Z-Axis calibration changes with shop temperature?

[Walt]

Hey Walt- you have me so curious now- what was axis you were moving while cutting?


Bernie

Bernie,

I was moving (feeding) the table along the x-axis while cutting. Do you think that weighs against my idea?

The weaknesses I see in my line of reasoning are 1)the error was suspiciously precise for me twisting a screw fitting by hand. If I was predicting what sort of errors I would see from a problem of this nature, I would guess there would be a variability in the error in the length of the cut piece that would correspond to the amount of force I was putting on the gib lock. 2) I was not able to produce the entire observed error in my test.

I don't know how to rule out some kind of issue with the DRO itself. I could have some problem, for instance, with degraded readings from the scale. What comes to mind is overspray from the WD-40 I was using as cutting lube, or metal dust. I'm not seeing chips on the scale, but fine dust could be adhering to the scale or getting inside the read head. Maybe my mounts aren't completely parallel with the column. (I only have a DRO on the Y-axis, sorry to have not made that clear earlier.

Walt

 

[itsme_Bernie]

First of all, nice vice!

Ok. So when you set your Z, or vertical column height, you lower the head below the height of your cut, and THEN raise it up to the correct height. (That's good, as it takes up the backlash out of the head leadscrew)

I only thought that even though you screwed the lock down tightly, that Z column might not have been originally set to take out the backlash, and the room getting cooler shrunk the column just enough to loosen the Z lock, allowing the head to drop down the .002 or .003.

Image guessing the "gas shock" idea pushes down on the headstock in vertical Z direction to keep it pushed down on the leadscrew (therefore eliminating backlash issues?)... and so feeding is not jerky and jumpy?

I have a Hardinge Mill and a little Sherline mill. The knee of the Hardinge is heavy enough to never stick and hang when feeding down.

But the Sherline is just so light that while feeding down in Z, it can hang a bit, and then jump.

Bernie

 

[Walt]

First of all, nice vice!

Ok. So when you set your Z, or vertical column height, you lower the head below the height of your cut, and THEN raise it up to the correct height. (That's good, as it takes up the backlash out of the head leadscrew)

I only thought that even though you screwed the lock down tightly, that Z column might not have been originally set to take out the backlash, and the room getting cooler shrunk the column just enough to loosen the Z lock, allowing the head to drop down the .002 or .003.

Image guessing the "gas shock" idea pushes down on the headstock in vertical Z direction to keep it pushed down on the leadscrew (therefore eliminating backlash issues?)... and so feeding is not jerky and jumpy?

I have a Hardinge Mill and a little Sherline mill. The knee of the Hardinge is heavy enough to never stick and hang when feeding down.

But the Sherline is just so light that while feeding down in Z, it can hang a bit, and then jump.

Bernie

Hi Bernie,

It appears that my guess about the z-axis gib adjustment being the problem was correct. There was a tiny bit of slack in the adjustment screws. When I adjusted that out, the z feed became more sticky, but there was no change in the readout on the z axis DRO when I used the lock.

Made some pieces last night and when I zero the tool and cut a piece to length using the DRO, it was accurate to +- 0.001". I did four cuts and they were all good.

Edit/ The gas shock would probably accomplish the same thing by ensuring the head was always "nodding" the same direction. /Edit

Edit2/ Your point about raising the head to the desired measurement to eliminate the gear lash is an interesting one. For me using my machine, it's nearly impossible to tell exactly at what point the head is fully supported by the gears alone vs. just being hung up on stiction forces. My current project has several pocket cuts, and it's tricky to take up the lash accurately inside a pocket. I try to always use the gib lock. /Edit2

Walt