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cutting 6BA thread on bronze bar

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Tony Martyr21/01/2010 11:59:46
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226 forum posts
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I am failing to cut the thread on 6BA bronze bar used to make screws to retain the regulator flange on my boiler backhead.
I am mounting the die in a tailstock die-holder and I have adjusted the die using a commercial screw thread so I know it is not too tight or slack.
I can only think that the die is tilted in the holder since the bar gets reduced to near root diameter at the end and then tapers to the head end where there is some semblance of a thread (they are 1/4 long U/H).
Is this my problem? I have made sure the die is seated correctly but it may tilt via the clamping/adjustment screws, or is bronze particularly difficult to thread?
Perhaps commercial hard Cap-head screws are a better choice in this location with a smear of copperslip?
Tony M
Rob Manley21/01/2010 12:24:03
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What make of die are you using and have you used it before on other materials?  Some new ones can still contain burrs from manufature.  It does sound like its not sat square though. 
There are several things to try.  Make sure that you have a chamfer on the end of the spigot before you start threading.  Also, on my die stocks - i think all of them are home made by someone or other - i have drilled and tapped 3 holes in the end and using shallow headed screws, pull the die back so it is always square.
If you wish to used stainless, im sure an internet search would come up with some 6BA stainless cap head screws.  

Edited By Rob Manley on 21/01/2010 12:25:51

Richmond21/01/2010 12:27:32
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Hi,
 
The other possibility is that your tailstock is out of alignment ??
 
Rgds
chris stephens21/01/2010 13:12:23
1049 forum posts
1 photos
Hi Tony,
There is sometimes a problem with using a tailstock die holder. The problem is that the cutting part of the die  is not central. This is not a problem in hand held die stocks but when the die is held off centre in the tailstock, a good thread is impossible, especially on small threads like your 6BA. There are designs for "floating" die holders about, I seem to recall one in MEW. 
chriStephens
Alan Worland21/01/2010 13:45:44
247 forum posts
21 photos
I was recently cutting a 7Ba thread on brass bar and using the tailstock die holder like you - I could not cut the thread, all I managed was to reduce the OD to about the core diameter with what looked like the thread torn off!
I eventually came to the conclusion that the die was not centred, so I left the taper out of the tailstock to allow it to centre - still no good!
Close examination of the die showed the thread didn't look very good (carbon steel new a few years ago)
I never managed the thread, I redesigned it in 6Ba
I feel a new die would have done the trick
Circlip21/01/2010 14:02:21
1723 forum posts
If the die is of far eastern manufacture, take to the local lake and hurl it as far as possible.  Bronze is a tougher material (Especially Aluminininium Bronze) and if you've checked everything the others have said, make sure the diameter of the bar is NO GREATER than the finished screw O/D and has a lead in chamfer.
 
  Got the die in the holder THE RIGHT WAY ROUND?????  Maker and size stampings on the OUTSIDE??  Yes, I know, but some don't.
 
   Regards  Ian.
 
  And just as an edit, AND likely to start a fire, instead of sticking the die in a tailstock holder that MAY not be truely on centre in all directions, when you get down to 6BA and such, why not a normal die holder and a pressure pad on the tailstock??

Edited By Circlip on 21/01/2010 14:06:13

Edited By Circlip on 21/01/2010 14:06:45

Edited By Circlip on 21/01/2010 14:07:10

KWIL21/01/2010 15:53:23
3681 forum posts
70 photos
Light a fire?  How about screwcutting it, works every time! If you do not fancy a long screwcut then cut an short length and then heaven forbid, run the die onto it!!
Rob Manley21/01/2010 16:07:41
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Would agree with circlip, a normal die stock against a pad in the tailstock is perfectly good.  Screwcutting BA threads can sometimes be tricky if you dont have the full set of gears can it not (not totally sure as im 100+ miles from my machine tables). 
If you can pick up a coventry die box from an exhibition i would, they are worth it and you can build up you die sets at every exhibition.  
KWIL21/01/2010 16:33:38
3681 forum posts
70 photos
Rob,
 
Fortunately I made DAG Brown's add on metric quadrant to my Norton gearbox equipped Myford S7, as published in ME some time ago, so almost any thread and certainly all the BA etc are there for the asking.
 
K
chris stephens21/01/2010 17:44:23
1049 forum posts
1 photos
Hi Circlip,
I have just come in from the Engineerium after cutting an M6 thread by the means you suggest, imagine my surprise to fire up the 'puter and find you and I agree on another matter. Well done, you.
chriStephens
Circlip21/01/2010 18:24:49
1723 forum posts
Find me a pin and an egg Chris and I'll show you a trick, can't do the loaves and fishes one though, ---- well not YET.
 
   Regards  Ian.
mgj21/01/2010 19:44:49
1017 forum posts
14 photos
there is no doubt that system is better for small threads - or make the bore in the tailstock die holder a bit slack and use the end of the tailstock mounted guide to push with.
Tony Martyr25/01/2010 12:28:13
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226 forum posts
45 photos
To end the story.
I never did get a good result with the tailstock die holder and resorted to using the same die in a handheld die-holder. I think the clamping/adjusting screws in the holder skewed the die an amount which is critical in such a small thread.
On a more positive note and following advice from this forum I used my brand new cyclone burner in the boiler firebox and silver soldered the tubes in place without drama.
Tony
Les Jones 125/01/2010 13:21:22
2292 forum posts
159 photos
Hi Tony,
                  I wonder which model cyclone burner you have and its heat output? I have been looking at adverts for the Primus  cyclone burner torch kit which is claimed to be 7.7 KW output but the picture looks like it is only fitted with a 3.3 KW burner.
Les.
Tony Martyr25/01/2010 13:49:19
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226 forum posts
45 photos
Hello Les
I am using a Sievert 3525 burner which I believe is rated at 10.3kW.
It is probably over-kill for a 3 1/2" loco boiler but I thought that it was better to be able to back off a big burner than struggle to get enough heat into the work as I had when I started the project. In fact the burner has a smaller OD than conventional burners.
Of course working with the flame inside the firebox, where a cyclone burner is essential, and with insulation all around the box, the heat builds up very quickly and although wrapping thin solder wire around every tube was fiddly the result when it all flashed was very gratifying.
Tony
Les Jones 126/01/2010 11:30:57
2292 forum posts
159 photos
Hi Tony,
                 Thanks for the information. I have just ordered a 3525 burner and 3486 handle to go with it. I suspect the burner will  not fit my existing torch which is over forty years old.
Les.

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