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I didn't want to write this, but I think I have to.
After a break from them, I've been tinkering with my 3D printers for the last few days. I have reluctantly concluded that what I've been claiming for the last couple of years isn't going to be practical for most users.
I did say that it was an experiment. Possibly I said it more than once.
My hope and intention was that the plug track chairs could be a dry press-fit in the sockets. But two factors are now clear:
1. maintaining the required tolerance dimensions on the plug and socket requires constant tweaking of the printer settings and the 3D export settings in Templot. Just changing to a different reel of nominally identical filament, or a different batch of resin, or a different temperature in the workshop, is enough to change the finished dimensions and require a tweak to the settings. Otherwise combined with the rounding effects of the typical printer resolutions, some of the chairs may be loose and liable to fall out, and/or some of them may be so tight that they need a hefty bash fit in the socket, risking damage to the chair and a bumpy ride if they don't seat fully home.
Finding some settings which result in the majority of chairs falling mid-way between these extremes requires constant measuring and adjustments. Some users may be happy to do that, but I'm thinking now that press-fitting is not a workable default option for the majority of users. Some of you may have reached the same conclusion, but didn't like to say so?
2. there is now evidence, at least for FDM timbers, that chairs which were originally a good press-fit will become loose after a few months. This is presumably caused by plastic deformation of the polymer under stress from the press-fitting. I ought to have expected that, but I didn't give it enough thought. As I said, an experiment. It can be avoided by applying some sealant into the underside of the sockets after assembly -- but in that case why not glue them in the first place?
What to do about it? Well the obvious solution is to say that the default option is for the chairs to be glued in place, and that dry press-fitting is an option for those who want it. It may possibly work ok for laser-cut plywood timbers (I'm not sure), or for CNC-milling in MDF (I'm a bit more confident about that) or solid wood (I'm quite confident about that). But since the default timbering option is FDM 3D printing, the default plug fit method obviously needs to correspond with that.
All of which is a bit dispiriting and disappointing. It's been a long road to reach this point, and the idea of dry assembly without the dreadful pong of butanone was the great hope which set me off on this experiment.
So if the chairs are to be glued in, how and with what adhesive? Most common solvents don't work on the resin chairs or the PLA polymer. An important consideration when using the loose jaws is that no surplus adhesive should get into the pin slots. For typical track construction, there may be several days between fitting the chairs in the timbers, and adding the rails and loose jaws. Ample time for any glue in the slots to set solid.
For some time now my preferred sealant has been a tinlet of old-style Humbrol enamel paint from the back of the modelling cupboard. It can be applied cleanly with a cocktail stick, is thin enough to penetrate gaps and cracks, and eventually sets quite solid. But eventually is the operative word. I doubt the modern acrylic paints would work in quite the same way. But they might do.
Enamel paint is obviously no good as an assembly adhesive, if you have to wait a fortnight for it to set. So I've been looking for something similar that might set a bit faster, but not so fast that it adds unwanted stress to the assembly process.
I've seen white typing correction fluid mentioned as a model-making aid for filling gaps. (It came up again recently on the Scalefour forum.) I had assumed that it had disappeared from use -- and so it has for office use, obviously. But it seems that it is still used in schools, so I had a look in the local pound shop and found this:
Doing some googling it seems that there are two versions of this. One is traditional solvent based and dries very quickly -- which means it would be of little use as an open adhesive. But it seems to have been replaced with this water-based solvent-free version, which being water-based takes a short while to dry -- and longer on non-absorbent surfaces.
So I've tried some, and it looks promising. After about an hour chairs which were loosely pressed into place are quite firm, and next day are firmer still -- they can be pushed out from below with sufficient brute force, but amply solid enough for our purpose.
The integral brush in the cap is just a bit large to fit easily in a socket (4mm scale), but it could be trimmed to a finer point. Or a fine paint brush used instead. The fluid can be brushed around the insides of the socket before the chair is pushed into place. The important point is that it is a liquid, not a sticky glue, with the result that it doesn't get into the loose jaw slots, or at least it hasn't done on any of the chairs I've tried.
A final point -- if you apply a bit too much so that it strays onto the timber surface, just wipe it along the timber. It then dries very quickly to a streaky effect with just a hint of texture -- perfect to represent sawmill marks on the timber surface (not actual wood grain, that's daft). So it might have a track-building use even if not successful as an adhesive/sealant.
The water-based version has a distinctive smell, but not unpleasant. Of course, for all I know it may be just some ordinary white paint!
If you want to try it, make sure it is the water-based solvent-free version and not old stock of traditional correction fluid. I think that will dry too fast to be usable.
Also, ideally for gluing it needs changes to the default socket sizes to create an equal clearance all round the plug. At present there is a clearance along the sides of the plug and an interference fit at the ends. I see yet more tick boxes.
Other suggestions for a suitable glue for chair fixing welcome. Preferably inexpensive and not messy to use. Where is Seccotine fish glue when you want it?
cheers,
Martin.
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