Glass screw
March 9, 2021 7:59 PM Subscribe
I have a Hemingray Glass Insulator like this one. It has threads on the inside, i.e female. What did it screw onto and what are the thread dimensions?
Response by poster: @stray:
That looks correct, thanks.
This leads me to the question ... how could I go about impressioning the insulator to get the screw dimensions, without destroying the insulator?
posted by falsedmitri at 8:11 PM on March 9, 2021
That looks correct, thanks.
This leads me to the question ... how could I go about impressioning the insulator to get the screw dimensions, without destroying the insulator?
posted by falsedmitri at 8:11 PM on March 9, 2021
Best answer: There are insulator enthusiasts out there, here's one such forum.
posted by stray at 8:16 PM on March 9, 2021 [2 favorites]
posted by stray at 8:16 PM on March 9, 2021 [2 favorites]
Impressioning: IMHO, plasticine or modeling clay; you don't need to impression the whole thing, just a big enough portion of it (eg: half of the circumference, full depth), remove carefully (pull away from surface, rather than trying to "unscrew"), grab a digital caliper and measure carefully. Shouldn't be too hard, really.
If you want to get completely crazy you could use metrology rubber, but that's large dollars for something that probably isn't highly precise in the first place.
posted by aramaic at 8:17 PM on March 9, 2021
If you want to get completely crazy you could use metrology rubber, but that's large dollars for something that probably isn't highly precise in the first place.
posted by aramaic at 8:17 PM on March 9, 2021
Specification for Standard Thread for Pins and Insulators, Proceedings of the American Electric Railway Association, New York, NY, United States, 1920, pg 161-163.
posted by RichardP at 8:19 PM on March 9, 2021 [12 favorites]
posted by RichardP at 8:19 PM on March 9, 2021 [12 favorites]
What I remember from years ago, the dowels that these insulators were mounted on weren't threaded, the wood was soft enough for the threads on the insulator to just cut in as it was twisted on.
And those little "nubbins" on the bottom? They were the cut into the crossbar, so they wouldn't come off due to weather.
posted by Marky at 1:05 AM on March 10, 2021 [1 favorite]
And those little "nubbins" on the bottom? They were the cut into the crossbar, so they wouldn't come off due to weather.
posted by Marky at 1:05 AM on March 10, 2021 [1 favorite]
It looks like the thread specifications were standardised as linked by RichardP above, but I'll answer your question about how to measure it anyway, because I'm interested in that sort of thing. Disappointingly, this thread is missing from my treasured and otherwise quite comprehensive machinist's screw thread reference, 1941 edition. But probably understandably so.
Firstly, I don't think you'd need to take an impression of it to get the dimensions of the thread, if you had the right measuring tools. It's wide enough that you can use a thread gauge inside the hole, to get the pitch. You'd want the sort where the leaves are held together with screws rather than rivets so that you can remove a single leaf for any given TPI and probe the hole with it.
You can measure the minor diameter of the thread with any set of internal calipers that will reach into the hole. If you measure this at the nearest thread, and at the farthest thread, and count how many threads there are in between, then since you know the thread pitch, you know the distance between diameter measurements, and you can calculate the taper angle.
The depth of the thread is a bit more tricky - ideally you'd use an inside groove caliper to measure the major diameter, but that's a somewhat obscure and usually overpriced tool. But you can get a rough idea by measuring the diameter of the hole just before the thread starts.
But let's say you don't have a thread gauge, or inside calipers, or if you do they're too big to get inside the hole. In that case it makes sense to take an impression.
To take an impression which is usable to measure the thread, you'd need the impression to fill the grooves of the thread, which means you'll either need to unscrew the impression to remove it, or have it be soft and stretchy enough to pull it out without damaging the impression.
A hard material would be a risky choice here because a glass thread is unlikely to be very accurate or consistent in shape, so it's likely that a hard impression would jam while trying to unscrew it. So you want something that's soft, but you also want it not to change shape in the process of removing it or your measurements will be inaccurate.
Metrology rubber, as mentioned above, is designed specifically for this - but it's expensive. Silicone is good enough for this sort of thing though. It's soft and stretchy but it's quite dimensionally stable - it springs back to the shape and size it was cast in very accurately if it's a reasonably sized piece.
You can get silicone as a two-part liquid mix - you mix the components together, then pour it in and let it set. Or you could use silicone potting compound, which comes in a single tube and starts to cure on contact with air, but this takes longer.
