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Identifying an odd thread.
- Thread starter Susquatch
- Start date
Another great idea!
Sadly, no dro on the lathe yet. It's still in the box waiting for a time when I can do without my lathe long enough to install it.
The only thing I've done with it is test it to make sure it and the scales are working.
Brent H
Ultra Member
My thoughts:
Measure OD of the thread, measure the TPI or check the pitch for metric. If things are “weird” then is the thread tapered? Straight? Application?? Is it hydraulic, gas, where is the contraption made?
The threads can get crazy - British Straight pipe, G1, G2 etc. typically you can find it, unless it is truly an odd ball - like my 1/2” x 1 mm pitch acme feeds on my “metric lathe”.
Measure OD of the thread, measure the TPI or check the pitch for metric. If things are “weird” then is the thread tapered? Straight? Application?? Is it hydraulic, gas, where is the contraption made?
The threads can get crazy - British Straight pipe, G1, G2 etc. typically you can find it, unless it is truly an odd ball - like my 1/2” x 1 mm pitch acme feeds on my “metric lathe”.
lucsimoneau
Well-Known Member
Last time I had to measure an internal thread without exactly knowing what it was I used the grandkid's playdough - pushed it in enough to create the thread impression and took it back out. No drama, works every time (other then using all the guages around in the garage 
It's not that hard. However, I think I have a better idea for both internal and hidden external. A 1" strip of 1/4x1/4 modelling clay can be pressed into the threads and then measured or counted or compared under a magnifying glass...... Best of all, it can be saved and re-used even if it gets a little dirty over time!
Thanks again @fixerup! Amazing what things can come from an idea!
Last time I had to measure an internal thread without exactly knowing what it was I used the grandkid's playdough - pushed it in enough to create the thread impression and took it back out. No drama, works every time (other then using all the guages around in the garage
Seems your post crossed mine in the interweb! Same concept though! I love it!
FatherWes
Wes
To get the pitch on both internal & external threads I’ve used a piece of paper to make a rubbing. For internal threads, I make a tube of paper and roll it out and expand it to fit.
The rubbing works great with a caliper and magnifying glass to get an accurate pitch measurement. I’ve measured the angle with thread gauges, taps or bolts. The angle is the iffy measurement.
The rubbing works great with a caliper and magnifying glass to get an accurate pitch measurement. I’ve measured the angle with thread gauges, taps or bolts. The angle is the iffy measurement.
Another GREAT idea!
I've used Bondo to make impressions. As long as the surface has a thin film of oil it will release OK. But don't get yourself into unchartered troubles, test it on a bolt or something beforehand. Not the best for threads because you could only get a half diameter impression, but maybe helpful if you don't have good access or facing a major teardown to evaluate the thread up close.
Some good ideas in all this, I kinda like the bondo one, plus can cut threads easy with a tap to check a shaft. But do need room to apply, which was part of the original problem.
I laugh at people “but the metric thread system is so simple, only course and fine thread”. Yeah, right! B.S.!! Not that imperial/standard is very great either, course, fine, extra fine, instrument thread, old imperial to new imperial, (some changed, others didn’t), screw gage threads, tapered pipe threads vs straight, Whitworth, and so on. Dam, I better stop. Oh, wait, collets, tool holders, ahh, but, no stop.
I laugh at people “but the metric thread system is so simple, only course and fine thread”. Yeah, right! B.S.!! Not that imperial/standard is very great either, course, fine, extra fine, instrument thread, old imperial to new imperial, (some changed, others didn’t), screw gage threads, tapered pipe threads vs straight, Whitworth, and so on. Dam, I better stop. Oh, wait, collets, tool holders, ahh, but, no stop.
So, I tried a paper rubbing. That worked GREAT!
The only problem is that both 20tpi and 1.25 line up just fine. I even used taps to evaluate. Can't decide which is better. I am leaning toward metric 1.25.
In fact, even tap to tap, you can barely see the misalignment over the length of the tap. Unless it's a custom thread (highly unlikely) the deciding factor will have to be the diameter.
Measured very carefully just now the OD Measurement is:
0.776in and 19.71mm.
For an imperial 13/16-20 thread, the pitch is 1/20=0.050
For a 60 degree thread, the thread height is cos(30) X 0.050 = 0.866x0.050 = 0.043. The crest removal is 1/8 of that which is 0.0054. For both sides, that is 0.0108 (call it 11 thou.) so the total diameter should be 13/16 - 11 thou. That = 0.802. But it measures 0.776. That's virtually no thread left.
19.71mm For a metric 60° thread, the crown is relieved by 1/8 of the thread height. The height is. Cos(30) x the pitch = 0.866x1.25. 1/8 of that is 1.25x.8666/8 = 0.135 mm. But for diameter reduction it happens on both sides. So the total diameter reduction should = 0.270mm. So the diameter of a 20mm thread should be 20 - 0.270 = 19.730 Very close to the measured 19.71.
So I agree with @MrWhoopee. But I have no idea why he thinks my math is a little off.
I ordered the tap & die earlier. But it won't arrive till much later this month. I wish it had turned out the other way around because the 13/16-20 I also ordered should be here tomorrow.
