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Thread Size Help With Starrett 196a Back Plunge Accessory

Bob Korves

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#1

I have a Starrett 196a back plunge indicator set like pictured above. I am missing the 5/16" post that screws into the bar that fits the tool post, the one with the ball formed on the end, second from top in the pic. I would like to make one. As near as I can tell using a thread pitch gage in the tool post bar hole, the thread is 32 tpi, but the hole is small and my eyes and fingers are old. Also, the size is bigger than a 8-32 and smaller than 10-32. I confirmed that it is a special Starrett thread by looking at archived posts on that "other" forum. They were not kind enough to say what the actual size is...

Could someone please confirm it is 32 TPI, get me the O.D. of the thread on the post, the length of the threads and the overall length of the post?

Thanks!
 

Ed ke6bnl

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#2
I just found that clamp in a pile of stuff and could not find any thread american or metric so I just retapped it to 10/24 not suggesting this but at least I can make use of this clamp. I can now make any bar with 10/24 to use with this tool
 

talvare

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#3
Bob,

I have the newer 196A Starrett set that has two different lengths of those posts and they don't have the ball ends on them. Mine are 12-28NF thread, 1/4" long. I don't know if they are a different thread than the older sets, but you could try a test fit with a 12-28 screw or tap. Or, you could try one of mine to see if it fits.

Ted
 

Bob Korves

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#4
Bob,

I have the newer 196A Starrett set that has two different lengths of those posts and they don't have the ball ends on them. Mine are 12-28NF thread, 1/4" long. I don't know if they are a different thread than the older sets, but you could try a test fit with a 12-28 screw or tap. Or, you could try one of mine to see if it fits.

Ted
Thanks, Ted. My thread is definitely smaller than that. It looks like it would be about a 9-32, if there was such a thing. This is old school Starrett, at least 50 years old, probably much older than that, so there is no doubt that it not metric. Starrett used lots of oddball thread sizes on their tooling, probably because they had always made them that way, so why change? It was also easy for them to get oddball tools because they owned Union Twist Drill, just up the river in Athol, Mass.
 
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Bob Korves

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#5
I just found that clamp in a pile of stuff and could not find any thread american or metric so I just retapped it to 10/24 not suggesting this but at least I can make use of this clamp. I can now make any bar with 10/24 to use with this tool
I could probably do that, Ed, but this is an antique tool holder in nice condition, with nice color case hardening. It would be a shame to deface it in the quest of a modern thread. I need to make the post anyway, and am quite sure I can make one that works well and looks near original, just would be easier to not be guessing at the size. Thanks for the idea, and I would probably do something like that if it was not an antique.

It is not the clamp that I am going to screw it into. My clamp has a permanently brazed rod on it. This is the bar that gets clamped into the tool post that I want to screw the post into. My old set is different than the newer ones.
 
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Tony Wells

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#6
I think I may have one of those posts, Bob. If so, you're welcome to it. I'll look this afternoon. I never cared for the back plunger indicators but have a few pieces of a set.
 

chips&more

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#7
Bob, I think something is amiss in your set. The rods you have should screw into that bar. The bar should say Starrett on it. The rod(s) should screw into the clamp and/or bar. It’s all interchangeable. If you need a replacement bar, give me some time to stumble across one in my shop and I will send it to you. If you want to make a bar anyway. I would just trial and error the thread until I got it correct. Shouldn’t take that long on the lathe…Dave.

PS: You can kinda date your Starrett tools by the kind of Starrett logo they used through the years.
 
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Bob Korves

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#8
Bob, I think something is amiss in your set. The rods you have should screw into that bar. The bar should say Starrett on it. The rod(s) should screw into the clamp and/or bar. It’s all interchangeable. If you need a replacement bar, give me some time to stumble across one in my shop and I will send it to you. If you want to make a bar anyway. I would just trial and error the thread until I got it correct. Shouldn’t take that long on the lathe…Dave.

PS: You can kinda date your Starrett tools by the kind of Starrett logo they used through the years.
Dave, this is an older set and it does not have the removable bars in the clamp. I have seen others like it on the web. The clamp is configured in the reverse of the later models. My bar does say Starrett on it. My old set is different than the newer ones. My set is exactly like the one I posted above that I lifted off the web, just missing the post. These are older sets with the slide top box.
 

Bob Korves

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#9
Bob, I think something is amiss in your set. The rods you have should screw into that bar. The bar should say Starrett on it. The rod(s) should screw into the clamp and/or bar. It’s all interchangeable. If you need a replacement bar, give me some time to stumble across one in my shop and I will send it to you. If you want to make a bar anyway. I would just trial and error the thread until I got it correct. Shouldn’t take that long on the lathe…Dave.

PS: You can kinda date your Starrett tools by the kind of Starrett logo they used through the years.
The tool post bar says "The L.S. Starrett Co./Athol, Mass. U.S.A." I think that only tells me it is made after 1896...
 

omni_dilletante

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#11
I am not quite sure what you are asking.

I measured the thread shown, and verified it screws into the bar shown.

32 tpi, 0.178" over the threads.

IMG_3742_zpsec7ujbgm.jpg

I always wondered what that bar is for. Is it supposed to fit into a lantern tool post? Come to think of it, that would be handy.
 

Bob Korves

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#12
I am not quite sure what you are asking.

I measured the thread shown, and verified it screws into the bar shown.

32 tpi, 0.178" over the threads.

