# Cutting a square tenon on the end of a round dowel



## Bodgers (2 Sep 2018)

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 (2 Sep 2018)

Bodgers":256micxs said:


> 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.
> 
> ...


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


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## MikeG. (2 Sep 2018)

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


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## MikeG. (2 Sep 2018)

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.


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## Bodgers (2 Sep 2018)

MikeG.":2km3e13c said:


> 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.


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## MikeG. (2 Sep 2018)

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


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## Bodgers (3 Sep 2018)

MikeG.":3s3k59fr said:


> 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.


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## Rorschach (3 Sep 2018)

Bodgers":2xv2hjvs said:


> MikeG.":2xv2hjvs said:
> 
> 
> > Well the grain is running the wrong way for it to resist much in the way of twisting.
> ...



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.


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## Bodgers (3 Sep 2018)

Rorschach":2xrfpenc said:


> Bodgers":2xrfpenc said:
> 
> 
> > MikeG.":2xrfpenc said:
> ...


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.


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## MikeG. (3 Sep 2018)

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.


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## Bodgers (3 Sep 2018)

MikeG.":2mrnbr4y said:


> 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 (3 Sep 2018)

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.


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## sunnybob (3 Sep 2018)

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.


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## Jacob (3 Sep 2018)

Bodgers":ri8g2v5g said:


> ..... 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!


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## Phil Pascoe (3 Sep 2018)

-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.


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## novocaine (3 Sep 2018)

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.


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## Bodgers (3 Sep 2018)

Jacob":34gf82dc said:


> 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.


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## Bodgers (3 Sep 2018)

novocaine":3nr37w1i said:


> 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 (3 Sep 2018)

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


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## MikeG. (3 Sep 2018)

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.


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## sunnybob (3 Sep 2018)

It COULD be strengthened with a wide steel ferrule heat shrunk onto the round a la wagon wheel steel treads, but when alls said and done, there are stronger joints.


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## peter-harrison (3 Sep 2018)

What I do for things like this is to cut a square piece of scrap, whose faces are as long as the diameter of the rod. Stick it on the end of the rod using superglue. Use the square as your reference against fences etc. When the work is done, cut it off.


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## Bodgers (3 Sep 2018)

Out of curiosity I posted the question on Lumberjocks with the following sketch. Consensus seemed to tend towards the square tenon 







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## sunnybob (3 Sep 2018)

look at the distance between the corners of the square to the outside of the round on the sketch, then on the picture posted.
The sketch shows at least twice as much wood than the pic.


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## Bodgers (3 Sep 2018)

sunnybob":3qluvrpz said:


> look at the distance between the corners of the square to the outside of the round on the sketch, then on the picture posted.
> The sketch shows at least twice as much wood than the pic.


Mine is probably somewhere between the two 38mm to 60mm dowels. Not sure if that makes much difference.


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## sunnybob (4 Sep 2018)

The weakness of the joint is in the distance between the corner and the diameter. Ignore the thickness of the square, imagine how much (or how little) force it would need to split that amount of wood, using a twisting lever as long as the cross bar youre fitting.

If this is a vice handle, then the first time you need to lean on it that amount of wood will give way. If this were made of metal, then it would be impossible to break, but made of wood.....


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## Bodgers (4 Sep 2018)

sunnybob":30na9opb said:


> The weakness of the joint is in the distance between the corner and the diameter. Ignore the thickness of the square, imagine how much (or how little) force it would need to split that amount of wood, using a twisting lever as long as the cross bar youre fitting.
> 
> If this is a vice handle, then the first time you need to lean on it that amount of wood will give way. If this were made of metal, then it would be impossible to break, but made of wood.....



By making the tenon longer and the mortise deeper should resolve that. I think that would be the same issue with the round tenon as well.


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## Phil Pascoe (4 Sep 2018)

Except you would not have four weak spots with a round tenon. Any stress/weakness would be on the the circumference (the whole glued area) and not the four corners outwards.


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## sunnybob (4 Sep 2018)

I think we've spent enough time trying to convince you.

its your work, do what you will.
We wont say "we told you so"
honest.


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## Bodgers (4 Sep 2018)

phil.p":2a1fr91y said:


> Except you would not have four weak spots with a round tenon. Any stress/weakness would be on the the circumference (the whole glued area) and not the four corners outwards.


I see what you mean there. But...if you think of the joint without any glue in it, the failure is immediate on the round tenon version as it would just slip around. 

The break on the square corners on the square version suggests the mechanical resistance that it offers breaks before the glue. And if that's the case the round tenon has no mechanical resistance so would fail instantly.





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## Bodgers (4 Sep 2018)

sunnybob":30kyb8sb said:


> I think we've spent enough time trying to convince you.
> 
> its your work, do what you will.
> We wont say "we told you so"
> honest.


I was just surprised by the initial comment/observation that's all, and genuinely interested in why. That's why I asked the same question on Lumberjocks (and got the opposite answer) 

As I say, it is already done now, although I will extend the depth.



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