Easy Servo Drive?

[Tenson]

One reason I like the idea of servo steppers is because I find the machine sometimes loose steps when pocketing circles. When it goes to the centre it needs to change direction really fast to keep constant velocity. It then loses position slightly. My current solution is to slow the feed part way through the operation as it nears the centre, but at the moment that means manual g-code edit for me. I might also want to machine a whole sheet of detailed nested parts and then finish off with the through cuts so vacuum is maintained until the final cuts, but doing so much work across the sheet and then eventually coming back to the first part I find I may be out of position very slightly.

[JAZZCNC]

One reason I like the idea of servo steppers is because I find the machine sometimes loose steps when pocketing circles. When it goes to the centre it needs to change direction really fast to keep constant velocity. It then loses position slightly. My current solution is to slow the feed part way through the operation as it nears the centre, but at the moment that means manual g-code edit for me. I might also want to machine a whole sheet of detailed nested parts and then finish off with the through cuts so vacuum is maintained until the final cuts, but doing so much work across the sheet and then eventually coming back to the first part I find I may be out of position very slightly.

Well you have under powered steppers or are overtuned if loosing steps or position.(or some mechanical issue) The closed loop system won't help in this case all it will do is fault rather than carry on out of position. Under powered or Overtuned it will be just same closed loop or not and Only way around this is correctly sized steppers or tune motors correctly.

My machine cuts Ali for 10hrs + with deep DOC at High feeds and still comes exactly back to where it started.
Like today it did a 4hr job with multiple tool changes and straight after a 2.5hr job with 5 tool changes. The last operation was a Finish pass on some pocket walls with smaller dia tool to clear out the corners etc. If I'd lost just few micro steps this would have shown on the pocket floors. And this was after 6.5hrs straight cutting Ali with at times high feed trochoidal toolpaths with 20mm DOC using 8mm cutter, 15 toolchanges and 200+ drilled holes.

So what I'm saying is Closed loop won't help a badly setup or under powered machine.! Or make it anymore accurate than a correctly setup plain vanilla stepper machine.

[Tenson]

You say it will 'fault' you mean it will just stop? I thought the whole point was it will get back to position and carry on.

How do you mill circular pockets? Say you want a 30mm pocket and you set a feedrate of 2000mm/min then it will be rather slow at the outside edge but at the center the direction changes will be super fast and the motors can't keep up.

[JAZZCNC]

You say it will 'fault' you mean it will just stop? I thought the whole point was it will get back to position and carry on.

Yes it will stop if it's outside of it's following error parameter. You are loosing steps for a reason not because steppers are inaccurate. That reason will still be there unless you change something like use larger motors or change the tuning setup. Closed loop system will correct for some error before it reaches the following error threshold but still error happend and if while in the cut material will be lost.!

In your case with only happening on occasional Rapid moves then Closed loop probably will correct the error rather than fault. But thats an awful expensive route when just buying correct size steppers/drives and/or tuning motors correctly will give you exactly same results.

[Tenson]

I think part of the matter is my gantry is super heavy. It's probably good for cutting metal but I mostly cut wood and want fast feeds. Actually fast feeds are not a problem, it's fast changes in direction that get it confused, like zig-zag ramping on a tiny detailed pocket.

I'd probably build a whole new machine to go along with the servo motors, it it will not be the only thing I do to try and improve matters.

[JAZZCNC]

I think part of the matter is my gantry is super heavy. It's probably good for cutting metal but I mostly cut wood and want fast feeds. Actually fast feeds are not a problem, it's fast changes in direction that get it confused, like zig-zag ramping on a tiny detailed pocket.

So the problem is the wrong size components not steppers are inaccurate. Wrong sized closed loop steppers will still do the same.!!
Thou It's more likely you have the accleration set to high. Or like often the case both velocity and Accel set too high.

How much does the gantry weigh.? Do you have any pics of the machine.
What drives and how much voltage are you running them with.?

[JAZZCNC]

How do you mill circular pockets? Say you want a 30mm pocket and you set a feedrate of 2000mm/min then it will be rather slow at the outside edge but at the center the direction changes will be super fast and the motors can't keep up.

The question is why can't the motors keep up.? 2000mm/min isn't fast and to be honest it's often acceleration that is the cause of most lost steps.
The way I do circular pockets is to enter with spiral entry to full depth then interpolate outwards with small stepover. There are no Centre direction changes, at the finish the tool retracts straight up or arcs out if programmed to do so.