Clearance on mortises and tennons?

[jeff_albro]

I'm working on a design I'm going to post to this forum that includes through and blind mortises in hardwood. My question:

Should I include clearance, and if so, how much?

How much larger should the mortises be than the tennons?

Thanks!

-Jeff

[Paul Amos (Unregistered Guest)]

Hi Jeff
It depends on the type of mortice and tennons you are producing...Cabinetry or Joinery, with cabinetry I would say you would need a clearance on the depth of a blind mortice only. The strength of the mortice and tennon is having a close fit, there are some exceptions though... such a very large joinery items having components with say 4"-6" long tennons, the friction on assemble can be very difficult. Have you done any trial cuts and what sort of results did you get.
The items I have done on the Shopbot did not have any allowance for clearance the results where good, the biggest problem is the setup i.e. working from one face side to give the best alignment and breakout on the shoulders of the tennons.
There is also the limit on the length of items you can tennon.... unless you are on an upper floor or have a hole to drop them in.

I hope you get on OK

Paul

[cnc_works]

Jeff, my experience in cutting mortise & tenon joints is that with no clearance for glue, number one, I fight the assembly and two, I starve the joint for adequate glue. .002" clearance seems about right for me.

[cnc_works]

Jeff, upon reflection, I suppose I should clarify. The .002" is joint in hand clearance. In other words, your machine tolerance may or may not produce the necessary clearance even if you draw the joints with your desired clearance.

I guess I am saying that some trial and error may be necessary to achieve the proper clearance for your tool/material variances no matter what you draw the joints at, but that, for me, a final "measured from the sample cut" .002" clearance works.

Donn

[srwtlc]

I find it much better to make my mortises with the bot to size and location, then make the tenons with a dado blade on the tablesaw for a slip fit.

[ron brown]

I've done mortices by hand with chisel only, chisel and drill "wasted" holes, on a dedicated mortice machine, mortices on a ShopBot, mortices on an old Greenlee hollow chisel morticer and tenons on tablesaws, ShopBot, twin-head shaper, bandsaw, hand-ripsaw and just plain old chisel. They all work and they all take a little adjusting to get the technique correct.

All joinery depends on skill and technique. The hardest thing to find for cuting mortices or tenons with a router is a long rigid bit. The longest bits I have used are only 3" or so in length for the 1/2" bits. This means the deepest mortice that can be cut is about 3" and a slightly shorter tenon for glue clearance in the bottom.

Ron

[jeff_albro]

Thank you for your input everyone!