Mini Mill - Ballscrew Z axis essential?

[megavolt]

I was planning on buying a Taig CNC-ready unit until I discovered this group. I think building the machine's retrofit myself will provide a valuable learning experience.

I want to convert the popular (Sieg?) Mini Mill sold by HF or Homier.

My question is: Is a new Z-axis moment necessary with this model? I can't tell from the documentation whether this Mill has an acme-screw thread type movement for Z, or only a rack/pinion. I'd really like to upgrade the table's screw drives later, when I have funds available for ground ballscrews.

Every picture I've seen posted here had a ballscrew adapter for the Z-axis.

Lastly, what size servos should I get at a minimum for direct-drive? I'm planning on Gecko drives. 200 oz/in? 300?

Thanks for your help!

John

[lancedulak]

I have the micro (not mini) cnc'd so take it for what its worth.
Gecko drives are fantastic but may be overboard for the mini unless you use servos. IF your willing to put in the extra money geckos and servos are the ultimate system for a mini. 200 0z/in are ok from what ive read. Remember its useless to have movement rates the mills motor/spindle cant handle. Ive seen Geckos and 300 0z/in or more.

For the micro Xylotex drivers and 150oz in are pretty standard. The mini has a much bigger motor. There is a schematic available for a linistepper driver that can handle very big steppers if you build the board yourself.

Ball screws are NOT essential at all. The advantage of ballscrews on the minis and micros is Zero backlash with dual ball screws. With a counterweight or gas shock installed theres no more pressure on the z axis than the other two.. except people install overpowered counterweights because it eliminates z backlash. With a 200 or 300 oz in system (or a geared down 150-200) theres no more reason to use a ball screw on the Z than on the X or Y. And with the money you saved you could buy a lot bigger stepper/servo.

[megavolt]

Thanks Lance. Yes, I was planning on using servos... not steppers.

What's confusing about the Sieg mini mill is that it appears to have a rack and pinion AND a "fine adjustment" as well. I can't tell if the fine adjustment is an acme-screw type arrangement (ok), or a worm-gear (bad, right?).

[lt paul]

I looked at the parts diagram here on the last page it shows that the fine adjust comes in to a casting above and at 90 degrees to the drill press type handle. this to me says worm gear adjustment. if i were you, which i am not, i would put an acme or ball screw on the z, maybe start with acme and switch to a ball screw when the machine earns it.

[RotarySMP]

The Minimill has a track and pinion drive on Z. The "fine" downfeed is a handwheel which drives a worm through a universal joint. The worm drives a worm wheel which then drives the racks pinion through a rough disconnect/ clutch mechanism.

You can picture the backlash.

Throwing out that system and installing some kind of leadscrew is considered essential. You may as well do it right and use a ball screw.

[lancedulak]

Everyone i know of who has a micro just Locks down the Z axis quill feed (which is handy to have sometimes when you just want to hand drill) and NEVER use the fine feed. It uses a switch that changes between the two so if you put it in handle feed and lock it you get zero backlash from that component. A few people do just htrow out the quill feed aparatus (i plan on it eventually).

The only real backlash due to mill design is due to the column setup. Its hard to get just the right counterweight so that you dont have to overtighten the Z gibs. Other than that its not a problem. I have mine set to less than .001 on all 3 axes and plan on getting it tighter. Just havent had time.