Linear scales on CNC?

[dogma]

How sensitive are linear (glass) scales to vibration and chip infiltration? Are they suitable for use when exposed to flood coolant or are rotary encoders on the motors a better choice for CNC?

 

[Karl_T]

glass scales should be protected, at least on top and sides.

Magnetic encoder strips will take the chips and coolant a bit better. Still best to get them out of the way as much as possible
just one vendor here https://www.sra-measurement.com/high-accuracy-magnetic-linear-tape

as to best place to put the encoder: It depends. Too much backlash or too dirty? put them on the motor for stability. My control allows them both places for both stabilty and extreme accuracy.

 

[Eddyde]

I have glass scales on my mill that give a 1μm resolution, sometimes during a cut the last digit flickers or drifts, if heavy vibration, the last two digits might. Locking the nonmoving axis usually cures it. I have the "open side" of the scales facing down or away from the action and they also have additional covers. After almost 7 years, I have never had any problem with chip fouling. That being said, I don't use flood coolant, just mist occasionally and I never clean the machine with compressed air.
Here's a link to my DRO installation:

Dro Install On Mill *Updated*

Hi All, I was going to wait till I have all three axises installed before posting this project. However, with all the recent inquiries about these Chinese DRO kits, I figured I would be best to post what I have done so far. I bought a Easson DRO on AliExpress the end of last year. The...

www.hobby-machinist.com

If you are planning to use the scales for CNC closed-loop feedback, you will actually need both rotary encoders on the servos and linear scales on the axises. Though, you might just need the linear scales if using steppers.

 

[dogma]

Though, you might just need the linear scales if using steppers.

I am thinking about converting my MR-1 from GRBL to Linuxcnc with closed loop control. This is largely because I'm tired of constantly rehoming the mill because I don't trust it not to loose steps. It currently has fairly generic open loop steppers on it and I'm trying to decide if I need to replace the motors as part of a control refit.

 

[dogma]

you will actually need both rotary encoders on the servos and linear scales on the axises.

I thought only one or the other was needed even with servos as long as there were limit/home switches?

 

[WobblyHand]

I am thinking about converting my MR-1 from GRBL to Linuxcnc with closed loop control. This is largely because I'm tired of constantly rehoming the mill because I don't trust it not to loose steps. It currently has fairly generic open loop steppers on it and I'm trying to decide if I need to replace the motors as part of a control refit.

Depending on your needs, you could use closed loop steppers. I use a closed loop stepper in a home brew electronic lead screw that I made. Servos are often a great solution, but not the only solution possible. An open loop stepper will not lose steps, at least up to its correction limit. It alarms at that point. In practice I have never seen it lose a step, but my usage is different than yours.

 

[Karl_T]

I thought only one or the other was needed even with servos as long as there were limit/home switches?

The issue is backlash. if its not an extremely tight machine its going to be hard/impossible to get stable servo tuning.

Now Galil cards can use both encoders on the motor and scales. The motor encoder makes it easy to tune. the scale is used to verify the machine got to the exact end point.

If Jim Dawson sees this, he can mention his machine with only linear encoders.

 

[dogma]

The issue is backlash. if its not an extremely tight machine its going to be hard/impossible to get stable servo tuning.

I have no experience with close loop control or servos. Part of what I am trying to figure out is if I can dip my toe in by adding scales but retaining the current stepper motors. I'm actually unsure if migrating from steppers to servos would be much of a gain for the machine in question (Langmuir MR-1). The factory GRBL control system limits travels to 100ipm and even then the ball screws are visibly starting to whip around.

 

[dogma]

I think (I need to confirm) that the lead on the ball screws is 4mm, which would make the driving rpm range something like 6-600rpm. I'm pretty sure that a conventional bldc motor has almost no torque at 6 rpm. Relacement drive would be bldc motor + gear box + encoder + brake + scale?

 

[JimDawson]

If Jim Dawson sees this, he can mention his machine with only linear encoders.

OK, I'm alive, but not enough coffee yet.

I love magnetic linear scales. I use 1 micron resolution units on my machines. They are pretty much impervious to chips, coolant, and oils. Keep magnets and acetone away from them. I have been using magnetic scales since about 2014 and have had very good luck with them.

Linear scales in general can be used with rotary encoders to create a dual loop system, but this is only normally found on really high end machines.

Using linear scales only to close the loop at the controller will automatically compensate for backlash to a degree, but there is no substitute for a tight machine. Tuning servos with linear scales is a little tricky if there is backlash in the system, but doable.

Linear scales can be used with steppers depending on the controller used. The software and operating logic gets a little complex. I'm currently working on two of those systems, one is a dual loop servo system, step & direction, with the inner rotary encoder loop closed at the drive and the outer linear loop closed at the controller. The other is a straight up stepper system with the linear scales closing the loop at the controller. Both machines are currently working fine.