What size stepper motor's to use?

[weldingtwopotatoes]

So I just read an article on how to calculate what size stepper motors to use for a CNC machine. Now I'm more lost than I was before. I originally thought that the bigger the better but the article went on to explain that this wasn't true and how having to big of motor is a bad thing.

This question is for Gadget but if anyone else has any experience on here with CNC plasma cutters please chime in.

Gadget: What size stepper motors are you using for your CNC? and how did you decide on which motor to use?

[tking]

weldingtwopotatoes,

I am new here and have recently read through all of the posts discussing CNC plasma machines. From what I read I recall Gadget used (or mentioned) Xylotex 425s stepper motors (see Drives and Steppers~Solsylva CNC Plans) on his machine, but he said if he did it over he would build a belt machine (see ~Belt Drive~ Solsylva CNC Plans). I don't know if that would change what stepper he used. There is some pretty good discussion on stepper and drive combinations/sizing on the first link. Also, there is some interesting discussion on the cogged pulleys and belts at http://solsylva.com/cnc/pulleys.shtml. The Solsylva site and their plans are for CNC tables that run routers which would require bigger forces than just moving a plasma cutter, so that might suggest that smaller steppers could be used. Reading between the lines, I am thinking that Gadget might be looking at putting together plans or kit materials sometime in the future for those of us interested in CNC plasma machines.

I am interested and will likely be looking at something that will be covered over as a work table when not in use and be equipped with a shallow removable water pan. These plasma tables would open up great possibilities to build things that would otherwise not be worth considering. Parts that have taken a day to fabricate using a torch, angle grinder and hole saws could be cut in a matter of minutes once everything was set up. Pretty much everything complicated that I have built is based on CAD drawings I have put together so it wouldn't be a huge step to transfer a dxf file to an older pc for driving the CNC.

Toad

[KHK]

Every thing I have read says to go with a 250 to 350 oz-in motor.
They also talk about steps per rev. A motor that has 200 spr which breaks down to 1.8 deg per step, may not have the accuracy to sute your type of grantry mechanics.


It is a complex question.
A few questions to ask your self;
What type of table drive are you going to build? belt?, screw?, ball screw?, Chain is not recommended.
How fast do you want the grantry to move?
What is the weight of the grantry?

This is the reasion that they sell plans, they have done the design work, and a lot of trial and error.

I am also thinking of making a cnc table. I have done this type thing before so it will be my design(I HOPE)

[tking]

Kieth,

I did a few hours of reading on this last night and I am now sure you are right on the 250 to 350 oz-in steppers with well matched drivers. One I read about was using 210 steppers on a 60" table using belts on the table and gantry and it was doing very well with lots of hours on it and the builder had even mounted rails on the back side of the gantry for a future drill.

The mechanical parts of the cnc table are seeming pretty simple at this point. I need to learn more about stepper drivers, power supplies, software, etc. Very interesting project.

Toad

[Gadget]

Hi all and sorry to come in late on this discussion. I was out of the country for a week and had no internet connection. I have the 305in steppers and they work ok. I can't seem to get the rapid speeds I would like but have got it adjusted now where I can get near 100 inches per minute speed. My motors have 200 steps per revolution and my lead screws are 5tpi so my table resolution on the X axis is .001 which is easily good enough for plasma and routing.
If I had it to do over I would go with the belt drive or rack and pinion to get faster rapids since much of the plasma cutting is with thin stock. Faster cutting means less warpage and of course higher productivity although as a hobby cutter only the warpage matters to me.
I would also go with servo motors and make them a larger size. The reason is the servo motors have a method of keeping track of motor position so lost steps is not an issue. The servo motors can go much faster without a loss of accuracy for this reason. Servo motors are more expensive though which is why I chose steppers in the first place.

[matteh99]

I bought the 425 oz motors and I am planning on using belt drive. I figured I could gear up the belt drive so it would go faster than lead screws so the more powerful motors would be better.

However writing this I just realized that half of the power equation is missing from the motor specs. It doesn't say how many RPM they are good for. The 425 oz motors might not be twice as powerful as the 205 oz ones because the 205 oz ones might be faster.

Oh well it should work any way and the 425 oz ones weren't that pricey.

Since there are no cutting forces with plasma I am going to try to keep my moving mass down to a minimum so I should be able to get higher travel speeds.

Gadget you said the steppers are accurate to .001. What is the actual accuracy that you can cut to? How much does the kerf vary?

Eric

[Gadget]

Eric,
The Sheetcam program makes adjusting for the kerf very easy. I think it is .0394 or close to it by default and that has been good enough for me. I made a CNC motor mount with the plasma cutter with a center opening of 1.5" I measured the final cut hole and it was exactly 1.500. Can't get any closer than that.
If your unit has a wider or narrower kerf you can create a new tool description in Sheetcam with the kerf setting and use that for all plasma operations.
Dan

[weldingtwopotatoes]

Gadget: Do you feel that the 305oz stepper motors that you are using would be enough to push a Bosch Colt Palm Router? According to Bosch the router weighs 3.3lbs. Are your motors running a 1 to 1 ratio to the table or do you have a gear or pulley system offsetting the motor spindle rate to table travel speed?

[Gadget]

Originally Posted by weldingtwopotatoes

Gadget: Do you feel that the 305oz stepper motors that you are using would be enough to push a Bosch Colt Palm Router? According to Bosch the router weighs 3.3lbs. Are your motors running a 1 to 1 ratio to the table or do you have a gear or pulley system offsetting the motor spindle rate to table travel speed?

I would be more concerned about the force needed to push the router through the material than the weight of the router WTP. I don't think 3.3lbs should be a problem. The only time the weight would come into play is when starting a rapid move and the software for the controllers will allow rapid speed acceleration adjustments to compensate for the weight. The gantry supports the weight and it is on bearings so the movement is easy.
I run a 1 to 1 ratio from the motor to my lead screws. The lead screws gear the movement down. On the X travel I have two 5tpi 3 start lead screws connected by a geared belt so 5 turns of the motor equal 1 inch of travel. On the Y I have 11tpi and on the Z 16 tpi. The reason for the odd threads between axis is they were all 16tpi at first and the table was SLOWWWW. I replaced them with what was most cost effective starting with the Y axis. I used 5/8 11 threaded rod for this. Then the X I purchased two 5tpi 3 start lead screws. That set me back nearly $200. The Z I left alone since the travel on this is only 5" and you don't move it as much, especially for plasma cutting.
As a side note, one of the reasons I suggest using belt drive is the lead screws on anything over a foot or two will start whipping at high speeds causing vibration and possible lost steps. Belt drives also can be driven faster without the loss of steps.