New manual to CNC conversion question

[familyguy]

I've ordered some of the gear for a manual to cnc mill conversion, having no experience with cnc I just looked at what most had done before me and started my plans that way - a pc with Linuxcnc or Mach3/4, steppers, breakout boards power supplies etc. but in my search for parts/prices I came across dedicated cnc controllers that replace the desktop pc, the software(Linuxcnc or Mach3/4) and also the breakout boards so I jumped in on impulse and ordered a DDCS 3-5 axis motion controller, only time will tell if this was a good decision.


I originally wanted to set the motors up so the drive could be disconnected and the mill used manually for quick one offs, however the DDCS motion controller comes with a handheld device called MPG CNC Control, this has a rotary encoder with a knob that looks much like the business end of the X or Y leadscrew, just wondering if any one has used this and is it a viable thing to take the place of the X/Y leadscrew handles used manually.

[snapatap]

I have 0 experience with hobby grade cnc, but the machines i use at work (FAGOR, FANUC & Heidehain controllers), are very easy to use manually. The MPG has select able resolution normally 0.001,0.01 & 0.1mm, or the are buttons to feed at a programed feedrate in the selected direction. not sure if the hobby controllers can do this.

[familyguy]

This is the MPG hand held control that is bundled with the offline controller

Universal CNC 4 Axis MPG Pendant Handwheel & Emergency Stop for FAGOR GSK System | eBay (https://www.ebay.com.au/itm/142157794998?hash=item211943ceb6:g:OWUAAOSwWpFfNGTY&frcectupt=true)

[snapatap]

that's exactly the same as on a couple of our machines.

[familyguy]

The mill I want to convert is an Arboga mill drill with a round column, the column is raised and lowered by a rack and pinion located by an adjustable gib with the pinion driven by a hand cranked worm mechanism - there are no graduations so all down feed for milling is by quill fine feed. The gib does a good job of locating the mill head, I've never had a problem with it even if I accidently leave the locking mechanism loose.
I'd like to leave the quill fine feed as it is and use the rack/pinion/worm drive that raises and lowers the mill head as the Z axis - I've worked out a 12Nm nema34 stepper has more than enough grunt if I drive it via a 3 or 4 to 1 reduction, I have checked and there is backlash is in one direction only when changing direction from raise to lower, the backlash is .004" (approx 0.1mm) but once I use a 4:1 reduction the actual backlash will not increase (I think) but the lost motion between the stepper and column will increase 4 fold so the effective backlash will be .016" or 0.4mm (I think) is this excessive?. My other option is to remove the worm and pinion and mount a chunky ball screw on the side driven by the stepper.

[OxxAndBert]

I'm not a CNC guy, so just spit-balling.....

If the head always slides freely down the column, then gravity should provide a preload until the force between the work and the head exceeds the head weight (ie heavy plunging/drilling). Sounds like its not currently sliding freely, so I'd say that would be the first thing to sort out.
Some form of rolling bearing at the top/rear of the head might be enough to free it up if there's a bit of stick due to the head tending to nod and bind on the column. Likewise a lighter lubricant may help it move more freely.
You probably wouldn't design a CNC mill from scratch to rely on gravity for preload, but in practice if its not an issue then why not run with it.

Steve

[familyguy]

I would have thought that gravity would do the job of eliminating any backlash but it doesn't seem to be the case, I did a test on the column - raised it a few turns of the handle - turned the handle back a fraction and the dial indicator under the chuck didn't budge gave the mill head a few good bumps still didn't budge so after giving a bit more thought I've decided that the worm drive/rack mechanism was never designed to be used as a Z axis drive, it has plain bearings, I'm not sure how they will stand up to repeated use so I'm going to mount a ballscrew on the side and drive it with stepper - I have a 12NM motor that I can use along with a low backlash timing belt reduction drive, probably the best thing to do. I probably have a good month or so to play with cardboard ballscrews and bearing mounts to see how it is all going to fit before I make a start with the real thing.

[familyguy]

This lot arrived today - the steppers were delivered a few days ago, now waiting for the ballscrews etc and I can make a start, I'd better try to clear the backlog of minor projects before they arrive.

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[familyguy]

Another package arrived today - 3 ballscrews - 2 x 2005 for the X & Y axis and 1 x 2505 for Z axis plus mounts, thrust bearings and couplers, I think this is going to one of those projects that initially looks like there is not that much to do but after a while you start wondering if you'll ever finish.

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