[RicknBeachcrest]

Thanks again for your great insight.

One of the first purchases with this project was the steppers and controller. I am going to use the g540 with 280 oz steppers sold by Gecko.
G723-280-4 Stepper Motor

Now my next learning exercise is matching up AC bearings to the fixed end of the ball screw. I thought that the end machining was pretty standardized. From my looking at the various sites it seems to vary from manufacturer to manufacturer.

[stevespo]

That is a versatile motor/driver package. You could go with fine or coarse screws and still have a nice balance. If it's important to have 1000 steps/inch resolution, then the .2" (or 5mm) is a great choice. If you wanted a bit more speed and lower RPM, then a .5" (or 10mm) pitch would be great. That combo has a nice torque curve and you shouldn't run out of headroom regardless.

I would not fabricate my own end blocks and AC bearings the next time around. I'd order a matched set of blocks, screws and anti-backlash ball nuts and focus on building and using the machine. Of course, there is some entertainment and educational value in doing that stuff yourself. The cost savings (IMO) is minimal when you consider the time and effort, sourcing parts, etc.

Steve

[RicknBeachcrest]

Thanks for the info on the steppers. I looked, but could not find a torque curve for this stepper. I kind of planned from way back to go with a .2" (5mm) lead. (I was originally looking at Roton threads which were just .2 lead for .5/8") But it makes sense to consider a 10mm lead. With a 5mm lead that would give the resolution to about .001" per step. Looking at it this way seems to be a little over kill on resolution.

I'll be using this machine 99% of time for wood working projects and .001" resolution per step seems a little much. So if I decrease the resolution by 1/2 (10mm lead) that would still give me a resolution of .002" per step, which I think is more than enough for my needs.

Am I figuring this right?

End block fabrication:
I was going to ask your opinion on this. I think I will plan on using the matched set of blocks, unless this just won't fit. I built the router mount which was a good learning experience, but I really was wondering about the wisdom of this decision, since I could have purchased one from K2CNC for about $68.
Thanks for all your help.
Rick

[stevespo]

I haven't seen a torque curve for that particular motor. Gecko might have one if you called them up. My impression (from looking at Keling specs, etc) was that the 270 oz-in and 380 oz-in motors had more available torque at higher RPM than my old (high impedance, triple-stack) 425 oz-in. Those *definitely* would have benefited from the .4"/10mm pitch screws, even running at 72VDC.

It's all a function of driver/motor/PSU and it's sometimes hard to know until you have it wired up and working. That is when theory becomes reality.

The limiting factor (in terms of accuracy) has probably less to do with the screw pitch (step count, etc) and more with the limitations of rolled ball screws (probably +/- .002" per foot), the structure of the machine, type of linear rails, runout on the spindle (router, lam trimmer), etc. You are right that .001" or .002" is plenty for woodworking. We're not doing aerospace work here.

I think the .4"/10mm pitch will keep the RPMs down, keep the pulse train down, give you very smooth, fast motion and probably keep the motors working in the strongest part of their torque curve. You really can't go wrong either way. It's more a matter of which way you want to hedge.

The end block fabrication not a big deal, but I used my CNC to do the work. So, you'd have to cobble something together (like I did) out of wood, plastic, etc and get the machine working. Then make the new blocks and swap everything out, reinstall, realign. That is the PITA. Optionally, you could have someone else machine them, but then you're probably better off just buying a matching set and having everything work.

The standard blocks don't often mount cleanly on the 8020, so some small adapter plates in aluminum might be needed. But that's just cutting /drilling and the accuracy doesn't have to be real high. Just my opinion, as always.

I'm happy to take this discussion over to your thread, I've been subscribed for a while.

Steve

[gera229]

I haven't seen a torque curve for that particular motor. Gecko might have one if you called them up. My impression (from looking at Keling specs, etc) was that the 270 oz-in and 380 oz-in motors had more available torque at higher RPM than my old (high impedance, triple-stack) 425 oz-in. Those *definitely* would have benefited from the .4"/10mm pitch screws, even running at 72VDC.

Did you supply the 425oz-in with enough power?
It also could have been that the inductance of it was higher than the other stepper motors.
I read that higher inductance means it loses torque quicker throughout the RPM range.
Lower inductance would be better.

From post #120:

You're correct that is is a bit of a trade off in terms of speed/accuracy. With steppers you may lose some positional accuracy because you're dividing each screw revolution into 200 steps. Fewer revolutions per inch means less granularity with motion.

My servos have 1000 CPR (quad) encoders, which I am only using in 200 step/turn mode, so I could definitely use a coarser screw and adjust the step count to keep the same accuracy I'm used to. The accuracy is a function of the encoder and not the motor.

By positional accuracy are you referring to resolution?
Or positional recognition?

As for the different size screws and keeping the accuracy that you're used to if upgrading to a larger pitch screw:
Have you considered micro-stepping with stepper motors?

Encoders for servos may have the ability to produce more steps per turn than micro-stepping for steppers, but you aren't using it that high anyway. And while the resolution will be high at a high amount of steps/turn, the speed would certainly lack. Who doesn't want speed? I most definitely do .

I am still new to CNC, but just pointing some things out.

Also in this thread I've heard you talking about how servos are closed-loop.
What exactly does closed-loop mean?

