So, I built the box joint jig in Shop Notes from a while back, but damned if
I can get the spacing to work out. I have a Freud Dial Dado Set (works
GREAT in laminates and ply, so far) and I have measured the thickness of the
stack at 3/8 and the space between the right edge of the dado stack and the
register pin - also 3/8... Yet after make a set of test cuts of more than
about 6" pins and sockets are out of whack enough that I've just made some
dentil molding kindling...
I have measured everything with calipers and over about 8" the dado'd
sockets get progressively wider, on the order of .015. I kept a tight hand
on everything while cutting, forcing the work against the registration pin,
so I am at a bit of a loss. I know that the pin spacing and dado width have
to be spot on, as any error is going to accumulate. The whole process seems
to be alot fussier than it needs to be...
Unfortuneatly, I've disassembled the dado stack, so I can't find out if the
stack "opened up" somehow, or what. I guess I'll try it all again in a bit,
but wanted to see if rec'rs had any suggestions.
Thanks in advance!
I know about the spacer, I haven't even made it that far.... I am still
working on getting the test cuts trued up and having fits...
My neighbor suggested box nails or corner brackets. Damn! Here I go
spending all this $$ on tools and I could have had the Ace Hardware guy
point me toward the bracket isle. Betcha this was covered in that most
secret of societies... HCA!
Try setting up your dado blades make a test dado. Then cut a strip of
wood to fit your dado cut and use that as your spacer block from your dado
blades and then attach your index pin on the opposite side of your spacer
block then the dado blades and then everything should work.
It sounds like your distance from your index stop is larger or smaller
that your dado width they have to be the same distance and width or you will
get some creep in your slots and pins.
My slots and pins both start out at .375. In two sample cuts, the slots
grew progressively wider by .010 to .0150.
I like your idea about using a strip that fits the dado cut, etc.... I'll
try that in the morning...
ONLY way for the SLOTS to get wider is the DADO blade changing it's
setting. Concentrate on the DADO blade, the problem you describe is
NOT a problem with your jig
On Sun, 30 May 2004 06:16:12 GMT, "John Moorhead"
Greetings and Salutations.
On Sun, 30 May 2004 02:34:42 GMT, "John Moorhead"
Hum...Are you measuring the teeth or the slot that the teeth
If the width of the dado cut out of the wood gets wider, then,
it HAS to be that the dado blade is "self adjusting" as the cuts are
made. After all, 0.015" is slightly under 1/64"...and it would not
take MUCH to get to that.
If the registration pin were moving, the width BETWEEN the
dadoes would be increasing.
I believe, though, that if the dado blade and the stop pin are
not at the "same" setting, that you will not end up with progressively
wider errors...but, a set of fingers that don't match the width or
alignment of the slots they are supposed to fit into. that, at least,
was my experience a bit ago.
I am not entirely sure that anything but a stack dado (or
router) will create finger joints without a bunch of fiddling and
trouble. I would recommend trying this...Set the blade to its
minimal thickness...say...1/4", and set the finger joint jig
to the same width. Cut a sample piece. If it shows irregularities,
then, perhaps it is a problem with the jig flexing. If it works
fine, then, perhaps the problem is with the adjustment of the blade
walking. Alternatively, set the jig to 1/8" and use your
regular saw blade. If all is "ok", that should produce perfectly
Of course, the problem SHOULD only show up if you have
adjusted the thickness of the dado by adjusting the dials away
Sorry if I seem to be wandering a bit...I am kind of
puzzling over this, and producing a stream of consiousness
note on things I would look at.
I measured the width of the dado head, (.375) then used the other end of
the caliper to set the alignment pin one "dado head" width away from the
right edge of stack. Now, the pin itself may 3/8 or narrower... I assume...
as long as I keep the work hard against the pin ( and I (*was* careful to do
so) everything should be okay...
I do like Chris' idea about cutting a sample dado, and then a pc of stock to
fit, one "cut width" away... That would certainly eliminate errors creaping
in from my measurements...
