On a 2 x 4 foot table, should i use a 1/2 or 5/8 4 start acme? any thoughts are welcome. They will be driven by 425oz steppers.

how many steps per rev?

I have a 200 step per rev stepper and I just bought a 1/2 inch 10 tpi single start. Getting a 4 start would reduce your jog speed by a factor of 4.

Heck I'm still a newbee here, I have a CNC foamcutter, and I know from that experience that speed is desired.

Glidergider,

I think you are mistaken.

1/2x10 single start screw turns 10 times per 1" of travel. At 100 rpm it would travel at 10 ipm.

1/2x10 4 start screw turns 2.5 times per 1" of travel. At 100 rpm it would travel at 40 ipm.

Stepper motors are much stronger at low speeds than at highspeed as there torque drops off very rapidly with rpm. With the 425oz motors discussed here, a long pitch lead screw would be most effictive. I would opt for the 4 start screw. The step resolution is still far less than the likely accuracy of the mechanicals, and the steppers would be operating in an rpm range best suited for them.

Royce Bunnell
www.obcues.com

Good catch. Thanks for the correction. I did buy the 1 start from ENCO. Great deal. 6ft for $7. Hope its fast enough for me.

Glidergider,

I have one machine with 1/2x10 single start acme. It works fine, but I can only rapid reliably at 70ipm. My second machine uses 5/8x5 ballscrew and it can move at 150ipm reliably. I am building a third using the 1/2x10 5 start and expect it to move at over 300ipm, but the rapids are only a small part of the story. I often cut at 45ipm and occasionally, I get stalling on the single start screw. The ballscrew works fine, but really isn't needed. I expect the 5 start to work very well with the nuts I got from www.dumpstercnc.com, and the dust will not cause any problems.

I use a Xylotex system with the 269oz motors. The 425oz motors are higher inductance, and although stronger, will lose that strength much quicker with speed. Therefore, the steeper lead screw will keep the rpm's of the motors real low where they are plenty strong.

Royce Bunnell
www.obcues.com

Hi Royce, I'm following this thread and can't help but ask. I'm building a 24 x 36 with the same 269oz Xylotex motors. Why, with your reasoning regarding the low rpm strength, would then anyone want single start leads? I'm quite a newbie but am trying to build a strong straight machine that will perform. Am I compromising any of the fine accuracy that a single start, slower lead would maybe deliver? If not, I'm thinking about the 1/2 X 10 4starts. The stuff at www.dumpstercnc.com look like I might be able to finish my shopping with some ease, including the fitted AB nuts.

Thanks,
Ken

Single start screws are cheaper. I now wish I had spent more then because I'm going to replace them now. I'll also use 1/2" 5 start. It will give me a resolution of .00025. As for accuracy, that depends more on how well the machine is built....IMO

keep an eye on ebay ,
i scored two 3/4" 10 start leadscrews 2''/rev , both five foot of thread for 45 bucks each

I can't find hex nuts for the 2 or 5 start 1/2-10 screws on McMaster Carr... is there such a thing?

...Why...would then anyone want single start leads? ... Am I compromising any of the fine accuracy that a single start, slower lead would maybe deliver? ...

Life is about tradeoffs. A higher pitch screw will give you faster speeds at the same RPM, however, it takes more torque to turn them at that SAME speed. The issues are Accuracy, speed, torque (and I'm sure I've missed others). It is unlikely you can increase one without decreasing another.

I can't find hex nuts for the 2 or 5 start 1/2-10 screws on McMaster Carr... is there such a thing?

Sacrifice about 10" of one end of what you have and tap your own into delrin (or other soft products). See mvaughn 's page here (http://www.cnczone.com/forums/showpost.php?p=61073&postcount=88).