Hi all,

I am planning to built a cnc mill. My plan is to convert an X1 mill to a nice cnc mill. My question is how strong my stepper motors should be. I read and saw a lot of things on the internet but I am still not sure how strong my stepper motors should be. Is there a simpel calculation for it or is it just experience? An other question is if I should drive my axis direct or indirect. I prefer direct because it not so complicated, but I also see a lot of indirect drives? What are the advantage of both. Thanks you a lot.

Greets,

arthur

I would like to see an answer to this too.

iwould also love to know the answer to this question any help anyone ? (chair)

I don't know the answer, but it would be helpful to answer a few questions. Are you using the stock screws and bearings?, and are you going direct drive or reducing/increasing the gear ratio? You asked what that does, if you do inderect drive, it is often to change the gear ratios, for more power or for more speed, it all just depends on what you want. If you aren't going to gear it down, than you will need a more powerful motor, but you will also save cost, because you can direct drive it and not have to purchase the gears/sprockets and other components.

You start by getting answers to 2 questions:

1) What speed do you want?

1a) What feed-rate IPM would make you deliriously happy? Fill in the blank__________
1b) What rapid IPM would make you deliriously happy? Fill in the blank__________
1c) What is your leadscrew pitch in TPI? Fill in the blank__________

2) How much "thrust" do you need? This is a hard one. Pretend the machine is a manual one and it has a handcrank.

2a) What is the diameter of the handcrank? Fill in the blank__________
2b) How many lbs of force would you put on the crank handle before you figured you are just not doing things right? Fill in the blank__________

You now have enough information to pick motors.

Let's say (1a) is 60 IPM, (1b) is 180 IPM, (1c) is 5 TPI, (2a) is 6" and (2b) is 10 lbs. These are the most common answers I get when I do application support. Let's use these numbers.

First thing is to calculate power required. Power is what gets things done and depending on the amount needed, determines if steppers or servos are required.

3) Power (Watts) = RPM times in-oz / 1351

We need RPM, we need in-oz. Let's get them.

a) RPM = IPM times TPI. Easy; 60 IPM times 5 TPI = 300 RPM. One down, one to go.

b) Torque (in-oz). 10lbs is 160 oz. 160 oz on the end of a 3" moment-arm (6" diameter = 3" radius or moment-arm) is 160 oz times 3" or 480 in-oz. That is the screw torque needed.

Let's plug the numbers into (3) now to see what power is required.

Watts = 300 RPM times 480 in-oz divided by 1351. I get 106 Watts. That's squarely in stepper territory. A good square NEMA-34 can give you 600 to 800 in-oz of low-speed torque. More than enough.

A 180 IPM rapid is 3-times faster than 60 IPM. Figure on having 1/3 the torque. Doesn't matter though because it is a rapid. It only has to have enough torque to get out of its own way.

Motor pick: Select a NEMA-34 square motor (avoid "round" motors like the plague) in the 4A range.

Power supply: Use a 48VDC power supply voltage.

That should get you the 100W or so you need or I'll eat my hat.

Mariss

Motor pick: Select a NEMA-34 square motor (avoid "round" motors like the plague) in the 4A range..

Mariss[/QUOTE]

i am running nema34 4amp round motors why should they be avoided ?

I have heard to not get the round steppers as well, and was wondering why?

You start by getting answers to 2 questions...............................Mariss
Wow thats the best explanation I have found in avidly reading everything I can for the last month on this forum.
Sorry if others have done the same but I have not found them yet :)

I'm just a uneducated chippy and don't understand all the formula maths that physicists use :D

Mariss I don't s'pose you could give a similar type guide come "rule of thumb" to the other big section of beginners.
You know the type - that want to build a JGRO size - sort of - moving gantry router with acme or
cheaper style ground ballscrews say 4 or 5 tpi. You know what I mean :)

Greolt

Square motors:

1) Have about twice the low-speed torque of a round motor.
2) Are designed for microstepping; can turn as smoothly as a servo.
3) Cost the same as a round motor.

Mariss

Hi Mariss
Your explanation is very good and helpfull. But I think definition of the force as manuall handcrank force is defficult. I want to design the driving mechanism before I have the maschine complete. I interested in how I can define the force more exactly. Does anybody know the free programm or Excel table or something similar, where I can enter the values and then I can get the results simply?
Thanks Palik