Router bit diameter tolerance - Router Forums
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post #1 of 6 (permalink) Old 03-12-2017, 06:33 PM Thread Starter
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Default Router bit diameter tolerance

Today I needed to do some router table work and found that I didn't have a 1/2" shank with 1/4" cutting diameter to put in some slots. So, off to the box stores and find they really don't have much of anything in 1/2" shank. The blue one had a set of 3 bits with 1/2" shank and one of them was 1/4" cutting diameter. Set it up and cut the first slot. To check it I used a brass gauge bar. It wouldn't even start to fit in the slot. Took calipers and measured the slot at .020". That is with whatever run-out is in the router! That is 20% undersized on a 1/4" bit.

Is that normal for a lower end bit? (The whole set of 3 was $60.)

I use Whiteside and Amana up-cut bits in my CNC and always establish the true diameter of every bit by cutting a rectangle and measuring it. More often than not they are 0.002" to 0.004" under size. On a CNC it is easy to enter the true cutting diameter.

I'm going to have the bit checked at a machine shop this week that has the right equipment and then contact the vendor.

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post #2 of 6 (permalink) Old 03-12-2017, 06:41 PM
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off shore bits are notorious for this...
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post #3 of 6 (permalink) Old 03-12-2017, 07:57 PM
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Shanks can be a problem too. I got a cheap set on sale from Canadian Tire which are guaranteed to be Asian and I couldn't get one to go in the collet. I've heard of others that were so loose in the collet that it wouldn't grip them tight enough. Just a few days ago a new member mentioned checking an Irwin, which I'm also certain is of Asian origin, and the cutting edge was tapered from top to bottom. I have a couple of old Taiwanese ones that the carbide is only about as thick as a dime.

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post #4 of 6 (permalink) Old 03-13-2017, 07:20 AM
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You mentioned a "set of 3" (Rockler?). Undersized plywood bits maybe?
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post #5 of 6 (permalink) Old 03-13-2017, 07:41 AM
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I as you sir, have learned to look twice and buy once. I, for what it is worth, do like the Freud bits. I have to order on line as we have no Woodworking stores here. I too like the Whiteside bits. I order from Woodcraft and found their products to be good. Like I said, I have to order on line.

Good luck.
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post #6 of 6 (permalink) Old 03-13-2017, 10:45 AM Thread Starter
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Originally Posted by FreeTime View Post
You mentioned a "set of 3" (Rockler?). Undersized plywood bits maybe?
Not Rockler, Lowes.

But you are a genius though, looked up the part number on Lowes site and it says "Creates cutting diameter to match thickness of 1/4-in, 1/2-in, and 3/4-in plywood" which explains the under size. Looking closer on the rear of packaging in fine print it lists the 1/4" cutting diameter as 7/32". There is a blurb on the front about "Plywood Mortising Set" in one of the three languages.

On the front prominent part of packaging it has 1/4", 1/2" and 3/4" plainly marked. Not the bits fault, just weak information by Bosch packaging people. Cut was great, just not dimensional what I expected or need. I'm sure they are of value to those who do plywood mortises. I don't do plywood mortises so they will be returned today. RBS0245SXW

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