Special techniques to cutting a non-90 degree mortise? - Woodworking Talk - Woodworkers Forum
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post #1 of 8 Old 12-17-2019, 09:41 AM Thread Starter
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Special techniques to cutting a non-90 degree mortise?

I'm in planning stages of building a small sitting bench. The legs will be M/T into the top, and I'd like to splay out sideways at about 5-7 degrees from 90.

Using traditional hand tools, the methods I've seen are:
1. Mark the top, then follow around the edges at angle to mark the bottom, and carefully chop until both sides meet.
2. Make an angle block to guide my chisels.
3. Set a bevel gauge to x degrees, and use that to check progress inside the mortise.

Are there other methods?

Geoff
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post #2 of 8 Old 12-17-2019, 10:33 AM
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I use method 2 for 90 degree mortises. I think it would work well for non-90 as well.

pat
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post #3 of 8 Old 12-17-2019, 08:42 PM
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Chop the mortise square and angle the tenons
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post #4 of 8 Old 12-18-2019, 12:34 AM
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I made some chairs and did exactly what Terry suggested.

e.g. The mortise is square to the leg.

There are two issues with the tenon. First the shoulders need to be on angle and the tenon needs to be on an angle.

The first step is to set the table set the table blade at the angle and don't change this during the process. Set your fence to the appropriate distance for the length of the tenon. Use this set up to cut the shoulders of the tenon along with the miter gauge set to the appropriate angle. I used the Rockler miter gauge to adjust the angles. It is a poker chip style of angle mechanism and you can easily switch from left to right angles with complete accuracy.

The tenon is cut with a dado blade. There are spacers between the outside blades and no chippers. Typically the outside blades of the dado set have a 1/64 set. Meaning that the spacers between the blades need an extra 1/32. DO NOT use wood as spacers but rather metal. (As the blade is tightened the wood will compress unevenly and your tenon will be off size.

You will need a tenon jig for your table saw. I found that using a home built jig that rides the saw fence is better than the commercial ones that use the miter gauge slots.

You need to sit down and really think about these rails with tenons are going to fit into your bench. Then with your equipment on how to machine the tenons. A clue is that your rails are NOT going to be parallelograms.

Rich
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post #5 of 8 Old 12-18-2019, 01:00 AM
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The absolutely best way I know to make mortise and tenon joints is with a Leigh FMT Pro. Makes angled tenons too.

After using a mortiser and dado blade to make over 100 mortise and tenon joints for a bunk bed I figured there must be a better way.

I discovered the Leigh FMT Pro. From tiny mortise and tenon joints in wooden match sticks to double or quadruple tenons in bigger joints.

A few minutes setup and you can cut perfectly fitting mortise and tenon joints in less than 2 minutes. Check out the videos.


https://www.leightools.com/super-fmt-overview/
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post #6 of 8 Old 12-18-2019, 08:52 AM Thread Starter
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Quote:
Originally Posted by Terry Q View Post
Chop the mortise square and angle the tenons
Angling the tenon would require the tenon to be thinner, or am I misinterpreting. Can you explain how this method is a benefit?

@NoThankyou and @Terry Q, I'm using hand tools. No table saw or router involved here.

Geoff
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post #7 of 8 Old 12-18-2019, 09:58 AM
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Special techniques to cutting a non-90 degree mortise?

Just try laying it out on a board the same dimensions as the legs you are proposing.

Draw your 5 to 7 degree lines on a face of the board, and draw a second parallel line spaced the thickness of the mortise. If a through mortise, the thickness of your work piece.

Take a square and draw 90 degree lines to the previous drawn lines as wide apart as you can, yet remain betwen the lines. It will show you how much wood needs to come off the tenon to fit.

If you are cutting by hand these angled tenons should be no harder to do than straight cut tenons.

Last edited by Terry Q; 12-18-2019 at 10:01 AM.
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post #8 of 8 Old 12-18-2019, 01:36 PM
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I think Terry's suggestion is better than my original one. It seems simpler.

pat
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