Now, about accuracy - it seems that the adjustment of any box joint jig
would have to absolutely spot on.... If your spacing was off by even 1/2 of
a thouandth, by the time you can cut a dozen "step and repeats" you could be
out by to 10 thou, a noticeable amount, especially since I'd be that .0005
tolerance would be a pretty high standard for WWing.
Gotta be an easier way....
There is. See my post about box joints that gets rid of the accumulation
problem. And laugh while you read the responses that tell me there is no
accumulation problem with the method you're using.
First, the best way to do woodworking is to fit the pieces to one another,
not by measuring with micrometers on tools.
This is why I prefer the router jig to the tablesaw, which works on the same
principle, but allows me to set my distance to the fence (pin in tablesaw
jigs) by cutting and fitting any two pieces of scrap. The router jig taps
the fence closer to make a narrower pin (looser joint) and farther to widen,
and I don't have a thing to remake. These guys sell one in slick plastic,
or you can make your own. http://www.oak-park.com/ If you're interested,
reply here and I'll drip you some pictures of the jig in operation that I
use to teach middle-schoolers.
As to "accumulating" errors. You are making a pair of pieces. If you
accept that, you can see that the errors of one are compensated for by the
other. Plus, you are working with a material which allows a bit of creep,
but also correction, based on compressibility. The one thing you must do is
ensure that you have a pin equal to or slightly less than the gap, which is
your dado or bit width. I find this easier to achieve with the router table
than the tablesaw,though I've used both. I have even, a long time ago,
used the spacer method, so it was neither recently invented, nor, given the
popularity of non-spacer jigs, is it in any way superior to them.
Oh yes, if you persist with the saw jig, remember that you don't need to
vary (or measure) anything except the distance from the groove cut in the
backer board to the part of the pin closest to it. Best jig I had was one
that used a brass pin, not a full tooth.
Greetings and Salutations....
On Sun, 30 May 2004 06:13:27 GMT, "John Moorhead"
Well, I do prefer to cut a sample dado then use THAT to adjust
the width over to the pin. I just don't trust the dado head not to
either splay out, or suck in a bit when it actually starts spinning
Spot on there!
Well, this is a point. However, I have cut a fair number of
box joints in my time, and, while most of them have been on pieces of
wood 4" or less in width, they have worked very well. They are VERY
strong joints, and, look pretty good (IMHO).
Now...one thing that I did that helps quite a bit is that I
standardized on a couple of widths (1/8" and 1/4"), then, built
a jig for EACH of those measurements. I had tried the adjustable
route in the past, and, while it worked ok, I always ended up spending
about half an hour making a test cut and adjusting the jig over and
Frankly, for pieces much over 4" or 5", I tend to use
dovetails anyway, so I did not run into the progressive errors
you were hitting.
First off, You mention that your slots are getting wider. Are your pins
getting narrower? If the pins are getting narrower also, it sounds like
your indexing pin may not be exactly the width of the slot being cut. It
has to be exactly the width of the slot so that the work will not slide left
or right. Also your indexing pin must be exactly the width of the dado
from the blade. If only your slots are getting bigger, it sounds like your
dado set is not staying properly adjusted.
Silly as it sounds at first reading, that error does _not_ accumulate in a way
that affects the fit of the joint. It may affect the cosmetic appearance,
but not the _fit_.
The 'critical dimension' is the measure from one side of the register _pin_
to the same side of the saw blade (i.e. left side to left side) this is
the spacing between successive cuts, "whatever it is" in actual dimensioning
units. If you want 3/8" fingers on both pieces, and get the 'pin left'-to-
'blade left' dimension at 0.8003" -- well they're going to fall at multiples
of .8003". If the blade is cutting out 0.375 dados, then you *DO* get an un-
equal amount remaining, 0.4253".
At which point, you have two options:
1) set blade to make a _wider_ cut -- to wit, 'half the difference'
between what it is cutting off, and the 'wider' part that is left.
then make a 2nd pass. You'll now have something very close to
.400 cuts, and .400 pins. Not what you were originally aiming for,
2) The other option is "don't worry about it".
