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Ross77
21-04-2013, 08:48 PM
Hi All.

Looking for some advice on an idea I had to use 2 steppers on each axis to get over the high resolution and low rapids vs the low resolution and high rapids problem.

To start with this is for a small mill that will be used for stainless steel and Ideally Alu, the spindle is limited to 4000/5000 rpm so cant get near optimal surface speed for Ali so don't need crazy fast cutting speed but would like decent rapid speeds.

Initially I was looking at belt drive and picking a set of pulleys to give 1:1,1:2, 2:1 that had the same centre spacing so it would just be a case of changing the pulleys to change the machine performance depending on what I was cutting.

Then I thought that it could be possible to have dual motors, one geared at 1:2 for high resolution cutting and then the other geared at 2:1 for fast rapids. both motors would be driven for one driver and have the motor wires switched by relays on a rapid command from the controller.

So good idea? all ready done or pie in the sky?

Cheers

C_Bubba
21-04-2013, 09:47 PM
IF I am reading you correctly, I think it is a bad idea.
1. It is extremely bad to have switches (the relay) between the driver and the stepper. Inductive kickback can let the magic smoke out:{(
2. Even disconnected, I would think there would be wasted power turning the disconnected stepper.
3. Two different "steps/move" to contend with, how to handle that?

Just my thoughts

Jonathan
21-04-2013, 11:02 PM
I tend to agree with the previous post. You would have to switch off the stepper driver when changing, or add a circuit to better absorb the back-emf from disconnecting the motor. It seems impractical to me since how do you switch between motors accurately? When one motor is switched off and the second one switched on, the second one is likely to have to start on a micro-step position, so the driver would have to somehow start the motor without the rotor moving, even when the rotor isn't aligned with a pole tooth.

Are you sure you get the machine fast enough with the right motor and 1:2 for high resolution and do you really need a 1:2 ratio? A servo motor would make this easy.

Ross77
22-04-2013, 12:07 AM
ok stupid idea then :hopelessness:

Cheers, got a bit carried away there. Think I will just go for multiple or changeable pulleys then.

Jonathan
1:2 was more for the cutting speed as avg. stainless is 75mm/min so with a 5mm pitch ball screw that would only be 15 rpm. 1:2 brings this up to 30rpm so a bit smoother and as I'm after 0.1mm accuracy that will give me 8 steps head room.

Downside means that rapids are only 2500mm/min which seems a bit slow.

Servos would be best and I have quite a few motors but no drivers. hopefully this will be building the pcb mill to make servo drivers........

JAZZCNC
22-04-2013, 07:32 PM
Downside means that rapids are only 2500mm/min which seems a bit slow.

You say this is for small mill so probably 300mm max travel so at 2500mm/min thats 41.6mm in 1sec so 7.2s to cover full travel.!! Seems like you'll be going to lot of trouble and expensive with using servos when machine will probably spend 90% of time cutting at 75mm/min.?

Personally I'd go for 2.5mm or 3mm pitch 1:1 for the torque and resolution and use Lead shine closed loop steppers.! This will make a nice and very accurate machine plenty fast enough for SS.

m.marino
22-04-2013, 11:41 PM
You say this is for small mill so probably 300mm max travel so at 2500mm/min thats 41.6mm in 1sec so 7.2s to cover full travel.!! Seems like you'll be going to lot of trouble and expensive with using servos when machine will probably spend 90% of time cutting at 75mm/min.?

Personally I'd go for 2.5mm or 3mm pitch 1:1 for the torque and resolution and use Lead shine closed loop steppers.! This will make a nice and very accurate machine plenty fast enough for SS.

+1

Michael

Ross77
23-04-2013, 01:37 AM
You say this is for small mill so probably 300mm max travel

450 ish from memory (too cold to go out and measure :0D) I see your point thought. I was looking on U tube to get a feel for what speeds people were using and 5m/min looked about right but 2.5m/min seems abit slow. Im not aiming for 30m/min like some of the routers but I remember back to when I got the other x-y table up and running (4mm pitch and 3:1, so 1.3m/min) it was painfully slow waiting for it home.

The other minor problem is that in the interrim I need this to make the parts (in Al) for the other machines, hence a changeable resolution



Personally I'd go for 2.5mm or 3mm pitch 1:1 for the torque and resolution and use Lead shine closed loop steppers.! This will make a nice and very accurate machine plenty fast enough for SS.

do you mean the encoder based systems or the digital hybrid ones? the encoder ones look as pricey as servos! the hydbid are much cheaper and some seem to be able to reach 3000rpm! if thats correct then problem solved.

With so many on the net could you point in the right direction:biggrin:

2.5 or 3mm pitch? is that belt tooth pitch or ball screw?


Thanks for your input, most appreciated.

m_c
23-04-2013, 05:31 PM
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JAZZCNC
23-04-2013, 06:07 PM
Jazz means the leadshine closed loop encoder system - Hybrid closed loop Stepper system - Zapp Automation Ltd (http://www.zappautomation.co.uk/en/123-hybrid-closed-loop-stepper-system).

Servos can be had for not much more (DMM-tech are pretty well priced, but you need to allow for import tax/vat), or you might luck out and get something suitable on ebay for less however that requires you knowing what you're looking for. Also servos can be a bit of minefield to set-up, which is where the Leadshine closed loop steppers have a major advantage.

Yes I do mean those but they are considerably less than the DMM servo system and can be bought off Ebay for not much more than Good digital drives and decent motor setup.
New Leadshine 300W 3-phase Hybrid Servo Drives Set HBS57 Drive + 573S20EC Motor | eBay (http://www.ebay.co.uk/itm/New-Leadshine-300W-3-phase-Hybrid-Servo-Drives-Set-HBS57-Drive-573S20EC-Motor-/300757428536?pt=LH_DefaultDomain_0&hash=item46068a2938)

They are fantastic for the money and very accurate. The 2Nm 1.2deg 3phase motors give finer resolution than the common stepper motors, the combination of these motors and the very good Hybrid Digital drives are a perfect match for a small machine that doesn't require high speeds servo's offer. They easily match cheap servo's for accuracy and repeatability.