Mortice & tenon glue line thickness

[Louise-Paisley]

Humour me, I am new to all this :roll:

What sort of gap should be left for the glue when cutting a mortice and tenon in MDF?

The intention is to join parts end to end and instead of half laps I am thinking of using a mortice and tenon which will not really take a great deal more time using the woodrat, one of the reasons being as the parts make up a circular rim it would be easier to clamp up without the circle being distorted due to the fact that the M&T will help keep the part located in two dimensions if that makes sense

I have done a few M&T's with the woodrat (new toy) to practice with some bits of hardwood which turned out quite well - not absolutely perfect but pretty darn good compared to what I would end up with doing it the hard way! then I had only got the thing mounted on the wall an hour earlier so practice will improve that even more I think - but the fit was pretty tight, and I am assuming that with MDF there would be a good chance that the hydraulic pressure would blow the MDF apart.

Also the MDF soaks up the glue like it is going out of fashion, should I coat the MDF and let it suck it up then apply more before assembly or is it OK without?

 

[Louise-Paisley]

156 views and not a single offer of help or opinion, outstanding, overwhelmed by the encouragement =D>

 

[Jacob]

OK! I wouldn't M&T mdf as it hasn't the strength - no fibrous grain like real wood, or even plywood. It might make more sense to insert loose tenons made of real wood (biscuits, dominos. make your own). Might make even more sense not to use mdf at all!

 

[Louise-Paisley]

It does not really need a great deal of strength, the reason I am cutting the rims in four parts is (a) the outer diameter is bigger than the sheet, and (b) I can get upteen rims from one sheet if cut in quarters and, well, none if done in one part.

Really it is more or less just to keep them in one piece while I true up the rim and glue in the running surface then the running surface will take most of the little stress there is

 

[RogerP]

156 views and not a single offer of help or opinion, outstanding, overwhelmed by the encouragement =D>

I think it would encourage more help if you could post a simple diagram (a scanned sketch) of what you are trying to do.

 

[SBJ]

If i understand you correctly, what you are trying to achieve will be quite weak, M&T in mdf is never a good idea - the core is softer than the faces. If it's just alignment you need then a biscuit jointer is probably the answer for speed and accuracy, or a domino will add more strength.

A sketch would help a lot.

 

[petermillard]

FWIW I use loose tenons in MDF when I make cabinet doors; I use a 1/4" (6.35mm) groover for a 6mm panel & tenon which gives a looser fit, but makes assembly very easy. That implies a glue-line of 0.175mm (or perhaps 0mm on one side and 0.35mm on the other, lol) and I've never had one fail; wouldn't want them to be any looser, though, personally.

But yes, I'd agree that M&Ts in MDF are a poor choice when there are many better options available, and no, I really can't visualise what it is you're trying to make - a picture would def. help.

Pete

 

[WellsWood]

156 views and not a single offer of help or opinion, outstanding, overwhelmed by the encouragement =D>

I think it would encourage more help if you could post a simple diagram (a scanned sketch) of what you are trying to do.

Agreed, or at least give us a better description of what it is you're trying to make. What thickness MDF do you want to join, what are the other dimensions of the peices you want to join and what are the "rims"?
Your original post sounds like you want to make long strips by end joining with M&Ts and then bending them into an arc. If so you'd be much better off laminating the whole thing out of as many layers as possible. M&Ts under these circumstances would not be suitable even with solid timber as the forces applied to the joint will be trying to split the material along the grain - I'd expect MDF to fail immediately.

 

[CHJ]

If I understand correctly what Louise is trying to construct, then I believe it's some side cheeks/members to form the sides and contour for a drum.

 

[Louise-Paisley]

Chas is correct.

That rim/cheek will have a groove cut in it which the surface the cat runs on will be glued into. The wheel will not be running on the rims, they are just maintaining the roundness and preventing the wheel running off the base..



The base unit has rollers which will run on the outside of the flat track, between the rims and on the base sides there will be some PTFE or other low friction plastic skids which will keep the wheel on track so to speak.

