Bit/mill sizes - maybe not as advertised

Some time ago, there was a post describing how to check the actual angles of V bits. Also, some time ago, Bill Korn and I found that some 1/8" mills that we were using on a joint project were not really .125" in diameter. Some were closer to .118".

Frequently, on this forum, there are questions about how accurate the Mill is. Users create gcode for a 4" square for example and find that when they mill it out, it is not precisely 4". Many Mill owners are looking for accuracy down to .001" or better, which, with wood that moves with humidity and stress relief is a tall order at the best of times. Combine that with bits/mills that may not be precisely as advertised and it’s pretty much impossible to achieve.

Today, for giggles, I tested 3 different 1/4" end mills - a down cut, an up cut and a single flute. I simply fired up the router and used the jog keys in gSender to route two grooves with each mill - one in X and one in Y. None of the grooves was precisely .25" wide. They ranged from .235" to .246".

I just thought that I would throw this out there for anyone seeing that they are not getting the precision that they would like from the Mill. When Bill and I were doing our project, we both simply entered the correct mill diameters into our respective CAM applications and things worked out.

I assume that can be done in any CAM application, but I can only speak for VCarvePro and Fusion.


@gwilki - Ha Ha Grant. I’ve been telling you we are sharing the same brain waves. One of my sons-in-law and I were cleaning up my garage/workshop today and I was showing him my bit colletseum. I also made the statement that bits weren’t always accurate in measurement. So I picked up a 1/4" upcut and it measured at .248. Then I tried a 2 flute straight bit and it measured at .246. I’m beginning to think there are very few that are 100% accurate. I do know I will be going back through and remeasuring all my bits. Thanks for throwing that info back out there.

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@Heyward43 I knew that I was channelling someone.
Long before starting up with CNC, I would drill a 1/4" hole for example with three different bits to find one into which a 1/4" dowel would fit snugly. I don’t buy cheap drill bits, but as you say, and as we both have learned, I don’t believe anything other than laboratory-spec is spot on. It really does not make a heck of a lot of difference in “ordinary” woodworking. The normal expansion and contraction of wood has far more influence on joinery than the plus or minus a few thousandths of an inch in drill bits. However, in the CNC world a couple of thounsandths can really throw some things off.

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Thats why I measure relevant faces after a first finishing toolpath. If there is still to much material, start a new finishing toolpath with negative allowance.
Usually the allowance depends on the bit, but also on material, and if wood, also on the direction of the grain.
Think there is no 1-click-to-100%accuracy when cnc-milling wood.

Btw. undersized drill bits are common in metal working industriy, since the hole is always larger than the drill bit.

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@JHahn Tks, Jannik. Your process clearly will work, but more often than not for my jobs, there is no finishing tool path. I am either cutting pockets or cutting what Vectric refers to as profile tool paths. Once the piece is cut out, it is too late to learn that the bit size was incorrect. By entering an accurate diameter for the bit in my CAM software tool database, I avoid the problem.

As you say, accuracy still depends on the wood itself, in addition to feeds and speeds that can introduce inaccuracies.

I have to admit, that I have no clue about Vectric. But if the bits are undersized (all of these you addressed), then an additional finishing toolpath should always be possible, isnt it?
So first do a pocket operation (maybe with some positive allowance) to rough out the material and afterwards do a profile-operation to finish the surfaces. But… as I said, I dont know if this is possible in Vectric.

However, measuring the actual diameter of the bits is done easy and fast and will increase the accuracy as you mentioned :+1:t2:.

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@JHahn Pretty much anything is possible in VCarve. I think that it’s just a matter of terminology. Roughing and finishing are terms uses in VCarve for 3D carving. Pocketing is just that, and profile is actually cutting the part out of the work piece. (That said, one can use a profile tool path and a V-bit, for example, to cut grooves on a line.) I could go on and on to add to the confusion, but you get the point.
I guess for me, measuring my bit diameter by one means or another is preferable to using a bit incorrectly-categorized, then using offsets to try to fix the error.
However, I think that we are agreed that one way or another, if we want to strive for .001" accuracy, accurately knowing the diameter of our mills/bits is absolutely necessary.

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The majority of the end mills are built with metric 3mm= .118

Definitely a good point about wood, and repeatability. I am very spoiled and work with a few high dollar cnc’s for a living.(Homag’s and Felder’s, I’m definitely bragging a little) Hardwoods will measure differently by a few thou depending on different board density. Point being even with the best cnc, perfect repeatability is difficult at best with real woods.

@Lawrence Welcome to the group, Lawrence.I agree that some end mills are sold as metric 3mm and 6mm, for example. However, depending on the manufacturer, many are sold as imperial. I don’t believe this is a case of a US manufacturer making 3mm end mills and marketing them as 1/8". Certainly, my email exchanges with Amanda and Whiteside do not indicate that. YMMV, though. :grinning:

So as a n00b, if I wanted to measure the bits that are coming with my Mill, or that I buy later, what is the proper way to measure them? I use digital calipers, so if someone can explain the positioning, or technique, I would high appreciate it. Hell - even a pic of you doing it would work.


@Grimmy2016 Welcome to the group, Scott. The members here are the best - all willing to answer questions and help in any way they can.

You will see the way that I measure end mills in the first post in this thread. I find that it’s difficult to measure the mill itself because I have trouble making sure that the caliper is perfectly perpendicular to the mill when it is measuring spiral flutes. Measuring a cut made by the mill is simpler and more accurate, IMHO.

Thanks @gwilki I totally missed you just measured the groove and not the bit itself. I will definitely go that route. Cant wait to get it all setup and dialed in.