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2 start thread on fountain pen

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steve kendall 115/09/2010 09:49:56
3 forum posts
Hi
 
I have lost the cap for my stainless steel fountain pen and I am trying to make a replacement.  As near as I can measure the thread seems to be square form, major diameter 12.7 mm, minor diameter 12.1 mm, pitch 1.6 mm per rev. number of starts 2.
 
I have read Screw cutting in the lathe, and found it nery helpful.
 
I have ground a HSS tool 0.4 mm wide, and cut an internal thread on a piece of scrap brass.  This almost fits, but does not run freely.  Does anyone have any advice, particularly on measuring and identifying the thread form?
 
I am using a SEIG C4 lathe using 16 TPI screw cutting feed, and I move the compound slide 0.8 mm to make the second start.  I have made a tap from silver steel and hardened it but I am having difficulty using it to cut a thread, I think because it is square form.
 
Can anyone help?
Will anyone with cutting threads be at the Myford open days so I could show you my problem and efforts so far?
Regards
Steve
ady15/09/2010 10:28:30
612 forum posts
50 photos
This is a handy site for threads.
 
http://homepages.tesco.net/~A10bsa/bago.htm
 
Searching on fleabay can also help find weird stuff which may be relevant.
 
http://cgi.ebay.co.uk/1-2-X-16TPI-BSF-Tap-DALZEN-/180559718479?pt=LH_DefaultDomain_3&hash=item2a0a32a84f

Edited By ady on 15/09/2010 10:41:35

Clive Foster15/09/2010 13:32:59
3630 forum posts
128 photos
Steve
 
Starting with the obvious are you sure that the thread is 1.6 mm pitch rather than 16 tpi, the two are very close and will be hard to separate using a normal thread gauge on a square thread.  Difference is just enough to prevent smooth running.
 
Normal practice with square threads is to make the tool a fraction narrow and feed longitudinally once the thread is down to depth to clean out the corners and get clearance.With two start threads its a right pain figuring out which start is at fault if things don't fit.
 
Might be worth doing a single start properly after clearing out the space occupied by the second start so you can play about with things and sort out exactly what settings are needed to get the thread right.  Once you know what's going on its not too hard to transfer relevant details to the real job.
 
That's a very small thread so its hard to see what you are doing.  May be quicker all round if you do a practice run at a size and pitch you can easily see.  Say 1" diameter.  Use friendly material too, delrin is ideal if you have it.
 
Generating the offset by moving the compound always seems error prone to me, especially on a baby thread.  Given the choice I'd shift the spindle through 180°. 
 
Clive 
 
Stub Mandrel15/09/2010 21:45:11
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4318 forum posts
291 photos
1 articles
Steve,
 
If it doesn't quite work (e.g. it's 1/16" rather than 1.6mm) you could just try only having a small number of threads and having alarge runout groove. (OK ,it's a bodge but...)
 
Neil
steve kendall 115/09/2010 22:37:31
3 forum posts
Hi Neil
 
That's a good idea, it will solve my problem and I can start using my pen again!
 
Thanks
Steve

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