The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
So how to machine these little lost wax castings ?
Well there's not a lot to come off - measuring the bore it's undersized by 0.25 mm so what happened to the expected 3% contraction? Obviously the bore has to be correct as it fits onto a shaft, but frankly little else is critical.
I decided first to fettle the outside of the 'barrel' so that I could chuck it up and clean up the drive dog end. This way I could (very carefully) chuck up on the dog clutch end to tidy up the bore. Now this set up is far from ideal, with too much overhang, so very light cuts !
So, dog clutch end is cleaned up, casting gripped by the newly machined bit, and the barrel 'trued up' using my roller nudge tool.
awemawson:
So far so good!
Then the remaining artifacts of the pouring sprues where the PLA model was fixed to a wax 'tree' had to be turned off the end. These had protruded slightly down the bore.
Again a delicate operation, very easy to rip the casting out of the chuck :bugeye: Eventually I got down to the bare casting and could consider truing up the bore. Before that the shank needed cleaning up, then on to boring! First thing, set the boring bar accurately on centre using the "6 inch ruler" method. Then it was a case of tickling the bore down to a nominal 14 mm plus a tad of clearance - I bored to 14.05 - the shaft is 14.000 - so hopefully a nice push fit.
awemawson:
A little bit of cleaning up and de-burring the edges, and it could be reversed in the chuck (far safer this way round!) for the seating of the fixing washer to be cleaned up
awemawson:
So now the keyway needs broaching. This is a standard 4 mm broached keyway - I have a 4 mm broach but no 14 mm bush suitable, my bush was too wide. Never mind - shim it! After all broaches are shimmed to get the right depth, why not the bush :clap:
The keyway as cast was a tad over 3 mm so not much to remove - two passes did it, second with a packing shim.
awemawson:
So that wasn't too bad was it - one down and one to go. :thumbup:
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