You'd also need to make sure that whatever you used didn't stick to the inside of the hole. So before putting your casting material in it, you'd want to apply a release agent. Some PTFE lubricant would do. The proper stuff comes in spray form, but a spray wouldn't cover sides of the thread that you can't see looking into the hole, so I'd want to spread the release agent around with a brush - a toothbrush would be ideal as it points sideways.
I would hang a flat or cross-shaped stick of some sort into the middle of the hole while casting, so that it gets set into the cast and can be used as a handle to remove it once cured. Even with the release agent, there will probably be a fair amount of friction, so I would use the handle to pull the casting away from each side of the hole in turn and drop a little oil into the gap to help lubricate it out, and try to unscrew it rather than just pulling it out if possible.
From the silicone impression, it should then be possible to measure the major and minor diameter with a regular outside caliper. You'd normally still want a thread gauge for the pitch, but if it's coarse enough you can get a reasonable measurement by eyeballing one ridge to the next with calipers and checking that the result is close to a round number of TPI.
At which point you'd hopefully get the same answer that's already been linked: 4TPI, 1/8" depth, a 1" major diameter at the bottom of the hole, and widening from there at a taper of 1.111 inches per foot.
Evidently I still have more obscure screw thread standards to discover.
posted by automatronic at 1:15 PM on March 10, 2021 [4 favorites]
Firstly, I don't think you'd need to take an impression of it to get the dimensions of the thread, if you had the right measuring tools. It's wide enough that you can use a thread gauge inside the hole, to get the pitch. You'd want the sort where the leaves are held together with screws rather than rivets so that you can remove a single leaf for any given TPI and probe the hole with it.
You can measure the minor diameter of the thread with any set of internal calipers that will reach into the hole. If you measure this at the nearest thread, and at the farthest thread, and count how many threads there are in between, then since you know the thread pitch, you know the distance between diameter measurements, and you can calculate the taper angle.
The depth of the thread is a bit more tricky - ideally you'd use an inside groove caliper to measure the major diameter, but that's a somewhat obscure and usually overpriced tool. But you can get a rough idea by measuring the diameter of the hole just before the thread starts.
But let's say you don't have a thread gauge, or inside calipers, or if you do they're too big to get inside the hole. In that case it makes sense to take an impression.
To take an impression which is usable to measure the thread, you'd need the impression to fill the grooves of the thread, which means you'll either need to unscrew the impression to remove it, or have it be soft and stretchy enough to pull it out without damaging the impression.
A hard material would be a risky choice here because a glass thread is unlikely to be very accurate or consistent in shape, so it's likely that a hard impression would jam while trying to unscrew it. So you want something that's soft, but you also want it not to change shape in the process of removing it or your measurements will be inaccurate.
Metrology rubber, as mentioned above, is designed specifically for this - but it's expensive. Silicone is good enough for this sort of thing though. It's soft and stretchy but it's quite dimensionally stable - it springs back to the shape and size it was cast in very accurately if it's a reasonably sized piece.
You can get silicone as a two-part liquid mix - you mix the components together, then pour it in and let it set. Or you could use silicone potting compound, which comes in a single tube and starts to cure on contact with air, but this takes longer.
You'd also need to make sure that whatever you used didn't stick to the inside of the hole. So before putting your casting material in it, you'd want to apply a release agent. Some PTFE lubricant would do. The proper stuff comes in spray form, but a spray wouldn't cover sides of the thread that you can't see looking into the hole, so I'd want to spread the release agent around with a brush - a toothbrush would be ideal as it points sideways.
I would hang a flat or cross-shaped stick of some sort into the middle of the hole while casting, so that it gets set into the cast and can be used as a handle to remove it once cured. Even with the release agent, there will probably be a fair amount of friction, so I would use the handle to pull the casting away from each side of the hole in turn and drop a little oil into the gap to help lubricate it out, and try to unscrew it rather than just pulling it out if possible.
From the silicone impression, it should then be possible to measure the major and minor diameter with a regular outside caliper. You'd normally still want a thread gauge for the pitch, but if it's coarse enough you can get a reasonable measurement by eyeballing one ridge to the next with calipers and checking that the result is close to a round number of TPI.
At which point you'd hopefully get the same answer that's already been linked: 4TPI, 1/8" depth, a 1" major diameter at the bottom of the hole, and widening from there at a taper of 1.111 inches per foot.
Evidently I still have more obscure screw thread standards to discover.
posted by automatronic at 1:15 PM on March 10, 2021 [4 favorites]
This thread is closed to new comments.
posted by stray at 8:06 PM on March 9, 2021 [1 favorite]