# Incra LS loose box joint problem



## PhilBa (Sep 25, 2014)

Sorry this is so long but I want to be as complete as possible.

I have an Incra LS25. It's a joy to use however I have run into a problem making 3/8" box joints with one of their templates. The fingers are fairly loose.

Prior to this I've made a number of box joints in Baltic Birch plywood with the same template using a cheap Chinese 3/8" straight bit and they all came out perfect. Exactly tight enough for a firm dry fit that would stay in place and take just a bit of force to pull apart. Pretty much a classic box joint. Glue ups were perfect with no gaps showing. 

So, I went to make the same joint with Maple and Walnut using a Freud 12-110 3/8" double flute straight bit but the fingers are loose. Using a micrometer, the slots are .381-.383" and the fingers are around .369. So there is a bit more than 1/100" gap which means a somewhat loose joint. I could get several thicknesses of paper in the gap. All the fingers appear to be exactly the same amount off so even with the incorrect width, it is very precise and repeatable.

My first though was the Freud bit is too wide but it measured out at exactly .375". Then I checked the Chinese 3/8" bit and it measured .371". Also, the Chinese bit in BB left a rougher finish than the Freud did in hardwood (beautiful clean crisp cuts).

This leads me to believe that there must be some slop in my LS setup and that the "return" stoke when cutting a slot must be shaving the extra width off. Because the Chinese cutter is undersized, it compensates for the problem (though the rougher finish probably helped a bit too). I can make the LS flex a tiny bit if I push it hard enough but that is a lot more force than I use to make a given cut when sliding the right angle fixture along the fence. I looked at the right angle fixture and it was tightened down pretty well. I tightened it to the point were it wouldn't slide and then backed off just enough for it to slide.

Any suggestions? I know I could keep using the Chinese bit but it's not that well made and leaves a rougher cut. I would prefer to have the higher quality cut from the Freud bit. I was thinking about how to avoid the return stroke but that seems problematic - wouldn't lifting the right angle fixture off the fence each time introduce more slop? Should I stop the router each time for the return? That seems like a lot of wear on the router. Is it possible the entire unit is flexing a tiny amount that I can't see? Should I try clamping the fence down to the table?

I glued up the box and it came out OK but I can see and feel the slight gaps that indicate a lack of perfection.

Any LS users out there seen this problem? There's a similar thread about dovetails but that was fixed by changing the DT bit height and that's not going make a difference here.


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## graycat (Oct 14, 2013)

Contact INCRA. They are very helpful.


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## Nickp (Dec 4, 2012)

Sounds like you're on the right track with the sloppy return...although it may be the combination of the harder wood and better bit that might be making it flex. The bit may be biting better adding to some side pressure while cutting. Try more router speed and/or slower feed rate...


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## 4042 (Sep 30, 2004)

Please post any suggestions Inca gives you. I have not had that problem but have not made that many boxes. Thanks


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## PhilBa (Sep 25, 2014)

Thanks Nick, I was running the router (PC7518) at top speed.

I had already contacted Incra but wanted to get board wisdom as well. Incra got back to me this morning and suggest that bit runout is often a problem. And sometimes a combination chuck and bit runout adds up. That seems kind of high runout for PC and Freud (I buy that stuff because it's supposed to be high quality) but I'll do a single pass cut and measure the results. If it's runout, they said to turn the bit 90 degrees to cancel each other's runout (though I would have thought 180).

If it's not runout, I'm thinking I need to improve my use of the right angle fixture. Incra did mention that as possibility (but said it was less likely).


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## Shadowrider (Apr 1, 2015)

Incra beat me to it. But if you turn the bit 180 in theory it's just going to the other side. Since it spins in a circle you get no change. If it gets worse after spinning it 90 degrees, then turn it 180 and should get better.


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## MorrisonCharles (Feb 13, 2009)

Do you have the right template installed. I use the Incra BOXG #7. I have the same bit and the same router and I get no gap.


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## PhilBa (Sep 25, 2014)

Problem solved. Operator error, sorta. It was the bit but not exactly what was suggested. I was able to reproduce the wide slot (.381") with a single pass after cleaning the shank, the collet and the router threads. But, no matter how I turned the bit, it still came up .381. Since runout was the problem, I looked a bit closer at how I was using the bit. It turns out the Freud bit has a line on the shank that says "Min insertion". See the picture. I had been inserting the bit in the collet a little past that but it was about 1/8" shy of the end of the collet. Then, I tried the old "bottom it out and pull it back just a smidge". Bingo, .375" slots on the money. Min insertion is not correct.


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