The only problem is that both 20tpi and 1.25 line up just fine. I even used taps to evaluate. Can't decide which is better. I am leaning toward metric 1.25.
In fact, even tap to tap, you can barely see the misalignment over the length of the tap. Unless it's a custom thread (highly unlikely) the deciding factor will have to be the diameter.
Measured very carefully just now the OD Measurement is:
0.776in and 19.71mm.
For an imperial 13/16-20 thread, the pitch is 1/20=0.050
For a 60 degree thread, the thread height is cos(30) X 0.050 = 0.866x0.050 = 0.043. The crest removal is 1/8 of that which is 0.0054. For both sides, that is 0.0108 (call it 11 thou.) so the total diameter should be 13/16 - 11 thou. That = 0.802. But it measures 0.776. That's virtually no thread left.
19.71mm For a metric 60° thread, the crown is relieved by 1/8 of the thread height. The height is. Cos(30) x the pitch = 0.866x1.25. 1/8 of that is 1.25x.8666/8 = 0.135 mm. But for diameter reduction it happens on both sides. So the total diameter reduction should = 0.270mm. So the diameter of a 20mm thread should be 20 - 0.270 = 19.730 Very close to the measured 19.71.
So I agree with @MrWhoopee. But I have no idea why he thinks my math is a little off.
I ordered the tap & die earlier. But it won't arrive till much later this month. I wish it had turned out the other way around because the 13/16-20 I also ordered should be here tomorrow.
Pitch IMP = 20 TPI = 1/20 = 0.0500"
Pitch MET = 1.25/25.4 = 0.0492"
So 0.00079" pitch difference assuming both are the same thread angle. Can you turn whatever screw you are operating a maximum number of turns & measure physical displacement? Even 10 turns would amount to 0.008" difference between the two. You probably already consulted tables, but here is how the 2 sizes compare +/- thread class
Pitch MET = 1.25/25.4 = 0.0492"
So 0.00079" pitch difference assuming both are the same thread angle. Can you turn whatever screw you are operating a maximum number of turns & measure physical displacement? Even 10 turns would amount to 0.008" difference between the two. You probably already consulted tables, but here is how the 2 sizes compare +/- thread class
It's a very short stub thread for a collar Peter. But I agree with you. The difference between 20tpi and 1.25 is very small. It would not be a problem till you get out there a ways.
It's worth doing what you are doing even though I now know (based on the thread OD) that's its a metric 20-1.25 virtually for certain. The purpose of this thread is not to solve my problem (although that is nice), it's just an example. What I really want is to define a generic method that everyone can use.
What tool are you using to develop those charts. It sure beats my math!
Downwindtracker2
Super User
At a glance, is it too fine to be UNC and too coarse to be UNF ? Of course it could be 1/2 x12tpi Whitworth or maybe old John Deere.
Nah, we have established that it's M20-1.25.
the left one IMP (although it also does MET)
UN imperial screw thread calculator
the right one MET
as I mention in my other post about metric threads, there can still be differences, presumably based on defined or inferred fit class, but for 'common' threads these tools work pretty good.
When I put the power feed on the knee of my mill, the adapter that came with the Align kit would not fit. I recall making my own and having to do it twice because it was a metric dia with an imperial thread pitch, or something like that.
My first encounter with an old British lathe where I found no bolts would fit in certain places. I used an internal threading tool held tight in the threaded bore, with a small clamp, clamped flush and blocks to determine that it was 1/2x12tpi, not 13tpi by withdrawing the tool from the bore 11 clicks till the 1" block fit under the clamp. It took some faffing about to get it worked out, but it worked. Later on i just removed another bolt and measured it at 12tpi. Duh. But in fairness to me, it was a bastardized contraption, with a mix of 1/2-12, 1/2-13, and even some metric, and I'm sure some of the newer 13tpi bolts were probably installed via self tapping .
My first encounter with an old British lathe where I found no bolts would fit in certain places. I used an internal threading tool held tight in the threaded bore, with a small clamp, clamped flush and blocks to determine that it was 1/2x12tpi, not 13tpi by withdrawing the tool from the bore 11 clicks till the 1" block fit under the clamp. It took some faffing about to get it worked out, but it worked. Later on i just removed another bolt and measured it at 12tpi. Duh. But in fairness to me, it was a bastardized contraption, with a mix of 1/2-12, 1/2-13, and even some metric, and I'm sure some of the newer 13tpi bolts were probably installed via self tapping .
My Hartford Bridgeport Clone is half metric and half imperial. I don't pretend to know all the logic behind that. But I know it was made in Taiwan and I know they were famous for mixing and matching metric and imperial as required. Thank God that the mill's operating interface is 100% imperial. I guess I would have been ok with a fully metric mill, but I still think in imperial and convert so I'm better off this way.
Might interest you to know that the part I've been using to facilitate the discussion is the thread where an adapter would be required to attach a power feed to the knee drive. It's a 5/8 shaft with an M20-1.25 thread at the bearing.
Good on you for remembering the details of that. I'd be hard pressed to remember it was a mill I'd been working on!
MrWhoopee
Well-Known Member
Perhaps it was your measurment rather than your math, .787 is almost exactly 20mm.