IMG_3742_zpsec7ujbgm.jpg

I always wondered what that bar is for. Is it supposed to fit into a lantern tool post? Come to think of it, that would be handy.
THANKS, omni, that is exactly the information I wanted! My best guess was .174 O.D. and 32 TPI, based solely on the idea that machine screw sizing is based on .013" increments per size, and that seemed to be about what a nonexistent 9-32 thread would measure. Your measurement is of the real deal, so I will use it. Nice photo, by the way. Tells the whole story without words. I will get busy and make one. Thanks again!

Yes, the bar is intended to go in a lantern tool post. Mine has quite a few marks on it from being tightened down by one. Mine also does not have the extra hole in the side of the bar, just the vertical one.
 

4gsr

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#13
If it's of any help, I have one set that both 4" and 6" post is 10-32 thread and the other set is different. One post the 4" is 12-28 and the 6" post is 10-32. The 4" post can only be used with the 3/8" x 3/4" tool post bar. The 6" post can only be used with the c-clamp. I've made a couple of extra post to fill out one of other kits I have.

EDIT: I forgot to mention, the threads are slightly undersized. I believe that was done to compensate for the shrinkage of the internal threads from heat treat on the c-clamp frame and tool post bar.
 

Bob Korves

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#14
If it's of any help, I have one set that both 4" and 6" post is 10-32 thread and the other set is different. One post the 4" is 12-28 and the 6" post is 10-32. The 4" post can only be used with the 3/8" x 3/4" tool post bar. The 6" post can only be used with the c-clamp. I've made a couple of extra post to fill out one of other kits I have.

EDIT: I forgot to mention, the threads are slightly undersized. I believe that was done to compensate for the shrinkage of the internal threads from heat treat on the c-clamp frame and tool post bar.
Ken, the male thread needs to be about .178" O.D. and 32 TPI to be the right one, with a ball on the other end, and I think it is about 4-1/2" OAL. I have some 5/16" O-1 drill rod, and threading that small thread close up to the shoulder would be a good challenge. I have a form tool that with an little modification could rough out the ball.
 

talvare

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#15
Is there a purpose for the ball on the end of that rod or is it just decorative ? The rods in the newer sets don't have the ball.

Ted
 

Bob Korves

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#16
As far as I know, the ball is just decorative, but then so is the color case hardening on the bar. Classy looks along with "best" quality tools is what has always separated Starrett from the rest of the pack. Sadly, it is going away, becoming just a another commodity. Starrett will not be able to hold on to their lead without producing tools that have class. In fact, they seem to have been losing ground for at least several decades already.

There could be some type of swivel clamp that fits that ball, but I have not seen one.
 

4gsr

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#17
Ken, the male thread needs to be about .178" O.D. and 32 TPI to be the right one, with a ball on the other end, and I think it is about 4-1/2" OAL. I have some 5/16" O-1 drill rod, and threading that small thread close up to the shoulder would be a good challenge. I have a form tool that with an little modification could rough out the ball.
The ones I have measure .181 and .182" x 32 TPI. Yeah, it is about .010" under for 10-32 but I bet you they tap them 10-32 and you get that much shrinkage from the carburizing process they used to use. Of course, Starrett used lots of special threads, too. As for threading up to the shoulder, there is a slight counter bore at the start of the thread on the C-clamp and tool post bar. The one's I made, I was able to get up very close to the shoulder when threading.
 

Bob Korves

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#18
Thanks, Ken. Yes, I saw the relief inside the bar. I would like to turn it using a tool on the far side of the work and thread out from the shoulder. I will probably harden it after making it, so I will make the corner from shoulder to thread with a slight radius.
 

Bob Korves

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#19
The ones I have measure .181 and .182" x 32 TPI. Yeah, it is about .010" under for 10-32 but I bet you they tap them 10-32 and you get that much shrinkage from the carburizing process they used to use. Of course, Starrett used lots of special threads, too. As for threading up to the shoulder, there is a slight counter bore at the start of the thread on the C-clamp and tool post bar. The one's I made, I was able to get up very close to the shoulder when threading.
Thanks for your measurements, Ken. Your numbers are a little higher than omni_dilletante measured, so perhaps I should shoot for around .180" or so. I would like to get pretty close if I am going to harden it, but definitely not too big.
 

Bob Korves

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#20
Here are some pics of the rod I made, the longer rod is off one of my surface gages, used as a pattern to make the ball:
SAM_1436.JPG SAM_1437.JPG SAM_1438.JPG SAM_1439.JPG
The project came out fine, and on the first try! The 32 TPI threads would not enter at .184" diameter, but it felt like they wanted to, so I lightly filed the thread crests to .182" and tried again without cutting the threads deeper. Nice fit. The threads were cut using a boring bar type HSS hand ground threading tool, with the cutter on the far side of the work, spindle running backwards, kept the half nuts engaged, jogged the carriage back close for the next cut and then turned the chuck by hand the last few revolutions up tight to the shoulder to start the next pass. 460 RPM. The ball was made almost entirely with hand files on the lathe. I think it turned out OK. Thanks to all for the helpful ideas and suggestions.

Edit: I thought about adding the tommy bar cross hole down by the threads for tightening the rod on to the bar, but this set did not have that feature when new, so I left it off. I can always add it later if I think I need it...
 

Tony Wells

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#21
Good job. IMO, those "tommy bar" holes are ONLY for removing it when it gets stuck. Most instruments are made to be tightened by hand only.
 

Bob Korves

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#22
Good job. IMO, those "tommy bar" holes are ONLY for removing it when it gets stuck. Most instruments are made to be tightened by hand only.
More likely, I will add the tommy bar hole when the rod is stuck solid in the bar, making the setup more "interesting."
 
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