Slideway locking screw bottoms out

Concerning the risk of gib distortion it depends on the style of gib.

A conventional thin gib can and will distort if the locking force is applied over a small area such as by ball or radius end. The action is also poor as not only do the very localised locking forces tend to pick up on minor imperfections in the machined surface but the whole action is inefficient. With a tapered end to the pusher the gib the gib should be firmly pushed up into the top of the female dovetail as well as against the male dovetail on the slide. Mechanically much firmer and the locking action needs much less force on the screw.

Not theory. I have seen and replaced gibs that sprung into a visible bend when removed due to small area locking. I have also re-worked gib systems so the locking screw, and adjustment screws in the case of ordinary plain gibs, act to push the gib up into the top of the dovetail recess. After such effort everything works much better with reduced working clearances and the slides lock more firmly with less force. Considerable effort needed to get things right but, I felt the improved performance well worth it when re-furbishing an older lathe or one where affordability to the first buyer meant some engineering infelicities had to be accepted.

Pete says that the angle presented by the outside of the gib is only 1 degree, much less than the dovetail angle. This can only be so if the gib is much thicker. Obviously a thicker gib is far less likely to bend but pushing on a reasonable area seems a much beter way of going about things.

Clive

I beg your pardon Clive you are quite right. The 1 degree is the (approximate) taper angle of the gib, not the dovetail angle.

I should allow more wake-up time on a Sunday morning before punching the keys.

A plain brass cylinder is all that you want, it pushes against a flat surface in this type of mill.

Thanks a lot for everyone's further replies.

John [Journeyman], your experience of things is pertinent in my case.

The M8 hole for the front gib adjusting screw penetrates about 25mm beyond its meeting point with the anterior y-axis locking screw hole coming in at right angles to it, and the locking screw hole is M6. This means that if the brass pusher is, say, 2cm long, the 8mm of empty space [at least] that it has to bridge unsupported before it contacts the gib strip leaves at best only 12mm of the pusher supported in the M6 hole. The chance of the tip flipping left or right and the whole pusher dropping into the M8 channel perpendicular to it seems a distinct possibility.

If this were to happen, hopefully the pusher is slim enough to enable a pick to be inserted down the M8 hole that can then hook the pusher out for reinsertion.

The positive news from my point of view is that peering down the gib adjuster hole reveals no sign of a displaced brass pusher, so we can, I believe, safely assume the pusher is simply missing.

That cavity would suggest either a blowhole in the casting, or a pin fitted from the inside with a much larger inner end. I would remove the bed, it would only take a few minutes, but if you are not sure about doing it yourself, then it will not do much harm to leave it alone and make do with one lock.