The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)

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awemawson:
So a bit more progress. The other day having increased the depth of the soft jaw retaining bolt counter bores I was able to fit them and skim them to a close fit for the billet. I still feel that they are rather skimpy, lacking thickness outwards, so it'll be light cuts only. To use a new set of jaws and bore them is obviously the best option but taking them out far enough to hold 170 mm vastly reduces the chance of them being able to be used for other things, and at £60 per set to replace,I don't want to waste the ones that I have.

I've still to form an undercut in the corner of the jaws, but as this again considerably weakens them I may bevel the billet on the manual lathe so that it can sit snugly without the undercut.

I've made some progress on the CAD side, downloading a DXF of a Kitigawa A2-6 chuck and importing it into FeatureCAM and deleting all but the A2-6 back end. I've got a working model, but need yet to match the tool crib on FeatureCAM to what I have on the lathe.

I'll put pictures of this up later when it's progressed a bit more .

awemawson:
Now today's task was to produce a suitable 'test billet' of machinable wax to save trashing the extremely expensive EN24T final one.

The pre-cast slabs of M/W that I already had are a bit too thin but I decided to cut one anyway on the band saw, but also cast one to size thereby using up the bits from bandsawing the slab.

I am slightly concerned that the clamping force of the lathe chuck will be too much for the machinable wax, and this thinner billet can be used for tests. The lathe has two pre-set clamping levels - high and low - I need to investigate if I can reduce the pressure of the low setting even further. (Selection between the two levels is by M codes)

So taking my life in my hands I raided the kitchen and found a nice 7" cake tin with a 'drop bottom' Ideal at 177.8 mm diameter as it is just over the 170 mm of the steel billet and the M/W shrinks considerably on cooling.

Then I went hunting for the pot that I had previously melted the M/W in - turns out that it was a Kennedy 'Red Hot Dip' pot for their version of Crocell tool dip. Evidence of previous use was obvious - there was still some M/W in the bottom!

So, pot set to melt - it's thermostatically controlled and I took the M/W up to 150 C very slowly giving frequent gentle stirs to get in homogeneous. I wanted to pour it at the lowest possible temperature to avoid major shrinkage.

Now it is poured I'll leave it over night to cool and shrink-release from the cake tin. I do hope it comes away cleanly or I'll be in trouble :bugeye:

awemawson:
Inspecting the 'cake tin billet' which has now cooled to 50 Centigrade it has already shrunk considerably.

But I'm in the workshop to investigate high / low setting of chuck clamping on the TC 20s not cooling wax!

M81 selects 'high clamping force', M82 selects 'low clamping force' and the hydraulic pressure meter in the headstock end cupboard displays the selected pressure, and this cupboard also houses the pressure setting and selecting valves. Easy really - twiddle the pressure knob and watch the gauge!

So putting in the band sawn billet and selecting low pressure it crushed the billet :bang:

So I reset the low pressure to 'very low' (see picture) and still the chuck took another bite out of the billet - argh !

It looks like I'm going to have to think again about what to use for my trial cuts :scratch:

So I video'd the PC screen as FeatureCAM was chewing out a demonstration A2-6 'chuck back end' as a consolation prize:

tom osselton:
Can you place something between the jaw and wax to get more surface area on the edge?

awemawson:
I was wondering on similar lines, but I think it will be marginal whatever I do with the m/w. I may end up casting aluminum ‘prototype disks’ but there’s quite a volume of metal there.

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