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View Full Version : Can linear scales be used in conjunction with mach 3 software?



Ady37
13-07-2015, 12:14 AM
Hi, I am new here and also new to cnc. I am buying a mill to convert to cnc and have been doing a lot of reading and research into how to go about everything. I know that software like mach 3 once set up will work to drive the equipment on desired paths using the reference data you input into it. Obviously backlash for instance could put the machine somewhere other than where mach 3 believes it to be, but mach 3 cannot know that so will just carry on its instructions and complete its routines presuming all is correct. So my question is whether it is possible to have linear scales on a mill like would be used with a DRO which could be used as a reference for exact position by mach 3 ? If this is possible then backlash would not matter because the software wouldn't be presuming it was on target but would actually know in real time its exact position and could work from it.

Ady.

njhussey
13-07-2015, 09:12 AM
Mach 3 has backlash compensation in it so you won't need a DRO unless you want to also be able to use the mill manually.

Clive S
13-07-2015, 09:56 AM
Ady Welcome to the forum have you thought how you would do the conversion ie ball screw etc.
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Before you buy a machine see if you can have a look at one to see if it is suitable to fit the screws under the table. Check what others have done to do the conversion.
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Edit Have you seen this:- http://www.mycncuk.com/threads/8842-For-Sale-CNC-Converted-Super-X1-MicroMill-from-SIEG-as-sold-by-Arc-Euro-Traders?p=70859#post70859

komatias
13-07-2015, 12:46 PM
Hi Ady,

Mach3 does not have a closed loop system built into it. You can interface the glass linear scales with the DRO but the engine inside the software will not be able to correct for deviation in position. There is a plugin somewhere that will stop the program if the deviation of measured position vs estimated is too large.

As long as you use good electronics and actuation hardware, backlash and lost steps are minimal. Backlash is manageable anyway through Mach3. So you can get professional quality results with an open loop system. On my machine with a Newall 5micron accuracy/resolution scale, I am happy that my DRO and Mach show an almost identical number.

To avoid lost steps from the stepper motors, you will need to consider a motion control board and not use the parallel port. There are many motion control boards out there so choose the best you can for your budget. (ethernet ones are better) Cabling and cable routing is also cruicial.

If you want to close the loop, you will need to buy a KFlop board and learn to program it. If you have the skills it is a nice project and gives good results.

Ady37
13-07-2015, 12:56 PM
Ady Welcome to the forum have you thought how you would do the conversion ie ball screw etc.
.
Before you buy a machine see if you can have a look at one to see if it is suitable to fit the screws under the table. Check what others have done to do the conversion.
.
Edit Have you seen this:- http://www.mycncuk.com/threads/8842-For-Sale-CNC-Converted-Super-X1-MicroMill-from-SIEG-as-sold-by-Arc-Euro-Traders?p=70859#post70859


I have just purchased a machine literally this morning. I don't think initially I will bother will ball screws but it may be something for later. I have gone for a machine which is as big as I could afford, I first looked at the X2 type machines but the one I have purchased has bigger table and more travel and other features for just a little more money. I want to do the conversion myself so I can learn about cnc and all its workings so in the future I will be clued up and able to modify other machines etc.. I have the Clarke cl500m 3 in 1 machine which i bought a couple of years ago and I find quite accurate and a reasonable machine for what it is. So I can make brackets and fittings on it for my cnc project.

Ady37
13-07-2015, 01:03 PM
Hi Ady,

Mach3 does not have a closed loop system built into it. You can interface the glass linear scales with the DRO but the engine inside the software will not be able to correct for deviation in position. There is a plugin somewhere that will stop the program if the deviation of measured position vs estimated is too large.

As long as you use good electronics and actuation hardware, backlash and lost steps are minimal. Backlash is manageable anyway through Mach3. So you can get professional quality results with an open loop system. On my machine with a Newall 5micron accuracy/resolution scale, I am happy that my DRO and Mach show an almost identical number.