When you go to making the mating part, set the blade so it cuts the
exact width of the 'unequal' tooth in the first piece. that '0.4253"'
** ========= ** Assuming you've designed your jig so that the register pin is on the
** 'nut' side of the dado blade, the 'critical dimension' (from the
** _fixed_ side of the blade to the near side of the pin) is *UNCHANGED*** by changing the cut width. i.e. it is _still_ that 'silly' 0.8003"
Now, some test cuts to get the dado with set right (0.4253") and you
start cutting. Every 0.8003 inches. And you're chopping out .4253"
sections, leaving a 0.375" tooth between each one.
Which happens to _exactly_ fit against the 0.4253" 'tooth' and
0.375" gap in the opposite piece.
Miscellaneous requirement -- you've got to process both pieces
starting from the -same- logical edge of the joint. i.e, from
'top' to 'bottom', or 'front' to 'back', or 'stage left' to
'stage right'. Whether you go 'top to bottom' or 'bottom to top'
does not make a difference, as long as you're consistent about
the handling of both pieces.
Note: only one side of the register pin is 'significant', thus it _can_ be
made very thin. A _metal_ one, that is thinner than the kerf of your
regular blade offers some real convenience for setting up.
Set the 'critical distance' to what you think is right for the joint
you're going to attempt. grab a piece of _scrap_ stock (as big as
the joint), and run a full series of test cuts. If things are
'exactly right', you end up with 'half the critical dimension' from
the last cut to the far end of the piece of scrap. If you actually
have more (or less), adjust the 'critical distance' and try again.
ONLY after you've got that 'right' do you need to concern yourself
with the dado blade. set it to 'half way', and "Awaaaaaay we go!"
Only if you don't understand the process. <grin>
_Effective_ woodworking is -not- 'science'.
You can -start- with science, but, in general, what is 'really important'
is that the things 'fit together', so you 'build to fit'. :)
Once you understand that, and approach matters from the standpoint of
'making them fit together', things look a _whole_ lot different.
_What_ the dimension is (in some arbitrary 'units of measure') is not
nearly as important as the dimension being "right" -- i.e. that it fits
where it goes.
It's the same principle as underlies the use of a 'story stick' rather
than relying on written dimensions and a tape measure.
There are several things that 'could' have been happening. Your description
isn't detailed enough to identify which one.
First off, if you're measuring from the *end* of the joint, and noticing
that 'everything' seems to be 'slipping more-and-more out of alignment' with
where you _expected_ them to be, the answer is "don't worry about it", The
fact that the fingers are every .753" instead of every .750" _doesn't_matter_.
As long as the other side of the joint is built with the *same* jig setting,
it _will_ mate up.
Second, as you measure from the 'left edge' of one finger to the 'left
edge' of the next finger, does the measurement change? If so, either the
register pin moved in relation to the saw-blade, or one was not getting a
consistent fit against the pin. 'Slop' in the mounting of the jig to the
saw table-top could also contribute. If the measurement _is_ constant, you
can eliminate register-pin issues from the possible problems list.
Third, if the finger-to-finger dimension is constant, but the width of
the cut-out is changing (with, obviously, a matching change in the opposite
direction, of the finger itself), then the _blade_ must have actually
been changing in the width of the cut it's making. Many possibilities
here -- ranging from a slightly loose blade, to excessive play in the
Comment: if a _visually_ 'exactly balanced' set of fingers is what
you require, then the 'step and repeat' methodology is -not- appropriate.
you need to use a 'divide and conquer' methodology instead. It takes
a bunch more time/effort, but has minimal 'error' and that error is
_visually_ evenly distributed across the joint, rather than showing
at one end or the other.
'divide and conquer' methodology. find a 'simple fraction' of the
entire joint that will have some number of complete finger sets (a
notch and a tooth). Mark off those fractions (a half, a third,
whatever) of the overall dimension. Repeat the process, for each
of the sections thus created. Repeat again, as needed, until
you have each individual tooth market out. Now, cut out all the
marked notches. You do have to align, individually, each saw-cut
to the marks.
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