I don't think biscuits or loose tenons would really add a great deal to the strength in this case, once the running surface is glued in it would give it good rigidity at right angles to the face which is (I think) where a biscuit would also provide the greater strength.

If need be the rims can be made from ply, although at this stage I don't think that extra strength would be needed.

Is that any clearer?

 

[Wouldchuk]

Biscuits work well in my experience, then use a band clamp to apply pressure to the entire circle (and therefore all four of your joints) while the glue dries. Would take no time at all - that is, as long as you have access to a biscuit jointer. Mine was 60 quid online and has been fantastic - a Clarke, the Pro range (yellow in colour, not blue, which is their DIY) - i can't remember the model number. I've used it for loads of MDF related tasks.

 

[Louise-Paisley]

Biscuits work well in my experience, then use a band clamp to apply pressure to the entire circle (and therefore all four of your joints) while the glue dries. Would take no time at all - that is, as long as you have access to a biscuit jointer. Mine was 60 quid online and has been fantastic - a Clarke, the Pro range (yellow in colour, not blue, which is their DIY) - i can't remember the model number. I've used it for loads of MDF related tasks.

I think it entirely possible as things progress a biscuit jointer will be added to the toys, at the moment though I am spending money like water so don't tempt me even more LOL

I have a couple of really long luggage strap things kicking about which would serve as the clamping method nicely I think

 

[MARK.B.]

I have a couple of really long luggage strap things kicking about which would serve as the clamping method nicely I think

If they are not big enough then you could use ratchet strap tie down's these come in various widths and lengths and the best bit is they only cost a couple of squid each.

 

[Eric The Viking]

If you're making these to sell, even in small quantities, then a glueing jig might be the solution. Could you, for example, re-deploy the curved offcuts to hold the quadrants in place while they glue?

I'd agree about biscuits being a good choice for this, too, as long as the MDF is strong enough in the first place. You don't need an actual joiner, although it would be a lot easier with one - you could use the router cutter version. Although the slots are hard to cut accurately that way, and the profile doesn't match the biscuits as well (the cutter diameter is too small), it's a cheaper solution.

E.

 

[dickm]

Picking up on an earlier suggestion - would it possibly be better to go for MDF of half the proposed thickness, make 8 pieces for each circle and then laminate the sets of 4 together with the joins staggered by 45degrees?

(Mind you, the diagram does look as if you are planning to make the sort of treadmill that was once used with a dog inside to turn roasting spits - should we be reporting you to the SSPCA in advance? )

 

[Louise-Paisley]

I have a couple of really long luggage strap things kicking about which would serve as the clamping method nicely I think

If they are not big enough then you could use ratchet strap tie down's these come in various widths and lengths and the best bit is they only cost a couple of squid each.

That is what they are, a bit like thinner versions of the strap they tie loads down on lorries with These are about 25mm wide straps I think

 

[Louise-Paisley]

Picking up on an earlier suggestion - would it possibly be better to go for MDF of half the proposed thickness, make 8 pieces for each circle and then laminate the sets of 4 together with the joins staggered by 45degrees?

(Mind you, the diagram does look as if you are planning to make the sort of treadmill that was once used with a dog inside to turn roasting spits - should we be reporting you to the SSPCA in advance? )

The only problem with that is clamping it all together, it would be a lot more messing about to get a good lamination of the layers. This was actually the first plan, but it was back burnered because the added strength it would create for the joins was really just an overkill - probably - based on the tests we did. But as I said nothing is set in stone so it is still an option if needed.

When my cats get running in their wheel (different design) they run hell for leather, the idea of a small dynamo with lights in the wheel powered by it has crossed my mind LOL

 

[SBJ]

How big are they?

 

[Louise-Paisley]

the internal diameter of the rim is about 47.5 inches, the outside diameter is about 53.5 inches

 

[SBJ]

In that case, use an 8 x 5 sheet and cut it from 1 sheet. The amount of time you will spend making the pieces, then sanding to make good a joint that will not be invisible you may as well just pay up. The cost of a sheet of MDF will be much less than the cost of your time.