Wow! Harder than I imagined

[Chuck Hoffman]

I finally got a stacked dado set and for my first project it was my intent to construct a box jointing jig. Turned out to be much harder than I imagined.

Oh, don't get me wrong...it was easy enough to make an aux fence for the miter gauge, cut a slot for the guide pin, cut and insert the guide pin, etc. The problem was reading the measurements closely enough on my folding rule so that the joints lined up precisely over a span of several inches. The first few seemed to line up OK but then the gap between the fingers and sockets would start to grow and by six or seven inches it was off several thousandths...enough to see plenty of light through.

Suffice it to say that I've delayed the box jointing jig until my new dial caliper arrives by UPS. That'll permit me to get the spacing right.

Geez! Why izzit every tool purchase necessitates one or more additional new tools???

[Robatoy]

Ain't life grand? *VBS*

[Mike in Mystic]

You absolutely don't need a dial caliper, and I can't see how it will help you make a box joint jig anyway.

The way you do this is to cut the notch in the backer board and then cut some stock to fit the notch to be used as the key. You need more than just enough for the key, cut probably 12" of it and cut 3" off for the key. Use the other 9" as a spacer to set the location of the key to the blade. Here's the trick, don't just put it place and screw the backer board down. Clamp it securely and then cut a test joint and check the fit. Adjust the position of the backerboard until it is just right and THEN screw it in place securely.

It only takes a few tries to get it perfect, and you don't need anything approaching the accuracy of a dial caliper to get it that way.

Mike

[Chuck Hoffman]

Uh huh, that's exactly what I did...several times...and it was off a few thousandths, which added up to many thousandths over a span of 6 or 7 inches. What the dial caliper will do for me is permit me to make the guide pin exactly the same width as the dado and verify that using the spacer (the offcut from the guide pin) will result in evenly spaced sockets and pins.

The other thing it will do is permit me to shim my dado set to exactly 1/2 inch. I'm making a drawer unit with 6-1/2 inch high drawers and don't want a partial pin or socket on either top or bottom. My first few attampts were off by about 1/8 inch over a span of 6-1/2 inches.

[TWS]

Mike, while I'll agree with your suggestion, the problem I've found is that it is very easy to accumulate, after several inches, an error the creates mis-fitting joints. For example, with a 3/8 inch box joint over a 9 inch span you make 12 cuts. If your workpiece can move even

0.005 inch on each cut you can have an error as great as 0.055 on the last finger. While this gap is probably an extreme, a 0.005 inch 'slop' on the key is probably not unreasonable. The question is how to avoid this from becoming a problem.

One way is to ensure a steady pressure in the same direction on the key for each cut. What ever accumulated error there is will be duplicated on all pieces.

Another technique would be to stack all four sides and make the all the finger cuts at once. This requires a long and strong key so the key doesn't bend slightly to prevent the outer pieces from being slightly displaced. You also need to offset two of the pieces if you have already cut the sides to dimension (I would dimension after cutting the joints).

Thoughts? TWS

[Mike in Mystic]

I see what you're saying. 1/8" over 6.5" seems extreme, though. You might want to try for an adjustable box joint jig, but probably too much hassle.

Anyway, it sounds like you're after perfection, which is an admirable goal, if not a frustrating one. Good luck!

Mike

[Slowhand]

I use my dial caliper all the time. I'd be lost without it. Can't say I've used it on my dado but I sure use it alot on other things. SH - The "gotta be exact sometimes" woodworker

[Mike in Mystic]

Me, too. I have one and use it. I didn't mean to imply that the caliper wasn't useful. After Chuck's response, I better see how he intends to apply it to this problem, and can see his point. I still think it will be a frustration point, but he probably will get it as accurate as he needs with it.

Mike

"Slowhand" used it on my dado but I sure use it alot on other things.

[Pounds on Wood]

Sounds like a man looking for tool justification. I respect that. Mike is correct however, but we will keep that our little secret.

-- Bill Pounds

[Lew Hodgett]

RE: Subject

Making an accurate box joint jig is a piece of cake if you follow Fred Bingham's approach.

It is shown in his book, "Practical Yacht Joinery", which should be available at your local library.

Lew

[Chuck Hoffman]

I have a detailed diagram of Lynn's box joint jig, which uses 3/8"-16 all-thread for indexing. It will move the carriage exactly 1/16" for each full rotation. In theory, it should easily supply the kind of precision I want. I will probably build one in the near future but I wanted to try the more primitive one first to see how accurate I could make it.

I'm not after perfection but I wanted to do better than I was able to achieve on my first attempt.

I don't have to do box joints. I can make rabbet and dado drawer joints that are very precise and very strong when glued up. But I need to challenge myself to keep adding new skills.

[Charles Spitzer]

[Brian in Vancouver, BC]

Lew Hodgett wrote in news:%ABDd.253$pZ4.215 @newsread1.news.pas.earthlink.net:

After seeing your reply, I realized I might have that book. Upon looking in my library I found the book "Boat Joinery & Cabinetmaking" by Fred P. Bingham (ISBN: 0-07-005307-3), which has the Box Joint Jig construction explained on page 94. Now I just have to figure out some of his explanation - a few more re-readings perhaps will do it.

Brian

[Andy Dingley]

I'm not a fan of the guide pin approach - it comes of having been an experimental physicist. When you cut a slot, you have a position error in it. When you cut the next one you have about this same error _plus_ the error from the first slot. By the time you've gone right across the board, you're screwed.

Much better is to make a jig like an Incra jig. There is an accurate comb or rack (1/32" spacing for Incra) and all the adjustments you can possibly make get corrected back to a position based on this rack. So every time you cut a slot you're measuring it relative to the original datum, not just to the previous slot. Slots are no more accurately cut than your first slot was, but the differences don't add up successively.

[George]

Make one for the router table. It's a straddle fence on a board.

Sixth graders use it with success.

[Chuck Hoffman]

OK, the dial caliper arrived and I re-did the jig. The pins and sockets are now consistent across a span of about six inches...no cumulative gap error. They fit very snugly together and I would imagine I'd have to sand them lightly or there'd be no room for glue.

[Lew Hodgett]

A little tip.

You are stuck with the width of the dado cut so you make the pin equal to the dado width.

Good use for a dial caliper.

Mount a piece of say 4" x 12" x 3/4" to the miter gage with a couple of screws I use melamine type wood chip and glue board.

Take a 2nd piece same as above and mount a pin along the bottom edge.

Assemble the pieces back to back such that the pin points forward and clamp with a C-Clamp.

You can now adjust the position of the pin relative to the dado such that you will get a dead nuts fit.

When you get the fit, screw to two pieces together and remove C-Clamp.

HTH

Lew

[Silvan]

Popsicle stick wrapped in sandpaper works wonders. Or an emery board.

[Chuck Hoffman]

[TWS]