Basic CNC

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Thanks guys, I now have more of an idea of what I Don't know. Before I just knew I didn't know anything about it, now I have an idea of the way the system works. The one thing I did not get is this. Is a ball screw a feed screw that uses balls instead of a threads on the "nut"?
 

JPigg55

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Okay, couple more questions on stepper motors.
First is to do with physical size and weight.
What is the typical physical size and weight of NEMA 23, 34, and 42 stepper motors ?
For example, I have a Clausing 8520 mill that I’d like to install stepper motors on for direct drive of the axis, but really don’t want to have a 50 lb stepper motor that’s 12” long sticking out in 3 different directions that I'll have to engineer some sort of mounting system.
Second, how would you decide if going direct drive or geared drive via gearbox or belt & pulley as far as required motor size ?
If math is correct and works here, I could use a NEMA 23 CNC kit with 382 oz-in motors ($369) with a NEMA 23 5:1 planetary gearbox ($200 ea) for a total of $969 assuming 382 oz-in at 5:1 gear ratio would be the same as 1910 oz-in of torque, but with smaller, lighter, and cheaper motors.
I know this would not be a direct correlation since the motors would have to operate at a higher speed, hence lower torque, for the same feed rate, but I think you get my drift.
Third, what factors would you consider for sizing a 4th axis for say connecting to a rotary table or dividing head ? Since these type accessories have some degree or gearing integral to them, I’d think a small oz-in stepper could be used, but how would one decide on oz-in sizing ?
Forth, is there an easy way to direct control stepper motors without using a CNC program and/or computer ?
I want to add power feeds to at least 2 of my axis (getting tired of cranking, LOL).
Accordingly, my research has shown that I’d need something like a Servo Type 140 power feed for my X & Z axis. Looking on eBay and other suppliers, a Servo Type 140 power feed runs anywhere from $650 to $800 each. That totals somewhere between $1300 and $1600 for adding power feeds to 2 axis.
Now, looking at Automation Technologies, I can buy a 3 or 4 axis NEMA 34 CNC conversion kit with 1805 oz-in stepper motors and drivers for $1022 (3 axis kit) or $1350 (4 axis kit).
If I needed larger steppers, they have a NEMA 42 kit with 2830 oz-in motors for $1193 (3 axis kit) or $1712 (4 axis kit) or, as stated in the second item, using smaller motors with gearboxes.
From a purely financial aspect, it seems to make more sense to install manually controlled stepper motors than buying power feeds, plus being most of the way towards a full CNC conversion, if I so desired later.
Considering this since I know nothing about CAD/CAM other than what I’ve read here, but not wanting to rule out the possibility for later. If I knew I wanted to go CNC for sure, I’d more likely sell what I have and just buy a Tormach or a Precision Mathews mill that’s already a CNC machine.
 

JimDawson

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I don't have time right now to compose a full answer, but 1200 oz/in NEMA 34 motors in direct drive would be more than enough. They weigh about 5 lbs, and are about 5 inches long. I'm using these same motors to run a 48x96 wood router at 150 IPM, and 300 IPM rapids. The router table probably weighs as much as your entire machine. Nothing wrong with Automation Technologies, but look at Automation Direct for steppers and drives also.
 

JPigg55

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Nothing wrong with Automation Technologies, but look at Automation Direct for steppers and drives also.

Any particular reason for suggesting Automation Direct over Automation Technologies ? Just curious.
 

JimDawson

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Just for price comparison and my preferred vendor. Both vendors are good.
 

