16er inserts?

[guineafowl21]

I see what you mean about getting the Whit form just right.

In amateur use, and there are far better amateurs on here than me, I find that taking a few spring cuts, and maybe running a file over the crests or even some scotchbrite, will achieve a ‘decent’ fit, especially if you can keep trying the internal thread on until it fits nicely.

One more thing - and this is from the book ‘the amateur’s lathe’ - I thought the compound slide should be set over to one degree under half the angle, ie 29deg or 26.5deg for metric/American and Whitworth respectively. This is apparently to avoid steps on the trailing edge of the thread.

 

[J-G]

One more thing - and this is from the book ‘the amateur’s lathe’ - I thought the compound slide should be set over to one degree under half the angle, ie 29deg or 26.5deg for metric/American and Whitworth respectively. This is apparently to avoid steps on the trailing edge of the thread.

Ah.. Well done for spotting my 'short cut'

There are (at least) two schools of thought about that. One is that there should be a 'clearance' between the tip and the Right hand Thread Flank, the other is that there should be a small 'interference' such that the Right Flank will always be 'skimmed'. I'm in the second camp, since I think that the first method is more likely to leave a 'stepped' finish.

I wouldn't go anywhere near 1° though - - - at most I'd consider a 0.1° interference acceptable.

One thing I didn't mention is that it is imperative that the Tip (or single point) is mounted EXACTLY at 90° to the workpiece - (unless cutting some pipe threads but that is taking the complications too far even for a pedant like me) - I use a Thread Angle Gauge held against the work to achieve this.

 

[TFrench]

J-G, thanks for those explanations. Makes a lot of sense, and explains why I've had trouble getting metric threads to cut nicely in the past.

 

[Scruples]

I use a hand stock and die to cut threads. I've always thought that machine thread cutting is more for mass-production.

 

[guineafowl21]

I wouldn't go anywhere near 1° though - - - at most I'd consider a 0.1° interference acceptable.

A bit tricky using the scale on my little Boxford! It seems the best I can do is approximate a Whit or whatever thread form with a single point tool. But with careful fettling and checking, that approach seems adequate for the odd custom piece that I need. Thanks for the explanation.

I use a hand stock and die to cut threads. I've always thought that machine thread cutting is more for mass-production.

I tend towards the opposite - single point for a custom thread that I don’t want to buy a tap/die for, but if I need 10 bolts made I’ll buy a die. Mainly because single-pointing them would take forever.

Best example was a brewing gas fitting, 5/8” (14 tpi) BSPP both ends, but one end needed to be taken down to 21.8mm, but still with 14 tpi that had to synchronise to and deepen.

I doubt I could have found a die for this strange b’stard M21.8/14tpi Whit form thread for a sensible price.

 

[J-G]

I use a hand stock and die to cut threads. I've always thought that machine thread cutting is more for mass-production.

Hmmm... Mass production would call for a 'Die Head' - whether that was a 'Coventry', 'Namco', 'Fette', 'Versotool' etc. or specialist machine (Flat Thread Rolls) would depend upon many factors though.

Screw-cutting would generally be done where prototypes with 'Special' threads are being developed or, of course, in the 'hobby' workshop where 'one-off Specials' can be the 'norm'.

 

[chaoticbob]

First I'd like to thank J-G for explanations / graphics - in the past I was as perplexed about thread cutting as the OP is now, researched, but never came across anything as clear (for a beginner) before, despite having been on many engineering sites.
Second, to the OP, I think you might be trying to run before you can walk in a way. You've got a nice lathe but maybe it would be better to worry about tooling and technique when you need to make something specific, rather than trying tool up for perfect thread forms at any pitch your lathe can make. Which will be very expensive and likely unnecessary.
Myself, I mostly single point with a V ground tool if over 12mm or a strange pitch. That leaves a stress raiser at the root, but I'm not making F1 engines!
Bob.

 

[J-G]

Unless you wish the threads that you make to be capable of mating with other peoples work, there is no real reason why you have to stick to any [Standard].

You could simply decree that YOUR threads will be YOUR standard which may well mean that the width of the flat at the root (and/or crest) will be (say) 0.5mm (or 0.020" if you wish) irrespective of the pitch - though naturally that might make a 1mm pitch thread somewhat weak and preclude the use of 0.5mm pitch or below. You could even decide that you would prefer a 40° or 50° form.

You would of course then have to create the 'nut' as well