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How close can I get? Indexable turning inserts....

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Rob Manley14/05/2010 21:03:18
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Ok, I'm currently trying to work out if this can be done.  I have been asked to produce a small batch of parts for an old motorbike which have a 5/16" BSCy thread on them for about 1-1/4".  Ill be doing them on the cnc as theres too many to do by hand sensibly but have not had any luck trying to find some coventry chasers and dont really want to use a die for the obvious reasons.  BSCy being quite a rare thread these days,  I wondered if there was a close indexable tip about which would doo (remembering that its 60° and NOT 55°).
26tpi roughly equates to 1mm (0.97mm)pitch and there is a 1mm pitch tip available - this I believe is the closest I could get. Would this be OK to use?  In real terms how much of a difference is there, I havn't looked into the radii differences but in such a small thread size would that matter.  It would need to screw into a BSCy nut. 
 
Rob  

JasonB15/05/2010 07:42:52
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I've seen 55deg inserts but not sure what the "partial profile" means as opposed to "full profile". They are the three pointed type so the pitch would be set on your machine.
 
Jason

Edited By JasonB on 15/05/2010 07:45:40

Rob Manley15/05/2010 08:44:36
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I have found  somewhere that does american thread inserts which are the correct 60° but have either flat crests and rounded roots or the other way around which wouldn't be too awful BUT they do them in 27tpi or 24tpi and not 26 which is a little annoying...
Is there anything closer?
 
using this website for reference (I have no connection etc.)
http://www.cutweltools.co.uk/home/threading-tools/indexable-threading-tools
 
 
 

Frank Dolman15/05/2010 11:36:50
106 forum posts

     I hope in due course to become a beginner, but I have not learnt enough
   yet.  I find that it is useful for me to chase up anything I see that I do not
   understand and this thread is a good example.
     The "thread chaser" that I know is a lathe tool like a short bit of saw blade
   that is used just like an ordinary tool in screwcutting, except that the thread
   can be cut all in one pass ( or nearly ).  This thing will do any diameter but
   only the pitch and thread form that it is made for.  The other thing that I have
   heard called a chaser is the interchangeable cutting insert that goes in the
   enormous stocks used by electricians for threading conduit.
     What puzzles me is how either of these can be "indexable".  Is there
   another kind of chaser that I don't know about?
     If its too complicated to explain on here, could I please be directed to a
   learned tome?
Frank Dolman15/05/2010 13:10:01
106 forum posts

     Don't bother!  I have looked at a catalogue of indexable tools and I see
   a range of what I would call "single tooth chasers".  The catalogue didn't
   call them chasers at all.
     Sorry if I have caused you to start head scratching over how to reply.  Many
   thanks anyway.
Ian S C15/05/2010 13:15:19
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The only type of chaser I know of consists of a steel blade sort of similar proportions to a wood chisel wide enough to accomodate 6/8 thread profiles on the end, the other end has a wooden handle, the thread formed tip is brought into contact with the previously formed thread, this forms the radii on the top and bottom of the thread, some sort of rest is required to support the toolsimilar to that on a wood lathe. I have read of this method being used in the distant past (100 + yrs ago)for forming threads (on a treadle powered lathe), both internal and external. Today the initial thread is cut with a single point tool. The chaser is hand held unlike the single point tool in the lathe. Frank the stocks used by plumbers and electricians have dies, either 2 piece or round button dies. Rob you'll proberbly be OK doing the threads with a single point tool and just leave the top with a little flat instead of a radius, the lathe tool takes care of the root of the thread. Ian S C
Tony Pratt 115/05/2010 13:16:21
2319 forum posts
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I would go for the 1mm pitch tip and set the lathe to cut 26TPI, you shouldnt have a problem as long as you have a nut availiable to use as a gauge. I have turned a lot of threads on CNC lathes and even when you are using the correct tip you may have to move away from therectical depth settings to get an acceptable job.
Tony
KWIL15/05/2010 14:22:17
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Jason, partial profile is just that, a simple pointed tool, full profile, will, if used for the correct pitch, "top" the thread peaks correctly, just the same as if you used a chaser.  As it is "just for an old motor bike", the tip profile is of no real worry. I would run a die down to chase them to size.
Rob Manley15/05/2010 14:35:53
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The reason I dont really want to do anything manual is I have 30 to do!  With a chaser/indexable tip I would get a mini automated production going.
Sorry fellas, the chasers i was thinking of were for the coventry die head and looks like this
The nuts I am also making but BSCy taps are easy to get hold of so thats not an issue.  The total error between a true 26tpi thread and 1mm is tiny (about 1thou per tooth) and the radii is out by 1thou also! so I think i would get away with this - just wanted to see what the masses thought. 
So, in conclusion if you need to cut a 26tpi BSCy thread - use a 1mm pitch form tool :D
(But dont take my word for it)



