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post #1 of 11 Old 04-20-2013, 11:43 AM Thread Starter
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Box Joints - Please Help!

I'm working on a boat box for a friend, and my original plan to dovetail went south when I realized the box was larger than my 12"- and accessed the numerous jig-building instructions, all of which said the box joints will be perfect. Not so! My joints are fine until I get about 13" from the first cut, and the joint spacing goes haywire. I cannot figure out what's causing that. I'm using 1/2" plywood that is not giving me any tearout or delamination problems. Any helpful suggestions from long-time box joint experts?

Last edited by jerryholcombe; 04-20-2013 at 11:53 AM. Reason: Needed to give more info.
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post #2 of 11 Old 04-20-2013, 12:55 PM
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Quote:
Originally Posted by jerryholcombe View Post
I'm working on a boat box for a friend, and my original plan to dovetail went south when I realized the box was larger than my 12"- and accessed the numerous jig-building instructions, all of which said the box joints will be perfect. Not so! My joints are fine until I get about 13" from the first cut, and the joint spacing goes haywire. I cannot figure out what's causing that. I'm using 1/2" plywood that is not giving me any tearout or delamination problems. Any helpful suggestions from long-time box joint experts?
What jig are you using/made?
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post #3 of 11 Old 04-20-2013, 01:56 PM
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What do you mean when you say "and the joint spacing goes haywire.?"

Be specific.

George
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post #4 of 11 Old 04-20-2013, 05:12 PM
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your jig's fence is too short?

If the spacing goes to "hell", you've lost your registration to the key. That tells me you work piece is quite long and your fence is not giving you support all along the way. Try a longer fence on the miter gauge or sled and see if that helps.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #5 of 11 Old 04-20-2013, 08:34 PM Thread Starter
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I built my own jig from a post on the Internet. As a matter of fact, I've built two. The one I'm currently using is approximately 8" wide & 6" high. It extends to the right of the blade a couple of inches and to the left of the mitre slot about 3". When I say the joint spacing goes haywire, I mean that the spaces tend to get wider as I go along, even though I believe I have a tight, snug fit over the template peg. I'm using 1/2" spacing and 1/2" stacked dado set.

Woodnthings ... I suspect you're correct, although my mind cannot grasp the concept of things changing somehow when I'm slipping a notch over a peg with no discernable "play". I do notice, however, that sometimes the slot fits over the peg handily, and other slots tend to fit really snugly.
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post #6 of 11 Old 04-21-2013, 12:31 AM
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If your slots are changing width, that may mean your dado blade is wobbling. The first thing I would do is actually analyze the error. I would use a dial caliper, digital or analog, don't care. Measure the actual space widths, starting from one end and progressing to the other. Write down the space number and the dimension. Next do exactly the same with the pins. This will tell you what problem to fight.

If, indeed the space widths are changing, look for run out on you blade. Your dado may also be changing width if the arbor nut isn't tight.

It may be that your pins are changing width due to inconsistent pressure against the key, either by your hand or because the saw blade wants to push it over. This can look like varying space dimensions.

Another thing you really want to check is to make sure your miter gauge is absolutely solid and perpendicular to your blade. If it is moving it will cause the space width to change. Miter gauge slot / bar slop could cause this, too. You start out with the board "tensioning" against the miter slot counterclockwise and end with it tensioning clockwise because of the length of the board and how you hold it and push the miter gauge.

Last edited by RogerInColorado; 04-21-2013 at 12:39 AM. Reason: add second quote mark around "tensioning"
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post #7 of 11 Old 04-21-2013, 10:28 AM Thread Starter
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Thanks, Roger. What an invaluable response from you! Most of my work thus far has involved free-form bandsaw boxes and a huge bedroom set (no dresser or chests involved), so I do probably need to look closely at all the alignment issues you noted. I know that yesterday I did a first-ever analysis of my measuring tools, only to discover that only two were in agreement. One was off by almost 1/4". I do have a digital caliper, and will follow your suggestion to the "T". I really appreciate your taking the time to give me your response.
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post #8 of 11 Old 04-22-2013, 01:31 AM
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Let us know what you find. Feedback is more valuable than theory cast into the void.
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post #9 of 11 Old 04-23-2013, 11:58 AM Thread Starter
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Well, I whipped out the old digital caliper (rarely use it), and found that my slots varied by as much as 9/10 of a millimeter. I concluded, as you suggested, that the variation is due to side slop in mitre bar to which my gauge is attached. That came with my saw, and I can't seem to adjust it so that it fits the mitre slot tightly. So, I've ordered an Incra mitre gauge. The pins were with 1/100 mm, so the dado stack must be working well for me.

One other question, though. What's the proper way to loosen up the pin/slot fit just a tad so all the glue isn't forced out of the joint. Should I insert a spacer in my dado set?
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post #10 of 11 Old 04-23-2013, 02:07 PM
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There are some adjustable box/finger joint jigs on here....

This one is pretty clear how to loosen or tighten the fit:
http://www.cabinetmaking.com/pages/fingerjig.htm

here's a microadjustable one:
http://www.popularwoodworking.com/ar...nger_joint_jig

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #11 of 11 Old 04-24-2013, 01:00 AM
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Originally Posted by jerryholcombe View Post
One other question, though. What's the proper way to loosen up the pin/slot fit just a tad so all the glue isn't forced out of the joint. Should I insert a spacer in my dado set?
Yes, you can use a shim. Start small. A piece of copy paper is about 0.003 inch. Start there and you can increase or decrease depending on your test results. Run a pair of parts that are about the length of your project so you can actually apply some glue and do an actual glue up. In the real world, the glue has some time to be absorbed by the fibers and swell the wood, so allow some real dwell time. It doesn't take a lot of glue, by the way, on a good fitting joint. A box joint has a huge amount of glue surface.
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