New Machine Build- Stepper motor size calulations for plasma

[midworld08]

Hey guys,
I've been lurking in the background and started a mill project. I've just had a friend see my mill and wants to build a plasma cutting machine. He has a 180A plasma at the shop. So to cut down to it, the bed size he wants is 2.4 x 1.2 metres. We are not looking at using ball screws, and using belts for the positioning with gearboxes on the motors. So looking at all this I am assuming I will not need nearly as much power in the stepper motors as I would do compared to my mill. So how would I start figuring on this. I know bigger is better, but bigger is also much more expensive. I can get a 1.2Nm 1.8 deg step motor for AUD$270. Is this a good place to start, and will it do?
Cheers
Colin

[TOTALLYRC]

Originally Posted by midworld08 Hey guys, I've been lurking in the background and started a mill project. I've just had a friend see my mill and wants to build a plasma cutting machine. He has a 180A plasma at the shop. So to cut down to it, the bed size he wants is 2.4 x 1.2 metres. We are not looking at using ball screws, and using belts for the positioning with gearboxes on the motors. So looking at all this I am assuming I will not need nearly as much power in the stepper motors as I would do compared to my mill. So how would I start figuring on this. I know bigger is better, but bigger is also much more expensive. I can get a 1.2Nm 1.8 deg step motor for AUD$270. Is this a good place to start, and will it do? Cheers Colin

Hi, Colin,
When building a plasma table of the moving gantry type, there are zero cutter forces involved and the speeds required for thin materials can be quite high. I think you need to find out what the feed rate he need sfor the materials he is going to cut will be before you can do anything else.
If he says he will only cut thick materails only then the top speed may not have to be as fast as possible and then you will be able to make a better choice. I have heard as much as 400 ipm on thin materials but I would check and do some seaching on that. Also since there are zero cutter forces involved, the gantry does not have to be as heavily built as for a router. This will help keep the mass down and the acceleration up.

As an outside guess I would say 2 motors to drive the x axis and 1 each for the y and z. Based on the size of the machine you describe.

But will we need the feed rates to be of more help.

Mike

[midworld08]

Mike,
I have had a look at the machine he is using. It is a 150A CEA. It can cut Stainless 40mm thick at 10ipm. The lowest thickness of material he will cut is 1/4" mild steel which has the specs of 100ipm. He will use it for 40mm stainless, and 50mm ally same speed. The fastest cutting speed on the chart is 170ipm for 1/8" mild steel!!! Like you could get that by hand! So with that in mind I suppose its time to put pen to paper and do some maths on the gearing and so forth.
Thanks for the help so far.
Cheers
Colin

[TOTALLYRC]

Originally Posted by midworld08 Mike, I have had a look at the machine he is using. It is a 150A CEA. It can cut Stainless 40mm thick at 10ipm. The lowest thickness of material he will cut is 1/4" mild steel which has the specs of 100ipm. He will use it for 40mm stainless, and 50mm ally same speed. The fastest cutting speed on the chart is 170ipm for 1/8" mild steel!!! Like you could get that by hand! So with that in mind I suppose its time to put pen to paper and do some maths on the gearing and so forth. Thanks for the help so far. Cheers Colin

Hi Colin,
I am glad to be helpful. I am learning a lot by answering people questions, like how much I don't know.

If he was to cut .030 alum, the rapids would be much faster than 100ipm.
I love to cut alum with plasma, A wild green shower of sparks if I remember correctly.

I would shoot for a cutting speed of at least 125, which means a rapid of 150ipm to be safe. You don't want the machine to just be fast enough because if you have any increase in drag the machine will stall or loose steps.

There are several threads about building a plasma table. Do a search and see what they are using for motion.

Can't wait to see the pictures of it cutting.
Mike

[millman52]

I built a cutting table all steel construction (5' X 10') primarily used for Ox/Fuel. I used (2) 740 oz/in NEMA 34 steppers on the long axis & an additional 740 oz/in on the gantry. NEMA 23, 500 oz/in on the Z. Power supply is 60+V. , 15A.

I have belt reductions of 4:1 & rack & pinion drives. Pinion DP of .750".


Although I use slower speeds of Ox/Fuel. I have test run the machine with rapids of 400+ IPM & cut speeds in excess of 150 IPM. With no loss of position (steps).

I have not crowded the acceleration up to a point that I begin to loose steps. So I can't really say how crisp of a corner or quick reverse of direction it will achieve at those speeds. Just watching it run over top of a piece of plate though it looks good.

It seems everyones situation is different. It takes a balance of the operating weight of your gantry, Motors with torque to adequately do the job you want, Power supply & stepper drivers of the correct size to drive the system to it's full potential.

It's easy to think "I'll use BIG steppers & monster power supply" to be sure of enough power but you end up wasting money & maybe even worse loosing the speed you need.

I am amazed just how strong the newer double stack steppers are for the size & weight of them.