View Full Version : rolled vs. cut threads
06-15-2011, 09:18 AM
In another thread, HJ observed that a basket strainer had 'rolled' threads instead of machined threads. He said this was disadvantageous because a slip nut will not tighten properly to a rolled thread. I'm curious about this. First, by observation how can one see the difference between a rolled thread and a machined thread? Why is a rolled thread harder to make water tight? Do experienced plumbers generally avoid rolled threads in all threaded plumbing applications? Are rolled threads present in only male threads, or are they present in female threads as well? Thanks in advance.
06-15-2011, 11:40 AM
the rolled threads are rounded whereas machined threads would be sharp at the edges
06-15-2011, 01:21 PM
Rolled threads are formed by running the blank through a die, and forming the threads by moving the metal, this changes the metal structure which does have an effect on strength and corrosion, but these issues are not relivant on a plumbing level. The other is that the roots and peaks of the threads will be rounded as it is uch more difficult to move the metal with a forming die that much.
Cut threads are formed by cutting the metal away. These can be either straight or tapered threads, generally the roots an peaks of the threads will be sharp as it is easier to cut...
Machined threads have a sharp apex, rolled threads are rounded and therefore since they lose the metal which would create the "point" they are slightly smaller, and a slipnut with a shallow thread, especially if it also has a rolled thread, may NOT have enough engagement to tighten.
06-17-2011, 10:51 AM
Rolled threads are how 99% of all bolts are made. They have an advantage of some work hardening, and can have a rather sharp apex.
A cut thread, in the case of a acme or whitworth thread can have a flat and large apex.
Strainers that cost less that 25$ are why the nuts do not hold. Rolled or not, the material is too thin and the nuts are die cast junk.
Bolts do NOT make "rolled threads" by distorting the "wall" of the metal to create the thread. It is IMPOSSIBLE to roll 'thin sheet metal" into a thread without creating weak spots if the "bend" is too severe. THAT is why they are "rounded" and also why they seldom tighten into a conventional slip nut. I use cast brass strainers; they do NOT cost $25.00, the metal IS thick enough to machine threads, and they DO tighten properly.
06-18-2011, 12:30 PM
The cheap ones seem to be rolled on a mandrel, that works. The cast ones of course are best.