Author Topic: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20) (Read 369676 times)

awemawson · « **Reply #650 on:** December 10, 2018, 02:11:13 PM »

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 · « **Reply #651 on:** December 10, 2018, 02:23:45 PM »

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

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 · « **Reply #652 on:** December 10, 2018, 02:27:56 PM »

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 · « **Reply #653 on:** December 10, 2018, 02:35:39 PM »

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

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 · « **Reply #654 on:** December 10, 2018, 02:38:01 PM »

So that wasn't too bad was it - one down and one to go.

nrml · « **Reply #655 on:** December 10, 2018, 03:21:33 PM »

Those castings turned out very nicely indeed. Whoever did them has done a great job.

awemawson · « **Reply #656 on:** December 10, 2018, 03:44:04 PM »

Gary Allen did the superb lost wax castings - a very helpful chap

http://www.geallencastings.co.uk/

awemawson · « **Reply #657 on:** December 11, 2018, 04:58:02 AM »

Then it was a case of 'rinse and repeat'. Second drive dog machined up and broached. Quicker this time as all the decisions had been taken!

Again the bore only just cleaned up.

PekkaNF · « **Reply #658 on:** December 12, 2018, 07:58:47 AM »

Looks great.

Awfully little material over the key way. Is that fusible or just no more space?

awemawson · « **Reply #659 on:** December 12, 2018, 10:41:05 AM »

I thought the same Pekka, but we are not talking huge torque, just light milling.

Between a visit to the dentist and a friends birthday lunch I managed to fit one of the bronze drive dogs - went in as a nice snug fit

PekkaNF · « **Reply #660 on:** December 12, 2018, 04:40:42 PM »

Quote from: awemawson on December 12, 2018, 10:41:05 AM

I thought the same Pekka, but we are not talking huge torque, just light milling.

Between a visit to the dentist and a friends birthday lunch I managed to fit one of the bronze drive dogs - went in as a nice snug fit

Sweet!

awemawson · « **Reply #661 on:** December 12, 2018, 04:50:08 PM »

Were I doing it again I would beef up the shank, leaving more meat by the keyway, but also make the PLA model's bore far smaller leaving more the bore out on the lathe. Both bronzes only just cleaned up, at 14.05 mm, leaving slight 'witness marks' in place - perfectly serviceable but not ideal.

. . . but I do have not only a bronze second one as a spare, but several ABS and PLA ones that actually seem to work OK !

nrml · « **Reply #662 on:** December 12, 2018, 05:08:54 PM »

Is the crown end going to be cleaned up or will it wear into place?

awemawson · « **Reply #663 on:** December 12, 2018, 05:22:42 PM »

The plan is to let it wear itself in. It has room to settle in a bit, but really it's just a case of watching and waiting as it is used.

awemawson · « **Reply #664 on:** December 14, 2018, 05:48:09 AM »

After weeks of wandering around various carriers and transport hubs, both in the US and UK, and having paid the HM C&E ransom demand to release it, my Euchner Precision Switch has finally made it all the way from San Antonio in Texas. It's amazing how many places it's been on it's journey

Hopefully I'll never need it, it can stay as a 'shelf spare', but being rare as hens teeth it had to be had

awemawson · « **Reply #665 on:** December 14, 2018, 10:31:23 AM »

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.

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

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 · « **Reply #666 on:** December 14, 2018, 10:39:09 AM »

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

seadog · « **Reply #667 on:** December 14, 2018, 11:04:42 AM »

Do the bearings sit in an oil bath and is that oil also used to indicate on the gauge?

awemawson · « **Reply #668 on:** December 14, 2018, 11:53:16 AM »

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 · « **Reply #669 on:** December 15, 2018, 07:09:22 AM »

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 !

WeldingRod · « **Reply #670 on:** December 16, 2018, 11:09:14 AM »

I'm amazed your date has any stamp left after that! ;-)

Sent from my SAMSUNG-SM-G891A using Tapatalk

awemawson · « **Reply #671 on:** December 16, 2018, 11:58:28 AM »

Yes I got carried away

awemawson · « **Reply #672 on:** December 17, 2018, 09:38:44 AM »

I decided that the Rohm rotating centre was too vulnerable rolling about on the bench. I don't want it in place in the tail stock except when in use, as it's a nice spiky thing to catch yourself on, when setting up tools. I had intended just to pop it in a suitable hole in one of the Versatol Cabinet shelves, but due to it's length I'd have to lower a shelf and waste a lot of space.

Reconciling myself to making a wooden box that stopped when I realised that my bottle of 'Resin-W' PVA glue was congealed beyond rescue- so what to do

Then I remembered an Acramill collet chuck box - no idea what happened to the chuck, I think this came as part of a job lot with no chuck, just the collets. So the remaining collets were ejected into my box of Acramill bits and the box 're-purposed'

A simple wooden divider with a hole in it to take the business end, an other with a slot (so it would go in) for the 4 MT shank, and a third loose bit to stop the centre sliding up and down. Then the two shaped dividers had a recess cut in them to hold the nose spanner.

I'd intended to screw through the box into the wood, but realised that it would very likely split. So rightly or wrongly I chose to use 'structural adhesive' Horrid messy stuff and I totally failed to tidy it up neatly. Not really happy with the result but at least the centre won't now roll off the bench!

Long term I will probably re-make the dividers in an engineering plastic that will take a thread, so can be safely screwed from the outside of the box.

seadog · « **Reply #673 on:** December 17, 2018, 09:53:08 AM »

If you'd pilot drilled they wouldn't have split, Andrew.

awemawson · « **Reply #674 on:** December 17, 2018, 02:08:10 PM »

I'm not convinced, it's only softwood.

Time to sort out the coolant: There is an external motor / pump unit rated at 20 litres / minute that sits on the coolant tank, draws up through a mesh filter, then passes coolant to the machine though a 125 micron hydraulic filter.

I started by undoing the filter housing - it was well choked with plastic swarf. It will clean up reasonably but I'd like to put a new one in. The actual element is a UC.R.76115 and googling so far has found that for what they are, they are a silly price (£30 plus the VAT). Hopefully I'll turn up a cheaper alternative. The only 'special thing about it is that the inner mesh
perforated tube is stainless to avoid rusting.

I intend to prove the pump and filter 'off line' using a bucket rather than fill the tank with it's 95 litres and then find the pump doesn't work!

All the nylon re-inforced hoses will be replaced as they've gone rather sticky with age.

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Author Topic: The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20) (Read 369676 times)