Jack Miter Jig

basswood

I made a jig to make cutting jack miters easy. Jack miter cuts can be done with power tools... but the head casing miter cut made with a power miter saw leaves a fair bit of hand saw work to get into the inside corner (like finishing stair stringer cuts by hand).

The jig I made holds the casing on edge with just the profiled portion to be mitered showing. This is similar to the jigs the old timers used to cut jack miters... I just added the hand miter box. This lets you cut the miter with a hand saw very precisely and quicker than cutting with a power miter saw and switching to a hand saw to finish. This only makes sense it you regularly cut jack miters (of course if you have one you might regularly cut jack miters ) )... but here it is anywho:

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Paulie

I've watched your posts and expertise in the trim work. They are always informative. I've done a few jobs but basic compared to your experience. But I'm interested in the jig. Can you by chance take a pic of a piece in the jig? I get what your doing but can't wrap my mind around the actual workings.

basswood

Here you go... an end view pic of the jig with a piece of casing at the end... just a long slot miter box. Also a pic of the jig in use:

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Paulie

Most excellent! Thanks all clear now. Nice work.

Hope I didn't make you cut a piece short on my behalf.

I see a small cut on the front board that holds the work vertical, is it hard to no when to stop your cut?

Gus Dering

Pocket screws in the joint?

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basswood

Thanks Paulie,

The short piece was already in the waste bin... so no worries. I make the table saw rip cut first... so when the cut is finished the piece just drops out.

basswood

Ya sure, you betcha!

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Gus Dering

Are we relying on the painter to caulk the little spot where the relief cut runs out the end?
You know how those painters are.

Looks good Bass

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basswood

Gus, this will be backbanded so the relief cut will be covered.

You didn't think I would expect that much from the painter did you?

BobsLandscaping

What is a jack miter and what is it used for?

rayh78

Its when Jack the carpenter makes a miter cut.
Jack drinks a little, thats why the tilted miter saw.

basswood

A jack miter is a combination of a butt joint (of the flat portion of a molding) and a miter (of the profiled portion of a molding). It is used to avoid very large miters which tend to open up with seasonal changes. In this day of conditioned spaces it is less important, however it is still a good idea in my area (Minnesota) due to radical differences between summer and winter.

In the age of hand saws jack miters were rather easy to do... with power tools, not so much. Now they are considered difficult to do well, and time consuming. My jig is a way to make them easy to do.

neolitic

Only had to do that for one house.
Had to build an old fashioned wood
miter box.
Is that a Jorgensen miter box?
That rig would be a tad awkward
with my old #358 Stanley.

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basswood

Neo,

That is a Jorgensen. The saw is decent, but small. Pretty short stroke... not a real pro miter box. It is good for cutting base shoe though.

The small size makes it good for this jig... small 45* ramp, and attaching it to the big slot miter box jig makes it nice and stable. I like doing hand saw work, makes me feel connected to craftsmen of the past.

neolitic

Sometimes it's better.
Sometimes it's faster
to set up.
Sometimes it's just more fun!











It's always more quiet.

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S Winklepleck

Very nice bit of info

I would have cut the mitre with my chop saw set to a 45 bevel, then the straight cut on the table saw. I wouldn't have even thought to have a jig like this. I'm sure this produces much better results. I may need to talk my boss into letting me put one together at the shop

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basswood

Got rained out last week (jobsite shop in the yard) so I took the measurements to the shop and precut as much as I could. Each doorway has 15 pieces of trim and 24 cuts (compared to 3 pieces of trim and 4 cuts for a basic door casing set):

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basswood

Two of the doorways met at an inside corner and the casings had to be ripped down. One of the doorways was not plumb so the rip was tapered... and the heights were 3/4" different. I cheated the head reveal up on the low door (to the garage) and made up the rest on the flat of the casing.

Even with the trim mostly precut, I only managed to trim 4 doorways in a day.

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BillD

Nice set up Basswood, thanks. Do you change the jig to accept different moldings or do you just make a different jig?

Bill

basswood

Bill the slot of the jig could have fillers added for smaller moldings... or the miter box platform could be moved up or down (it is easily attached with pocket hole screws):

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