View Full Version : 208OZ.IN stepper motor

09-03-2012, 01:53 PM
is a 208OZ.IN stepper motor ok for a 3 by 4 foot plasma table

Robin Hewitt
09-03-2012, 02:18 PM
Need more info... :tup:

That's about one foot pound of torque, so it will accelerate a mass of 32.16 lbs at 1 ft/s/s

Is that enough?

09-03-2012, 03:31 PM
to be honest i dont know much adout accelerate rate i need. Im making the machine and the beam that holds the torch will be a s lite as possable with two motors runing it.

Robin Hewitt
09-03-2012, 10:44 PM
When you come to reverse the gantries direction of travel you have to stop it then start it again.

How quickly you can do this depends on...

The weight of the gantry
The weight of any rotating components
The feed rate
The power of the motor

If made friction free a tiny motor could drive a heavy gantry but the accelerations/decellerations would be disappointing.

It won't be friction free so the motor also has to overcome the rolling resistance.

Two motors on one axis is asking for problems, suggest you run a light weight aluminium tube side to side and use it as a drive shaft.

Don't blow all your budget on monster steppers and high voltage drivers, you could end up with a 4.2 litre milk float.

10-03-2012, 10:10 AM
so do you think theses motors will work or do i need bigger ones

Robin Hewitt
10-03-2012, 12:11 PM
Quite possibly the motor is fine and dandy. Fitting a larger motor might be better, could make things worse. You can either have a wild guess or you can do the maths. The wild guess is standard practice but I cannot say I would recommend it.

Let me pinch a stepper power graph at random from Gary's ZAPP Automation website. Gary is helpful because he gives you graphs. It is unwise to assume the motor holding torque number tells you much about what the motor will actually do. Gary has the right idea.

Notice how by 300 rpm the torque has fallen by about half and is just about to plummet should you dare to go any faster.

(HLAF should read HALF, meaning 400 steps per rev)

You don't actually require much torque for a plasma cutter because you aren't pushing against the tool. OTOH, should you find yourself at the wrong end of that 4 foot table with this motor fitted, you might have enough time to make a cup of tea before it was ready to start cutting.

Can you find a graph for your motor? Only then can you start to decide how to gear it. It is always a compromise between step accuracy and rapid movement. There will also be a maximum speed where you can slam the gantry in to reverse without losing position, it is very useful if that is near your proposed cutting rate.

John S
10-03-2012, 12:50 PM
Balls screws ? trapezoid screws? or timing belt ? the more friction you have to overcome the more power you need.
personally I'd feel better with a larger motor, 3Nm sounds a lot better.

As regards graphs, they do give a lot of information IF and that's a big IF you KNOW that graph refers to that particular model of motor. I'm very sceptical on what gets released from China as regards what it applies to.

Like those test sheets issued with rotary tables and the like , everyone passes with the exact same results ?

Robin Hewitt
10-03-2012, 02:10 PM
That would be an F1 Ferrari plasma cutter then? :naughty:

Let's make it work hard, 1" travel per stepper rev, massive 20kg gantry.

If I've got the maths right, 3Nm would do nought to 83 mph in one second.

I know you need some degree of overkill to compensate for the losses, but a smaller, faster revving motor would seem to be a better bet.

10-03-2012, 05:57 PM
Like John S says more info required.!! All that John asked plus gantry weight or material/design plus type of linear bearings used.
Robins reasoning is sound enough but really without the full picture then can't be answered accurately.

Personaly I know of a 8x4 plasma that use's 3Nm motors geared 2:1 without any issue's but that use's R&P which hisn't the most efficient, also use's some crappy home made linear bearings.!!

10-03-2012, 07:38 PM
i was goin to build a kinda test plasma table as im new to cnc to get it all working and make a proper machine in the future.
my plan is to use chain a sproit to run the x and y axes they will run on channels and v pulleys.the torch will be run with ballscrew.it wil be built using steel box but i will make it as lite as possable.

10-03-2012, 08:08 PM
i was goin to build a kinda test plasma table as im new to cnc to get it all working and make a proper machine in the future.

Personnaly I wouldn't take that route, it's just as easy to build it correct the first time and a lot less money.
It's hell of a lot easier these days electronics wise due to modern drives etc and there's more than enough knowledge available thru forums to easily build a fully working machine straight off.
Best thing you could do is draw up an idea for the machine and start a thread and ask as many questions as needed untill you feel fully satisfied you understand whats needed then go for it.
Best advice I can give is don't rush and don't try taking the cheap route regards components it doesn't work.! You just end up doing it twice getting frustrated and spending double the amount it would have took to buy the correct components in the first place.!

Robin Hewitt
10-03-2012, 09:14 PM
my plan is to use chain a sproit to run the x and y axes they will run on channels and v pulleys.

Chains are good, I suggest 1/4" pitch and at least 2" sprockets to keep cogging down. Not so keen on channel and Vee pulleys, presume you are worried about dust getting in to bushings.

A 2" pulley would move you about 6" per rev (pitch x teeth). That's about 0.015" 0.38 mm per half step.

Gear it down 2:1 and you have 0.0075" 0.2mm per half step. Then quarter step for 0.1mm resolution, sounds good for a plasma cutter.

Being 1" radius the 208 oz.in motor would give you 208 oz of pull direct drive. 416 oz, 26 lbf at 2:1 down.

Guesstimate 2lbs of rolling resistance with some better bearings, leaves 24 lbf to move it.

4 rps on the motor would still have plenty of torque and get you rapids at 1 ft/s.

Let's assuming the gantry and chains weigh 32 lbs. 1 lbf will then accelerate them at 1 ft/s/s.

24 lbf will accelerate them at 24 ft/s/s. Equivalent of 0 - 60 in under 4 seconds, it's a motor cycle.

My maths is not guaranteed any more, brain aging rapidly :naughty:

11-03-2012, 12:29 AM
so you think my setup with theses motors will work for me.
i just want to say thanks so much for taking the time to explain it to me .