Centralizing Acme inserts

Okay, I can find Acme thread inserts, but for some reason I can't find inserts for a "centralizing" Acme thread. They are very close, except that
Centralizing threads are not quite as deep on the thread height. What we've been doing is using a regular insert and carefully grinding about .010 or .015"" off the top and stoning a radius in the corners. Yeah, it's working, but jeez.
Anyway, Sandvik doesn't list them, nor does Vardex or Valenite. Anyone out there run into this before?
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Tom Accuosti wrote:

Tom,
Check out the Vardex or Carmex Stub Acme inserts. This might be what you need.
JR
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JRWheels wrote:

Aren't the stub Acme even shorter than centralizing?
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Tom Accuosti wrote:

A stub acme is half the height of a standard acme.
John
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| Tom Accuosti wrote: | | JRWheels wrote:
| | | Check out the Vardex or Carmex Stub Acme inserts. This might be | | | what you need. | | | | | | Aren't the stub Acme even shorter than centralizing? | | | | | | | | A stub acme is half the height of a standard acme.
Yeah, I just looked it up. They won't work.
I think that I'm going to call my customer tomorrow to see what they did when they made them in-house. I do, though, have a suspicion that they did the same thing: used regular inserts and stoned off the top.
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On Wed, 07 Jun 2006 20:33:46 GMT, "Tom Accuosti"

Why not take a finish cut on one of the flanks?
--
- JN -

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Jan Nielsen wrote:

Because it would wipe out the radius that should be at the corners of the top of the thread. But it's a good question, I wonder if there could be some size range in which it might work.
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On Fri, 09 Jun 2006 15:32:42 GMT, "Tom Accuosti"

We often grind custom inserts for acme thread profiles with radius on both the top and bottom profile. A chamfer on the top will often be enough, but a radius looks more snazzy. If the insert is for multi start threads with a pronounced helix, we often put different cutting angles on left & right flank. Typically, the tolerance is + -0.02mm, but if the customer is willing to pay, we can do + - 0.01mm or better. --
-JN-
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On Fri, 09 Jun 2006 18:40:25 +0200, J. Nielsen

===========where are you located and what is your web site url?
Thanks
Unka George (George McDuffee)
There is something to be said for government by a great aristocracy which has furnished leaders to the nation in peace and war for generations; even a democrat like myself must admit this. But there is absolutely nothing to be said for government by a plutocracy, for government by men very powerful in certain lines and gifted with the "money touch," but with ideals which in their essence are merely those of so many glorified pawnbrokers.
Theodore Roosevelt (1858-1919), U.S. Republican (later Progressive) politician, president. Letter, 15 Nov. 1913.
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On Fri, 09 Jun 2006 11:48:09 -0500, F. George McDuffee
Denmark, an obscure little contry, by many believed to be the capital of Sweden. <g>
www.ka-slib.dk Sorry, it's in danish, but it has a few pictures. Clicking "kontakt" should pop-up an email page. --
-JN-
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On Fri, 09 Jun 2006 21:35:21 +0200, J. Nielsen

===========Very nice look site. I am stuck with only a 28.8 Kb POTS dial-up connection and I appreciate the omission of music, dancing girls, etc. as these take forever to load.
You have made very good progress from 1986/88. Do you have problems finding qualified Danish employees?
Thanks
Unka George (George McDuffee)
There is something to be said for government by a great aristocracy which has furnished leaders to the nation in peace and war for generations; even a democrat like myself must admit this. But there is absolutely nothing to be said for government by a plutocracy, for government by men very powerful in certain lines and gifted with the "money touch," but with ideals which in their essence are merely those of so many glorified pawnbrokers.
Theodore Roosevelt (1858-1919), U.S. Republican (later Progressive) politician, president. Letter, 15 Nov. 1913.
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On Wed, 07 Jun 2006 20:33:46 GMT, "Tom Accuosti"

