Drive and leadscrew questions...

[SatanKlawz]

I am attempting my 2nd round at a DIY cnc router. I have bought some parts already for the x and z axis and am now down to picking out electronics. I once owned the 3 axis setup from Xylotex but sold it before I ever used it. I am thinking about buying it again. Does anyone have any god input on this package? Also I will be using the table to mainly cut wood and a little plastic. The table size is 60 x 24 x 6. What pitch acme screw would be appropriately sized for the x and y axis on a table this size. SPEED is NOT a big concern to me. I value resolution more than speed. At the same time I do not want a turtle. Just a good medium with it maybe geared a tad to the side of accuracy.

[ger21]

1/2-8 2 start and use a Gecko G540.

[SatanKlawz]

I want to use NEMA 23 sized motors on the machine. The z axis I have already has a 1/2 - 10 acme screw and a nema 23 mount on it so I do not want to change it. 1/2 - 8 seems ok for the other 2 axis. If I use the g540 drive what sized steppers would be good for it? I see that it will mount right into the PC, what and where does it get power to run the steppers? I would not think that it runs from the PC power.......?....?

Thanks

[BobF]

It needs its own power supply.
These Kelings are a good match to the 540.

KL23H284-35-4B (1/4” Dual shaft with a flat) 387 oz-in
( in Stock, Can be used for G540, G251, G250
or any other driver)

http://www.kelinginc.net/NEMA23Motor.html

[stevespo]

http://www.geckodrive.com/product.aspx?c=3&i=14469

There is a terminal block on the side to connect power, ground, inputs, outputs, etc.

[SatanKlawz]

Gotcha, I think I will go with the Gecko drive, they seem to be priced right for what you get. Also I can not find 1/2 - 8 acme rod. Who sells it in custom lengths and will turn down the ends for the bearing blocks and also for the stepper motor hookup?

[BobF]

Mcmaster sells 1/2-8 ACME in 2 start. Don't know if they can turn it for you, you may need to get a local machine shop to do that.
I used it in my build. I used 1/2" bearings and dumpster couplings - no turning.

http://www.mcmaster.com/#about-acme-...d-rods/=1i5p6u
http://dumpstercnc.com/

[SatanKlawz]

Originally Posted by BobF Mcmaster sells 1/2-8 ACME in 2 start. Don't know if they can turn it for you, you may need to get a local machine shop to do that. I used it in my build. I used 1/2" bearings and dumpster couplings - no turning. http://www.mcmaster.com/#about-acme-...d-rods/=1i5p6u http://dumpstercnc.com/

This 2 start is new to me. The last time I bought acme thread it was 1:1. What is the difference? 1/2-10 in 1:1 will be slower than 1/2-8 in 1:1 right?

[BobF]

2 start there are actually 2 threads parrallel to each other. The effective pitch of 1/2-8 2 start is 4 tpi, since the nut travels 1 inch in 4 rotations. There is a 1/2-10 5 start available and a 1/2-8 8 start.

[SatanKlawz]

Originally Posted by BobF 2 start there are actually 2 threads parrallel to each other. The effective pitch of 1/2-8 2 start is 4 tpi, since the nut travels 1 inch in 4 rotations. There is a 1/2-10 5 start available and a 1/2-8 8 start.

What is the benefit of using the 2 start compared to the 1:1?

[ger21]

They are more efficient. 5 start is ~70% efficient. Single start is ~40% efficient.

[stevespo]

If you are using stepper motors you may want to use a screw with fewer (effective) TPI, which will allow your machine to reach faster cutting speeds at lower RPM.

One tradeoff is less resolution, but with most microstepping drivers you have very high resolution to begin with. Another tradeoff is losing some mechanical advantage, but because the motor is spinning more slowly it's probably going to be generating a lot more torque than it would be at higher speeds.

It's all a matter of balancing what your needs are in terms of speed, precision and the motors, drivers, PSU you have available.

For instance, I'm using 5 TPI ballscrews, so to reach cutting speeds of 100 IPM, I require 500 RPM. For 150 IPM, it requires 750 RPM - or about the limit of what my motors seem to produce with adequate torque. With 10:1 microstepping, I have 10,000 steps/inch or way more precision than what I need.

If I had chosen 1/2-10 5 start, I have an effective pitch of 2 TPI. I could generate 150 IPM at a speed of 300 RPM, and possibly go much higher, even to 250 or 300 IPM because the RPM would be in the 500-600 range. My precision would drop to 4000 steps/inch, which is still a lot more than is required for most woodworking applications.

Steve