Retrofit of Mill with programable DRO and power feeds

[Paraprop]

Hi,

I bought an old mill.

Now I am considering to put a Digital Read Out system with linear scales on all 3 axes.

The X and Z axes are originaly link to a gear box for the feed. the Y axis is manual only.

I am thinking of disconnecting the mechanical gear drive and fit some electrical drives onto all 3 axes.

To proceed, I found that Fagor from Spain makes a DRO, the NVP300MRS which allows to program steps and possesses outputs which switch when a given axis is reaching a given setpoint.

My idea is to use this output to stop the related electrical feed with this signal and let the DRO with the next step and so on.

Now I am wondering what kind of drives to use. Further I am not sure if between the time the output switches to stop the drive and the moment the axes is immobilized, won't the setpoint be overun ?

If you think it can be done what kind of drive would you use ?
Any brake required ?

[Al_The_Man]

I would say you are going to get some error and will vary with speed/feedrate being used.
I have retrofitted machines like the old Cincinatti horiz. mills that had point to point motion and the screws were driven by hydraulic motors, but this system had an anticipation feature so that the motors would slow down when the desired position point was reached. There was no servoing action and it is fairly crude.
You could use common analogue input servo drives and set the feedrate by pot. the signal would pick up the servo brake and enable the drive at the same time. But you should not expect any really precise machining. Things like face milling and through milling where you clear the part at each end would probabally be OK.
Al

[Paraprop]

You confirm what I was worried about.
Finally I dropped the programable DRO for a normal one with a RS232 interface.

Has anybody experience with using linear scales to provide the feedback for servo motors instead of the rotary encoders which are fit onto their axis ?

If this is possible I could use the DRO's linear scale to provide very accurate position feedback to a future CNC control system.

What is your opinion.


Daniel.

[Al_The_Man]

Heidenhain are currently using this method on their Knee Mill retro-fits. You may need precision ball-screws with pre-loaded nuts for low backlash.
Al