Sunday, February 3, 2013

A Snap-Fit Plastic Hinge Repair

[When I began this, I was not at all certain that I could pull it off, but it turned out remarkably well.]

I have a very fine tire pressure gauge with a plastic case. The gauge is superb; the case leaves a bit to be desired. Here's a view of the two 'halves' of the case.

The problem is with the black portion of the upper hinge half. That's the 'female' portion of the hinge, and its socket wall is chipped and won't retain the 'pin' of the clear half of the hinge. It doesn't photograph well, but here's a close-up.

Those hinge halves are about 5/32" 'diameter' -- substantial enough to be drilled through 1/16" diameter for a pin. Drill 'reach' with a standard 1/16" twist drill is likely to be marginal on the black portion, and setting up the pieces on the drill press with acceptable axial alignment with the drill may be challenging.

The drill reach problem, fortunately, is something I solved quite some time ago with this technique. By lengthening a standard drill's shank with brass tubing, I'll have no trouble with reach. As for axial alignment, all I can do is do the best job I can of aligning the work to the drill by eye, and hope for the best. Here's the black portion's hinge half successfully drilled through.

(In the above photo you can see very clearly where the hinge socket is chipped.)

The clear portion's hinge half was rounded on top. I had to file that flat so I could dimple it with an awl for a reliable start to the drilling. That worked out ok -- here's the clear portion's hinge half successfully drilled through.

Both drilling operations went well; here's the hinge assembled with a length of 1/16" rod trial-fitted as a hinge pin.

It dawned on me at this point that a cotter pin would make an ideal hinge pin. Here's a 1/16" x 1" cotter pin installed.

Not a bad outcome at all.

That clear lid is begging for a paint job -- it's pretty shabby looking, and I don't need to be able to see through it to know what's inside.

- - -

I had grey primer, and a bit of leftover metallic silver enamel on hand. Here's the lid with the second coat of enamel just applied and still wet.

When that paint has hardened and I have this all done, I'll photograph it again along with the gauge that resides in it.

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All Done -- FRIDAY, FEBRUARY 8, 2013

Here's the case reassembled with its painted lid.

(Some blemishes on the clear plastic 'telegraphed' through the paint. I don't know if there's a sure way to preclude that from happening -- sanding, possibly.)

And here's the gauge back in its renovated home.

Update -- SATURDAY, AUGUST 20, 2016

The lower hinge developed exactly the same fault that the upper hinge had, and I repaired that in exactly the same way. Here's a view of the case now that both hinges are pinned with cotter pins.

Having to repair both hinges in this manner introduces a slight complication -- both hinges' pin-bores ought to share the same axis, and that's a difficult condition to meet.

I've managed to get close enough that the repair does work, but there's a slight binding condition when the cover is fully opened. That binding is an indication that the two hinges' axes are not perfectly in line with one another.

As long as the hinge bodies can tolerate the stress that the slight misalignment introduces, the repair will continue to function ok. If the stress should result in a cracked hinge body, then it will be game over for this plastic case. We'll see if this repair lasts.

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