How could it be 1.5 mm? The only reasonable pitch for that screw is 2 mm (a four-start screw).tm america wrote:By my caculations the tr8*8 has a 1.5mm pitch
This is how it works. Say you have:
Tr 8 x 12 (P2)
- Tr -- metric trapezoidal screw (30°);
- 8 -- diameter: the screw is 8 mm thick across the peaks of the threads;
- 12 -- lead: one turn advances the nut 12 mm;
- P2 -- pitch: the spacing (along the axis of the screw) of consecutive threads is 2 mm;
- The lead and the pitch together give the number of starts: 12 / 2 = 6 starts.
So for Tr 8 x 8, one turn moves the nut 8 mm; for Tr 8 x 12, one turn moves it 12 mm -- and that's regardless of the pitch. In both cases, the pitch is almost certainly 2 mm (and should be specified), so you have a four-start screw, and a six-start screw. Both of them would be much faster than single-start 1/2-10 Acme screws.