acme Screw Too steep?

[DDM]

I was looking at using some 1/2-10 acme screw with 5 starts so it would be 2 turns per inch. Is that going to be too steep to get decent accuracy with a preloaded system? with a 54 inch screw at and the nook industries calculator I could get 450 ipm but with what kind of linear accuracy.

Carl

[2muchstuff]

Not taking backlash into account, 2 turns per inch @ 200 steps per turn = 400 steps per inch or .0025"

I would consider using some kind of reduction unit between the motor and screw.

[DDM]

I was looking at doing a 2.5:1 reduction so that one turn of a stepper would be .001 on the lead screw. I was looking at making my own delrin nuts, I work in a machine shop so it shouldn't be too much of a problem making the nuts and just about every thing else. Anyone have any input for the linear resolution?

Carl

[ger21]

Originally Posted by DDM I was looking at doing a 2.5:1 reduction so that one turn of a stepper would be .001 on the lead screw.

1/2-10 2 start is .001 per step. 1/2-8 2 start is .00125

What resolution you need depends on what your making with it.

[DDM]

I'm looking at a steel frame machine for mainly wood and possibly a little bit of aluminum but not anything too heavy, those nuts on Nook industries are only rated for about 625lbs static and dynamic for the 1/2" 10 5 start rod so that's probably going to be my limiting factor. Stepper driven system at least 400 oz in steppers with probably gecko 201 drives and like I said earlier a 2.5:1 belt reduction.

Carl

Anyone have any more comments?

[fyffe555]

Any reason you're gearing down with a belt reduction and then gearing up using a high lead screw? at 2.5:1 reduction and a 1/2-10 fivestart you'll get 0.001 per step(?) and a direct drive on a 1/2-10 two start will be the same 0.001 per step - without the cost and complexity of a reduction drive and a more expensive screw?

The spec for the nook screws is the same for linear accuracy.

Do you have a target speed in mind? might want to compare the torque curve of your motors to the gearing of what ever drive system you choose..

[DDM]

The goal in my design is to have a gantry that can handle something 4 feet wide and have a decent rapid above 200 in/min. With most of the screws out there I have to run the screw at such a high RPM that whipping of the screw would happen or thats what the calculator on nook industries said. With the lower threads per inch I can run a faster feed rate, I'm shooting for a production machine and I don't want to be twiddling my thumbs waiting for the thing to rapid home. Rack and pinion need a lot of reduction and I don't want to use a multiple belt reduction per Axis. This screw just seemed like the answer.

Carl

Hopefully this weekend I'll be able to jump on solidworks and get something drawn up so it can be ripped apart

[2muchstuff]

Instead of belts you could use a harmonic drive on the end of your motor. They can be a little pricey new but can be found on Ebay for a reasonable price.

[ger21]

Is this a router you're building? A production router should be able to cut at least 200ipm, with much faster rapids.

[jeepn4life]

Originally Posted by DDM I was looking at using some 1/2-10 acme screw with 5 starts so it would be 2 turns per inch. Is that going to be too steep to get decent accuracy with a preloaded system? with a 54 inch screw at and the nook industries calculator I could get 450 ipm but with what kind of linear accuracy. Carl

I am confused. 1/2" 10 threads per inch should mean 10 revolutions to get 1 inch of travel. Why do you think it is 2 turns per inch?

[ger21]

Originally Posted by jeepn4life I am confused. 1/2" 10 threads per inch should mean 10 revolutions to get 1 inch of travel. Why do you think it is 2 turns per inch?

It's a 5 start screw, which is 5 seperate threads on the screw, each thread being 2 turns per inch, giving you 10 threads per inch, but only 2 turns per inch.

[DDM]

It'll be a wood router with the possiblity of doing some aluminum and maybe some softer exotic materials for limited runs. I'm looking at a completely steel frame with a working area of 24" x 36". I want to be able to do production runs so 200 ipm is going to be too slow on a rapid. With this screw it would allow me to have 400 ipm or more rapids. I hope to have some drawings done up later today or at least the linear bearings that I plan on using.

Carl