Cutting a square tenon on the end of a round dowel

[Bodgers]

I need to mortise a 38mm dowel into the end of a 60mm dowel.

The 60mm dowel will be the 'head' end of a large wood screw with a handle drilled through it.

Any ideas on how to accurately scribe a proper square on to the end on the dowel? I have used a combination square to attempt it, but I don't seem to be able to get the lengths of the sides correct. Hard to explain. Is there a jig for this?

I could just drill a round hole into the 60mm dowel and push it in, but I don't have a 38mm forstner, and there will be a fair amount of twisting force and I want to have some mechanical connection.





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[Jacob]

I need to mortise a 38mm dowel into the end of a 60mm dowel.

The 60mm dowel will be the 'head' end of a large wood screw with a handle drilled through it.

Any ideas on how to accurately scribe a proper square on to the end on the dowel? I have used a combination square to attempt it, but I don't seem to be able to get the lengths of the sides correct. Hard to explain. Is there a jig for this?

I could just drill a round hole into the 60mm dowel and push it in, but I don't have a 38mm forstner, and there will be a fair amount of twisting force and I want to have some mechanical connection.





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Cut out a square from something and scribe around it with a scribe pin?

 

[MikeG.]

Print it out on paper and prit-stick it to the end of the dowel.

 

[MikeG.]

I can't see why it needs to be square. It would be much easier to make it round, especially if you have access to a lathe.

 

[Bodgers]

I can't see why it needs to be square. It would be much easier to make it round, especially if you have access to a lathe.

I don't have a lathe. I don't want it round, as I need some mechanical strength as it will be subject to some turning/twisting force.

 

[MikeG.]

Well the grain is running the wrong way for it to resist much in the way of twisting.

 

[Bodgers]

Well the grain is running the wrong way for it to resist much in the way of twisting.

Not sure what you mean by that, but having a turning force against a square captured inside a square hole has to be a lot stronger than a cylinder in a round hole.

Anyways, got it cut now, Just need to cut the mortise to match.

 

[Rorschach]

Well the grain is running the wrong way for it to resist much in the way of twisting.

Not sure what you mean by that, but having a turning force against a square captured inside a square hole has to be a lot stronger than a cylinder in a round hole.

Anyways, got it cut now, Just need to cut the mortise to match.

Not really, the shapes do not pair well together leaving weak areas at the corners of the square. A circle within a circle would be better with a cross pin for a mechanical lock.

 

[Bodgers]

Well the grain is running the wrong way for it to resist much in the way of twisting.

Not sure what you mean by that, but having a turning force against a square captured inside a square hole has to be a lot stronger than a cylinder in a round hole.

Anyways, got it cut now, Just need to cut the mortise to match.

Not really, the shapes do not pair well together leaving weak areas at the corners of the square. A circle within a circle would be better with a cross pin for a mechanical lock.

So the mortise in tenon joint was wrong all along?

I had considered a pin (which I can still do with this method) but thought driving a hole through the whole thing would introduce weakness.

 

[MikeG.]

No, what I meant was that long grain doesn't resist twisting well. It's nothing to do with the mechanical shape of the joint, but the grain direction in the finished article. If you took that dowel with the square tenon on the end and stuck it in a vice, then cut a mortise in the middle of a piece of 2x1, placed that over the tenon, and applied a rotational force, what is going to break? It won't be the piece with the mortise.

With the design of this joint (end grain to end grain), you are putting together two pieces each orientated in the weakest possible manner. If the purpose of this construction (you haven't said) involves regular or strong rotational forces through the joint, it will have a good chance of failing.

 

[Bodgers]

No, what I meant was that long grain doesn't resist twisting well. It's nothing to do with the mechanical shape of the joint, but the grain direction in the finished article. If you took that dowel with the square tenon on the end and stuck it in a vice, then cut a mortise in the middle of a piece of 2x1, placed that over the tenon, and applied a rotational force, what is going to break? It won't be the piece with the mortise.

