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Messages - stirling

1791
General Mach Discussion / Re: Noobish Question On Motors & Drivers
« on: April 29, 2008, 03:48:16 PM »
Hey - no problem. If I had a quid for everyone that thinks steppers lose the occasional step here and there I'd be a rich man. The question to ask is why would they do that? Maybe they loose concentration and forget what they're doing or something ;D

Seriously though - I'm not dissing servos - far from it. For a start I don't know an awful lot about them - but I do know steppers, and I know there's lots of cr*p talked about them. Steppers generally don't live up to expectations because they're either not sized or driven properly or both. As a rule of thumb (FWIW) I use this: if the application demands more power than you can get from a (properly driven and properly sized) stepper then use a servo. But first - know the power you're going to need. So far I've never had the need for a servo.

Cheers

Ian


1792
General Mach Discussion / Re: Reset
« on: April 29, 2008, 09:17:24 AM »
I think the problem may be that we're looking at limits from different perspectives. I'm going to take a flyer and guess your machine is smallish because if you hit a limit on a large heavy machine you really don't want to do it again. I see them as the equivalent of airbags in a car - I really don't want them to go off but if the sh*t hits the fan I'd rather they did. Forgive me - but you seem to see them as a tool to aid positioning and perfectly ok to hit whenever you like. Try that too often on a large heavy machine and you'll most likely wreck it. All sorts of nasty things happen when you stop a machine dead. Inertia - both mechanical and electrical has to be dumped big time - not nice on anything involved. That's why Mach has soft limits - at least then the potential for damage is reduced. Just my opinion.

1793
General Mach Discussion / Re: Reset
« on: April 29, 2008, 08:17:46 AM »
Hi Ken - Let's draw the distinction between limits and home - As has been said, they perform two totally different functions regardless of whether you're using the same physical switches or not.

Homing: As Chip has already said you can set home speed on the Home/limits dialog - this is totally independant of jog speed.

Regarding your home twice idea. If you could do that, it simply wouldn't make any difference. The home position as far as I know is set on the backoff stroke - not the approach stroke.

Limits: As has also been said - if you hit a limit - something has gone wrong - the only thing that is safe is to stop all motion. You're then free to check out what's occurred, jog off the limit and then ref all axis. IF you could get the system to ref all axis after you hit a limit what would it tell you? - you come back to the machine, find the job incomplete and the system homed - what was the problem? - you'd have no clues.

1794
General Mach Discussion / Re: Noobish Question On Motors & Drivers
« on: April 29, 2008, 07:44:45 AM »
Please forgive me if I am wrong, I thought that in the closed loop system, ie. an encoded motor, if it were to lose steps, then encoder would provide the feedback, and the system could do error correction to get back into position.
OK a fair thought - and a very common one. Let's try to answer by asking a couple of questions...

Question: Why does a motor (stepper or servo) lose steps/position?

Answer: Because it doesn't have sufficient power to overcome the resistance it's being subjected to.

Question: So how many steps does it lose?

Answer: all of them for ever and a day. (or maybe it vibrates the obstruction sufficiently to eventually get going again - this is typical for example of a "sticky" leadscrew)

Here's a pretty good read: http://www.torchmate.com/motors/electronics_selection.htm

1795
General Mach Discussion / Re: Noobish Question On Motors & Drivers
« on: April 29, 2008, 05:11:41 AM »
I'll make a couple of observations that may or may not help. You say you're happy to use steppers if you can use feedback/encoders because you're concerned about lost steps. Steppers only lose steps if they are under-spec'ed for the job or they are incorrectly driven. Adding encoders does absolutely nothing to change this. If motor x loses steps without encoders - motor x will lose steps with encoders.

But guess what - an under-spec'ed servo will also lose steps/position regardless of the fact it has encoders. Conclusion - encoders will do nothing whatsoever to stop either motor from losing position if it isn't powerful enough for the application.

Your spindle motor.... 1/3HP - 7000 revs - I think you need to reconsider this. Considering the massive construction of your machine this seems to be waaaaaaay too puny - 1/3HP is ~250Watts - that's about dremel power!!! you'll be lucky to cut plastic. 7000rpm - not for wood at any sort of feedrate - you'll need way more.

Just my 2c

1796
General Mach Discussion / Re: Noobish Question On Motors & Drivers
« on: April 28, 2008, 01:10:55 PM »
Hope this doesn't over flame your noobness - but.... can I ask a simple question? what is your rig designed to do? I ask because I'm thinking 1.5inch ballscrews 36inches long - 125 lb gantry - this is for serious high speed deep milling of carbon steel - yes? :)

1797
General Mach Discussion / Re: Stepper motor power supply
« on: April 28, 2008, 11:28:08 AM »
The 60% (actually 66%) rule of thumb applies only to chopper drives. I think yours are LR drives. Check your documentation. If they are LR drives then your PS current will be more than the combined current of your motors - in some cases, substantially more.

If you can up your voltage to 24V then your motors will stall at roughly twice the speed thay do at 12V. However your PS current will also substantially increase. Without specs it's impossible to give figures. But you need to know these specs to avoid damaging your PS, drives and/or possibly your motors.

Uni-polar LR drives are ok for some jobs (often used in printers for example) but generally speaking they are cheap because - well they're cheap!

For your next project I'd recommend bi-polar chopper drives. There are many good makes available.

1798
General Mach Discussion / Re: Tangential control manualy?
« on: April 26, 2008, 05:28:07 AM »
Hi Dennis - You can use g68. With this you can rotate your coordinate system to match the angle of your knife. Depending on which angle you choose to rotate it against you can then jog with either x or y.

So - if your knife is at 30 degrees (to x or y) and at x10 y20 then code g68 x10 y20 r30 then jog (x or y).

1799
General Mach Discussion / Re: Stepper motor setup questions
« on: April 25, 2008, 01:23:16 PM »
Hi Sam - what problems do you get with large numbers of small lines?

1800
General Mach Discussion / Re: Stepper motor setup questions
« on: April 25, 2008, 12:02:30 PM »
So, does un-checking "Stop CV on angles >" cause CV to be enabled all the time then?
Indeed it does - unless of course you affect it by your CV Distance setting. For example setting CV Distance to 0 would effectively turn off CV. The trick is to decide firstly what angle you want to turn CV off at (if any) and then for those that are less than this how tight do you want the blend to be via CV Distance. Personally I don't bother with CV Feedrate - more trouble than it's worth IMHO.