====================A tangential question on acme threads. Does anyone know why the included angle is 29 degrees?
A quick question to JN in Denmark -- how much problems do you have keeping the 29 degree acme tools/products separate from the 30 degree iso/din trapezoidal tools/products.
Does anybody know why Whitworth threads have a 55 degree included angle? [It took a while but I was able to dig this out of a very old engineering text.]
Unka George (George McDuffee)
There is something to be said for government by a great aristocracy which has furnished leaders to the nation in peace and war for generations; even a democrat like myself must admit this. But there is absolutely nothing to be said for government by a plutocracy, for government by men very powerful in certain lines and gifted with the "money touch," but with ideals which in their essence are merely those of so many glorified pawnbrokers.
Theodore Roosevelt (1858-1919), U.S. Republican (later Progressive) politician, president. Letter, 15 Nov. 1913.
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On Fri, 09 Jun 2006 17:12:49 -0500, F. George McDuffee

Nope!

I can't recall doing any 29 degree profiles. 30 degree seems pretty much the norm over here. (and an occasional 40 degree) When I mentioned acme it was to avoid confusion - we refer to the profile as trapeze thread.

Do tell... Whitworth is my hero due to his work on small bore rifling. (back in an era when .45 cal was considered small) --
-JN-
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On Sat, 10 Jun 2006 17:23:29 +0200, J. Nielsen

========================The Whitworth screw thread system including standardized thread form, diameters and pitch per diameter was originally published in 1841. Obviously, Mr. Whitworth had been conducting work on this for several years previously. At that time there was no good and easy way to calculate stresses, and no good data on materials. Thread forms, pitches and screw sizes were whatever the blacksmith felt like making that day, (and what he had tools for.)
The Whitworth threads were empirical before they were imperial, in that he collected samples of every thread that had been in use for a while from every part of England (I think Scotland, Wales, Ireland, etc. were excluded) assuming that if it had been in use over time, it was most likely an acceptable thread. He then "averaged" the screw thread specifications. Thus the 55 degree included angle of the Whitworth is the average of the included angles of long-use threads in use in England from about 1835-1840.
The relationship of pitch to diameter was established in the same way. A complication was that the lead screw pitch for lathes still had not become more-or-less standard, and there were lathes with 4,5,6,7,8,9 etc. (and some fractional) threads per inch and each of these would result in a different sequence of [easily cut] thread pitches.
It appears the only reason we don't have Whitworth pitch specifications such as inch diameter X 26.75 t.p.i. is that these were difficult to cut with the lead screws and gears in common use at that time. He did however compile the pitches and diameters in long-term use and picked the most common one for a given diameter after establishing a mathematical relationship or ratio of t.p.i to diameter. [I don't think the concept of helix angle had yet been introduced.
Considering the state of engineering at that time, this was perhaps the only practical approach. Given the materials [highly variable cast iron, brass etc.] in use at the time, the coarse thread series was entirely functional.
From a theoretical standpoint the rounded root of the Whitworth thread has much to recommend it, as it avoids a stress riser. The UN series of inch threads includes a similar rounded root thread for high stress aerospace use. In actual use, a 60 degree HSS [and the early carbon steel] thread tools tended to rapidly wear from a sharp corner to a more rounded root and rolled threads also have a rounded root.
I do not know if anyone with access to a computer with finite element software has evaluated the 55-degree Whitworth v. the 60-degree thread profile for use in the soft and variable materials in use at that time. There may be a slight theoretical advantage.
Now if I can just find out where that 29 degree included angle for the U.S. Acme came from
Unka George (George McDuffee)
There is something to be said for government by a great aristocracy which has furnished leaders to the nation in peace and war for generations; even a democrat like myself must admit this. But there is absolutely nothing to be said for government by a plutocracy, for government by men very powerful in certain lines and gifted with the "money touch," but with ideals which in their essence are merely those of so many glorified pawnbrokers.
Theodore Roosevelt (1858-1919), U.S. Republican (later Progressive) politician, president. Letter, 15 Nov. 1913.
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