Hi everyone,
I am building a cnc plasma cutter and I'm ready to order my screws. The table is almost 4x4 and I'm planning to use 1/2 x 10 screws. My question is the amount of starts. I know what starts are and how the speed up the travel, but what are most of you using? Is one start enough or is two better?

Thanks

two starts can make you faster, which is good on a plasma table. We need to know more about your setup. Servos, steppers, belt drive (ratio) or direct drive, drives etc. etc.

Eric

I was planning on steppers, 1/2-10 acme screws, geckodrive. My table will only be used for plasma cutting. Is this enough info?

thanks again.

My screws are 1/2-8, my nuts have two starts. This equals 4 turns per inch. Look at the torque curve of your motors. Try to design so that you are cutting at an rpm in the "meaty" part of the torque curve. Steppers don't have much torque after a certain rpm point. My motors turn 120rpm @ 30ipm, where I do most of my cutting (oxy/acetylene). 120 rpm is right in the middle of the "fat" part of the powerband. Plasma may need to go more rapidly (more starts, less turns per inch, "taller gear"). Your rapids end up being as fast as it will go without stalling (& losing steps). My gantry is 55#. With 187oz motors I can get 360rpm (90ipm) on the X and 424 rpm (106ipm) on the Y. Nice thing about Torch vs. Router, with no tool loads, I could probably cut this fast. With power at cutting speed, it will push popped-up pieces out of the way though. I would venture a guess that 2 starts on a 10tpi (5 turns per inch) would be OK, maybe 4 starts (2.5 turns per inch might be better). Don't sweat resolution. At 4 turns, 1/4 step & 200 steps/rev my resolution is .0003125"/step. My accuracy (without plate shift) can be .0025" (on a good day). Even in full-step, resolution is 1/2 of accuracy. I wouldn't be afraid of 2.5 turns per inch ( resolution of .0005" in 1/4 step, .00025 in 1/8). Good luck ! MIKE

Thanks Mike. Now my head is spinning! I have not purchased my motors yet. Am I working this backwards?

You say your nuts have two starts, does this mean your screw has to have two starts as well?

If I am reading your post correctly, you think for my setup 4 starts might be better?

CNC is all new to me (obviously) so your help is great. Thanks

Don't think in number of starts, think screw lead, or turns per inch. A 4 start screw can have the same turns per inch as a 1 start screw. 1/2" -10 acme only comes in 1,2, and 5 start. A good choice for you would probably be either 2 start 1/2-8 or 1/2-10. Depending on the weight of your gantry, you'll probably want at least 250-300 oz motors, maybe more. O don't know how fast a plasma can cut, but if you want to go really fast, go with the 1/2-10 5 start. But then you'd probably want 400oz motors or mor. There is a thread in the DIY router section, by InventIt, of a router using 1/2-10 5 start and 400+ oz steppers. I'm pretty sure he can get over 250 ipm rapids, maybe more. I've also read on some other groups, that that is the way to go, if you have strong enough motors. Remember, the higher the lead, the more torque you'll need.

Look at the manual that came with the plasma cutter. It may have a chart of feed rates for various metals/thicknesses. Pick the feed rate that you are likely to use most. (IE: 30ipm for my torch, plasma may be higher). Multiply this by your number of turns per inch.This will give you an RPM. (IE: 30 x 4 = 120rpm) Find a torque curve for your prospective motors. Compare the rpm's that the various turns per inch would yield, to the torque curve. Pick the one that is in the "fat" part of the torque curve. My rods and nuts have 2 starts (I think). As mentioned, it is turns per inch that count. Do a little math and compare the options, there is a best combination for YOUR application. MIKE

Great info guys, thanks!

Ok, I got another question. Please tell me if I am thinking correctly.

My machine will mosty be used for cutting sheet metal in the 20 to 16 gauge range. I will occasionally cut 1/8 to 3/16 material. The ipm range on my plasma is 350 ipm for 20 ga and 45 ipm for 3/16 stock.

Should I plan my ipm and torque curve for the higher end of this range? Like 275? I assume it would be easier to slow down the cuts for the thicker material than to try to get more out of a motor for faster cuts. Am I thinking correctly?

Set up your machine for the fastest speed you want to move at. With steppers, the slower you go, the more torque you'll have. So going slow won't be a problem. If it cuts best at 350 ipm and that's where the majority of your work will be, than make sure it can cut at least that fast, if not a little faster.

Ok, based on Mike's equation:

A 1/2-10 screw with 5 starts has 2 turns per inch. If 400ipm is my max thats 400ipm x 2 tpi = 800 rpm

Or a 1/2-10 screw with 2 starts has 5 turns per inch. 400ipm x 5 tpi = 2000 rpm

So i need to find motors that produce their highest torque in one of these rpm ranges. Correct? If so, which screw would be better?

Thanks for all the help.

No, all steppers produce their highest torque when not spinning. If you're going to use steppers, you'll want to use the 1/2-10 5 start screws. Just guessing here, but you'll probably want a motor that has at least 150 oz-in of torque at 800 rpm. This would probably be at least a 400 oz-in motor.

To get 2000 rpm, you'd probably have to go to a servo system. But with higher rpm's, you have to worry about shaft whip. Even at 800 rpm, you might start to see the leadscrew whipping. Nook makes a 1/2-8 8 start acme, which would lower your rpm needed to about 400. But remember, the higher the lead, the more torque you'll need.

Another option that's used a lot for plasma (and routers), is to use rack and pinion. You can probably get away with a little smaller motors for the same speed.

http://www.usdigital.com/products/ms23/graphics/speed_torque_curves.gif
This is a typical set of torque curves for steppers. You want to be in the plateau rather than the slope. (500-1000 rpm for this one). By 2000rpm there isn't much steam left in them. MIKE