Sunday, 3 May 2020

Mini-Lathe - Cross Slide Gib

From new, the cross slide has shown itself capable of lifting at the gib strip, causing over-long turning or milling tools to judder. Adjusting the gib has been tricky - it's a balance between being too tight and having too much play. This is the cause:

Another shot:

You can see that the gib is very poor - it is too wide (the gib should be tall and thin, not short and thick), but we can't do very much about that without making a new cross slide. Secondly, there is too much clearance above and beside it.

The main problem that this causes is apparent when milling, boring or general turning with a long overhanging tool. Obviously the cutting load going into a tool should be passed into the tool post and then into the slides, but if the tool is long, the forces will generate a turning moment which will lift the right hand side of the cross slide. If the the gib is too thick, or there is too much top and bottom clearance, the gib will rotate in it's slot allowing the cross slide to lift on the gib side.

We can do something about this, by making a new one which fits; a better solution is to make a new cross slide with a much narrower gib slot, but for now I will file up a new gib strip from brass. Brass is self lubricating and easy to file, so it will prove the principle.

This is a bit of 1/4" x 3/8" brass bar - conveniently, the 1/4” dimension should be a perfect fit in the slot, leaving me to file chamfers in the 3/8” dimension:

We can file it to shape quite easily, by coating it with Dykem and using the odd leg calipers to mark the line to file to on each side.

Once close, we can fit it in position working from one end until it slides in.

Next job is to spot the dimples for the screws through the existing holes.

I've actually used dog-point grub screws this time, in deep 2.5 mm holes to improve support for the gib. This arrangement feels very smooth now, and there is very little clearance in the vertical or horizontal directions.

Next step is to test it, but I can tell it will be much better.

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