With the design of this joint (end grain to end grain), you are putting together two pieces each orientated in the weakest possible manner. If the purpose of this construction (you haven't said) involves regular or strong rotational forces through the joint, it will have a good chance of failing.

Hmm, not sure, but it sounds like I haven't been clear with what I am doing, because that doesn't sound right to me.

The only difference between the two will be the shape of the tenon. I am proposing one end to be square and the other end to be a square mortise. There is still a long grain connection in the same way as if they were kept round.

The pictures below show exactly what I am doing. These aren't my pictures, these are from Chris Albee on Lumberjocks. He added a pin as well.

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[Jacob]

I reckon that joint will be far weaker than a straight forward round to drilled round hole. It's over complicated and weakened with a lot of wood removed and a lot of weak points to fail.
A joint is only as strong as its weakest point. Round to round doesn't have a weakest point and also has a good meeting surface for glue.

 

[sunnybob]

from that picture, the lower square is cross grain, so how is that fitted to the piece below?

Its a very complicated way of joining two pieces of wood (looks more like a training exercise for apprentices), and if there is any major force on the cross handle I wouldnt give it much chance of lasting long.

EDIT; Just had another look and seen more of what is happening, still hasnt changed my opinion of strength though.

 

[Jacob]

..... having a turning force against a square captured inside a square hole has to be a lot stronger than a cylinder in a round hole. ....

Definitely not if it fits well and is glued. The square shape has four weak points (corners) where there would be maximum leverage

I'd add - the cut thread on that shaft is far too fine for a wood screw vice. It would fail in no time.
It would do for a one off fixing (commonly used for cupboard knobs etc) but not for frequent use.
He's also using a Workmate which means he is an amateur without the proper kit. Best not to follow his example to closely!

 

[Phil Pascoe]

-As long as it fitted well and was glued properly I would think it far stronger if it were kept round. Can you find someone with a lathe? Or it could done with a router, a bush and a bit of a jig.

 

[novocaine]

you could have marked this out with dividers and a centre finder (your combination square would have done). mark the centre. use Pythagoras to work out your diagonal (a^2+b^2=C^2), half it and draw a circle of that radius. stick a point on the circle (doesn't matter where), set your dividers to the length of a side, work round the circle from your point and connect the dots, 1 square, your done, now drill it and cut it.

not really needed (more relevant for metal work) but if you want it, then so be it and it's nice to know you've done something a bit different.

 

[Bodgers]

I reckon that joint will be far weaker than a straight forward round to drilled round hole. It's over complicated and weakened with a lot of wood removed and a lot of weak points to fail.
A joint is only as strong as its weakest point. Round to round doesn't have a weakest point and also has a good meeting surface for glue.

Point taken on the weakning - makes sense. I am mitigating this in that my hub side is a lot thicker than this one (60mm diameter) so there is a lot of spare material left.

Glue surface area is surely greater with the square tenon though. Surface area of the circumference of a circle is 3.1415 X diameter Vs 4 X square width of the cube...

Maybe starting to overthink it!

Ultimately, as I crank the handle of the screw, I just want to avoid it shearing off when it is being used to clamp something.

Yeah, I noticed this was his first bench build. This is the issue with internet woodworking, you have to look beyond the surface. Not sure why his threads are so shallow as he is using the same type of die cutter as I have, and it is definitely capable of going deeper.

I notice he is still using it a few years later after he built it, so he may have redone it by now.

 

[Bodgers]

you could have marked this out with dividers and a centre finder (your combination square would have done). mark the centre. use Pythagoras to work out your diagonal (a^2+b^2=C^2), half it and draw a circle of that radius. stick a point on the circle (doesn't matter where), set your dividers to the length of a side, work round the circle from your point and connect the dots, 1 square, your done, now drill it and cut it.

not really needed (more relevant for metal work) but if you want it, then so be it and it's nice to know you've done something a bit different.

That's the kind of thing I was after! You came a day late

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[novocaine]

Yer, sorry about that, at least you know how to do it next time now.

 

[MikeG.]

No, I knew perfectly well what you were intending to do, and I stick by my point: that is a weak join because of the grain orientation.