Tapered gibs on a mini mill (SX2.7)

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?

Hi Piotr you can check the gap with feeler gauges and if there is sufficient gap, 0.25mm or more, you can fit a shim to the non sliding face. Cut a shim if the corect thickness and shape, pin by drilling and fitting silver steel pins to prevent it moving and then replace as new.

David

A quick and dirty fix if you want to see the effect of a thicker gib is to superglue a shim on the back of the gib.

Paul.

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

Piotr Gertz:

There was a thread about taper gibs over on the Practical Machinist forum which might be of interest.

It looks like the gib is too thin.

Tony

That does seem quiet a long way down, by way of comparison on the SX2.7 I have here the top of the strip is approx level with the top of the dovetail and with a lever dti at right angles to teh spindle that puts it 100mm or so away I get less than half a division on a 0.01 scale so probably 0.003mm when lock is tightened yet head still easy to wind up and down when unlocked.

Strip of shim between the sloping faces would get round it.

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.

A factor you may not have considered is the question: is the column truly perpendicular to the base? if it leans outwards, you will get the effect you describe - machining towards the back of the measured position, if your measurement was done with the head higher up than the machining (as may be likely with a longish finder and a shortish cutter).

And I agree that the gib you have does seem to be thinner than it should be. If the mill is fairly new (as it appears to be, you could try asking the supplier for help. Goods are supposed to be fit for purpose during use, not just when brand new, and faults like this are not obvious until everything - including the machined surfaces of gib and ways - has settled.

A strip of shim which overlaps the gib at both ends, with the ends bent back to locate it, would be worth a try.

Cheers, Tim

Edited By Tim Stevens on 09/02/2019 11:43:29

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.

OP's drawing shows the gib retained at each end by the adjusting screws.

My vintage Warco Economy has taper gibs with a single cheese head screw operating in a slot in the gib meaning that the gib is free to move lengthwise by the amount of play in the slot. The system seems adequate for this small and basic machine but I have occasionally wondered whether I ought to try to restrain any such movement.

Any helpful comments, please?

The answer for you is to re-make the adjuster screw with a fatter head that fills the slot. If the slot sides are not properly parallel they will need filing or machining first and the new screw made to fit very closely. I had to do this with my own Warco milling machine.

Edited By Pete Rimmer on 09/02/2019 16:24:44

Thanks for this helpful suggestion.

The taper gib is attractive because of its ease of adjustment; apart from this the parallel (preferably block-type) gib seems preferable.

Edited By ega on 09/02/2019 22:00:49

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