"Formula" for hand cut mortise depth? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 3 Old 08-28-2020, 02:38 PM Thread Starter
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"Formula" for hand cut mortise depth?

I'm doing my first project with hand-cut mortise and tenon joinery. For the most part, it has been surprisingly easy to cut the tenons - not to say mine are super clean or great, just that I thought it would be even worse than it is. However, one thing I didn't realize was that there seems to be a point of diminishing returns in how deep I can chop the tenon in terms of being able to chop and lever out waste wood with the chisel. At a certain point, you naturally run out of room to angle the edge in or lever back without dinging up the shoulders (is that the right term? - the short ends of the rectangle that are 90-degrees to the grain) at either end. Of course, the more narrow the tenon is, the less room you have. So, you have more room to operate in a 1 1/2" mortise than a 3/4" one.



At the same time, I figure the deeper the tenon, the more gluing surface there is and the stronger the joint. But at some point, you just can't cut the mortise any deeper without a mortising drill, so the tenon then has its corresponding limit. So, is there a particular formula or rule of thumb that should guide how deep a mortise/tenon needs to be for a given width?
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post #2 of 3 Old 08-28-2020, 04:12 PM
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Quote:
Originally Posted by RParker View Post
I'm doing my first project with hand-cut mortise and tenon joinery. For the most part, it has been surprisingly easy to cut the tenons - not to say mine are super clean or great, just that I thought it would be even worse than it is. However, one thing I didn't realize was that there seems to be a point of diminishing returns in how deep I can chop the tenon (mortise) in terms of being able to chop and lever out waste wood with the chisel. At a certain point, you naturally run out of room to angle the edge in or lever back without dinging up the shoulders (is that the right term? - the short ends of the rectangle that are 90-degrees to the grain) at either end. Of course, the more narrow the tenon is, the less room you have. So, you have more room to operate in a 1 1/2" mortise than a 3/4" one.

At the same time, I figure the deeper(longer) the tenon (mortise), the more gluing surface there is and the stronger the joint. But at some point, you just can't cut the mortise any deeper without a mortising drill, so the tenon then has its corresponding limit. So, is there a particular formula or rule of thumb that should guide how deep a mortise/tenon needs to be for a given width?

Rule of thumb is 1/3 the thickness for depth of mortise and length of tenon ... plus 1/16" for glue build up to prevent hydraulic pressure spring back .... or skive a "V" on the tenon to allow squeeze out of surplus glue.


The wider (not longer) the mortise, say 1 1/2", the more work to get it all cleaned out, that's on the width.



I have started using a router with a self centering jig I designed, rather than chop out my mortises by hand. More power to you if you can chop mortises by hand! I made a Mission Quilt rack with 32 mortises and tenon joints, too many for hand work for me..... just sayin'


https://www.woodworkingtalk.com/memb...on-quilt-rack/

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #3 of 3 Old 08-28-2020, 04:34 PM Thread Starter
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Thanks. So, an example in summary would basically be a 9/16" deep mortise for a 1 1/2" tenon.

I have a mortising jig for a drill press, but unfortunately the frame on my current press is too big for the collar on the jig. I think I've got something like 24 mortises on my current project and found that it has been more satisfying to cut them by hand than trying to drill or route them out. Not sure I'll retain that feeling, but it has been a fun learning experience.

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