Trials and tribulations with a 7x12 lathe

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modeldozer:
Hi all,

Spent some time today to invetigate the play, me thinks me found it. As I had to lap the saddle quite a bit to get good contact the "v" in the saddle ended up wider at the edges than in the middle, an unfortunate side effect of lapping the actual guiding surface while using the same to guide.

For cost comparrison have been in contact with Amadeal and at the cost of a new saddle and having no mill myself I think it might be time to bite the bullet and get a new saddle.

Meantime am racking the grey matter :scratch: to come up with an external guiding system for the lapping process.
Any sugestions welcome.

Abraham

lordedmond:
the saddle should be scraped to a known prism and camel back , not lapped with wet and dry stuck on.

have you checked the squareness of the saddle to the single axis

BTW you are using far to much blue it should only be the faintest smear and the saddle only moved a few mm not slid the entire length

modeldozer:
lordedmond,

Thank you for the advise. Unfotuanatly scraping is out of my capabilities due to my disability. Out of interest though what is a camel back? Using only very litle blue I found out today trying to detect the problem, learning as I go along (only used it before for setting tooth contact in differentials).

Abraham

lordedmond:
A camel back is a long straight edge
http://www.schsm.org/SCRAPING.pdf

the blue should only be just discernible on the master part ( non worked bit ) degrease both parts apply blue and wipe 99% off to get an evan smear the lower the worked part on vertically ,then move it a few mm lift off vertically .
your pic show that you have run it end to end and the blue has been shoved off the end , more like a coat of grease ,remember very little and move a little
incidentally you do the ways on the bed with a camel back then you fit the saddle to the ways, not the other way round , if there are dove tails then you use a prism

well I need wheels and I can still use a scraper but I was trained when I was an apprentice Large old DC motors ( 100 hp plus ) had brass/babbit bearing but hey that was when I was 15 now 65

good luck with the refurb , IMHO with the cheaper end of machine tool its best to work round the problems as the cast iron is very soft and nearly could be remove with a dumb nail

don't get me wrong I had a 7 x12 it lasted a week ( dangerous electronics , it started up on its own and motored against a hand push ) now my lathe is £10k not touched at all just lube it and USE it to make loco's etc

Stuart

loply:
Abraham,

I'm not sure if I understand how it ended up wider at the edges than in the middle - though I can see exactly why that would cause rotation.

Since mine is the same and I have not (really) tried to lap the saddle to the ways, I wonder they tend to be like that from the factory. If it was milled on a pretty poor mill the bit could flex inwards as it makes the cut, thus resulting in the V being wide at the edges and narrower in the middle?

Maybe you didn't make it like that, maybe you need to lap it some more in order to fix it?

I am almost tempted to buy a or make a precision V and bond it in there to replace the original. If only I could get a precision V which was only 1 or 2 mm thick, and a matching flat piece to jack up the opposite side, which could do with more contact area anyway.

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