Measuring Router Bit Diameter

Advice on how to verify whether tooling diameter is correct within proper tolerances. January 12, 2015

Question
I ordered some new bits and the diameter as best I can measure is not the called out diameter. I am getting 3.72 for a 3/8 bit. What is the best tool to measure router bit diameter. I am using a Digital Caliper. I would hope out of the box it would be closer.

Forum Responses
(CNC Forum)
From Brian Personett, forum technical advisor:
You should be getting .375, I think you're getting .372. You're talking about 3 thousandths of an inch. I seldom get bits that are spot on, but my vendor typically supplies bits that are consistent in OD. Since I typically use spiral bits I chalk it up to my imprecise measuring method or the calipers are off a smidge. I know they're consistent because I never have to jack with the OD in my tool tables.



From Contributor H:
Are you talking about a solid carbide bit with a 3/8 shank? If so, the carbide rod is .375 +.0000 -.0005. So when the tool is made, the diameter must come under the diameter some to clean up the cutting edge.


From Contributor B:
If it is a 3 flute bit then you are not going to be able to get a measurement using calipers, but if it is a 1 or 2 flute calipers will work. If I am unsure of the diameter of the tool I put a piece of 1/4" scrap on the table and bore a hole through it and then measure.


From contributor F:
One easy way to measure is to cut a panel from MDF that is exactly 10.000x10.000 square on your router and measure it. If it is not 10x10 you know how much offset to add or subtract. If youíre working with wood you will never find the .002 or .003 in your construction, cabinetmaking is just not that close and other construction factors will cause much larger changes in dimensions. Run the tools and move on it isnít a milling machine.


From contributor M:
The best way to measure diameter on helical tooling is to use a V-block with an indicator. Set a zero off of the shank or round portion of tool then measure cutting edge by rolling the tool in the v-block. The difference doubled minus the shank size will be your diameter. It works great for three flute tools as well. As Contributor H mentioned, new tools will/should be slightly under the shank size. We control ours with a tolerance so they will never be more than -.003".