The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
I've been keeping an eye out for a suitably robust and good quality rotating centre for this lathe. It needs to be 4 MT and preferably be dismantlable for servicing, unlike most of the import versions that are about.
I could just use a 'jump up' sleeve to fit one of my centres from my 3MT manual lathe but it adds length and a bit of flex, so I rejected that in favour of a dedicated one. The Beaver tail stock is equipped with a screwed ring arrangement to eject the centre, so the one I chose didn't need to have one fitted.
Recently a VERY nice Rohm 504 AC Mk4 Indicating Centre popped up on eBay in the UK as 'spares or repairs'. 4 MT, rated at 3500 RPM, and 900 kg end thrust, it has a pressure gauge indicating the end thrust while in use. Slight snag - the tip didn't look too good, and the gauge assembly was in bits with the glass and possibly other bits missing. This is a VERY pricey rotating centre, cheapest I've found new is 800 Euro's plus VAT and carriage. :bugeye:
Having successfully negotiated away from the sellers optimistic asking price this let it fall into my clutches arriving this morning by DHL.
A quick appraisal showed it was far better than I had expected. The tip was just discoloured and not worn, the reportedly 'stiff to rotate' feature was the front oil seal, and the gauge looked OK although there was no seal for it's input, nor as mentioned any glass.
I assume that there is a piston / cylinder arrangement acting on a very strong spring within so as the load increases oil is forced out to deflect the gauge.
Peering into the hole in the body of the beast where the gauge spigot engages, it looked obvious to me that an O ring should be in it to be squashed by the spigot to form a seal. About 6 mm x 3 mm at a guess.
It just so happened that years ago I made a mistaken purchase of 1/4" x 1/8" bore O rings from RS Components, not realising that they came in bags of 100, so I got 100 times as many as I'd expected, and they've been falling out of my 'O Ring Tote' ever since :ddb:
Popped one into the hole, offered up the gauge, screwed on the bezel, and it seated nicely. The bezel bears on two flats on the gauge on a step in it's internal bore, so that were the glass in place it would take no load - handy for replacement, but also means that I can test it without the glass.
awemawson:
So how to test it?
I could just put it in the lathe and crank the tail stock forwards by hand, but I had no idea if anything would fly off or break. I decided to make up a simple adaptor to pop over the 'point' to allow me to load it up in my hydraulic press under controlled conditions and me a fair way away holding the umbilical controls !
Amazingly it works - spot on, I took it to half and then full load, no leakage, no nasty noises, all fairly undramatic I'm glad to say.
So now I just need to source a glass. I've decided to use actual glass rather than acrylic, as it will otherwise easily be scratched. So where do you buy a 1.5 mm thick by 32.3 mm diameter tough watch glass - well eBay of course and should arrive before Christmas :ddb:
seadog:
Do the bearings sit in an oil bath and is that oil also used to indicate on the gauge?
awemawson:
I'm honestly not sure - it's quite likely I expect.
I need to make a pin spanner to dismantle it from the front. There is a cross section of a non indicating Rohm centre on their web site but I've not found one yet of this specific indicating one.
awemawson:
This morning I made the pin spanner to enable me to unscrew the front seal and bearing retainer from the Rohm centre.
Works very well, but I must remember that it only needs 1 ton on the hydraulic press to get a nice deep impression of my date stamp - that was 2 tons and it's too deep and has spread the centre bit of the spanner so it looks swollen !
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