Fadal Z tool offset upward drift

1992 Fadal 4020.
All -5C Cards
DC amp/servo
Recently replaced Z BS mount bearings and shims within last year

My Z axis on my Fadal 4020 is the most consistently accurate axis with less than 2tenths error. Today I noticed that after setting the offset to do some engraving the tool was not engaging the material. The depth of cut was only 2thou. I ran this program yesterday with no issues but today I had to increase the tool offset -2 thou to even start making a mark. After the program ran I did some a simple test program with a dial test indicator in the tool holder. Setting zero on the indicator and setting the TL offset at 0. Ran a test program that just drives the Z down to that tool zero. The result was 3tenths error which it returned to every time I ran this simple test program.

After running the test I though maybe my height gauge was out of adjustment. It was not. I then ran a program that I had run yesterday that faces the part. The part had already been faced and remained in the machine overnight so I knew if I had any z axis drift I could stop the program and jog the X over to see if the tool tip was offset the top of the part. It was more than 2 thou above the part. I decreased the facemill offset 2 thou and ran the program which did skim the top of the part. I waited 20min and ran the facing program again. Again I had to lower the tool offset 2thou to get it to skim the top of the part. Im thinking maybe the resolver? I find it strange the test program with the dial test indicator would NOT give the same 2thou error I am seeing when actual cutting program is ran. Before anyone mentions contraction of warm spindle I did have to do this same error correcting procedure this morning with machine cold for the engraving so I am ruling out heat growth at the moment.

sounds like you dealing with tool pressure, a face mill usually will cut a little more stock the second time around

Delw said:

sounds like you dealing with tool pressure, a face mill usually will cut a little more stock the second time around

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Now I'm curious, did you even read his post?

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Are you getting lube to the Z-axis on all lubed faces? The ways should be wet with waylube, if not you may be having stiction problems.

sirAIG said:

Now I'm curious, did you even read his post?

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Click to expand...

yeah I did, specifically this part

After running the test I though maybe my height gauge was out of adjustment. It was not. I then ran a program that I had run yesterday that faces the part. The part had already been faced and remained in the machine overnight so I knew if I had any z axis drift I could stop the program and jog the X over to see if the tool tip was offset the top of the part. It was more than 2 thou above the part. I decreased the facemill offset 2 thou and ran the program which did skim the top of the part. I waited 20min and ran the facing program again.

Click to expand...

actually theres a whole bunch of factors it could be thrust bearings , head movement saddle oil issues more oil or less oil raises or lowers the saddle, tool holders not seating correctly, because or warmth cold oil/collant on them, level of the machine etc etc.
I generally run mine a few cycles for warm up when I need critical dims in the Z as when your running a 4th .002 = .004 and the part could be out of tol. all day my machine repeats with in a few tenths.

generally a face mill(depending how big) is a pisspoor tool for for recutting or drypass's especially on a old machine , 1/2" endmill would be much easier

Delw said:

generally a face mill(depending how big) is a pisspoor tool for for recutting or drypass's especially on a old machine , 1/2" endmill would be much easier

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I understand using a facemill to gauge z tolerance is not the best. I know my machine and there is something definitely wonky going on. The part is mic6 and faces beautifully and accurately also the face mill in question is really a 1" ripper mill.

The major giveaway to me that something is wrong is the engraving operation I executed. Setting the tool height zero on the plate's surface with a 2thou DOC would surely cause the engraver to engage the material, but the engraver was always right at the part surface or a couple tenths above it even after re-zeroing TL to the part surface.

I'll check the oiling of the machine. Usually my Z boxways are dripping with oil, and its probably been a couple of years but I should probably clean/replace the metering units. I dont run the machine everyday and it will sometimes sit for weeks, but when I do run it it is usually for a couple of weeks of continual 8hr daily use.

I am leaning towards an electrical problem because the issue popped up over night and did not propagate gradually.

kstone said:

I understand using a facemill to gauge z tolerance is not the best. I know my machine and there is something definitely wonky going on. The part is mic6 and faces beautifully and accurately also the face mill in question is really a 1" ripper mill.

The major giveaway to me that something is wrong is the engraving operation I executed. Setting the tool height zero on the plate's surface with a 2thou DOC would surely cause the engraver to engage the material, but the engraver was always right at the part surface or a couple tenths above it even after re-zeroing TL to the part surface.

I'll check the oiling of the machine. Usually my Z boxways are dripping with oil, and its probably been a couple of years but I should probably clean/replace the metering units. I dont run the machine everyday and it will sometimes sit for weeks, but when I do run it it is usually for a couple of weeks of continual 8hr daily use.

I am leaning towards an electrical problem because the issue popped up over night and did not propagate gradually.

Click to expand...

I doubt its electrical, I am thinking more along the lines of mechanical.
have you adjusted your gibbs lately? check those out. also your motor to ball screw coupler can be loose.
check your backlash on your z when ts cold then when it runs for a few hours.
do you have anyway to check the draw par tention?
do your pullstuds have little dimples in them? if so and possible replace them with new ones, that way you no its pulling the same.

one last thing to check is the preload in the spindle bearings. put an indicator on the end of the spindle, pull the tool out put another tool in. if the proload is bad that can and will change heights between tools. My Hass did that at about .002 for a few weeks I couldnt hold better than .002. after a month it went away. Matter of fact I even pm'd wheelieking when I was having this problem. its been over a year and parts are back to repeating with in .0003 thought for sure I need to think about a new spindle.
we used to engrave with ball endmills and go .005 deep. just being off .002 in depth you can notices a huge difference, I know where your coming from.
you also should be ok with that 1" face mill was thinking your were talking a real one like 3 inch .