To make new feet for an Underwood 5; a simple plastic cylinder was made to take original, salvaged bolts taken from disintegrated Underwood 5 feet. These could have been 3D-printed in rubber (TPU), but in this case printed from rigid PLA and given a pocket to take 3 or 4 mm stick-on furniture felt. The felt was given a rubber-coating for anti-slip.
These foot-bolt inserts are turned from square stock and have a ~6.15 mm threaded stud for mounting on the machine. Re-using these bolts is a good thing, because this is today an uncommon screw-thread. It's almost the same as UNC 1/4" with 20 tpi, but not quite. With tolerances of manufacture, a modern quarter-inch UNC screw may fit the machine, but it's about 0.2 mm too wide. (Outer diameter of UNC 1/4" is 0.250" (obviously) and #14 diameter is 0.242".)
An Underwood 5 has American N 14-20 threaded holes for mounting the feet; this thread was removed from ASME standards already in the 1940s. This used to be a fairly common screw size as I've read. To add to the diversity of threads, it exists both as 14-20 and 14-24 (i.e. 24 threads per inch). In either case, today nearly unobtainable.
So it's a good thing to re-use the original bolt-inserts of the old, original feet if possible.
Sizing of the feet themselves is a guess - estimating from the remains of the old feet, chose a diameter of 20 mm (25/32 inch ?). On photographic evidence of the 1920s the feet do look similarly thin and cylindrical, but not quite as thin as all that.
Giving it some thought, a diameter of 7/8" is more likely. As we're 'anoraking' anyways and these being easily printed; manufactured a new set with the larger 7/8" diameter.
These look a bit more substantial - and less ridiculously flimsy on the machine. Now the Underwood 5 typewriter with new cylindrical feet that are probably similar to the original when-new feet.
It remains to be seen how well these feet hold up over the years. The PLA material is very strong at room-temperatures, but will weaken when hot. At temperatures over 60 degrees C they will probably slowly 'collapse' and be squished (actually, just like the original feet did! Very authentic :-). That's where the felt-pads come in, they will prevent the bolts from protruding and scratching the table.
For now, this common Underwood is on a stable footing again. And with the cushioning felt, also much quieter on the table! :)
