Stepper Size Re-think - Are they too Small?

[marc_the_kiwi]

Hi there...

I am well on my way to building a small desktop cnc, predominantly for pcb milling. My interface is done, as is much of my machine, but I am currently waiting on nuts for my leadscrews and a few other bits and pieces.

I have 3 of these:-
Vexta PK243-03AA Unipolar Stepper motors

Yes, unipolar have their dis-advantages, but the main advantage was that they seem to work well with my interface (uln2803 board based on Etch CNC), which was CHEAP to make. I am now wondering if they are likely to be underpowered.

My design is a simple moving bed (ie. X-Axis bed moves, Y-Axis and Z-Axis move overhead). My linear rails consisting of Aluminium Equal angle and skate bearings are very free moving, and fairly light (12mm mdf bed weighs not more than 500grams, total Y & Z Axis arrangement including dremel shaft is less than 1.1Kg).

Leadscrews are 1/4 inch (6mm) 20tpi teflon coated, approx 12inch (30cm) long, are fairly light. Nuts coming are of good quality and should be relatively friction free.

Looking at my motors, I think I may have made an egregious error in the size of motor I have chosen, as they only have 22oz-in / 0.16N*m holding torque.

Have I bit off far more than I can chew with using such small motors?

I will be going ahead and mouting everything and testing anyway, but if anyone has any opinions I would appreciate them. Specifically, if you have experience using small motors, or those with a similar holding torque I would be most interested.

I am not a maths guru, and have judged it by eye as to whether the motors will actually be strong enough. Even though I have my doubts, I still think I'll get a useable machine (just won't be breaking any speed records)

Anyway, to those who chime in with their $0.02, thanks in advance.

[gmfoster]

Hi there...

I am well on my way to building a small desktop cnc, predominantly for pcb milling. My interface is done, as is much of my machine, but I am currently waiting on nuts for my leadscrews and a few other bits and pieces.

I have 3 of these:-
Vexta PK243-03AA Unipolar Stepper motors

Yes, unipolar have their dis-advantages, but the main advantage was that they seem to work well with my interface (uln2803 board based on Etch CNC), which was CHEAP to make. I am now wondering if they are likely to be underpowered.

My design is a simple moving bed (ie. X-Axis bed moves, Y-Axis and Z-Axis move overhead). My linear rails consisting of Aluminium Equal angle and skate bearings are very free moving, and fairly light (12mm mdf bed weighs not more than 500grams, total Y & Z Axis arrangement including dremel shaft is less than 1.1Kg).

Leadscrews are 1/4 inch (6mm) 20tpi teflon coated, approx 12inch (30cm) long, are fairly light. Nuts coming are of good quality and should be relatively friction free.

Looking at my motors, I think I may have made an egregious error in the size of motor I have chosen, as they only have 22oz-in / 0.16N*m holding torque.

Have I bit off far more than I can chew with using such small motors?

I will be going ahead and mouting everything and testing anyway, but if anyone has any opinions I would appreciate them. Specifically, if you have experience using small motors, or those with a similar holding torque I would be most interested.

I am not a maths guru, and have judged it by eye as to whether the motors will actually be strong enough. Even though I have my doubts, I still think I'll get a useable machine (just won't be breaking any speed records)

Anyway, to those who chime in with their $0.02, thanks in advance.

The real problem is yes the motors are small and they have to turn real fast to drive 20tpi lead screws and get any speed at all. It seems backwards but stepper motors work best at real slow speeds. From what I can tell even little ones. So I have no doubt you can drive the cnc but it may well be at 10 inches per second to be reliable...

The good thing is steppers have really came down in price lately. I have to buy some soon.

Try what you have it may work well enough.

[marc_the_kiwi]

So I have no doubt you can drive the cnc but it may well be at 10 inches per second to be reliable...

It may well ?need? to be at 10inch/s to be reliable? Is speed more of an issue than I anticipated? If my machine is accurate will that help compensate for the lack of speed?

The logic in my head says that within my set tolerances the only number that will increase by a slow machine is the length of time needed to mill something. Time is not really an issue, as this is a hobby machine, really only to make custom pcb's.

I do realise 20tpi is a high thread count. This was in part because I wanted accuracy, but mostly because the haydon kerk leadscrews were free from a friend.

BTW, thanks for the quick reply.

[gmfoster]

Well it all depends on what up are milling. Routing burns if you move to slow. But my largest problem is the time it takes to get set up. You won't believe how slow a 10 inch per minute jog is.

I say this because it is what I did on my router. And now I am getting ready to try and remedy the situation. I used it for a few months but then have let it sit for a few years. Never really lost interest but just didn't have the time. I am not sorry I built it but would do things differently. I actually more or less ended up using my machine mostly as a cnc drill, I would center drill with it then drill using the center drilled starter holes. As a router it was just to slow to do good work. Now I did have about a 12 x 18 work area.

If you have good bearings then for PCB milling the motor leadscrew combination should work well. I could actually get up to 60 ipm jogging when my linear bearings worked well but then they would grab a little and the motors turning at such high speeds would start missing steps. I was using totally unconventional slides. If you have good linear bearings I suspect you will work out very well.

I used all thread but do have 3 kirk screws and nuts at 20tpi that I am sure would make a huge improvement....

Garry

[RomanLini]

For $59 each they are very expensive for tiny weak motors. You would be better off buying some much larger and stronger size 23 motors surplus, and since you would save money on that you could put the spare money to larger motor drivers.

[marc_the_kiwi]

For $59 each they are very expensive for tiny weak motors. You would be better off buying some much larger and stronger size 23 motors surplus, and since you would save money on that you could put the spare money to larger motor drivers.

Heh, I didn't buy them for 59 dollars, they cost me 4 euro each.
Underestimated size requirements; maybe. Pay a ludicrous amount for such a small motor; never!

[James Newton]

I put up a simple little calculator for motor size at:
http://techref.massmind.org/techref/...htm#Estimating which you might find useful. It's based on "rules of thumb" from a leader in this area and makes it pretty easy to figure out what specs you need given some simple torque or weight and speed measurements of the thing you are going to drive.

[MrWild]

Roman, the calculator s not working for me.

[James Newton]

Terribly sorry about that MrWild. It's fixed and working now:
http://techref.massmind.org/techref/...htm#Estimating