How much does your table move after you stop moving it?

[Shotgun]

Good news: I'm getting better and more precise at machining.
Bad news: Being better at it allows me to experience the idiosyncrasies in my machine.

I'll dial in exactly 2.5000 on my milling machine's DRO. But then if I hit down on the top of my vice with the side of my fist, it might jump to 2.5010. Or, tightening one of the X locks will shift it to 2.4995. I've been playing with it, and if I apply both the X locks at the same time, it won't shift at all sometimes. Or stopping a thou short of my measurement, applying the locks will bring it the rest of the way, no matter which way I approach the measurement.

I've been trying to figure out where this slight play is happening. Is it stickiness in the DRO scales? Is the gib not quite tight enough so that there is a little wiggle when tightening the locks? Maybe it has something to do with the geometry of the ways?

Any ideas on what might be causing this behavior? Has anyone else experienced it? What to do about it?

 

[woodchucker]

backlash in the nut. And don't put your fist down on the vise, seriously. I avoid touching the table while machining or finishing a cut.
Also, you may want to slightly tighten the locks toward the end to prevent any looseness from allowing the part to be pulled in.

 

[RJSakowski]

I have a DRO on my 40 y.o. RF30 clone and see about .001" on the z axis and about .0005" on the x and y axes. I always set my position to read correctly after the axis is locked if I am going to lock the axis. I would expect that it is play in the gib that causes this and wouldn't expect problems with the DRO scale.

 

[Janderso]

Dang RJ, that's a tight old machine.
Bravo

 

[Inferno]

0.0005?

One half of a thousandths of an inch? And you're not happy?
DANG!

 

[G-ManBart]

I don't have a Z readout on my Bridgeport, but do for X and Y. I see something like .0005" change when I tighten the locks...but not always. Just realize that even brand new and perfectly adjusted there is a tiny bit of space that allows the table to move and tightening the locks removes that.

 

[mmcmdl]

Put a piece of tape over the tenths on the readout , unless the part is going into space with NASA .

 

[Jim F]

We have 5 knee mills at work with XY DROs, all do the same thing..........

 

[RJSakowski]

Dang RJ, that's a tight old machine.
Bravo

Actually, she's a tired old gal.

That's the difference between free and locked. I keep the gibs fairly tight. The backlash on the x and y is much greater, on the order of .025" to .035". There is no adjustment for thrust bearing preload on the machine and the lead screw nuts are worn. But with the DRO, backlash isn't a concern as I machine by the DRO coordinates.

 

[Winegrower]

If the least significant bit switches in a digital system, the implied movement may be much smaller than the resolution. If two counts change, then there is at least one minimum resolution movement.