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Author Topic: Backlash not applied during manual JOG?  (Read 10627 times)

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Offline rcaffin

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Re: Backlash not applied during manual JOG?
« Reply #10 on: January 10, 2016, 04:10:02 AM »
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There is a school of thought, which was prevalent during my time in Mach3, that backlash
shouldn't be corrected and that backlash should be mechanically fixed or accepted, ...
Its like asking your cars software to help with a loose steering wheel, could be done, but probably
shouldn't be..it would affect other safety issues right down to the tires.
Doubtless there will be howls of anger, but I have to agree 100% with Art.

For a start, if you build a sloppy car or machine, you get what you have paid for. Don't expect anything more. You won't get it and you don't deserve it. Bubba need not apply.

But second, you can not correct for backlash completely. It just is not possible. Oh sure, in theory you could look ahead and see what is going to happen IN THEORY and how you might correct for it in software, but that won't work when you come to real hardware in the real world.

Any experienced machinist knows that when you are using a manual mill with ACME threads on the X & Y axes (always with backlash), you never climb mill; you always conventional mill. Why? Because if you try to climb mill your mill table is suddenly going to jump by a distance equal to the backlash in the ACME thread. That way lies smashed cutters and wrecked jobs - and possible injury to the machinist.

The same applies to a CNC mill or lathe. For sure, your software can pretend to correct for backlash, but it cannot handle those sudden jumps the table makes when forces reverse and the backlash is taken up in the other direction. Having seen a 20 mm HSS cutter snap and go flying in my early days ...

Basically, backlash compensation is a dream which cannot come true. Get real, and fix your machine. If there is wear, fix it!

Cheers
Roger

Offline dude1

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Re: Backlash not applied during manual JOG?
« Reply #11 on: January 10, 2016, 01:39:46 PM »
100% correct rodger
Re: Backlash not applied during manual JOG?
« Reply #12 on: January 20, 2016, 12:19:14 PM »
I've been looking for something to replace my flashcut pro series cnc box for my G0704, which does have backlash compensation and it works well, it's the other other parts of the software/hardware i'm having problems with. So looking for an alternative too, i saw mach3 had it, now reading up about it and disappointed to see this, if there is a mach3/4 +controller that does support backlash compensation, and we have worked our hardest to get mechanical backlash to a minimum, but there is only so much we can do and i'd like to be able to keep fitting bearings after an endmill and not with a boring bar.

As for the car analogy, that is what a closed loop O2 sensor does, it looks for changes in the fueling so it can feed back to the engine controller and trim for fuel changes, small air leaks, and a whole host of other things that are correctable by software for a small hardware issue which will occur on every car eventually. That seems a lot closer to backlash than a loose steering wheel.

should i just look at mach3 and not 4?

Offline Chaoticone

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Re: Backlash not applied during manual JOG?
« Reply #13 on: January 20, 2016, 01:26:15 PM »
Hello Charlix, welcome to the forum.

We all know that allowing misinformation to be repeated is the stuff that turns myths into legends right? Well, this is the only reason I'm going to hit on this topic that has been discussed many, many times already.  

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As for the car analogy, that is what a closed loop O2 sensor does, it looks for changes in the fueling so it can feed back to the engine controller and trim for fuel changes, small air leaks, and a whole host of other things that are correctable by software for a small hardware issue which will occur on every car eventually. That seems a lot closer to backlash than a loose steering wheel.

This is where you are dead wrong. While your analogy of an O2 sensor is fairly accurate the only way it can work properly is if it has full control of all elements involved. Should the ECM decide the mixture is a little rich and needs to adjust the air/fuel ratio by lowering the pulse width of the injector. It does so but the actual time the injector is open for does not actually change because there is a piece of trash in the injector keeping it open. This is something the ECM assumes it has control of but actually does not. What now? How does the ECM fix something it has no control over? Does it cut off the fuel pump to lean it down resulting in starving the engine for fuel and ultimately shutting it down? Does it inject air to adjust the A/F ratio and in the process raise the RPM of the engine? Do you add a pile of telemetry sensors and mood recognition to read the environment and drivers face so you can take all of that information to make an educated decision about what is the best option? These things and lots of others could be done to compensate but at what cost and what sense does it make to even try to. The best possible outcome would be piss poor at best.

You can avoid reality but you cannot avoid the consequences of avoiding reality.

