Backlash compensation in CAM Workspace

[Anonymous]

Backlash is an annoying thing that interests every low-mid cost milling machine, and a lot of systems don't support the compensation in real time.

The backlash is the "air" between every change of direction of the tool.

Actually, I made an editor of gcode who analyses the strings and do the modification, but sometime something goes wrong and some lines don't work well causing errors in the movements and crashing tools and others unpleasant things.

Should it possible to add a function to include this setting for every axe, where every cam user can add his personal quantity of backlash, easily measured with a dial indicator, in order to postprocess a gcode who take account of it?

Thanks

Alberto

[daniel_lyall]

backlash should be taken out by mechanical means, it changes with the weather or it's not the same from day to day 

[bmxjeff]

Mechanical means might not be an option. Or, can be expensive in most cases. On my machines, I can compensate for Backlash on the control. This is a machine problem and they should incorporate this compensation on their end.  

[Anonymous]

Fusion 360 is made for hobbyists too, so it is possible that a machine is made from an user that couldn't incorporate this compensation on the electronics who control the machine movements. And how bmxjeff said, mechanical option could be expensive.

I only think that put this compensation as a setting in the postprocess is the easiest way to do it.

As I say, i made a routine that does it. But i'm not a programmer, a programmer can do it better.

[Laurens-3DTechDraw]

Yes fusion is available for hobbyists, and start-ups but this would make the software go the wrong way if you ask me.

Also this indeed is something that should happen on the control. I'm pretty sure you can do this on Mach for example.

It would also make the code(program) unreadable since the outputted value's don't represent the actual path anymore.

[lederer.ca]

Backlash should be compensated by the CNC controller or mechanically if needed not in the CAM software.

[RandyKopf]

Backlash should be dealt with at the machine through mechanical means and software on the control. If it is not supported on the control then you need to live with the quality you can afford. I've never seen backlash addressed through CAM and to me it's really a strange idea.

[Anonymous]

Personally I think the opposite of most of you.  My memory is practically unlimited, and my CAD/CAM computer is up to the task so let the CAM do everything and make life for my interpreter as easy as possible. 

[al.whatmough]

I can fully see your need here.   That said, this issue is the job of the CNC controller, not the CAM system.

[Anonymous]

Sorry for digging this out again. But while many of the things mentioned above are true, I disagree in one thing: That Backlash should be the job of the CNC system. I might be wrong, but I think, the CNC system just doesn't have enough information for this. Imagine the CNC system having to drive a circle. How does it have to compensate for backlash? From which side does the material push against the mill? Is it a pocket? Or a pole? The CNC system just doesn't know, but the CAM system does. So, backlash should be either eliminated or handled by the CAM system in my opinion.

[Laurens-3DTechDraw]

@Anonymous Backlash is a mechanical thing of the ballscrew or other drive systems.

It occurs when the ball-screw changes rotation direction. It has nothing to do with what side the tool is pushing against.

[Anonymous]

I think it is safe for us to conclude that all who have posted to this point have some idea what "Backlash" is. Now as for who's job it is to fix or offer compensation. Because the problem originates at the machine and it's mechanical inability to create movement that is perfectly consistent with the programs idea of it's position in space, it is easy to say "fix it mechanically on the machine". Anyone who has built a machine either as a consumer product or for personal use will know first hand how prohibitively expensive it would be to try to remove most backlash mechanically from a small machine without feedback based movement. I say most because it is not possible to remove it all. It is part of most larger and more expensive machine control software to account for backlash and offer a menu for the operator to regularly update with the real time mechanical backlash the machine has at that time. Many small hobbyist or DIY machines are based on an Arduino UNO board and use GRBL machine controllers. The UNO board does not have enough memory to expand the code enough and include backlash compensation. The parts in these small DIY machines are cheaply made and have a lot of backlash. For a big software company to offer a plugin or a third party to create a intermediate program modifier to include  backlash compensation is not at all an unreasonable request nor would it be a big job for them. It would be a great help to all of the hobbyists and entrepreneurs. 

[Anonymous]

@Laurens-3DTechDraw

While you are right that backlash is a mechanical thing of the ballscrew, I disagree that it has nothing to do with the the side the tool is pushing against and that it ONLY occurs when the ballscrew is changing the direction.

I'm sure you know about the difference between conventional milling and climb milling. While climb milling is better for some reasons, it is bad for milling machines with huge backlash. Why is that? Because the milling tool bites into the material and pulls the head forward. While the head has been pushing against "one side of the backlash length" it suddenly pulls against the other side. Before, it was only friction of the head movement that "decided" where exactly the head was, it is suddenly the tool pulling it to the other side.

My example with the circle that could be a pocket or a hole is just about the same. The machine knows in which direction the tool rotates. But it dosn't know on which side there is material. Therefore it doesn't know if the milling tool is pushing to the left or to the right. But the CAD knows and could compensate for that.

[Anonymous]

Actually I think you would be right if we had, say, an FDM 3d printer. Here, the ball screw always has to "push" the head forward. And if the direction changes, it pushes again, but in the other direction. So backlash has to be compensated. I'm behind the donkey pushing its butt so it will move. And if I want to change direction, I go to the donkey's head and push in the other direction.

But with a mill, the screw is not always "pushing", but sometimes "pulling". Sometimes the donkey wants to run. Then I have to pull its tail so it doen't go too fast. And while I let go of the butt and hold on the tail, the donkey makes one big step --> backlash. But only the CAD, neither the CAM nor the machine itself will ever know if the donkey  - the tool - is pushing or pulling.

[Laurens-3DTechDraw]

@Anonymous

When I said it doesn't matter what side it is pushing against I was referring to " is it a pocket? Or a pole?".

I think you interpreted as it doesn't know if it's climb or conventional milling. I interpreted it as if both were climb cut, a pocket or a pole it shouldn't make a difference compensation wise.

But it is tricky to ask when is the climb cutting force stronger than the friction to move the table so it actually pulls it to the other side of the nut? Because that is what you are getting into now. The whole compensation would be different for a light cut vs a heavy cut on such a machine. 

What I also wonder now if the machine doesn't have backlash control how do you compensate for backlash into a drilling position? The CAM doesn't actually know where the machine is before it pre-positions(You could have moved it out of the way to clear chips for example) so the CAM can't compensate there and could be a serious positioning error.

To be honest the CAD doesn't know anything. Since it doesn't know what the CAM is doing the CAD is kind of the dumb information block at this moment.