Lathe Spider

by John

What!  More dangerous wildlife in my workshop? (see the previous post  about the tiger snake).

No.  Not this time.  Thank goodness.  No more highly venomous snakes wriggling between the lathe and the milling machine.  Mind you, we have red back spiders and white tail spiders in abundance here.  (both very nasty, to explain to the non Australian readers).

But in this case, a lathe spider is a tool.  Used to repair worn lathe chuck jaws.

I wondered why my beautiful Colchester lathe would not part off thin brass rod.  Should have been a doddle.  Closer examination revealed that the jaws in the Colchester 3 jaw chuck, were “bell mouthed”.  That is, worn in their outer extremities.  That is where lathe chuck jaws wear initially.

Solutions?   Buy new jaws….   none available,. anywhere that I could find.   Buy a new chuck….  a new quality chuck of this size (200mm diameter) costs between $500 and $2000.  A second hand chuck might have the same problem.

Another solution, which I have used successfully previously, is to regrind the jaws with a tool post grinder.  I have a tool post grinder, not used with this lathe, but should be suitable.

So, I spent a half day fitting the tool post grinder to the Colchester lathe.  No big deal, but it needed 2 complex bush-washers and a new tool post bolt.    It also needed the internal grinding spindle to be fitted to the grinder, a first for this grinder.

I had made a lathe spider a few years ago, for a different lathe and chuck, and it fitted the Colchester!


The 200mm Colchester lathe chuck, and the spider, made years ago, which fitted!

I spent another few hours fitting the spider, grinding the jaws, and the measuring the run out of the chuck.  The spider permits the jaws to be tightened inwards, against pressure, then the jaws can be reground using the tool post grinder.

Using the tool post grinder to resurface the jaws.

Using the tool post grinder to resurface the jaws.

A very frustrating few hours!!!

Despite multiple runs, grinds and measurements, I could not get the runout to acceptable levels.  The best was 0.1mm which is totally not acceptable.

I wondered whether the spider was just not accurate enough.

I also noted that the runout was better (0.05mm) if the jaws were not tightened heavily in the measuring phase.  I wondered if the chuck scroll was badly worn, which would mean a new chuck!

So, I searched the net.  And found a picture of another style of lathe spider,  and I determined to give it a try.

Today, I used the CNC mill to make the new spider. Actually, 3 parts which are fitted individually to the chuck, to give the same effect.


CNC Milling the lathe chuck spider components.

The new version lathe spider bits, clamped to the lathe chuck

The new version lathe spider bits, clamped to the lathe chuck.  This spider is made from 6mm thick aluminium.  This photo was taken after the jaws were reground, as you can see.

I ran the grinder in and out a few times, re-measured the runout.   Zero, zilch, nada, niente.  No movement of the dial indicator.  So the indicator must not be touching the test piece, or pushed in so hard that it cannot move.

So I checked the positioning of the indicator, and ran the test again.


So the runout, at least at this diameter, is zero!

So I tried a smaller test piece.  Same result.

So I tried really tightening the jaws hard.   Again no movement.

I must point out that when I tighten the jaws for measuring, I always tighten the jaw which is nominated by Colchester.  I tried tightening the other two jaws, and found 0.1mm runout.  So, the nominated jaw tightening really works.  It is not bull shit!

And this lathe chuck spider method really works!  It just needs to be made really accurately!  (did I say before that I really love CNC).

Next step.  Totally disassemble the chuck, and carefully and fastidiously clean every component, then reassemble it with new grease.  To rid it of every trace of grinding wheel dust, which could destroy it in no time at all.

I was delighted to see that the internals of the chuck looked perfect!   No signs of wear at all.  Very happy.

The disassembled lathe chuck

The disassembled lathe chuck