Box Joint are too tight - help!

[Jim Broestler]

Trying to finish up Christmas gifts and I'm having a problem. Making some canisters using box joints, but the fit is really tight - have to use a rubber mallet to put them together, and they still won't quite seat all the way. Here's the set up I'm using:

Incra jig (original), mounted on a 3/4 piece of MDF, held by two C-clamps to my router table top. 1/4" spiral upcut bit to make 1/4" box joints. Material is 4" wide ash, 1/4" thick. I move the fence in exact 1/4" increments. All fingers seem to be equally tight fitting, so I've ruled out discrepancies in body position and hand movement as I move the pieces along the fence using the right angle jig that comes with the Incra set.

I've checked the bit with a set of calipers, and it's 1/4" dead on. I'm wondering it it's the type of bit I'm using, but spiral up-cut seems to be the type mostly commonly used for this operation. I'd really much rather do this on the router table than the TS just because of the precision factor, but I'm actually wondering if that's part of the problem: too little tolerance. I don't have time to order Rockler's box joint jig for the router table. Basically I want to stick with the method I'm using, but need to figure out why it's not 'working like the video'. Thanks.

[roman fedyk]

When I make box joints on my Akeda jig I use straight bits and they work fine. My Leigh also provides straight bits for this purpose. I guess you should try those as well.

[Jim Broestler]

Yeah, I was a bit worried about tear out, but with the backer board it should be okay. Hate to change the set up, but no big deal really. I'll see how that works and let you know.

[roman fedyk]

If you use a 1/4" straight bit you probably won't have to change the set up...

[Anthony Whitesell]

How wide are the fingers? How wide do your other spiral bits measure versus their stated size? It definately sounds like "too much precision" or the router bit is just a hair undersized.

[Jim Broestler]

tried a 1/4" straight bit and it works great! Imagine that - a cheap Skil bit ends up working better than a pricier Amana. Maybe it's just the type rather than the brand.

[Jim Broestler]

just the router height. no big whoop, though.

[roman fedyk]

Good deal.....now those gifts can get out on time....do you need my address??;)

[Jim Broestler]

For only $25 + S&H, I'd be more than happy to send you a set! Thanks for the tip. Looks like I'll be buying a straight bit tomorrow. The Skil bit probably won't be able to handle the 1,100+ cuts I'll have to make.

[Gary Chester]

Could you adjust the fence a couple of thousands and run the pieces thru again? That would open the joint just a bit...

[Jim Broestler]

Problem is it's an Incra jig. Only adjusts in 1/32 increments. I could use a conventional fence, but that costs me the precision that makes the router table preferrable to me over the TS jig method. The bit change seems to have solved the problem with resorting to a lot of trial and error fiddling. If it was only one or two boxes, probably wouldn't matter. I'm making 18. That's 1,152 cuts!

[roman fedyk]

If you can find Whiteside locally, they are excellent bits.

[Rod Sheridan]

Trying to finish up Christmas gifts and I'm having a problem. Making some canisters using box joints, but the fit is really tight - have to use a rubber mallet to put them together, and they still won't quite seat all the way. Here's the set up I'm using:

Incra jig (original), mounted on a 3/4 piece of MDF, held by two C-clamps to my router table top. 1/4" spiral upcut bit to make 1/4" box joints. Material is 4" wide ash, 1/4" thick. I move the fence in exact 1/4" increments. All fingers seem to be equally tight fitting, so I've ruled out discrepancies in body position and hand movement as I move the pieces along the fence using the right angle jig that comes with the Incra set.

I've checked the bit with a set of calipers, and it's 1/4" dead on. I'm wondering it it's the type of bit I'm using, but spiral up-cut seems to be the type mostly commonly used for this operation. I'd really much rather do this on the router table than the TS just because of the precision factor, but I'm actually wondering if that's part of the problem: too little tolerance. I don't have time to order Rockler's box joint jig for the router table. Basically I want to stick with the method I'm using, but need to figure out why it's not 'working like the video'. Thanks.

Hi Jim, I make box joints on a table saw, and the jig has an adjustment to vary the spacing of the fingers.

For example if I'm making 1/4" box joints, I set the dado blade (bit size) to 1/4", and adjust the jig so that it cuts a finger every .248".

(OK, that's a guess, however by cutting the fingers less than .250" apart, it means that the fingers are very slightly smaller than the slots they fit into.

That way when I put a PVA glue on them, and they swell, they're still hand tight.

Regards, Rod.

