was cutting air because it was 'on spec' at 0.250" +/- 0.001" I haven't gone back to check my other rougher EM's but might explain a few past mysteries I had chocked up to setup.

Is it reasonable to expect roughing end mills to be within similar diameter tolerance as regular end mills, or maybe that comes with brand name tooling? I just naively assumed they would be pretty close to reference size.

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Always measure your endmill before using it. I learned this early in my CNC hard knocks education. :) HSS are normally pretty close to nominal size, carbides tend to run a bit small, and many roughers are a bit oversize as you found out.
 
Finish endills are usually very close to their nominal size. After sharpening, they will be somewhat smaller. Roughing endmills vary in size. As others have stated, standard procedure is to drop down one size and make multiple passes to sneak-up on your final dimension.

It's good that you've discovered roughing bits. It's like getting 25% more free horsepower from your machine. Let them do the dirty work (which they do very well) and switch to a finish cutter for the last 10 thou.

Ray
 
Roughing mills have different standards than close tolerance (keyway) finish mills. The typical ANSI/ASME B94.19-1997 standard for a small diameter rougher is +.0025 to - .005. The typical ANSI/ASME B94.19-1997 standard for a small keyway finish mill is -.00015 to +.0000. A standard finish mill like a roughing mill can be as much as .005" oversized. Having said that there are different standards for single and double ended mills. In addition there are different standards for larger diameter (over 1.500") and long, and extra long mills.

Keep in mind these are the ANSI/ASME standards. There is no way to know whether a manufacturer is adhering to these standards unless they somehow identify that they do either on the box, the tool, or in their advertising brochures. This is one of the main reasons I try to purchase tooling made in this country as opposed to off shore. Every US based company I have purchased from uses these standards. I have no Idea whether or not off shore companies even attempt to adhere to these standards. I do know the one time I purchased a number of 3/8" ball end mills made off shore they weren't even close. They were returned after a single attempt to use one.
 
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I had to go back & check. I don't want to throw the company under the bus. OTOH, I know how to measure a diameter with a micrometer & it is what it is, ISO label notwithstanding. The moral of the story is don't assume, measure.

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