Recently, I've found that my lathe isn't turning parallel. In fact it's out by about 0.05mm (2 thou) over 30-40mm near the headstock. In a different post I tried to resolve this by shimming the headstock. Everyone told me to level the lathe. So I'm going to try.

I have some ideas and I've started work on them, but before I go too far I wanted to see if I was totally mad!

Oh – my lathe is a CJ618A a 7×14 minilathe pretty much the same as the SC3 (I think)

This was my basic idea.

The bit at the top represents the lathe bed. Obviously it's not that shape, but I just wanted to model the feet. Normally it's bolted to a thin tray through 4 M6 bolts which secure from underneath.

My original (current?) thinking was to provide a baseplate (the bottom bit of steel) which I secure to the bench (home built wood) which I would level and then bolt the base (the middle plate) to. This would leave the base level and make it easier to level the lathe bed against it with shims.

I thought that a simple base of 20mm steel may not be rigid enough to level against, but 2 lots of it would make the lathe (normally about 40k) far too heavy to move about for servicing, but I could probably cope with the extra 10k from the base. So the baseplate stays on the bench nice and level but I can remove the base and lathe without having worry about twist.

I have two concerns. Firstly, my original (current?) plan was to put some m6 holes in the feet of the bed to bolt it to the base (with shims as needed). The problem with that is that the control box and motor make access to the feet at the headstock end difficult. I wondered if I should use the existing M6 threaded holes from underneath, but then I wouldn't be bolted down to the baseplate and that would lose some rigidity..

I'm also wondering if the baseplate is actually useful.. The recent article about levelling in MEW talks about straightening a lathe up without it being level (Dollies Dad.and so on).

Any views welcome!

Iain

As mentioned above I have already started on this lunacy.

First was to find some steel to make the base and baseplate from. A visit to the local scrap yard found a piece that was far too big, but I thought, 'what the hell – cheaper than buying a smaller new bit'.

The piece in question is about 1.7m x 77cm and weighs about 170kg. I case you are wondering, this considerably exceeds my bench press capability!

2 strong you men and a forklift got it in the back of my car and I was sure I'd work out some way of getting it out!

Yup – that's a big bit of steel!

After panicing mildly for a while (to say nothing of the views of SWMBO) I got the end lifted up with the aid of my son and pulled out a bit. then I cut off the first 200mm (for the base) with a angle grinder (and several blades).

Having done that I pulled out another 300mm (for the baseplate and cut that off). By now I'd burned through my 8 inch blades and was going through my small angle grinder! But in the end i got it off.

This left a piece only (!) about 100kg and we were able to slip that off the boot onto the ground and then marched it over the end of my shed where it will stay until I can get round to chopping it up a bit more (I do have some ideas for it, in fact).

Iain

I have got to the point where I've built my bed and bolted it to the lather

I've then run a series of tests as follows:-

MT3 test bar in headstock, min and max deviation of distance from toolpost.

Fine cut of 40mm bar in 3 jaw chuck. Diameters and devation from 0 along one side.

Fine cut 20mm bar in ER32 collet. diameters and min and max devation from 0 along along length.

TEST BAR

20mm bar in collet

Here are a couple of graphs. the first is the diameter measured at a number of points along the length of the test bars with a 20mm bar in a collet chuck (ER32) and a 40mm bar in a 3 jaw). the distances shown are the distance from the end of the spindle rather than from the chuck. the collet chuck is about 47mm deep and the 3 jaw around 75. 20mm bar and test bar measured with a micron digital micrometer. 40mm bar with an analog vernier micrometer.

Diameter changes

The one below is the min and max distances from the tool post to the test bar

Test bar min and max from toolpost

The test bar diameter is within a couple of microns all the way along the length, but I can't guarantee it's straight (though I would hope it is, it's new!).

If you remember back to my first post, I found that a bar held in the 3 jaw cut to be narrower at the headstock end by a thou or two over 50mm.

As far as I can see this information supports it. It would appear that as it moves from the head, the crossslide moves OUT until around 150mm from the spindle. It then starts to move back in a little.

IN short the bed appears to have a dogleg in it.

All of this (long) blurb ends up with a simple question… 'how can I fix this?'.

The base is probably reasonably flat (I don't really have the kit to test it over a long distance). The bed bolted down to it without any obvious stress implying the feet may already be planar.

If the bed was just twisted I would have expected to see the movement (change of diameter) in one direction not in two. So I can't see how shimming the feet will help – I could possibly twist out the deviation at the headstock end, but it would make the later deviation worse.

Help!

Iain

Dave. Duh! That makes a lot of sense. The mill would allow me to clock the prism along the area of interest. Pain to disassemble the lathe again, but needs must.

Benedict. I would expect to cut wider away from the chuck due to reduced rigidity. However, this is starting to cut narrower + I took a few of spring passes and there was no material being removed (or microns) after. Also, the 40mm bar should be rigid enough to avoid that.

Martin. Looking at the two sets of results, I realise that they don't quite make sense. The test bar should show a closer distance to the toolpost, then further then nearer. I need to have another look at that.

Thanks, guys.

Iain

Hi, Benedict. That is what the second graph shows.

And the headstock has no natural way of being adjusted. It bolts directly on the bed, onto the prism.

Iain

hm. Things don't get much simpler do they?

The base is complete (and heavy). Before bolting it on, I put the lathe on the mill table to measure how straight the edge of the bed was. My first concern was that the lathe rocked somewhat and I needed to put a 0.15mm shim under the back left foot to chock it up. Clearly the bed, at some level is twisted!.

I used a 2 micron test indicator attached to the spindle to measure along the edge of the bed. I recorded the results in 10mm increments. Being ocassionaly careful, I ran back in a second pass and ended up in a different place . results are below.

TWO Micron Indicator results

Blue is the first run forward, orange the dun backwards after that and grey a 2nd run forward.

I suspect that either the indicator is being swayed by being pulled sideways along the bed or that the spindle is being pulled slightly.

To try this in a more controlled way, I bolted the lathe to the new base, locked the spindle and broke out my one micron compac indicator.

Micron Indicator on mill

The results of that are rather more consistent:-

Lathe bed deviation in Microns

I took one run at the top and one at the bottom of the edge.

As a reminder, the graph below (now normalised for distance from headstock), shows the deviation from an average of the diameter of the 20mm and 40mm bars cut in the collet and chuck.

Bar deviation in MM

This all makes some mechanical sense if the headstock is angled slightly away from the bed (that is furthest away at the tailstock).

No at all sure what I can do with this.

Iain

Hi, Neil. If I can paraphrase:-

Set the headstock end of the bed via the cross slide to be level across the bed. Move to the tailstock end and tune that until it's level (across the bed). Rinse and repeat until both ends are level …

I assume that I can set the headstock end level using the adjustment feet on the base. I will then make sure that the base is stable at the tailstock end and use shims between the base and the lathe feet to level the tailstock end.

I will give that a go.

Thanks

Iain