Fixing-up a Shore hardness tester (Type "T")

When hobbying-around with typewriter platens, it's about rubber and the hardness of that rubber. To give that a number, usually the Shore hardness or Durometer hardness gets mentioned. To be able to measure that myself, tried to get a Durometer - and instead of one of the common, widely available items I tried to get a period-correct specimen to match the pre-war typewriters.

So already a few years ago, I bought a relatively cheap, used Shore Durometer on the local classifieds site. That proved to be a a bad purchase. The listing pictures hadn't shown it, but it was a wreck; missing window, broken pointer and somehow jammed halfway the scale. The dial was so heavily pitted, that it was hard to read anyways. The item was put away on a shelf, written off as a 'fail'.

However, having gained some more confidence and because of the platen-testing with the Blickensderfer, it was taken down again and opened up. Normally wouldn't dare take apart such an instrument, but it was broken anyways so nothing to lose.

Very quickly a new window was cut from some plastic-sheet (packaging) and a thin strip of card added to the curved, shiny distance bracket. This is needed to compensate for the difference in thickness of the plastic sheet with the original glass. With a fine camelhair brush, the black pitting of the dial was retouched with gold watercolour paint.

The spring-assembly was taken out and the small bearing in the lower-left corner that holds the axle of the pointer was screwed-out. That allowed the pinion to slip a tooth with the rack-segment on the horizontal 'weighing-bar' that holds the pin. (No idea how that got out of sync, it must've had quite some mistreatment!)

Re-assembly is -as they say- the reverse of disassembly.

This leaves however the adjustment - this specimen will not ever be a reliable tester again, but wanted to make an attempt to at least get it 'reasonable'. Still more reading-up to do on Durometers (a lot more!), but it boils down to setting/checking pin-exposure, adjusting spring-constant and setting/shifting the force-level.

First-off, the pin should be exposed 0.1" from the base (abutment). This is unlikely to be out of adjustment, but astonishingly this one was. The pin can be screwed in or out and is fixed with a tiny set-screw (orange). Vernier-calipers should be fine for now. Then the spring constant should be adjusted to be about 74 gf per 10 on the scale (that is for medium-spring Durometers - there's a world out there!).

To be able to measure, read the scale and adjust at the same time, a testing stand was made from Meccano. Fortunately the Meccano standard BSW 5/32 thread is a beautiful fit for the threaded hole in the knob of the Durometer - made for it!

To adjust the ratio of pointer-movement per force applied, the link can be moved over the beam. To reduce movement per force, move to right and vice-versa. Fix in place with its screw (green).

Then the absolute level of force for the values has to be set. That can be done by changing the pre-load with the distance-bush. Use small pliers or a tiny wrench to rotate the bush until right (purple).

All this having been done (a couple of times), the Shore Durometer again reads close to 60 when pressed against the check-block that was in the box. There may well be a better way, but so far this seemed the logical order of things. (Maybe the factory adjusting instructions for these instruments will shortly show up online!)

After reading more about Durometers, it's become clear that this set is a hodgepodge - it is wrong. The check-block has serial nuimber 6622 and the instrument is 13764 - the check-block should be with instrument 6622 and 13764 lost its own check-block. The black leather box likewise is wrong for the 1940s crinkle-black instrument, probably it's the box of the 6622 instrument.

Originally I had assumed that the "T" on the dial was some confusing 'freak' corrosion, making it look like Type "T". But this is actually correct! This is really is a Shore Durometer Type "T". It has a spherical presser-pin, the '2-pound' spring and a concave abutment plate - i.e. a Type "T" tester. 

The Type "T" Durometer is probably pretty rare, as it was made for a very specific use-case in one specific industry. (Lucky find.) It is also not at all suitable for assessing the rubber of typewriter platens. (Less lucky.)

Shore Durometers are a whole subject in and of themselves, as I'm discovering - more Durometers to come I'm sure and perhaps an agelist to be compiled too!

(rabbit-hole :-)