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Inconsistent "Z"

F

Flatjwl

Plastic
Joined
Mar 28, 2007
Location
South Georgia
I would appreciate some trouble-shooting/correction suggestions for my machine. I have a NASA knee mill with Fagor 8025M control. On a recent job, with rather important drilling depths, the Z was wandering .030 or more. Never too deep, but often not deep enough. I have a bunch of holes to rework and it won't be much fun since they are only .047 diameter. The mechanical connections at "Z" are darn good. It must be a problem with encoder? Thanks for your help.
 
How have you verified that the "mechanical connections at "Z" are darn good"? The reason I ask is that just yesterday I worked on a Bridgeport knee mill with approximately .030" of wandering. I tried pushing the quill up and down as hard as I could by hand and did not detect movement, even though as it turned out it was the problem.

The weak point appears to be the bolts loosening that go from the Z-nut-mounting block to the quill. This exact same problem occured on a different brand of knee mill a few months prior.
 
I have to agree with super dave the same thing has happened at our shop 2 times in the past.

You normally have to take the ball screw part way out to get at the cap screw that holds the ball screw mounting block to the quill. If it isn't that i'd be really shocked. If it isn't that (which i doubt) check the thrust bearings real good.Really don't think the encoder would be intermentent the error should repeat.
any questions don't be afraid to shout!!!
good luck danny
 
why machine problem ?
why not toolholder /tool problem ??

MJM
maintenance

OK spoken like a true maintenance man.. ( they never agree that there is a problem with the machine !!)

But this attitude really helps sometimes.
when we attend to a problem we tell the operator that we are here after all to solve machine problems but before we plunge into checking the ball nut housing screws etc,could you maybe recheck the toolings etc one more time ?? because it may be a while before we can give the machine back to you ...

I've found that on quite a few occassions they solved it without my help.

So At the risk of irritating you flatjwl
Can you check toolings/tools again ??

MJM
 
We had similar problems on our haas machine
it turned out it was the muck in the spindle taper


But our problem was with same hole on different jobs.

I think yours is different holes on same job ?

Do some extra special cleaning on the inside of the taper .it may solve the problem..

MJM
maintanance
 
I had that happen on a machine that had the servo connected directly to the ballscrew with a coupling. The coupling loosened, and would move about .025 or less, and only on a full speed rapid, so it stayed pretty stable while a tool was in the spindle but would change every time a tool was changed because of the rapid up to toolchange. So every tool was a different increment out of length. Drove me nuts until I figured out that it was mechanical and not in the control.
 
.047" diameter hole, kind of small, a bit hard to see in. Are you getting out all of the chips before measuring it? Even that little curly bastard that won't even come out with 120psi of air (OSHA approved of course).

Are you measuring with a .047 pin and the drill is a bit worn so the pin is getting stuck in the bottom? Is there a bit of coolant left so that you can't push the pin all the way to the bottom? I've seen that happen a lot.

One thing I have learned, don't assume anything. Start with the simple stuff, 99% of the time you will find the problem.
 
Some good thoughts and suggestions. Thanks.

When I observed the problem I changed the tool and tool-holder. I also put a test indicator base on the table and a pry bar under the tool holder where I set the indicator. I could not move anything except to flex the machine column. The holes go thru a .300 land and create a half hole for another .050. In other words, there is no doubt about the hole depths as they are clearly visible. And, one other bit of information: When I changed the tool-holder I was jogging Z down in .100 increments when the spindle went right past the increment and crashed into the holding jig. This made me suspicious of a machine problem. However, after clearing that Emergency stop it has not malfunctioned again except it is subject to stop short of full depth even though the screen indicates it is to programmed depth.
 
Just a thought when you pushed up with a prybar that was a great idea
, EXCEPT what if the spindle was already up and needed the play (if any) pushed down? doun't laugh I've seen it go either way meaning you have to find a way to push up and down to be sure.

danny
 
i have a machine in my area that is the same wat the z will either drop after comming out the other side of a hole or it wont go far enough it has a worn out spot on the ball screw on the spindle drive. we run most of our stuff in the same spot knee wise. so we drop the knee a few inches and it seems to help but a good test is to set up a peice of materal and make a step cut with a end mill with a rad on it set the tool drop the Z .030 or .040 and cut across the matl. if it comes up shallow then it has slack form somewhere. ours is an aniliam it happend twice, twice the same thing.
 
I just want to thank everyone for their thoughts and suggestions. I have not YET fixed the machine, but will advise this group regarding corrective actions, when taken. Yesterday I was face milling about 8" X 14" piece. 3/4" end mill with 5/8" stepover. I ended with 2 strips (X axis passes)adjacent about .005 deeper on Z than the balance of the piece. All passes were made at the same Z programming???? It was a cast iron piece which went to the grinder from the mill, so nothing was lost, but, another oddity.
 
Sounds like something loose to me, not electrical in my eyes
 
If the mechanicals really are all tight - have you checked the belt for bad or ripped out teeth?

Think Snow Eh!
Ox
 
Well, Do we have an answer yet??? (been 9 days)

we are all waiting
 
Sorry, no answer yet. Machine is sitting idle while I am off in another direction on another project. But, I will respond. Maybe next week, maybe next month, maybe---. Thanks again for your interest and suggestions.
 
Flatjwl,
OK, I know all about the other direction problem. I go in circles alot.

danny
 
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