Router Bit Accuracy

Mike Wingate

When is a 3/8" bit not a 3/8" bit. I made my first box using my Incra Ultra jig and an MLCS 3/8" dia 1/2" shank bit today. No big problems, considering it was a new home made table, first outing for the Jessem Router Raiser and the Incra, both of which I had lying unused for a number of years. The MDF box was good, the plywood box (both 1/4" thickness material) was a bit tight to fit. I measured the bit and it was 12 thou undersized, and had never been used before, another unused purchase (set of 8 for Incra from MLCS) of mine. I replaced the bit with a well used Trend 3/8" dia, 1/4" shank bit, did a test set and it was a precision fit, and only 2 thou undersized. I realise that I can tweak the jig. I realise that the Trend bit cost 3 times more than the MLCS, and an oversized bit will not cut fitting joints. This is the first time in 25 years that I have noticed this discrepancy. Probably as I was using a very precise jig set-up. Lesson learned - test and measure bits before using and make allowances. I am and always have been pleased with Mlcs products. The ability to make a box in 2 minutes was exciting as I did both boxes in front of my students at school. One set had spent a minimum of 10 hours making ther box frames with hand cut finger joints. Their teacher had got them to make an aluminium template, and they had FILED the excess wood away to fit the sides together.

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dutchman 46

Hello Mike, Thanks for your post. There are some bits that are not as precise as others, I had heard that there are some that are metric, and also, plywood is not exact anymore either . You almost goo to town with the micrometer in your back pocket

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RustyW

Welcome to the world of Incra Mike. I had the same problem when I used mine the first time. I had three 3/8" bits that all cut different. The Incra book said that most high quality bits would be slightly oversized and should be used. So I called Whiteside and asked what size groove would be cut with their 3/8 bit. They said it wouldn't be over or under, it would be 3/8. So I picked one up locally (under $20) and it cut a groove that measured .375,and the joints fit perfect.

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AxlMyk

I just had the same experience with a box I'm making. Incra jig and a Freud 3/8" upcut spiral. The fit was very tight so I filed the fingers a little on the end pieces. The fit was excellent after that. I measured all the fingers and spaces with my calipers and they were exact before filing. They just needed a hammer to get them to fit together.
I didn't try other bits to see how they worked.

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Mike Wingate

I have measured some of the straight router bits at home and they are all spot on. So it is just the one 3/8" 1/2" shank MLCS bit that is well off. Just a slip up. No real complaints when the bits average $6 each! Good value.

Mike

Did you guys make a test cut on scrap before making your finished pieces? This is one of the first lessons Bob and Rick teach... to check the fit before committing to your good project wood. This also brings up an interesting point: The Incra Jig depends on the bit size being accurate for good results. The Oak Park box joint jig would of handled the off sized bits with no problem since the cuts are determined by the bit.

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Mike Wingate

I spend more time checking things with my digital vernier, than making. Even the 3/8" plywood came up short. Paper makes a good spacer for adding thickness.

Mike Wingate

I bought a 1/2" shanked 3/8" cutter from Trend. Perfect, got a 1/2" x 1/2" at the same time.

dustmaker

Was it Reagan who used to say, "Trust but verify."?

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