Acceptable tolerances on router bits?

Question

I had having tear out problems with my box joint jig and decided to blame it on the craftsman 1/2" straight bit that came free with my router table.

I bought a MLCS 1/2" spiral bit. It worked great, giving me no tear out on the same wood that was a problem before. But when I went to assemble the box, it wouldn't go together, the cuts were too small. In desperation I recut it with the old bit and it went together fine. I was very surprised at that, figuring that the spacing would be off and I would just have a pile of scrap, but sometimes you get lucky.

Anyhow, I measured the cuts from the craftsman at 0.504" and the MLCS as

0.496". Presumably my router has no run out at all. (the spacing on the jig is 0.490").

I can accept that the craftsman is oversized, a bit extra for sharpening maybe; but is 4 thousanths undersized acceptable for the MLCS. Okay, it was real expensive, but it wasn't exactly cheap either.

Is this something to complain about, or an acceptable tolerance?

Bruce · Accepted Answer

On Fri, 12 Nov 2004 20:28:31 -0700, toller wrote (in article ):

What type of wood? Spiral bits give more of a shearing cut which can produce a smoother surface. Of course they do cost more than straight bits.

It would be best to measure the bit since wood can have "spring back" after a cut, but in general router bits that don't use carbide cutters brazed to a shank (probably like your spiral bit) should have accuracy close to mill bits. I'd call it an inspection error if the bit was more than 0.001" out, but I wouldn't be bothered by 0.004" if the bit was inexpensive. I seriously doubt any of your bits we made over sized to allow for sharpening, but it would be handy if you used the bit alot ;^)

I've dealt with MLCS before, but never had to work with their customer service. I'd wager they would correct the problem for you.

-Bruce

Routerman P. Warner · Answer

It is wide spread practice; wide target tolerance of bit specs except for shank diameters. What to expect: Nothing, measure all critical cutters. And to be sure, the high stress trapped cutting of box joints will bend the cutter, (less so at shallow depths). So even tho a cutter measures .500" for example, its cutter pathway can easily be .505-.510 or more! Practice on scrap first to learn the working nature of the boogers before you commit to project stock.

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Acceptable tolerances on router bits?

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