Edited By Rob Manley on 15/05/2010 14:38:19

Circlip15/05/2010 15:50:55
1723 forum posts
So if you're going to tap the nuts, why not pay a fiver for a die from Tracy Tools?? It was to make 30? Not forgotton any noughts?
 
  Regards  Ian.

Edited By Circlip on 15/05/2010 15:53:07

JasonB15/05/2010 17:40:24
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30 would not take long to chase with a die in a tailstock die holder with the lathe turning slow, just wind it in, select reverse and back it out.
 
Use a 55deg tool like your picture and its just tidying things up.
 
Tracy tools have 26tpi coventry type chasers listed in their paper catalogue
 
Jason

Edited By JasonB on 15/05/2010 17:42:49

Frank Dolman16/05/2010 00:51:54
106 forum posts

     Ian   Thanks for info.  I had heard about the hand chasers the Old Boys
   used to use, just plunging them into the work and letting them generate
   their own pitch!  Can't help feeling that in those days, trying to start in ones
   old age without any practical instruction would have been much more
   difficult than it is now.
 
     Rob   Thanks for comments and particularly for the pretty pictures.  You
   confirm what I had gathered from the catalogue.  I now have to find out
   about the Coventry Die Head.  A happy half-Sunday on the web but not
   until after the Grand Prix!
    
Ian S C16/05/2010 04:45:20
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Hi Frank, I don't suggest you use a chaser to cut the thread, just a light finishing cut. The threads on the bolts used by the manufacturer would proberbly be rolled rather than cut. The old timers use of chasers must have been a real art. Ian S C
Frank Dolman16/05/2010 16:35:09
106 forum posts

     www.oldengines has shown me pictures and descriptions of Coventry
   die heads, and all is much clearer.
     Ian, the electricians diestocks that I mentioned having seen were in
   fact very close relations to the Coventry die head.  The chasers that I have
   seen being used on a screwcutting lathe cut a lovely thread in brass in
   one pass.  I have not the slightest idea why they were used in preference
   to a tailstock die holder.
KWIL16/05/2010 19:12:09
3681 forum posts
70 photos
Because a tailstock dieholder does not always produce a clean correct thread.  It can esily be "drunken" if not started dead square.
jomac17/05/2010 11:14:00
113 forum posts

Hi, Trying to get the right indexable thread insert is not hard, what is hard, is trying to decypher the correct catalogue index even from Vardex who have dedicted thread cutting inserts, anyway Iv'e seen home made chasers, useing a cut up button die which is then brazed to a suitable holder or clamped on like an indexable insert is, its a thought ??? its also cheaper than buying Die head chasers.

By the way how did you measure the thread down to one thou, I'me flat out measuring 3mm threads over a multible of 10, and still get it wrong.

A question ??? can you get plumbers dies in fine threads to use as a chaser, as  I need to cut threads in alloy for the crankcases in small IC engines, or do some reading on threading.

PS down here in the southern highlands of OZ its the start of winter with temps -2 to 17. which is warmer than London, now I know why my parents and me and my young brother migrated down here in 1949.

Keep up the good work.

John Holloway

KWIL17/05/2010 11:55:40
3681 forum posts
70 photos
Jomac,  You use thread guages which are strips of teeth to size and you match up the pitch that fits

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