Mach3 does have backlash comp. that works as good as any but I wouldn't get Mach3 or Mach4 or any other controller expecting it to be able to fix all the woes introduced with backlash. If I knew of one that could manipulate physics I would sure point you to it.
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Backlash not applied during manual JOG?
« Reply #14 on: January 20, 2016, 01:36:35 PM »
sure but the difference is, its a best case scenario versus absolute perfection, it's trimming to overcome slight issues that occur versus fixing something that is a potential disaster. People seem to have difficultly with middle or mean ground. they hear , here is a solution, but it must fix all possible failure modes instead of, here are at the most common failure modes, ie the ones i listed for an O2 sensor, versus what you listed as a failure mode. It's a trim to overcome the slight issues, not a be all and end all, not having the option is worse. My O2 sensor wasn't able to correct for someone who swapped out an injector not realising that the latency for different injectors although with the same flow size type, is different and now my car runs worse etc..

with that logic next time i design an ECU, or work on a car thats got a non 0 trim. i should say,, oh you're not passing emissions because of the slight wear, change in fuel type, location or other things that an O2 sensor does compensate for and you should go and clean, and fix every tiny part that is out of spec. Sure you can make it perfect, but that is what feedback is for. i demand mechanical perfection.

The analogy is much more apt. you're just limiting yourself to "properly" as being totally accurate, it's flawed , but it works in some scenarios, not all, its a bandaid, fix your machine, we get it. but sometimes we can only go so far and having the option is nice. It's a trim , not a cure all.

Having an closed loop O2 sensor, is better than nothing.

Offline rcaffin

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Re: Backlash not applied during manual JOG?
« Reply #15 on: January 20, 2016, 03:38:20 PM »
If you are trying to correct for 25 microns of backlash, than maybe 'backlash compensation' could be of some limited value. Fair enough, I guess.

But then someone with 1 mm of backlash hears the magic words 'backlash compensation' and thinks it will fix his problems. And it can't. And a fight ensues.

Cheers
Roger
Re: Backlash not applied during manual JOG?
« Reply #16 on: January 20, 2016, 03:47:57 PM »
yep thats all i'm looking for not some global panacea. there will always be people looking for something magical, but those people shouldn't be the dividing line. 

with backlash comp off on my flashcut i can't fit a bearing without using the boring bar, with it on i can, so obviously backlash comp works in that scenario. if i'm doing some complicated climb milling my expectations are different. i don't think thats all that unreasonable, and i'm certainly not of the ilk of how dare X remove it from product Y, they likely know what they're doing.  i'm just hoping for a recommendation on a mach3/4 + controller with existing backlash support. i can only really run to the costs of double nut C7s and abec5 on a chinese mill screwing around making stuff in my garage.

but in the meantime i'm reading up on mach3 + suitable controllers now, thanks.

Offline rcaffin

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Re: Backlash not applied during manual JOG?
« Reply #17 on: January 20, 2016, 04:22:49 PM »
OK.
W7, mach3 .062, ESS
I am running with double-ball-nuts and double races, and with no backlash compensation applied. That is working just fine.
Could it be worth your while checking both the end ball races and the ball nuts to see where any backlash is? Dial indicator and some push and shove. Might pay dividends, even to just know where.

Cheers
Roger

Offline Chaoticone

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Re: Backlash not applied during manual JOG?
« Reply #18 on: January 20, 2016, 04:27:56 PM »
Charliex, you are totally missing the point here I was trying to make. I agree that the O2 sensor is a good thing and works well when it has control and overall is a good thing. However, if it does not have control the best it can do is indicate troubles with things that are beyond its control. Backlash is just that, a range of motion in which the controller has no control. This is not a best case, worst case scenario, it is not that complicated. Either the controller has control and can do all sorts of great things of varying degrees with the motion or it does not. There is no middle ground there. Backlash comp is asking the controller to control something it has no control over. By admitting the machine has backlash you are admitting there is nothing the controller can do and it is time to call in the mechanic, no two ways about it. Either the mechanics or your machine is capable of holding tight enough tolerances to do its job or it is not. If it is not asking the controller to control something it has no control over is non-productive.

The only effective way to combat the ill effects of backlash is programming strategy and it is not great. It does not fix backlash but does give some control over it.
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline rcaffin

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Re: Backlash not applied during manual JOG?
« Reply #19 on: January 20, 2016, 04:52:26 PM »
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Backlash comp is asking the controller to control something it has no control over.
What a neat summary! I like ti.

cheers
Roger