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
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
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
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.
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.
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!
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.
Sounds like a man looking for tool justification. I respect that. Mike is
correct however, but we will keep that our little secret.
On Fri, 07 Jan 2005 18:18:05 GMT, "Mike in Mystic"
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).
Making an accurate box joint jig is a piece of cake if you follow Fred
It is shown in his book, "Practical Yacht Joinery", which should be
available at your local library.
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.
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.
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
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
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.
I still have one remaining problem and that is the combination of one pin
and one socket are more than an inch wide. I was more concerned with
getting the relationship of pin to socket right than I was with the
dimensions. But I may just ignore that and build a version of Lynn's jig.
That should solve both problems simultaneously since both the dimensional
accuracy and the pin to socket relationship will depend on the accuracy of
the screw machine that cuts the 3/8"-16 all-thread..
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