[joeavaerage]

Hi,
microstepping does not give any great increase in resolution, as the article I linked to suggests that half-stepping (400 steps/rev) is the best achievable resolution with a two phase stepper.
What microstepping does give you is 'smoothness of motion'.

Astronomers first started exeperimenting with microstepping, no doubt with a view to getting better resolution. They were thwarted in that regard by the physics of steppers, but what they did find
is smoother motion which caused markedly less vibration of their telescopes. They took it as a win.

As you increase the microstepping smoothness increases, but in diminishing returns. Much of the 'smoothness' advantage is gained by 1/8th stepping (1600 steps/rev) and so there is little advantage in higher
microstepping regimes.

You may set your CNC machine to whatever seem appropriate, say 2000 steps/rev, as you have now and get the 'smoothness' advantage but you'll gain no more resolution than if you set it at
half-stepping (400 steps/rev). So your anticipation of getting 1000 steps/rev (at the stepper) resolution are dashed.

These are the physics of steppers, no getting around them.

You could use a gear or belt reduction, that does increase resolution. Let's say you used a good low lash (2<arc min) 5:1 gearbox and your stepper at half-stepping (400 steps/rev) then at the output of
the gearbox you'd get 2000 steps/rev or 5.4 arc min/step...that would be useful....but now your speed in in the toilet.

Craig

[crjohnson]

Hi Joe,
Have been trying to learn a bit (pun intended) about servo motors and their positioning accuracy. What resolution encoder do you use on your motors? 17BIT? My understanding is that a 17bit encoder has 131,000 positions, 20bit is 1,050,000? But is it reasonable to expect those resolutions or lower that expectation (like you talked about) as with stepper motors?

It seems as though my initial question on bearing size is going to be insignificant in the end and more attention could/should be paid to the drive system.
Do I go with a $150-200AUD closed loop, 4.5Nm kit and have a low max rpm? OR go with a $300-500 (stepper online) servo kit (400W or 750W). 400W only has 1.27Nm rated torque, I'm unsure if this would be adequate for my machine. Maybe it would as I only run a 2.2kw high-speed spindle type cnc machine and don't go ploughing into steel.

THEN I need to ensure the servo system will integrate with my existing AXBB-e and UCCNC system.

[joeavaerage]

Hi,
you are correct, I don't quite know why they call it 17Bit encoder, it is in fact 160,000 counts per rev. My guess is that they use a 17bit data word to communicate with the drive, the encoder is serially connected.

You are also correct....do I need or could I use 1/160,000 th of a revolution resolution. No I can not. Servos have 'Electronic Gearing' which is a software thing that allows you to program the encoder 'to behave like
an encoder of nnnn counts/rev, and you can choose the nnnn.......all very VERY convenient.

For instance my fourth axis I have programmed the drive/servo to 4615.4 steps per revolution, that after going through the gear reduction of the fourth axis gives me 6 arc second resolution. Not only does that give me
good resolution but also means that the servo can STILL do 3000rpm, and the fourth axis can do 150rpm as a result. Good speed at good resolution.

It seems as though my initial question on bearing size is going to be insignificant in the end and more attention could/should be paid to the drive system.

Correct. It is not quite as straight forward as you might think.

I am not a fan of closed loop steppers, having said that they are capable of good resolution, typically 4000 to 6000 steps per rev, and they would make a very good motor for your fourth axis.

OR go with a $300-500 (stepper online) servo kit (400W or 750W). 400W only has 1.27Nm rated torque, I'm unsure if this would be adequate for my machine. Maybe it would as I only run a 2.2kw high-speed spindle type cnc machine and don't go ploughing into steel.

I would be very cautious about cheap servos, not because they don't work, but because the cheap ones have no set-up and tuning software and many people struggle to get these cheap servos to run.
I use Delta B2 servos, a Taiwanese brand made in China, that are good quality, documentation, support, and most importantly good and free set-up and tuning software at fair prices. Despite the extra cost I would recommend,
if buying a servo at all, then get a good one.

A 400W 3000rpm servo has a torque of 1.27Nm. What might surprise you is that it has three to four times that torque as an overload, say 4Nm. You might think that sounds a bit anemic, but I can assure you a servo will just dig in
and do the business when a stepper stalls, open loop, closed loop, it matters not, when a stepper is overloaded it stalls whereas a servo just gets on with it. My servos are all 750W or 2.4Nm at 3000rpm. They cost $438USD plus shipping.
I've been buying them at that price for four years. A 400W Delta B2, from the same company costs $398USD plus shipping, so for an extra $40USD you can have 1hp (750W) rather than 1/2hp (400W)....whats your choice?

https://www.fasttobuy.com/flange-80m...er_p28084.html

Highly recommended. I think however closed loop steppers would be a fair choice and much cheaper, but about 1/3 the speed.

The bottom line is that if your are going to the trouble and expense of a harmonic drive fourth axis, then you will want a servo or stepper of something like 2000 to 5000 steps/rev resolution. It would seem a shame to make a really
good fourth axis with all the best gear only to put a substandard motor on it and get less resolution than you might.

Craig

[joeavaerage]

Hi,
another little trick with servos that have set-up software is that you can save all the parameters (many hundreds of them typically) to a parameter file. This would mean that you can use that file to
program another drive, or use it as a basis to program another drive, for say a fifth axis. You can also email the file to someone else so they can program their drive. This last feature is quite
handy if someone is new to servos and struggling to set them up and tune them......import a known good parameter file, and flash your drive.....and your in business.

Craig

[Azalin]

Didn't your HD come with a cross-roller bearing? Attached a photo of my 25 size HD. The cross-roller output bearing is good enough to hold a 125mm size chuck.

The other photos of my castings and 3d printed model.

[crjohnson]

No unfortunately mine is the type which does not have the cross-roller. But being brand new and for $103AUD, I'm not complaining!

Joe: I also picked up a 400w delta B2 servo and drive last night for a bit less than your price mentioned (and in AUD!). Bit less torque but I will survive considering the price..

Looks like the plan is to continue with the large THK tapered roller bearings, but now with a 400W servo motor. I'll figure out a design which allows me to put some nice preload onto the bearings and it should be happy days from there!
Cheers for the help guys.

[joeavaerage]

Hi,

I also picked up a 400w delta B2 servo and drive last night for a bit less than your price mentioned (and in AUD!). Bit less torque but I will survive considering the price..

Good news. Servos put stepper into the shade. I would suggest that you get a programming cable. The B2 drives have an IEEE1394 socket for programming. There are two different ways they can be wired, with one way damaging the drive.....
which I did not like the sound of.....so I bought a genuine Delta programming cable, cost $68USD. There are plenty of them, and even cheaper ones on Ebay, but you want one set-up and use your new servo/drive.

This is exactly the same as I bought (about 3.5 years ago) and look at the price, about 1/3 what I paid!!!

https://www.ebay.com/itm/31533010600...Bk9SR-jE0IWFZA

Craig

[crjohnson]

Thank you, I certainly don't want to damage it haha

Have you ever used those cable with a rs232-usb adapter? I'm not sure I even own a computer with a 232 port these days!

[joeavaerage]

Hi,
My apologies, my cable is USB to IEEE1394, but the box looks identical. Naturally you have one cable and you can program many drives.
I've bought maybe twenty B2 drives and servos over the last four years, some for me but the rest are for others, and I've used this cable to program all of them.


Craig