I hate my Z! (Axis... X2) Ideas?

[TroyO]

Rake60... I know that refreshing snap-crunch feel you are talking about!

Old thread... and I still haven't done much about the Z axis.

I have a DRO so it's a little easier to fiddle-by. Dial it in, lock it up and go. I still hate it... it's the least well performing axis on the machine and with it being a mini-mill that's saying a lot. ;-) It also sucks when trying to drill an exact depth... 30---35---90... WTF?

I did get the Making Gears the Easy Way video so maybe it will get a revamped worm drive which I think would help a huge amount. I do also wonder if a simple leadscrew with a big disk on top might work pretty well too. Turn slow for precise increments, and "slap-spin" it for quick adjustment using the flywheel effect... not sure.

 

[purpleknif]

I'm referring to a Bridgeport type feed for boring. Tough to get a good finish by hand. Yes, it would have to disconect for drilling and such.

 

[dgjessing]

...I keep mine tight enough so that there is no drop and no droop ...

Me too. If the thing will drop on its own accord Id say tighten the gibs :

 

[Wrist Pin]

Over the weekend I did a real fast, (half-fast?), job of tightening the gib on my Z.
It did improve the drop problem but it still drops when close to the top. Guess the next step is to shim the rack.
Thanks to everyone for the info and ideas!
Jim

 

[purpleknif]

Looks to me like the gas spring in the pic is inside the column. Couldn't do that on mine, I got a couple of cast ribs in the way. Hmmmm...

 

[TroyO]

My rack "shim" was quick and dirty... I used aluminum foil tape, like you use to seal up duct work. (But not duct-tape/duck-tape)

Stick it on, trim it with a razor and try it. I think I used 2 layers?

RE: Tightening gibs... that's not really what the problem is. Of course you can tighten the gib to make everything super stiff... which I tend to do but then you have to crank hard on it to move it.

IMHO the "drop" happens because the pinion gear in pushing the head down is doing so by applying contact to the -bottom- faces of the rack gear.

The bit, when it grabs and pulls down pulls the head directly and it won't stop until the pinion teeth contact the -top- of the next set of rack teeth. You also have a sloppy worm gear involved that lets the pinion move when it drops too.. making it even worse.

The only "fix" I can think of is different drive system entirely OR very careful fitment to minimize slop between the worm, and pinion and rack so the pinion teeth contact both the top and bottoms of the gear rack at the same time and the worm keeps the pinion from turning freely.

I'm kind of thinking a 10 TPI lead screw might work pretty well. 10 turns per inch, ~12" travel so no more than 120 revs from end to end..... not so hot for drilling though.

Anyway.. learned to live with it mostly. Still hate it tho... LOL. Still on the someday list...

 

[lensman57]

I really don't like the Z axis on my mini mill. I'm still in the process of getting set up/tuned but I get the feeling that good old Z and I aren't going to get along... LOL.

That whole worm gear/rack and pinion is just... sloppy.

Anyone have or seen a great idea for modifying that?

Thoughts that came to mind... just get rid of the existing system and put an Acme screw on it. Lots of cranking for big height changes though.

Next thought... same idea, but with a lathe leadscrew type half-nut system? I think the half nut has more slop though... that gets taken up and doesn't cause issue driving a lathe leadscrew, but may be a problem for a fine feed on Z.

Last crazy idea... what about a ballscrew driven/driver? I saw somewhere that you can either drive the screw, or the nut for linear motion. What if you had a ballscrew set up so you could turn the screw directly with a hand wheel, or using a (Gear? Belt?) drive from a larger crank turn the nut (@ 10 to 1 or similar)? You would have to lock whichever wasn't in use, though. Hmm, and also lock Z since the ballscrew won't hold it.

Or... maybe not re-inventing the wheel and just making another worm drive setup that actually fits nice and tight is the way to go?

Or just admit defeat and live iwth it, LOL?

Any thoughts/help/ideas or just brainstorming appreciated!

Thanks,
Troy

Hi Troy,

While we are all it you can add the Z axis on my X1 mill to the list as well.
I have sort of converted it to cnc and while the X and Y axis have about 0.04
~0.06 mm back lash the Z has about 0.9mm, I have not done anything really special to sort this out with the exception of hanging 12kg of my dumbbell weight to the column to keep it straight.
What a waste of effort and money this turned out to be.
I was thinking of making a second nut for the Z axis but I don't seem to be able to find one for M10 and 1.5mm pitch in the UK.
Regards,
A.G

 

[Wrist Pin]

In the Hi-Tork Mill from Little Machine Shop that I have, the Z is not controlled by a worm/rack. It is a rack/pinion arrangement. Until I tightened the gibs, it would drop when ever moved. Now it doesn't move except when at the top of its travel.

What I did before the gibs was to hold upward hand pressure on the head as I moved it down into position and before releasing pressure, would make sure the Z lock was tightened. This has worked very well in the past and I continue to do so now, mainly because I don't trust it and don't want to lose a work piece that I have a good amount of time in. I find this to be very reliable and accurate for this machine. I have been very happy with the results.

For what it is worth...

 

[TroyO]

Ohh yeah, there are tons of ways around it. I actually don't mind it so much when I am setting a height and locking down the gib. It's when drilling or feeding in Z when it's a PITA, lol. (Or when I forget to lock the gib...)

This one has a rack/pinion and the pinion in turn hooks to a worm gear gear drive for "fine feed". The drill press handle drives the pinion directly.