I've got a SIEG mini mill (SX2.7) and have recently been struggling with machined features ending up ~ 0.1mm off center (shifted towards the column) even though they were carefully picked up with a dial test indicator or a wobbler + DRO.

I then checked the column squareness and let's just say it wasn't square so the center picked up at 200mm would not be the same after lowering the head to make the cut. This issue is still not fixed but it's not what I want to ask about.

While troubleshooting I started checking the head movements with an indicator and discovered that when using the head lock, the entire thing would tilt up by as much as 0.12mm measured at 50mm distance. That's obviously unacceptable as squaring the column will be pointless with the head and therefore spindle being off anyway as soon as I use the lock.

I then tried adjusting the gib but it was already sitting quite hard. I put a piece of wood under the head, lowered it and removed the gib and both the adjustment screws. I cleaned it and put it back in without the screws. I then found out that the gib can be pushed almost all the way through, way beyond the adjustment screw travel, see drawing:

Am I not getting how a tapered gib should work or is already that worn after a few dozen uses, maybe it was bad from the beginning? In my understanding the bottom screw is there only to prevent jamming, and not stop it from falling out, the taper should do that.

I'm considering making a new gib, but before I go for that I want to make sure there is no simpler way. I tried searching for information about tapered gibs, but it's very sparse. I checked the MEW index and I don't think it was ever covered?

Thank you both! I will try that tomorrow evening and see if it makes things better.

Your measurements definitely look better. I can actually push the gib even further down than in my drawing without locking the head so it’s definitely too thin. I am glad that there is hope anyway.

The column is not square, I mentioned it in my first post. I will take care of that once the head issue is solved. Both problems need to be addressed before I can get decent results.

I’ve had the mill for ca a year, but it didn’t get much use. And it wasn’t until recently that I needed to do something with a tighter tolerance. I guess I should have checked things when I first bought it.

After a few hours of work on my shaper and surface grinder the head stiffness problem is now solved.

I didn't have enough shim stock to glue to the back of the existing gib as some of you suggested, but after measuring it on a surface plate I decided it was better to make a new one anyway. It was not only bent but also twisted. I could insert a 0.15mm feeler gauge under a few spots without even touching it. The scrape marks were almost 0.2mm deep in some places.

I made the new out of a piece of cast iron storm drain cover. I left some extra allowance for final fitting by scraping and recovering from possible mistakes. Even without tuning the head moves smoothly and there's barely any tilt when locking it, maybe 0.002mm measured at 140mm.

Tomorrow I will try to do the final fitting and when done file recesses for the adjustment screws. Then I can start working on squaring the column itself.

Edited By Piotr Gertz on 10/02/2019 21:46:44