[RustedOut]

I had the exact same problem and did what your doing by trying to readjust the gibb to the point i burnt out the z axis motor. the real problem is the bolt that holds the ball screw nut mounting bracket to the z axis slide is loose. this is a mod that aaron was doing where he added a washer and i believe a 1/2-13 bolt to lock down that connection. my bolt was too short and also the loctite i put on did not hold, so under the vibration, the bolt worked itself loose. i had to pull the head and then redo that connection with cleaning it out, heli coiling it and adding a longer bolt.

That issue has been in the back of my mind since last night. Seemed that the gib is loose enough now that it shouldn't be a problem. I think I have enough time on the mill where little things are starting to crop up. Funny thing is this problem is very repeatable. .0075" every time makes it seem like backlash, but with that heavy head and gravity I don't see any chance of that.

My mill is one of the first from IH Connecticut and I believe it has the improved connection there. I had to borrow a 12mm allen wrench when I did the conversion, so now I guess it's time to own my own. Well, I've never been opposed to a trip to the big tool store. I'll loosen the gibs some more just to totally eliminate them, then I guess I'll be pulling the head next.

Thanks for the advise.

Bob

[rusel]

Hi Bob
Just read through this thread and configuration on a fine build.
We have a similar mill and we where getting the weird z axis reading and found it to be in the connection between the ballnut mount and the head. We had a single bolt mount and no matter how tight this was it had movement in it. There a lot of mass hang of it whether you a driving it up or down. We ended up making a total new ridged mount. This has work out for us
Hope this helps.

Russell

[ksanalytical]

I've measured about 0.001" backlash and added the appropriate correction in Mach3. It's easy to do under the "Config" tab. I'd set the take-up speed to something a little faster than 20%, but that's just me. It give it a weird little hitch when it changes direction, but should make things more repeatable.

Ken

[Runner4404spd]

my z axis movement was very repeatable as well and once i had redone the connection between the ball nut mount and the head all was good.

also just as a side note... i just snapped a 1/2 HSS endmill with my IH mill and i never missed a beat, just kept rolling through.

[BobWarfield]

also just as a side note... i just snapped a 1/2 HSS endmill with my IH mill and i never missed a beat, just kept rolling through.

Been there done that!

Cheers,

BW

[RustedOut]

Well, I picked up a 12mm allen headed socket today and pulled the head. The bolt was only around 20 to 30 ft lbs. Not totally loose, but not tight by any means. Pulled the bolt and applied some red locktite and took it as much as I could which was right at 190 ft lbs. Really needed to be standing on the bed for better leverage, but never moved the head completely out of the way. My torque wrench goes to 250 ft lbs, but I guess I don't!!

I really wanted to get it back together tonight to see if that solved the problem. Not quite. I still got .003" to .0035" jogging from up to down. Continuos up or continuos down give me exactly what I'm jogging. So, starting loosening the gib some more. Now when I shut the motors off, the head comes down a slight bit, but then stops. No real slop in the head measured with the spindle all the way extended.

I'm wondering if there should be a anti backlash ballnut on the Z axis like is on X and Y. Or if further improvement to the attachment on the Z slide is needed. I think I'll pop the head back off tomorrow and get it out of the way so I can get real physical with it. Maybe should get a grade 8 cap screw and replace it as well. That is if I can find one around these parts.

Bob

[Runner4404spd]

Bob,

if you didn;t buy the preloaded ball nut then you very well may have some back lash in it. .003 is not out of the question. however, i have found when machining to depth that it really is not a problem. as long as your positioning moves are larger than .003 the machine repeats very well. if your trying to set tooling with .001 increment moves you may have a problem. i've set all my tools in the going down direction, and never move back up to compensate for this problem.

if you think about it...gravity is ultimately removing your backlash because its acting on the head and pulling it down whether the head is going up or down. the caveat to this is that if you run your gibbs tight like i do, then gravity cannot overcome the static friction of the gib and screw and everything else. once its moving however your working with dynamic friction and this all changes, however, you have to move more than what your backlash is in order for it not to have an effect on your dimensions. i usually use .100 as a positioning move for z and have not had any ill effects.

[RustedOut]

if you didn;t buy the preloaded ball nut then you very well may have some back lash in it. .003 is not out of the question. however, i have found when machining to depth that it really is not a problem. as long as your positioning moves are larger than .003 the machine repeats very well. if your trying to set tooling with .001 increment moves you may have a problem. i've set all my tools in the going down direction, and never move back up to compensate for this problem.

if you think about it...gravity is ultimately removing your backlash because its acting on the head and pulling it down whether the head is going up or down. the caveat to this is that if you run your gibbs tight like i do, then gravity cannot overcome the static friction of the gib and screw and everything else. once its moving however your working with dynamic friction and this all changes, however, you have to move more than what your backlash is in order for it not to have an effect on your dimensions. i usually use .100 as a positioning move for z and have not had any ill effects.

Runner,

I used what came with the partial kit I received and that was non pre loaded on the Z. I have been learning to set my tools on the downstroke and they seem to repeat pretty good lately. And I too use at least .100" positioning, but may have to take a look at some of my pocketing strategies.

Last night I was thinking I was done until i could fix it, but after sleeping on it I realized exactly what you're saying. I had some concern on kellering parts and that's exactly what is up next. I think I'm ok until I decide to replace the nut.

One thing that's been on my mind since this whole thing started is that I don't really have a way to get lube on the sides of the dovetails and gibs on any axis. I thought at one time that was what was causing my problem and I'm sure it contributes. I've drilled and milled the flat ways and they oil good with the one shot as do the ballnuts. Not real sure how to go about correcting that oversite. Right now I brush them with lube, at least the Z ways. I do need to change from the oil I use in the gear head to way oil so maybe it'll stay in there longer.

Thanks for the advise!
Bob