[Anthony Whitesell]

How big does the straight bit measure?

[Ken Higginbotham]

I have found that the fit of the box joints are a function of the distance between the key and the blade. In other words moving the key closer to the blade will make the fit tighter and moving it away from the blade make the fit looser.

[Myk Rian]

I have found that the fit of the box joints are a function of the distance between the key and the blade.
He's using an Incra jig on a router table.

Jim;
I use the exact same setup to do box joints with a Freud upcut bit. I find that most times they are very tight and I have to persuade them with a mallet. I have also used a file to open them up a little because I didn't want the wood to split.
If you have a micrometer or accurate caliper, measure the bit. It might be 2 or 3 thousandths small.

[Phillip Bogle]

I might be way off, but if you have used the bit a considerable amount or it has been sharpened or honed it may not have the "correct cut." The actual cutting diameter will change over time. On most applications the change is not worth the worry, but in a very critical fit like box joints or dovetails you will notice. The other problem I have here is the humidity or ambient air qualities. You can get just enough moisture in the air or a temp change that makes a joint not fit. I cut some joints for a jewelry box in the evening, and the next morning they would not fit. One whole 1/32" expansion across a 12" board is not much, but the interlocking dovetails did not fit.

Just some ideas.

[Alan Schaffter]

A couple of potential causes:

1. Bit is dull or undersized. Don't measure the bit- that doesn't matter, measure the width of the slot it cuts.

2. Once you make the cut the Ash fibers may be springing back, especially if the bit is dull. Nature of the beast, and if the set up is as precise as the INCRA will allow, will be a tight fit regardless. Make sure the spaces are thoroughly clean of chips and dust.

3. Slow down the speed of cut (feed speed) and make bi-directional passes

4. Is a dry fit as tight as during the glue up? When you apply glue the fingers will swell to some degree and you likely have too many fingers to do it fast.

There is hope on the horizon- but not for many months.

[Tony Bilello]

If your Incra jig is that accurate (I have never used one), then it is impossible to make an exact 1/4" peg fit into an exact 1/4" hole. Since most things are not exact, one would have to be slightly larger than the other even if only by a few thousandths of an inch. This is where luck comes into play. A few thousandths thicker in the finger and it wont fit. A few thousandths wider on the slot and a perfect fit. With box joints, generally it's the spacing that is easier to adjust than the thickness of the wood. Wood doesnt remain the same size from day to day or hour to hour if the climate changes, so adjustments will have to be made.
You can make a very accurate adjustable box joint jig for your table saw and a Dado blade in a few hours.
http://www.sawmillcreek.org/album.php?albumid=140

BTW, these photos were made after the fact so ignore the narrow kerf blade and let your mind insert a dado blade into the photo. A little Zen practice here.

[Alan Schaffter]

The one problem with Tony's and most adjustable box joint jigs that use replaceable pins and is intended for use on the tablesaw is the width of your dado may change from setup to setup depending on how careful you are and much you tighten the arbor nut.

Help is on the way.

[Ken Higginbotham]

He's using an Incra jig on a router table.

Is there no way to adjust the distance to the bit?

[Myk Rian]

Is there no way to adjust the distance to the bit?
When I initially set my Incra up, I adjust the cut for exactly in the center of the stock, with the jig set at one of the inch marks on the ruler. I cut once, turn the piece around, and cut again, adjusting the fence until the second cut is right on with the first. After that the ruler on the unit takes over.
The Incra jig gives very accurate cuts. Closer than any I have made with a keyed system.

I have a rousseau insert in the table and it has the slight hump on the inserts that many complain about. I make sure to clamp the stock to the push block with it sitting on a flat portion of the table. That way, when it rides over the insert hump, every cut is exactly the same depth. It works out very nicely.

[Alan Schaffter]

It must be the bit is undersized and not exactly 1/4" since the INCRA is very precise and the tightness is uniform and not a result of cumulative jig error.

The INCRA jig allows very precise and snug box and dovetail joints:

Joint cut on a router with an INCRA:

http://www.ncwoodworker.net/pp/data/500/medium/IMG_3025.jpg

[David Prince]

Could you use a heat gun to "over" dry the wood a little bit and give yourself a little bit of "play" in the wood? When it cools enough, glue it up and let it dry and acclimate back into a tight fit. I wouldn't think it would split the joints because they are pretty close now with persuasion. At least this could possibly save this project and let you deal with correcting your bits and jig for another time and not have to redo these.

