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Converting a bench top mill to cnc.

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Volans29/12/2021 11:28:03
4 forum posts

Following the demise of my mill which I had converted to CNC, I bought a new one. You could have knocked me down with a feather when the 'boss' said get a decent one - so I did. Although I hadn't considered it beforehand I ended up with one with DRO.

2 Questions.

I use Linux CNC. The 1st, which I will not do until the guarantee ends, is, is it possible to use the DRO to control the cnc?

And secondly ( I don't think they will, but thought I'd better ask) would the stepper motors affect the DRO?

Edited By Volans on 29/12/2021 11:28:49

Martin Connelly29/12/2021 12:40:35
2180 forum posts
227 photos

If you are using CNC doesn't that make the DROs redundant? I think in theory you could have a closed loop CNC that uses DRO feedback but it would be more bother than it is worth to set up since it is usually possible to put backlash figures into CNC controllers and in some cases screw mapping.

If you want absolute positioning then the use of a homing setup will enable that.

I think there was some discussion, that may have been in this forum, regarding the output from DROs and it gets quite complex very rapidly as most systems are not interchangeable as they use different protocols.

I have not had any issues with stepper motors and DROs on the same machine but as I pointed out above there are many different systems so there is no 100% guarantee there will be no interference issues.

If you are thinking that you want to use the machine in both manual and CNC modes then consider the fact that a cheap mini wireless USB keyboard can be used to operate a CNC machine. This means you do not have to keep going to the CNC computer to move an axis and so can effectively work like a manually operated machine.

Martin C

Volans02/01/2022 16:19:50
4 forum posts

Thanks for the reply. I will think on. I did think that using the DRO might be too complicated.

My old mill had so much backlash that I ended up replacing the screws with ball screws. The new one has far less - a maximum of 0.15mm, I read somewhere the max. feasible backlash that could be compensated for but can't find it now.

I used the keyboard for manual movement. A strange thing, every so often when I start the program the direction on the Z axis would reverse and would have to go into setup and invert it, never did find out why. I have also CNC'd my rotary table ( 1mm = 1 deg)

John Haine03/01/2022 17:43:51
4715 forum posts
273 photos

I have read somewhere that it is possible to use feedback from a position sensor into LinuxCNC, can't remember where. It certainly is NOT possible in Mach3. However I wouldn't recommend it because putting any backlash into a servo loop make it very tricky to stabilise.

In Mach3 you can specify any amount of backlash, I would assume that LinuxCNC is similar. On my mill it is compensating for something around 0.01mm and seems to do a pretty good job. However on my lathe the Z axis lash is something like 0.3mm I think and trying to get compensation to work is hopeless. Fortunately I haven't found a job yet where I need to cut in both directions... The X axis (cross slide) does have a ballscrew.

I think there are two issues here. First, yes in principle BC will "take out" the backlash so the mill should cut to the dimensions instructed - but of course any variation on backlash over time or depending on position will reduce the accuracy. Second though, BC can't do anything to make it possible to do climb milling, so it's still possible that the cutter force may pull the work through the backlash, possibly breaking the cutter, ruining the work, and at the least affecting the accuracy. I would strongly recommend if you do convert the new mill to CNC, fit ballscrews!

John Haine03/01/2022 18:27:07
4715 forum posts
273 photos

Just to add on the DRO point. The great thing about a DRO on the mill is that it measures actual movement, so backlash in the screws isn't an issue. The same is true of using the CNC "DROs" PROVIDED there's no backlash in the screws.

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