On Tue, 28 Jul 2015, Pete Turnbull wrote:
On 27/07/2015 23:13, Tothwolf wrote:
On Mon, 27 Jul 2015, John Wilson wrote:
The only thing here is, it's a *terrible* design. Those damn nubs
snap off so easily (I'm sure decades of outgassing haven't helped),
even when you're expecting it and being super careful.
Yes, even in the late 70s i recall them being considered fragile.
Are the original parts made of nylon? If so, that might explain the
tendency for them to break. Nylon 6/6 is hygroscopic, and over a long
period of time, it could very well be swelling up just enough so the
bulb portions are more likely to break off.
No, they're ABS - at least mine are, I just tested a few. I don't know
what grade, though, and perhaps something more flexible and forgiving -
a different grade of ABS, semi-rigid PVC, or nylon would be better.
My suggestion, however, would be to incorporate a vertical slit in the
ball and a couple of millimeters of the stem, so they're a less tight
and less critical fit. Might help, especially with a less brittle
plastic.
If they are made of ABS and the ball isn't critical, why not mill or drill
out a small bit of the broken stem and solvent weld in some round ABS
stock to make a new stem? Methylene chloride (dichloromethane) works
extremely well for ABS (methyl ethyl ketone in my experience does not work
very well with ABS). I've been using Plast-I-Weld (Flex-I-File) but in the
past I've also used Tenax 7R and Proweld (Ambroid), however Tenax 7R and
Proweld have both since been discontinued.
Another option worth considering is polyacetal / polyoxymethylene
(Delrin). It is tough, stable and inexpensive (but not as inexpensive as
ABS). PVC cold flows under pressure and Nylon absorbs moisture and expands
over time, so neither of those are all that great for this sort of
application.