[stevespo]

Did you supply the 425oz-in with enough power?
It also could have been that the inductance of it was higher than the other stepper motors.
I read that higher inductance means it loses torque quicker throughout the RPM range.
Lower inductance would be better.

Yes to all of the above. Wired bipolar parallel, the inductance is 6.8mH, so the ideal voltage would be more like 32*sqrt(6.8) = 83V. I was using an unregulated 72VDC supply at 10A, so adequate voltage and current were not the issue. Sure, I could have gone a little higher on the voltage, but I was pretty close to the upper limit with 72VDC (they already ran quite hot).

By positional accuracy are you referring to resolution?
Or positional recognition?

As for the different size screws and keeping the accuracy that you're used to if upgrading to a larger pitch screw:
Have you considered micro-stepping with stepper motors?

Encoders for servos may have the ability to produce more steps per turn than micro-stepping for steppers, but you aren't using it that high anyway. And while the resolution will be high at a high amount of steps/turn, the speed would certainly lack. Who doesn't want speed? I most definitely do .

I am still new to CNC, but just pointing some things out.

Also in this thread I've heard you talking about how servos are closed-loop.
What exactly does closed-loop mean?

I'm talking about resolution. Microstepping is helpful for smooth motion and fighting resonance, but it won't necessarily offer greater precision. There are MANY CNCzone discussions on the topic and I don't have the details committed to memory. Assuming greater than 200 step/rev accuracy for a stepper motor is problematic and I would leave the explanation to the experts.

I used 10:1 microstepping for years and it's a very good arrangement, provided you have matched your accuracy needs to the motors and screws. My issue with these particular steppers was more to do with max RPM and top speed than accuracy or resolution. They are fine motors, but would have benefited from a lower TPI screw to provide more speed (at the cost of resolution).

Closed-loop means that the encoder's positional output is fed back into the controller to create a feedback loop. If an external force causes the motor to lose it's position, the controller will sense this and adjust based on what the encoder is telling it. A PID algorithm takes the inputs and computes the movement require to maintain position or move to a new location.

Consider my 300W BLDC servo motors which provide consistent torque across the 0-3000 RPM range, with peak current of 20A. Another league of motor/controller entirely. The configurable quad encoders can provide whatever resolution I want from 1-4000 CPR.

My limitation now is ball screw resonance and the ability of Mach to generate a fast enough pulse train. That's one reason for running them at 200 steps/rev like the steppers.

It's a VERY nice setup and has been absolutely accurate and problem free. Given the amount of time I spent troubleshooting and upgrading my stepper setup, I wish I had gone to servos much sooner, but it's all part of learning.

[gera229]

I want to try servos now, but my budget is just too low for servos and my setup would be mainly for woodworking so servos might be overkill.

Still, the positional feedback you describe there just adds on to the accuracy and the resolution of the quad encoders. Plus eliminates the worrying of lost steps and guarantees accuracy.

I supposed I will learn along the way too. For me right now about 100 ipm is fine and that's faster than an old MacroMill CNC I used with 16 ipm max which is far too slow and rather limited on the size of material it can cut. *Might want more speed later*
I also used a plasma cutter with large servos on the X and Y axis (I think they were servos because they were cylinder shaped and quiet) and a smaller stepper (Might have been a small servo, but it was square shaped like a stepper) on the Z axis and there was a setting in its CAM program of up to 1000 ipm. Now that was amazing.

Servos are much quiet than steppers, and that's another great thing about them. They don't produce that high pitched noise that steppers do. Is that correct for your set-up?

[stevespo]

I want to try servos now, but my budget is just too low for servos and my setup would be mainly for woodworking so servos might be overkill.

Start with what meets your needs and fits your budget. DIY machines can be tweaked and upgraded because you'll know every nut, bolt, screw, wire, etc. Focus on a very solid frame and accurate mechanical parts and the motors and electronics become a "bolt on" upgrade later.

Still, the positional feedback you describe there just adds on to the accuracy and the resolution of the quad encoders. Plus eliminates the worrying of lost steps and guarantees accuracy.

Yes, it's nice - but more costly. Most brushed DC servos are geared down with a belt/pulley system to increase torque and keep the RPMs high. Extra complexity and cost there too. My BLDC (brushless DC) motors and Granite drivers would be considered on the high-end of the DIY market. There are lots of stepper combinations that will work very well.

I supposed I will learn along the way too. For me right now about 100 ipm is fine and that's faster than an old MacroMill CNC I used with 16 ipm max which is far too slow and rather limited on the size of material it can cut. *Might want more speed later*

That's what they all say!

I also used a plasma cutter with large servos on the X and Y axis (I think they were servos because they were cylinder shaped and quiet) and a smaller stepper (Might have been a small servo, but it was square shaped like a stepper) on the Z axis and there was a setting in its CAM program of up to 1000 ipm. Now that was amazing.

Servos are much quiet than steppers, and that's another great thing about them. They don't produce that high pitched noise that steppers do. Is that correct for your set-up?

Yes, the servos are very quiet - essentially silent. I never minded the stepper noise and it's not noticeable over the sound of a router/spindle and a dust collector in the shop. You can get round steppers and square servos. Many of the BLDC and AC servos are in a square package that look like steppers. The encoder on the end is the visual clue, but sometimes it's integrated and not obvious.

Have fun!