The latest posts on this topic refer to the important matter ( for “Mr Average” ) of cutting clean, well-fitting threads. I’ve always found this hard to achieve using the commonly available split dies, even when opened up to their maximum – especially on stainless steel. Incidentally, the expanding screws on die holders I have purchased usually had poorly machined tips which did not engage the split accurately, and required re-machining. Even with this improvement the results are still inferior and I am considering enlarging the die holder diameter by a few thou … any comments on the wisdom of this idea? There is obviously a risk of die fracture! Suggestions on a suitable cutting lubricant would be welcome.
Efr’s suggestion of roughing out the threads first on a lathe before using the die is appealing. My only reservation is the lowest spindle speed on my lathe is 160rpm – far from ideal. I’m going to give it try though. I can cope with the fiddle of change wheels in the interest of a better result.
The choice of die material, and manufacturing quality, are factors in the equation. My carbon steel taps and dies, purchased on the grounds of a low volume of work, may have been a false economy. HSS tools from the best UK suppliers would be very much more expensive.
I had occasion to inspect the threads on a set of Roger Moss heavy duty cranks. Those big-end and centre screw threads were superbly cut – clean as a whistle, they screwed in very smoothly with an almost imperceptible waggle. Roger admits to being considered expensive but you do get the precision that you pay for.