How much gap for glue?

[Deadeye]

Short version: how much gap is needed for glue? 0.1mm?? 0.25mm?

Long version:
I got excited and designed a jig/template to make interlocking joints. I then took the plunge and someone kindly cut it out of 5mm aluminium. It works brilliantly and looks lovely *BUT* I have to tap the joints together with a mallet; fairly firmly.

This is because my CAD work described perfectly matching curves - zero gap between the surfaces. I can adjust by some rather laborious sanding, but it would be better to recut the template allowing a micro-gap for glue. How big?

 

[Sgian Dubh]

Short version: how much gap is needed for glue? 0.1mm?? 0.25mm?

This is because my CAD work described perfectly matching curves - zero gap between the surfaces.

How big?

Intimate contact is what you're looking for in successful wood joinery using adhesive so, no gap. The need to hammer the joint together while the wood is dry indicates the joint is probably too tight. If you're going to adjust the template aim for a difference of no more 0.2 mm. This should allow easy hand pressure dry assembly, but by the time you add a water based adhesive (assuming this is your chosen type) which will swell the wood slightly, you'll probably need to add pressure to pull it all together, e.g., clamps of one sort or another, block and hammer, etc. Slainte.

 

[Deadeye]

Thanks Sgian Dubh

Sounds like I should take 0.1mm off my pins and add 0.1mm to the sockets. Back to CAD!

 

[Myfordman]

When I set my adjustable box joint jig, I use a scrap of paper for glue clearance. Usually this will be a piece of 80gsm printer paper. This is for a pair of joints
I do treat myself to a little bit more clearance when making a long joint with fine 4mm pins and sockets as the time taken to apply glue can cause swelling and the joint becomes a PITA to assemble and any force can risk breaking off the pins.

 

[Deadeye]

At risk of going off on a tangent, I'm a little perplexed by my box joint setup.
It's along the lines of Matthias Wandel's, but I swapped the wooden gears for a 2:1 90-degree cast set.
Now, I know my blade is 3mm kerf. And I know my threaded rod is 1.5mm pitch. So one turn should be one blade width.
But when I cut, I have to "pretend" the blade is narrower - i.e I have to make the gaps wider by stopping the turn earlier and starting the next later - otherwise the joint is too tight.
What's causing this? Is it that the blade is not in fact 3mm kerf? or does it deflect when one side is unsupported during a cut? Or what?

 

[johnnyb]

Is it slop in the mechanism? You have 2 gears and a rolled thread and nut. Also many sawblades I use are indeed 3.2 Kerf.

 

[custard]

The perfect PVA glue layer is just one molecule thick, anything thicker detracts from ultimate strength.

So don't worry about leaving space for PVA!

UF and Epoxy adhesives are slightly different, when doing lamination work I will sometimes leave a tiny allowance for glue thickness, but that's of the order of a few tenths for a twenty or thirty leaf lamination, so pretty tiny in the scheme of things.

You may occasionally hear cabinet makers talking about "leaving somewhere for the glue to go", be careful how you interpret this, often it's a tongue in cheek comment to justify speeding up a job by undercutting a joint.

 

[Deadeye]

Is it slop in the mechanism? You have 2 gears and a rolled thread and nut. Also many sawblades I use are indeed 3.2 Kerf.

Good point. I don't think it's the blade - it says 3.0mm and my calipers agree. And I'm careful to turn to the exact point to try to avoid backlash. It's not a big problem because I know how much less/extra to turn to compensate but just seems odd that, say, 12 cm of overall width does take 40 turns, yet the turns have to be made in a way that *ought* to make the holes too big and pins too small.

 

[MikeG.]

The perfect PVA glue layer is just one molecule thick, anything thicker detracts from ultimate strength.

So don't worry about leaving space for PVA!.........

The problem isn't the eventual thickness of the glue, but the fact that inserting one piece inside another pushes all of the glue out.

OK, that's probably not the actual problem, which is more likely to be worrying that you've pushed all the glue out. :lol: How do you persuade yourself that the dollop of glue that comes out on the end of the peg (for instance) doesn't mean it's cleaned the entire hole of its adhesive as it has been hammered home?

 

[peter-harrison]

I've designed a lot of pieces to be made on a CNC by people who do an awful lot of it, and they always say that when 2 pieces are going to fit into one another, you should make a .1mm gap. This is true for dry or glue joints. I think the glue is a bit of a sideshow here. If it fits dry, it will fit with glue on as long as you don't take too long over getting it clamped.

 

[peter-harrison]

As an afterthought, are you using a following cutter on your template? If so, are the cutter and the bearing exactly the same diameter?

 

[Deadeye]

As an afterthought, are you using a following cutter on your template? If so, are the cutter and the bearing exactly the same diameter?

At the moment I'm using an 8mm cutter in an 11mm guide bush - and the drawings have assumed this, so the "pins" are 1.5mm tighter radii and the "holes" are 1.5mm broader.
However, a disadvantage of the setup (one of) is that I can't see what I'm routing. So one thing I'm thinking about is putting an 11mm bearing on an 8mm cutter in the router table and sticking the template to the top of the workpiece so I can see the cutter bearing as I move the piece along. Is that a daft idea?

You'll correctly conclude from my various posts that I'm new to the game and my ambition is regularly outstripping my capability!

 

[meccarroll]

If a pair of matching joints slide together when dry with only minimal resistance and no play then they will glue nicely. When I make doors I tend to make the mortice and tenon joints so that the tenon will just slide through the mortice under it's own weight without any pressure being applied. It's the perfect joint really, just slides together when dry with no play at all. I have seen forced joints end up twisting a frame or door so think spend a bit of time setting up is the right thing to do. I've never really measured my joints to within 0.1mm at any stage in my woodworking career but maybe it's time to start thinking about it.

 

[Simon_M]

Is it slop in the mechanism? You have 2 gears and a rolled thread and nut. Also many sawblades I use are indeed 3.2 Kerf.

Good point. I don't think it's the blade - it says 3.0mm and my calipers agree.

Have you made a test cut and then used callipers/feeler gauge etc. to measure the cut? If it wasn’t cutting square or there was vibration, then the cut could be bigger than the thickness of the kerf suggests?

 

[Adam9453]

Guide bushes are rarely rigidly fixed into the base of the router enough to give better than 0.1mm accuracy. I’d suggest checking how much your guide bush wiggles before you recut your template. We typically work on 0.1-0.2mm difference to get bits to slot together using our cnc’s. dead fit is always too tight and that’s ignoring that your cnc will have a margin of innacuracy depending upon how well maintained and setup it is. Also is the cutter being used on the cnc brand new and been checked for actual diameter before cutting your template. Not casting aspersions but many people seem to think cnc machines are magical perfect woodwork producing wonder things but they take a great deal of careful setup, programming and testing to get accurate cutting, even then they will only cut as accurately as the cad design, you’d be surprised how many cad files produced by professionals which are less than useful. Good luck and post some pics of the finished article