Joint question

[heronviewer]

I have been asked to make a graders set square (for dressmaking) as there seem to be none available of the size my daughter wants.
She wants one with the right angled sides 24" each - which would mean a hyponenuse of 33".
I have some old thin oak which I could use and thought to make it from strips 1½" wide and 4mm thick. Problem is how to make the joints.

I have this old one, probably used about 1900, which seems to be made from strips of mahogony edged with ebony 1¼" wide. It is about 2mm thick. The right angled sides are about 9¾" and 5¾". It appears to be only edge glued and I don't detect any hidden iron pins. The discs on the joints are oak and I can't see that they serve any purpose other than as a decoration.

Are edge joints going to be strong enough for the increase in size and if so, what glue would be best ?

 

[Brentingby]

Since the legs will be the same on your triangle, the two joints adjacent to the hypotenuse will be nearly edge grain and fairly strong. The mitre at the right angle will be less strong. If you use an adhesive with structural strength such as epoxy, the joints will be quite strong. I think the disks over the joints is a nice look. I'd be inclined to drill very shallow recesses with a forstner bit and glue in disks with the grain running across the joint. Put them on both sides and they'll add a fair bit of strength.

 

[Racers]

I would be tempted to make the whole thing from thin ply or MDF, or double up the 2mm oak and brick the corners with over lapping planks.

I have made trivets from pine etc with half laps, they take some time to get the joints close by "kerfing in" where you run a saw down the gap to make it fit closely.

24 October by Racers, on Flickr

Pete

 

[heronviewer]

Thanks for the replies.

I should have said that the discs are staggered - and don't go right through - so I think Brentingby's suggestion is right on the mark. So I shall do that - now to get a small forstner bit !

Yes, I have made trivets with half lap joints and they are just glued and seem to last for ever. I'd prefer not to use half laps, really just to try and keep the set square as smooth as possible so there will be no catches on fabrics when it is being used.

 

[dzj]

I'd be inclined to drill very shallow recesses with a forstner bit and glue in disks with the grain running across the joint. Put them on both sides and they'll add a fair bit of strength.

Or perhaps a Butterfly joint.

 

[Jacob]

Interesting!
I googled and got this http://collectingme.com/drawing/Framed_ ... uares.aspx
There's some form of T&G or very thin loose tenon joining the pieces.
I doubt your dowels are decorative as they don't look decorative at all. End grain too?

A lot of technical stuff was made of wood (cameras etc) which I think accounts for the very thin mortice chisels which turn up.

 

[heronviewer]

That link is interesting ! Thanks. The left hand one seems to have pairs of pins on the joints - not as mine.

The discs on mine are not end grain - and as I said, they don't go right through. So I think they are there to strengthen the joints.

 

[Fitzroy]

If you use a forstner bit watch out the point doesn't break through. Some of my forstner bits have a point on them that is not far off the thickness of the wood you plan to use.

 

[heronviewer]

Thanks - a good point. Maybe a router bit with an end cut would be better.

 

[Steve Maskery]

2mm is very thin indeed. Veneer is about 0.7mm, so you could laminate up three layers to make a sort of bridle joint. It would be very strong.

 

[heronviewer]

Thanks !

I'm going to make it 4mm thick to make it more rigid for the increased size, so I shall have 2mm discs for the strengthening of the joints. That should be possible for me !

 

[Steve Maskery]

I am a bear of very little brain and I don't understand how the discs strengthen the joint. significantly.

 

[Jacob]

I am a bear of very little brain and I don't understand how the discs strengthen the joint. significantly.

I agree it's not obvious.
Maybe if the hole can be precisely drilled half way through on one side, then ditto the other side but in a different position not meeting the top one - the inserts then effectively laminate and key the two halves together - in a balanced way - equal on both sides hence more stable?

 

[Steve Maskery]

Maybe, Jacob, maybe. But I can't help thinking that there are better ways to make this.

 

[Jacob]

Maybe, Jacob, maybe. But I can't help thinking that there are better ways to make this.

It's like the trendy butterfly tie joint but circular, without the dovetails i.e. machined, very practical, not decorative.
One on each side for stability.
No ifs or buts, I've convinced myself that this is the answer!
If time is no object you could do the same by hand but with butterfly shapes

 

[Steve Maskery]

Yes, but if all you wanted to do was tie over the joint, a circle is not the easiest shape to make. You can't just slice off a piece of dowel, the grain is wrong. It would be easier to make a shallow cut across the joint and glue in a strip, like stringing.

Don't get me wrong, I think you are probably right, I just don't think it is a very good way to do it.

 

[Jacob]

Yes, but if all you wanted to do was tie over the joint, a circle is not the easiest shape to make. You can't just slice off a piece of dowel, the grain is wrong. It would be easier to make a shallow cut across the joint and glue in a strip, like stringing.

Don't get me wrong, I think you are probably right, I just don't think it is a very good way to do it.

Circle really is the easiest - drill hole with the right bit (no spur) - cut circle from veneer with a wad punch - lay it in the hole with grain across. Probably over size in thickness and planed down for a neat finish.

Wad punches basically a leather workers tool (and plumber in the old days) are very useful for all sorts of other jobs e.g. making dowels - punch the diameter in each end of the block and joint them up with a plane

 

[heronviewer]

Jacos's description of the joint is correct. A disc 50% of the thickness on one side and another on the other side - but at the other end of the joint. My ancient set square made that way is still perfect and the total thickness is only 2mm. Double the thicknesses and it should be fine for a much larger set square.

 

[Jacob]

Yes i think we've cracked it!
So all you need is a router cutter with a matching wad punch and you could be in production!
A useful way to join thin boards - for model makers , box makers etc?

 

[Jacob]

Another one here - both butterfly and circular: