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Machining a metric external thread

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LADmachining05/01/2011 21:03:36
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126 forum posts
11 photos
Hi all,
 
I have taken on a small job of making an adaptor for an oil pressure gauge on an engine.
 
It will basically be a brass plug with a hex head, threaded externally M24x1.5 to fit in the oil pump, then with a 1/8 BSPT threaded hole in the centre to take the gauge sender unit.
 
Here's the issue:-  I do not have access to the oil pump into which the adaptor will fit, so I am in a bit of a quandry as to how to size the thread so that it will fit first time when it received by the person I am making it for.  I have a tap for the 1/8" BSP thread, so that is not an issue.
 
Can anyone provide information as to external diameter and thread depth that I should work to to ensure the plug fits right?  I do not know what tolerance the internal thread in the pump is made to.   There will be a Dowty washer under the hex head of the adaptor to provide a seal, so any oil leaking along loose threads shouldn't be an issue.
 
Thanks in advance,

Anthony
Ramon Wilson05/01/2011 22:45:56
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1655 forum posts
617 photos
Anthony,
Assuming this is non standard by which you do not have access to another male fitting of the correct size then if it is going to have a Dowty seal possibly the thread could be made a fraction smaller - can you not turn up a test piece first, measure it carefully taking note of all settings then get it checked first.
 
A tad long winded I know but unless you know the exact dimensions probably the only way to be sure. The part can then be made based on the info you have recorded.
 
Just a thought - Ramon
 
 
 
 
ady06/01/2011 00:13:56
612 forum posts
50 photos
As a brass plug it will be soft enough to fit reasonably well, even if you aint exact.
 
A decent thread LOOKS good after it's been cut, and it sounds like you've cut a few before anyway. As mentioned, have a 1.5 bolt to compare it with after it's been cut.
John Haine06/01/2011 13:20:38
5563 forum posts
322 photos
  http://en.wikipedia.org/wiki/ISO_metric_screw_thread is a good start
chris stephens06/01/2011 15:05:31
1049 forum posts
1 photos
Hi Anthony,
Are you familiar with the three wire method of thread measuring? 
chriStephens 
Don Brymer06/01/2011 21:19:26
9 forum posts
Anthony,
Try the following for the various thread dimensions.
•    Depth of external thread = 0.6134 x pitch.
•    Depth of internal thread = 0.5412 x pitch.
•    Basic minor Ø = Major Ø – 1.0825 x pitch.
•    Root radius = 0.1443 x pitch.
•    Offset depth = depth x 1.1434.
I suggest the thread OD is turned to 0.05mm to 0.1mm minimum & as Chris suggests measure the thread by using the three wire method. See page 25, issue169 MEW.
Regards,
Don.

 
Clive Foster06/01/2011 23:24:08
3630 forum posts
128 photos
Anthony
I presume you don't have a full form 1.5 mm pitch threading tool as this makes the job very easy as you can work direct to standard dimensions. The big problem when using the usual home ground tool is getting the actual in-feed depth just so to cope with the inevitable errors in tip truncation and lathe characteristic induced differences between what the dial says you have fed and what actually gets cut.
 
Basically you need a way to calibrate these errors out so the actual thread flanks are in the right places. 
I'd tackle it by cutting a nice 1.5 mm pitch test piece to fit a decently long hole cleanly tapped with a nice sharp tap.  M 10 x 1.5 about 15 to 20 mm deep, including lead in and start bore, should do nicely.  Aim for at least 8 quality, full depth threads.
 
Best way to cut the test piece is with the "zero-2-zero" angular in-feed method as it allows you to directly calibrate the offsets needed to take out any error.  To use this method the top-slide is rotated to a little less than the thread flank angle, my top-slide is pretty much permanently set at 25° to do both 55° and 60° threads but arguably 28° is better for 60° ones.  Not that I've noticed any difference.  After the work-piece OD has been turned to size the tool tip is bought up to touch the work and both thread dials set to zero.  Use the cross slide to bring the tip just clear of the work and use the saddle to take the tool past the end of the work.  Now feed the cross slide forwards from zero by the depth of the thread you intend to cut and re-set the cross slide dial to zero.  Use the top slide to withdraw the tool just clear of the work then thread using the top slide to set the successive cuts and the cross slide to withdraw the tool to return to the start after each cut. Obviously the cross slide is moved back to where the dial reads zero for each cutting pass.  When both dials read zero and all spring cuts have been taken the thread cut is what you set up.
 
Ideally it fits the test piece.  In practice it is almost invariably needs an extra cut or two to get a nice fit.  Because your nominal depth setting is on the two zero's its easy to add known amounts of extra cut until the fit is right.  My practice is to use the cross slide, making the extra cuts by the straight in method, leaving the top slide set on zero.  When I get the right fit  simply adding the cross slide reading to the original "book" depth gives the correct cut to set-up next time.  Alternatively the top slide can be used and a note made as to the new "beyond zero" finishing point, don't attempt to re calculate things so the top slide finish point is shifted back to zero.
 
Slide locks and good feed stops make life easier but many small lathe feed stops aren't repeatable enough.  The SouthBend one being a case in point, 2 or 3 thou variation being common.  As ever with the angular in feed method extra thought and care is needed to create clean shoulders and the end of the thread.  Best to cut a suitable groove first.
 
Clive 

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