Hi Folks,
I’m afraid I’m in a bit of a pickle! (Quite a lot of a pickle, actually!) I’m hoping someone can help me out here. It’s a bit off-topic but also rather ironic that it’s happened now, given that I host the monthly Zoom meetings for West Lancs O Gauge Group and we were only taking an initial look at 3D printing for modellers last month - something I've not really got into yet. Hopefully, there will be someone on here who can help. It’s also forced me into a rapid ‘baptism of fire’ with FreeCAD after years of 2D designs on an ancient copy of AutoCAD.
Anyway, I’ve recently been working on my 1989 Volvo 740 where a variety of faults finally caused removal of the instrument binnacle. The odometer had been dead for many years (a regular problem with this model) and the addition of a completely erratic fuel gauge and, finally, intermittent loss of gauge lighting (both also well-known stock faults) finally forced me to take action.
I’ve re-soldered and flux-washed all the dry joints on the fuel gauge and lighting dimmer PCBs and successfully tackled replacement of the disintegrated odometer gear wheels using the after-market kit available for that problem. However, the final move of replacing the pointer on the speedo galvanometer shaft after reassembly, met with disaster. It was so tightly fitted that it required two diametrically opposite screwdrivers beneath to carefully lever it off in the first place. Consequently, replacement required a fair amount of pressure, which bent the ~0.6mm wire spindle. In attempting to straighten it, the inevitable happened, leaving the final 11mm of the wire separated where it emerges from the central shaft tube, complete with the splined end attached to the broken wire. (I’ve since wiped a needle file over the splined end and had it in and out of the pointer’s central boss a few times and it seems to have completely eased off.) The top sleeve bearing has been removed in this picture, for clarity.
Hindsight is a wonderful thing but I now realise I should have gently tapped it on rather than trying to press it on. Unfortunately, I’ve not had any success locating a replacement, which is hardly surprising after such a long time – it seems everyone was after them years ago as replacements when the odometer failed, rather than dismantling and replacing the gears. (It’s 'unicorn time'!)
Looking at it, the simplest way to repair would appear to involve cutting down the height of the tube by a few mm to allow a new 10mm extension to be placed over the existing tube with the upward extension reduced to similar dimensions as the original tube (space for a fractionally larger diameter) and with a centre hole to take a new shaft wire. (See turquoise component in right hand diagram, below.) The existing tube is only 1.2mm diameter and the central wire spindle is just 0.6mm. The splined end has proved impossible to get off the existing wire so will need to be replaced too.
Looking at some specs of resin printers, I’m not sure whether even those would get down to a fine enough resolution to do things like the splines, as the parts are so tiny (though any semblance of splines would probably work because the plastic of the central boss on the pointer already has the female shaped spline cut away and that would probably cut into the new plastic centre piece). Likewise, is it possible to ‘print a hole’ of just 0.6mm diameter for the wire without it filling up with resin? Perhaps the diagonal nature of the slicing would allow for that, as it effectively turns it into an ellipse whose longer measurement would be greater than that? (I don’t think a filament printer would get near that, would it?) Even a part-filled smaller hole would act as a guide for a tiny drill for hand finishing, if required. There's no torque placed on these components - they're just moving a pointer round on the end of the shaft, so I think resin 3D printed parts with a spot of LocTite to fix them would be perfectly ok in principle, so long as it's possible to make such small components.
These are images from the 3D FreeCAD files that I’ve just knocked up as my first attempt at 3D CAD. The extension tube is 10mm overall length, with a 0.6mm through-hole and a recess of 1.7mm diameter at the bottom end to slip over the cut-down original metal tube. The splined end is 7.6mm overall length with the splined part being 2.54mm, the plain part 1mm diameter and there's a 0.6mm through-hole for the wire.
I’ve yet to work out how to drill the brass rivets off the case without damaging the plastic it’s riveted to, as well as unsoldering the delicate coil springs from the supply leads. This is going to need a steady hand!
Any thoughts on the dimensional tolerances and feasibility etc. for 3D resin printing? Also, could anyone help me out by printing a few of them in the hope that they'd work (maybe 2 or 3 of each - in case I damage them!)? Obviously I’d be happy to pay a suitable sum towards resin and postage etc.
My rather battered but still much loved and loyal ‘daily shed’ is now off the road until I can get this sorted!
Thanks,
David.