Smoke wrote:
Whoa! variable "pitch" threads? How do you get those to screw into a
mating part?
They don't, generally. A variable pitch thread might work in wood,
however, if the pitch change is not too big.
But, they are more likely to be used in rotary screw compressors,
pumps for high viscosity liquids, etc. Two variable-pitch screws
that are mirror images of each other will turn together like helical
gears. It seems that one of the rotors always has 'fat' teeth and
the other one has 'thin'. That may have to do with making the
trapped pockets of air move through the compressor's inlet
and exhaust ports better, or something.
Rotary screw compressors make some of the best air compressors
on a number of characteristics. But, they are VERY hard to make
on manual machines.
I really don't think you'd make these with thread programs, however.
The rotors have VERY complex geometry in 3 axes, and you'd need
a fancy program to design the shape and guarantee that the trapped
air pockets will be sealed in all the way through the compressor, without
a gap developing at some point in the rotation that lets the air leak
backwards. You'd also need careful control, as the gap between the
2 rotors and the housing is just a couple of thousandths of an inch.
I thought about making one of these, mostly as a 'can I do it' type
of project.
Jon
