Threading accuracy?

[Andrew TinsleyParticipant @andrewtinsley63637]

All the textbooks and experienced people say screw cut threads. Dies are not as good (accurate?). I well believe this statement, but nowhere can I find any quantitative data that shows how much poorer (less accurate?) are threads that are cut using dies.

Does anyone have any idea? I am just curious that nowhere can I find such a comparison.

Andrew.

[PeteParticipant @pete41194]

I doubt there’s any real numbers available since there’s so many variables including the quality and accuracy of whatever die is being used, if it’s a solid or adjustable die. A new quality die, highly worn, or one where the rust has to be scrubbed off it first? Then how it’s guided or kept square to the work. There’s still many times where a die cut thread is adequate or good enough for the purpose. It can also be difficult to single point a lot of the small thread sizes we might use. Model size studs as just one example.

[JasonBModerator @jasonb]

Probably as many variables when screw cutting – Worn leadscrew, grind of the cutting tool & tip, full or partial insert, work deflection, depth of cut, operator. exact pitch or near enough with small %age error, etc

[old martParticipant @oldmart]

A lot depends whether the die suits the material being threaded. I always open up split dies as much as possible for a first cut and then close up slightly for subsequent passes until the female part screws on with minimum slop. Assuming there is a female part that can be used as a gauge. The finish is rarely as good as screwcut threads, but normally good enough for government work.

I remember an apprentice cutting a 3/8 BSF thread about 6″ long bringing it to me and I had never seen such a long drunken thread before. I did explain what had gone wrong and he never repeated that mistake.

[Clive FosterParticipant @clivefoster55965]

I suspect that threads produced with a Coventry or similar die-head are likely to be more accurate than anything screw-cut on a lathe of unknown history.

It is said that the quality of long die-cut thread suffers due to simultaneous variation in cutting forces and the force needed to pul the die along the three. With a spring loaded die head, like the Coventry system, thread cutting and traverse forces are essentially constant.

Feeding via the feed screw rather than by hand is likely to be best. Which is what I usually do if its a thread on the QC box. It helps that my S&B 1024 is metric and my P&W B is imperial so most things can be easily covered. Except 19 tpi.

Nailing down specifications is pretty difficult but Wikipedia AI says :-

The American Standards for Accuracy of Engine and Toolroom Lathes (ASA) says typical lead errors fall into the following ranges: [1]

1. Cumulative Lead Error (Linear Accuracy)
This measures the cumulative error over a specific length of the screw, usually measured per foot (300 mm): [1, 2]
Toolroom Lathes (High Precision): Maximum allowable error of ±0.0010 inches per foot (≈ 0.025 mm per 300 mm).
Engine Lathes (Standard Industrial): Maximum allowable error of ±0.0015 inches per foot (≈ 0.038 mm per 300 mm).
Hobbyist / Inexpensive Lathes: Typically range from ±0.002 to ±0.005 inches per foot (≈ 0.05 to 0.12 mm per 300 mm), as these leadscrews are typically thread-rolled rather than precision-ground.[1, 2]

2. Short-Distance / Incremental Variance
This measures local pitch errors over a short span to catch quick fluctuations or “drunken” threads: [1, 2]
Toolroom Lathes: Max variance of ±0.0004 inches in any 4-inch span (≈ 0.01 mm per 100 mm).
Engine Lathes: Max variance of ±0.0005 inches in any 4-inch span (≈ 0.012 mm per 100 mm). [1]

3. Backlash and Wear Realities
While pitch accuracy dictates the precision of the thread form while cutting in a single direction, overall operational accuracy is highly dependent on backlash: [1, 2]
New Acme/Trapezoidal Setup: Typically exhibits 0.05 mm to 0.15 mm of backlash between the leadscrew and the half-nut.
Worn/Used Setup: Easily degrades to 0.20 mm to 0.40 mm of backlash over time. [1]

Working out exactly what effects such errors have on thread properties is a nice exercise for the reader.

So lead accuracy is probably not a great worry on a lathe in good condition.

Form errors due to imprecise grinding or tool setting up seem the most likely reason for noticeable error in a screw-cut thread.

Clive

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