JimDawson

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Okay, couple more questions on stepper motors.
First is to do with physical size and weight.
What is the typical physical size and weight of NEMA 23, 34, and 42 stepper motors ?
For example, I have a Clausing 8520 mill that I’d like to install stepper motors on for direct drive of the axis, but really don’t want to have a 50 lb stepper motor that’s 12” long sticking out in 3 different directions that I'll have to engineer some sort of mounting system.
Second, how would you decide if going direct drive or geared drive via gearbox or belt & pulley as far as required motor size ?
If math is correct and works here, I could use a NEMA 23 CNC kit with 382 oz-in motors ($369) with a NEMA 23 5:1 planetary gearbox ($200 ea) for a total of $969 assuming 382 oz-in at 5:1 gear ratio would be the same as 1910 oz-in of torque, but with smaller, lighter, and cheaper motors.
I know this would not be a direct correlation since the motors would have to operate at a higher speed, hence lower torque, for the same feed rate, but I think you get my drift.
Third, what factors would you consider for sizing a 4th axis for say connecting to a rotary table or dividing head ? Since these type accessories have some degree or gearing integral to them, I’d think a small oz-in stepper could be used, but how would one decide on oz-in sizing ?
Forth, is there an easy way to direct control stepper motors without using a CNC program and/or computer ?
I want to add power feeds to at least 2 of my axis (getting tired of cranking, LOL).
Accordingly, my research has shown that I’d need something like a Servo Type 140 power feed for my X & Z axis. Looking on eBay and other suppliers, a Servo Type 140 power feed runs anywhere from $650 to $800 each. That totals somewhere between $1300 and $1600 for adding power feeds to 2 axis.
Now, looking at Automation Technologies, I can buy a 3 or 4 axis NEMA 34 CNC conversion kit with 1805 oz-in stepper motors and drivers for $1022 (3 axis kit) or $1350 (4 axis kit).
If I needed larger steppers, they have a NEMA 42 kit with 2830 oz-in motors for $1193 (3 axis kit) or $1712 (4 axis kit) or, as stated in the second item, using smaller motors with gearboxes.
From a purely financial aspect, it seems to make more sense to install manually controlled stepper motors than buying power feeds, plus being most of the way towards a full CNC conversion, if I so desired later.
Considering this since I know nothing about CAD/CAM other than what I’ve read here, but not wanting to rule out the possibility for later. If I knew I wanted to go CNC for sure, I’d more likely sell what I have and just buy a Tormach or a Precision Mathews mill that’s already a CNC machine.

One way to look at the torque requirement is to make a disk to fit the leadscrew and wrap a string around it and pull with a spring scale, a little math will give you the torque required.

I am going to use a NEMA 23in the 300 oz/in range on my rotary table when I get around to converting it. A RT is a 90:1 gear, so doesn't require much torque to turn the crank.

Without ballscrews more torque will be required to move the table which is the reason I suggested a 1200 oz/in NEMA 34. You can always turn the torque down if it's too much. On my mill the X and Y DC servos are in the 600 oz.in range, and I'm am using a 1280 oz/in stepper on the Z. I cut the torque back by 1/2 and it will drill a 1/2 inch hole in steel. My mill table is 10x54

One problem with coupling the steppers directly to the axis as a power feed is that they cog. It makes hand feeding kind of lumpy. A method of decoupling the motor would be suggested. Maybe something like a lever operated dog clutch.

A NEMA 42 is a huge motor, 20 lbs or so, about the same torque as a 3HP motor. Here is a picture of a NEMA 42 with a NEMA 23 sitting on top of it. Way overkill for your application.

IMG_0479.jpg

Stepper speed controls are cheap, I bought a couple of these, one to run a NEMA 42 stepper for a project, and the other just to play with. http://www.ebay.com/itm/9-24V-Input...59466c&pid=100338&rk=3&rkt=30&sd=281570110066
 

JPigg55

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Thanks Jim,
Any suggestions/links for a control method other than a PC for speed, direction, and possibly distance of travel including being able to switch between full, half, or micro stepping ?
For the purpose of using more like a power feed, considering using something like Rasberry Pi or Arduino through a keyboard interface for this purpose.
Was also considering some sort of adjustible limit stops to prevent powering into lead screw limits of travel.
Still trying to learn a lot of this stuff, but figure it's possible.
I was thinking of using some sort of coupling device to better allow manual operation, but I can't figure a way to do this for my Y and Z axes unless I use something like a belt & pulley cofiguration since there's only one handle on these axes vs two on the X axis. Thinking I'd have to use dual shaft steppers and connect the handles there.
 
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