Purchasing Steppers - Advice please

[andy_ck87028]

Hi All

I am going to Kuala Lumpur next week and want to buy my steppers.

I have attached a picture of my frame and have put a bottle on it so dimensions can be assessed visually. My CNC cutting footprint will be 800* 1200 and the z axis travel will be 200mm. The z axis assembly picture is also attached sitting in a plastic box because it is not very complete yet

My router is rated at 1050W.

I want to be able to mill aluminium and hardwood timber.

Given that I will be talking with people who may be trying to flog me steppers without me being able to see the specs or being able to research the stepper that I'm being offered, I need some guidelines.

From what I can make out it is best to have a 6 wire stepper. Am I right?

From what you can tell of the infromation above, what minimum size stepper should I aim for for x,y,z axes;

how do I confirm that I have achieved that minimum size (nema rating?,
physical size? standard information on side of stepper?, anything else?);

how do I ensure I have got good ones (wear?, freedom of movement?, binding?, heat marks?, manufacturer? (good/bad), country of manufacture?, anything else?);

good price (what range I can expect to pay I suppose clarifies itself when I have a handle on how big a motor I need).

Any information very much welcomed.

Thanks

Andy

[ger21]

Originally Posted by andy_ck87028 From what I can make out it is best to have a 6 wire stepper. Am I right?

What drives will you be using? You need to make sure the steppers will work with the drives you'll be using. Bipolar or Unipolar?

[andy_ck87028]

I've yet to decide. I assumed I could work backwards from the stepper. I guess you are asking as it is relevant. Am I creating a problem for myself by doing it this way? I'm not electronically literate and aim to keep life as simple as possible.

I guess that by making my very first step away from the mechanical it is inevitable that these issues will come up.

I will mostly likely follow the crowd unless there are compelling reasons to do otherwise. Does that imply my answer is bipolar or unipolar?

Andy

[Nono]

You can work this any way you want, from motors to controller to power supply I think is best. The motors that you want will be about 86mm square minimum and under 7amps, double or triple stacked is another term. Look for something in the range 282.5 Ncm that is a 400oz motor. Are you buying these motors used? No movement from side to side in the bearings and no wear is ideal, look for as new as possible and the bigger the better.

[dfranks]

I had a question and thought that I might bring it up so see if anyone had any thoughts on it. I have seen some stepper motors that are smaller in size (83oz.) but come with motor heads that 'up' the 'size' so to speak. The heads have gears with 12:1 and 60:1 ratios that supposedly allow the stepper to handle larger loads. Is this something that is normally acceptable? I have seen some similar designs here to where the stepper motor(s) has a small gear on the end and then a larger gear on the ball screw with a belt connecting the two. Is this basically the same or are you asking for trouble by using 'motor heads'?

[2muchstuff]

Steppers with gearheads are a great idea for getting great amounts of torque from small motors. You will get great accuracy but at a price, slow speeds. I would stay away from them and go with a stronger motor, belt it down with a timing belt in the range of 2:1 to 4:1. When using a threaded rod or ballscrew in conjunction with a belt reduction, you will get the torque that you need. To get the speed that you need, drive your motors with 10 to 15 times the nameplate rating.

[andy_ck87028]

Thanks all

I guess that I'm going to be looking for around 400oz steppers. It makes sense going for the larger motor... just gives me greater all round flexibility.

How do I know what the rating is? Can I do it purely on size? Do I look for 400oz/in on the label or could it be rated NM (what is NM equiv of 400?)

Should I worry about whether I have purchased a unipolar or bipolar or if faced with a choice which should I favour?

Finally, am I confined to Geckos as soon as I get to steppers of this size?

Cheers

Andy

[ger21]

Originally Posted by andy_ck87028 Should I worry about whether I have purchased a unipolar or bipolar or if faced with a choice which should I favour? Finally, am I confined to Geckos as soon as I get to steppers of this size? Cheers Andy

400 oz-in = 2.8NM

Whether bipolar or unipolar depends on the drives. Keep in mind, that if you get 8 wire motors, if using unipolar drivers, you'll get about 30% less torque than you will using a bipolar driver. Some 8 wire motors are rated at the unipolar rating, some the bipolar rating. You need to know how it's rated.

The only real way to know the torque is if it's printed on the side of the motor.

And you're not totally limited to Geckos with motors that size, but you'll be wasting your money on anything else. I've seen 1200 oz steppers that will run on a Xylotex, but they'll spin so slow that they'd be useless.

Ideally, try to find motors rated 2-3 volts, and stay under 7 amps if using Geckos. If you want to use something like a HobbyCNC driver, you need to stay at 3 amps or less.

[Nono]

http://www.convert-me.com/en/ is a great website for conversions for oz to nm and back. Ger is absolutely correct 2.825 and stick to the geckos for possible future upgrades. Keep us posted.

[andy_ck87028]

Thanks for that.

Gerry, when you say motors rated 2-3 volts, I don't quite understand. I thought the power supply was 24-80 for geckos.

Please humour me on this one as I suspect its a basic question. In my simple understanding the wattage is the amp drawn * rated voltage. If only 3 volts, that would make a 21 (7*3) wattage motor which seems tiny.

Just to add to my confusion, I did a google and came up with an MS23 motor rated at 3 and 4 amps showing a torque curve for 24v and 48v yet the thing only draws 7.65 watts.

Andy

[ger21]

Most of the motors we use are rated 2-6V. You drive them with 10-20 times the voltage to get a lot of speed from them. If you drive them at the rated voltage, they'll have full torque when not spinning, but won't spin very fast at all. This is the simple explanation.

[ViperTX]

Andy,

Steppers are kinda unique....you can generally use a voltage that is 15 to 20x the rated voltage as long as you current limit the phases. The uniqueness is because of how they are normally used....the phases are pulse width modulated....so they are on for a short span of time.

Yes Power = Voltage X Current.

Go back and look at the spec.s on that motor and how it's driven and you'll understand.