Steppers and Ball Screw Relationship

[Mr.Chips]

I’m starting over on the electronics of my CNC build. Have a couple of questions regarding the relationship between the ball drive screws and stepper motors, and possibly the Power Supply.

I currently am using Acme ½” X 10 TPI thread on my X and Y axis, but this is just my temporary machine that I will use to make a better one. It is a fixed gantry as it was easier to build with the tools I had. Don’t want to optimize for this configuration but do want to for the second machine as described below. And it will be a movable gantry configuration.

The steppers are BiPolar.

I have ball screws for the second machine, the X and Y axis screws have 3 start threads, 0.782” dia. and 1.75 turns per inch. The Z axis screw is a 1 start thread, 0.762” dia. And 5 turns per inch.

I will be cutting plywood, hardwoods, aluminum and possibly brass.

Naturally I want it all, Strong cutting power and FAST. But then there is reality sets in.

STEPPER SIZE:
I believe that there is a relationship between the TPI and motor size. I currently have very tiny motors 116 oz/in I think. For the second machine what motor size would be the best all around. Seems bigger (490) is not always better.

POWER SUPPLY SIZE:
I will use either Xylotex or Probotix driver boards. Both suppliers have a range of voltage something like 24 to 35 VDC with an recommended voltage of about 30VDC. I will use the recommended voltage rather than the minimum as it should perform better. Is this thinking correct? What about Amps, what part do they play?

Thanks
Hager

[Mr.Chips]

Hmmmmm. No Response. Guess I asked too many questions at one time.

I'll make it simple.

Is there a relationship between the TPI and motor size?

I am using direct drive 116 Oz/In Bipolar steppers and Acme 1/2" 10 TPI screws, what kind of fast forward speeds (not cutting) could I expect?

Thanks,

[Al_The_Man]

Originally Posted by Mr.Chips Is there a relationship between the TPI and motor size?

Yes obviously there is, not wishing to be sarcastic, but probabally the reason for no answers is it is a little like asking 'how long is a piece of string?'
I would suggest you look for any of the several motor sizing programs that are out there supplied free by almost all of the servo/stepper manuf.
I have used Kollmorgen, but I am not sure if it is still available since the Parker take over.
Most consist of Graphic display of different machine types, and at least by plugging in different scenario's you get a feel for what is involved in sizing CNC motion devices that depend largely on the mechanics of the machine.
Many factors come into play and I find at least these programs can put you in the ball park.
Al.

[ger21]

Originally Posted by Mr.Chips Is there a relationship between the TPI and motor size? I am using direct drive 116 Oz/In Bipolar steppers and Acme 1/2" 10 TPI screws, what kind of fast forward speeds (not cutting) could I expect? Thanks,

A very important factor is the motors torque curve. That will show you how much torque a motor has at a given rpm. Since torque drops off rapidly as rpm's increase, that will give you a good idea of how fast you can spin your motors. Once you have that info, you can choose a screw to give you the speed you want from that rpm. You do have to stay within the power range of your motors, though. The drives your using, and the voltage you're running them at also play an important part. Higher voltages will get you more speed.

A few years ago, when those motors were more commonly used, about 30ipm with 1/2-10 acme was a common number. Now, due to the availability of cheap chinese motors, most people are using bigger motors. Witha HobbyCNC board, and 200 oz motors running at 36V, you should be able to get close to 100ipm, or possible even more.

[cyrius]

Hagar,

I think that the reason your question is going unanswered is mostly due to a lack of information. Your screw thread pitch determines how far your screw nut travels per rotation of the screw. Threads per inch is misleading because multiple start screws change your realationship between rotations to linear travel. If you have a single start screw then 10 turns of the screw equal one inch of travel. Now, depending on your gantry weight, loading of the screws and hence the motor will vary. Also, acme screws are not very efficient. Combine that with the fact that stepper motors are albeit rated for torque, where that torque is created is dependant on the winding of the motor. It could be at 5 Rev/sec or it could be at 30 Rev/sec. Also, motor voltage / current has a lot to do with weather your motor can push your load at your required speed.

So in short. Lots of variables are required to be known to get an answer to your question. I would recommend a tested solution / design from another member using similar components or, get ready for some long hours of reading, and a goold set of online calculators.

[Mr.Chips]

Originally Posted by cyrius Hagar, I think that the reason your question is going unanswered is mostly due to a lack of information. Your screw thread pitch determines how far your screw nut travels per rotation of the screw. Threads per inch is misleading because multiple start screws change your realationship between rotations to linear travel. If you have a single start screw then 10 turns of the screw equal one inch of travel. Now, depending on your gantry weight, loading of the screws and hence the motor will vary. Also, acme screws are not very efficient. Combine that with the fact that stepper motors are albeit rated for torque, where that torque is created is dependant on the winding of the motor. It could be at 5 Rev/sec or it could be at 30 Rev/sec. Also, motor voltage / current has a lot to do with weather your motor can push your load at your required speed. So in short. Lots of variables are required to be known to get an answer to your question. I would recommend a tested solution / design from another member using similar components or, get ready for some long hours of reading, and a goold set of online calculators.

Ok I can give a little more information.

It is a fixed gantry.
the movable table is a piece of Nylon?? 1" thick and about 24 x 30" Weight???
The 1/2 10 Acme screw is a single start.
The Stepper motore are wired unipolar
The step rate right now is full step and 2000 pulse per inch.
The driver can do full to 1/16 step.

Does this help/

Thanks