[Alan Schaffter]

Judging by number he has to make, I suspect he needs a good fit without having to mess with it.

[Floyd Mah]

Here's a solution that I read in a machinist's website. You can make a bit slightly oversized by using a hardened surface, like a dowel pin, and drawing it over the cutting edges of the bit. Obviously you have to take care not to cut your fingers off, but by slightly deforming the thin cutting edge outward, you can make the bit cut slightly oversized. I guess that there are limits to how much pressure that you can apply before fracturing the edge, but it may be worth a try if you can't find the proper size bit.

[Myk Rian]

You can make a bit slightly oversized by using a hardened surface, like a dowel pin, and drawing it over the cutting edges of the bit.
Probably won't work on carbide, but will on HSS.

[Ken Higginbotham]

When I initially set my Incra up, I adjust the cut for exactly in the center of the stock, with the jig set at one of the inch marks on the ruler. I cut once, turn the piece around, and cut again, adjusting the fence until the second cut is right on with the first. After that the ruler on the unit takes over.
The Incra jig gives very accurate cuts. Closer than any I have made with a keyed system.

I have a rousseau insert in the table and it has the slight hump on the inserts that many complain about. I make sure to clamp the stock to the push block with it sitting on a flat portion of the table. That way, when it rides over the insert hump, every cut is exactly the same depth. It works out very nicely.

I looked through a couple of web sites and I think I understand how the Incra jig works. Seems you do a cut then move the guide to a mark on a ruler - ?

[Myk Rian]

I looked through a couple of web sites and I think I understand how the Incra jig works. Seems you do a cut then move the guide to a mark on a ruler - ?
Yes. For a 1/4" bit you move the fence 1/2". 3/4" for a 3/8" bit, etc.
When doing the other sides/ends you start with the fence offset 1/4" for a 1/4" bit. Then continue moving it 1/2" for each cut.

[Ken Higginbotham]

Yes. For a 1/4" bit you move the fence 1/2". 3/4" for a 3/8" bit, etc.
When doing the other sides/ends you start with the fence offset 1/4" for a 1/4" bit. Then continue moving it 1/2" for each cut.

So if your bit is a little under-sized, which would result in a tighter joint, could you dial-out a few .001's each time with the micro-adjust to compensate?

[Alan Schaffter]

So if your bit is a little under-sized, which would result in a tighter joint, could you dial-out a few .001's each time with the micro-adjust to compensate?

OP says he has the original INCRA jig- I don't think that has the micro adjust like the new INCRA LS systems, only the 1/32" indexing notches.

[Ken Higginbotham]

OP says he has the original INCRA jig- I don't think that has the micro adjust like the new INCRA LS systems, only the 1/32" indexing notches.

Oh. Hum....

Wonder if he could put in a couple post-it notes each time as shims...

Just brain-storming, sorry if it seems I'm beating a dead horse :)

[Myk Rian]

So if your bit is a little under-sized, which would result in a tighter joint, could you dial-out a few .001's each time with the micro-adjust to compensate?
No. The bit needs to be right on, or slightly oversized.
An undersized 6.5mm bit might work well.

[Alan Schaffter]

Oh. Hum....

Wonder if he could put in a couple post-it notes each time as shims...

Just brain-storming, sorry if it seems I'm beating a dead horse :)

That wouldn't work either because the whole board is moved each time via the jig. The only way it would work is if he needed 2 post-it shims for the first notch, he would need 4 for the second, 6 the third- you get the idea, since the INCRA increments are fixed. Unless, you mount and unmount the jig after each notch- that cancels any advantage of having a jig. The real answer is a bit of the correct or slightly larger size.

[Ken Higginbotham]

I see. I guess I'm hung up on self made jigs. And if you have to set it up everytime what good is the jig. Somebody mentioned a slightly oversize bit, that seems the right way to go...

[Myk Rian]

And if you have to set it up everytime what good is the jig.
You have to set ANY jig up.
It's the most accurate way to make box joints that I've ever used. It really isn't hard to understand how to use the Incra jig.

[Jon Amsden]

I hate to suggest this, because I hate the stuff, but I feel 24h epoxy is the best stuff to use when assembling box joints. Unlike waterbased glues it wont swell the wood fibers, and will actually lubricate them. Your clamps will be tied up for quite a bit of time though.

[Myk Rian]

I put the joints together just enough to hold the pieces, and use a small brush to put Titebond 3 in the openings. Then I tap them together and am good to go. Never have had one loosen up.