To avoid lost steps from the stepper motors, you will need to consider a motion control board and not use the parallel port. There are many motion control boards out there so choose the best you can for your budget. (ethernet ones are better) Cabling and cable routing is also cruicial.

If you want to close the loop, you will need to buy a KFlop board and learn to program it. If you have the skills it is a nice project and gives good results.

Thanks for the info on the motion control and KFlop boards I will do some research on that tonight. I guess the big professional gear runs off closed loop then?

dodgygeeza
13-07-2015, 01:35 PM
Never Used the Kflop, but i have used the CSMIO/IP-A and this is closed loop between the motor / driver to the controller.
You can also use external linear scales as long as the output from the scales are compatible.
But and this is a big but the IP/A outputs +/-10V so can only be used with servos, so forget trying to connect stepper drivers.

Whatever you do, you will need to use an external motion controller because there is no way to get the encoder signals into mach3 without one.
However, i think you are over thinking thing, if the machine is a small mill like you mention, just upgrade the leadscrews to ballscrews and add stepper motors and you will be fine.
Moving from leadscrews to ballscrews is a must for CNC.

komatias
13-07-2015, 02:45 PM
Thanks for the info on the motion control and KFlop boards I will do some research on that tonight. I guess the big professional gear runs off closed loop then?


Some do. The thing is that most professional high end machines will also run servo motors so you need the encoder feedback anyway. The feedback is mainly through the encoder on the motor with the linear scales in parallel.

Looked at length at the KFlop and it is a very nice piece of kit. Like you I wanted to tie in the linear scales with the stepper motors to have absolute accuracy.

Unfortunately for the cost and the faffing with coding lead me to use a different solution.

Considering that stepper motors are extremely accurate in their stepping, it is not usually worth the extra expense on the servo based system for hobby work.

There are also steppers that have feedback on them and know if the steppers have lost steps due to mechanical issues. See the "Leadshine hybrid servo"

Ger21
13-07-2015, 03:52 PM
To avoid lost steps from the stepper motors, you will need to consider a motion control board and not use the parallel port.

This really has nothing to do with lost steps. Thousands of people use Mach3 with the parallel port without losing steps.



Whatever you do, you will need to use an external motion controller because there is no way to get the encoder signals into mach3 without one.

You can get encoder signals into mach3 through the parallel port. You'd probably need a second port, though, as one probably won't have enough inputs.

But regardless of whether you use a parallel port or motion controller to get the signals into Mach3, Mach3 can't do anything with them, except display the values in DRO's.

The motion controllers that can close the loop with Mach3, do it on their own, and Mach3 never sees the signals.

m_c
13-07-2015, 07:09 PM
The others have covered the practicalities of Mach3 and closed loop, however closed loop using linear scales will struggle to deal with any excessive amounts of backlash.
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Lets say for example you have the equivalent of 20 steps backlash between the stepper and scale. You command a move of say 30 steps, and want to try holding within +/- 5 steps. The controller commands a move of 30 steps, but due to the backlash, the table only moves 10steps, still 20 steps short, so the closed loop control commands another 20 steps and you theoretically stop at your requested place. But then the cutter pulls the table, and you've suddenly overshot the target point, at which points the controller commands a move back within range, and the process continues.
Another example is the table moves overshooting the target due to deceleration, and you end up with a continual oscillation.
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Using the PID loop typically found within closed loop controllers, you can tune to handle each of these situations, but never really in a satisfactory way. Increase values to enable quicker corrections, you'll likely end up with a continual oscillation (which in the case of servos can be so severe you cause them to overheat in a matter of seconds), reduce the values to reduce how quickly errors are corrected, and you end up with an over-damped or very sluggish to respond system.
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And that's before you consider the mechanical problems from excess backlash. With excessive backlash, climb milling will likely be out of the equation, and overall accuracy will be reduced.
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Personally, I'd put the additional cost of adding scales/closed loop into removing the backlash in the first place. A on open loop machine with minimal backlash will always be better than a similar but closed loop machine with excess backlash.