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Extending an M16 thread.

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Robin Graham29/11/2019 23:38:08
755 forum posts
186 photos

I've had a request the extend the length of an M16 thread on the end of an 18mm bar. I think I would struggle with an M16 die and anyway I haven't got one - I've always cut anything above M12 on the lathe.

On the occasions when I've 'screwed up' (ho ho) whilst cutting threads and lost registration of tool and thread, I've not had much luck getting them properly back in sync - so I'm doubtful of my ability to extend an existing thread by this method. Am I being unduly pessimistic? Is this a viable technique with a bit of practice?

Of course the existing thread may have a relief groove before the shoulder (I haven't yet got hold of the part) which would further complicate matters, but let's assume not for the purpose of discussion.


Grindstone Cowboy29/11/2019 23:50:25
325 forum posts
27 photos

I think Joe Piecynski on YouTube has done a video on this very subject i.e. picking up on existing threads, I'll have a search and post a link if I find it


Well, that was easier to find than I thought!

Edited By Grindstone Cowboy on 29/11/2019 23:52:38

Pete Rimmer29/11/2019 23:53:34
774 forum posts
50 photos

You'll have to turn the 18mm dia down to 16mm anyway, so why not attempt it in a lathe?

With care, it's not usually that hard to pick up on a 60 degree thread. Set the compound over to 30 degrees. Blue up the threads and feed the cross slide in until you just start scraping the blue off the trailing flank. Now make more passes feeding in the compound until you start scraping the blue off the leading flank. You've now picked up the thread perfectly. Set your dials to zero, back off and start cutting the new threads. It's better to use the 'straight in' method for this to keep the load on each side of the tool equal, but it shouldn't really matter.

John Haine30/11/2019 07:53:28
3272 forum posts
175 photos

As a starting point, leave the work slightly loose in the chuck, engage the leadscrew with the tool at a convenient point say half way along the thread, and gently feed the tool in so it enters the thread. Jiggle the work as you do so the tip fits the thread snugly, then draw back slightly and tighten the chuck. That should give you a good starting point or could even be good enough.

Alternatively get a new bit of bar and cut the whole thread new.

Robin Graham30/11/2019 23:28:38
755 forum posts
186 photos

Thanks for replies - it seems this is doable, I shall practice on a bit of scrap.

Grindstone - thanks for the link. I'll follow that up.

The alternative of making the whole thing from anew from scratch is the most efficient way forward I'm sure, but I have to balance the customer's budget, expectations and ideas against what I can do for a few beer tokens. I don't mind doing stuff for next to nothing if it teaches me something, but don't I want to end up out of pocket.


PS - a chap in my town, having had sight of my workshop, asked me to make a camera mount for him. No problem, I've got a 3" round aluminium offcut which would serve. Made it, got treated to a couple of pints, then 'thanks for that, it would have cost me £75 to buy'. Nowt rummer than folk.




Edited By Robin Graham on 30/11/2019 23:38:58

old mart01/12/2019 15:25:44
1917 forum posts
151 photos

It is a fairly large thread, so should be easy to pick up visually. Just set the pitch and take out any backlash by turning the spindle, by hand if you want. Then bring the threading tool into mesh with the spindle stationary. Have a piece of white card behind to make it easier to line up. Start with the usual light cuts, and after several passes, the tool should just be touching the original thread, and you are close to the same size.

I added 4" of 3/4" X 5 ACME to a mill leadscrew with no bother. The other day, I had to make a 32 tpi thread on an R8 blank arbor to fit an optical sight which had an ISO 30 fitting originally. The actual pitch should have been 0.8mm, but 32 was near enough for a short length of thread. The tip rad was too big, so I had to change to a finer pitch insert. Even at 32 tpi, it was not difficult to get it re aligned.

John Reese02/12/2019 21:41:50
845 forum posts

As a 17 year old I chucked a tube too tight and deformed the end. It had a 40 tpi internal thread. I needed to pick up the existing threads and re-cut them.

If I could do that as a 17 year old with almost no shop experience, you can surely do it on an external thread. Good lighting and a magnifier help. Eliminate backlash in the half nuts by using the feed handwheel to preload the carriage against the direction of travel. Adjust the cross slide and compound until the tool is centered in the existing thread. Then back out the compound to the major diameter. I assume you will be flank feeding using the compound. Then go at it.

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