Mini Lathe problem

That's an interesting problem. As you say, it could be the crossslide out of alignment, but could it not also be the spindle out of alignment?

For advice, I'd recommned you try the 7x12 mini lathe yahoo group. It's mainly an American based group, but most of these lathes are very similar, and I'm sure they'll point you in the right direction.

moray

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Mount a dial gauge on the cross slide and run it across a known flat surface held in the chuck. The face of the chuck itself ought to be true so try that first. I think it's vanishingly unlikely that there'll be an error as large as the one you are generating on the workpiece though. It could just be that the material you are facing is cutting differently at the o/d and i/d due to the change in surface speed. Tool sharpness and mounting rigidity will both affect this. Try something that machines really cleanly like well heat treated aluminium. An old piston is ideal. I've got loads of em in the workshop if you want some.

What's also possibly happening is that at the periphery of the workpiece the cutting forces are pushing the tool back against any slop in the system and at the centre it's springing back in again. Check the play in the gib strips on the cross slide and adjust out as much play as you can without it going too tight. Use a sturdy cutting tool with as little overhang as possible from the toolholder.

Definitely don't go buggering about with dovetail cutters on anything. That's a sure fire way of shagging the machine completely.

-- Dave Baker

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Archie Are you taking the facing cut across the work by moving the topslide or the cross-slide? Take successive cuts using the alternative means and compare the results. First instinct is that 0.07mm in 20mm sounds like a topslide misalignment. How old is the lathe? Has it had a hard life?

Best regards --

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Assuming you mean 0.07 concave on a 20mm dia piece, if the headstock was out of alignment, you'd be cutting a half angle taper of 0.07mm per 10mm (ie, reducing the diameter by 0.14mm per 10mm). If you aren't getting that kind of error, then it's probably a problem with the carriage.

If the lathe has been badly made, then I would suggest returning it before going chopping bits of it up. Before either of those, I'd thoroughly check the alignment and set up (and cleanliness) of everything related to the carriage, from the lathe bed to the cutting tool.

I just checked with a DTI against the chuck face. To keep the DTI vertical, I run it just above the jaw slots positioned like a Y. It showed an error of

0.12mm over 55mm which more or less agrees with my firts check which I did on the surface plate on a piece of 40mm dia alu bar which I faced using light cuts. I have notice the concave effect before but I put it down to the fact that I didn't have a saddle lock and was using pressure against the half nuts to lock the saddle. I now have a saddle lock.

I also checked the head /spindle alignment using John Moran's method shown here

shows an average error 100mm out from the chuck of only 0.005mm.

What now?

I suppose I could try and talk nicely to Chester. I bought the lathe at Harrogate ME show 2004 so will be out of warranty but it is highly likely that this problem has been there since new. Maybe I should just buy a new saddle.

How much metal is in old pistons? I thought they were hollow all the way up to the crown.

Archie

The lathe hasn't had a lot of use but I did manage to get a parting tool jammed a couple of times which seemed to loosen up the headstock bearings and caused much chattering. I replaced them with angular contact bearing at the suggestion of Chester and they have been fine.

Also see my reply to Dave

Archie

See my reply to Dave.

Archie

Have you dismantled the carriage, cleaned all parts and reassembled?

Several times. Most recently it was to put marking blue on the V groove looking for high spots. It seemed OK.

I followed Moray's advice and searched through the Yahoo 7x12 minilathe group found others have had the same problem. The general consensus is to mill the dovetail.

Archie

Archie

For a beginner to have to contemplate milling the dovetail on a one year old machine is b******s! I know that we accept a few compromises in favour of the low cost on imported machines but this is unbelievable.

Have you spoken to Chester about this? Tell us their response. I'm (almost) lost for words, not a condition I'm known for! I thought Myford had plumbed the depths when they supplied me with a new lathe fitted with the wrong gears but this tops that by a yard.

Best regards --

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Call me foolish or what but with all the snow on Sunday and nowhere to go I had a go at milling the dovetail. To be honest, I made a bit of a pigs ear of it. I forgot to lock off the height adjustment and ended up with the head dropping and cutting the top of the saddle. This is not too much of a problem as I have a quick change toolpost with adjustable height. I think I took a but too much off the dovetail also. My cheap and nasty dovetail cutter must have one high cutter face judging by the sound it made. Anyway I got the error down to 0.04mm in 50mm. I was aiming for 0.02 since I didn't want to overdo it and turn it into a convex cut. I guess that the method of using a bit of drill rod in the dovetail to run the DTI along is prone to errors if the height changes slightly. A DTI with a cylindrical end instead of a ball would be better. Is there such a thing?

Filled with confidence after my success I am going to try and get it down to about 0.01mm in 50 mm. Is this reasonable? I suspect that I will also need to make a new gib strip out of brass as the dovetail gets thinner. I have seen a couple of websites where this has been done.

Archie

Archie -- 'festina lente' -- hasten slowly! This all sounds to have the potential for a right royal (expensive) cockup! Where are you located? Perhaps somebody else might help you assess the situation before things get any worse! If it helps, I'll willingly offer you a bit of support if you are within, say, a sixty mile radius of Shaftesbury.

Best regards --

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Central Scotland. Just a little bit too far eh? Thanks for the offer.

Archie

Archie

As a follow up to the foregoing, the acceptable concavity of a precision lathe to BSI standards is, I believe 0.0004in per foot or 0.01mm in 300mm -- known as the Schlesinger Limit.

Tubal Caine, who wrote the celebrated Model Engineers' Workshop Handbook, measured his Myford Super 7 at 0.0013in per foot back in 1987. Mine comes out at about 0.005 in about 6in but that was a very quick and dirty test in a freezing workshop!

This should put your target of 0.01mm in 50mm into proportion.

--

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

That's a bugger....! --

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Thanks Chris. Useful to know what can be achived but the mini-lathe isn't really in the precision catagory. I think that its beyond my measurement ability anyway. My travel on my cross slide is only 70mm and DTI is

0.01mm/division.

I had another go at milling the dovetail and my first facing cut on a 40mm alu bar looked pretty good checked against a straight edge. When I tried the DTI against the chuck I was getting unstable readings though. I think that a new gib will help.

Archie