Sieg C3 lathe stiffness

isn't it supposed to do that?
resulting in a ridged finish on the face of the work. probably - locking the saddle with the leadscrew is a bad idea though, you want a proper saddle lock which locks to the bed, not the leadscrew.
There are lots of home-made ones on the 'net, or if you can't find anything suitable I can post a picture of mine - made with one nut and bolt and a bit of scrap.
-- Peter Fairbrother

It may be that the saddle clamping plates (gibs, really) need adjusting. Limited advice is just over halfway down this page:-
Bear in mind that the gibs of all machine tools need occasional adjustment, so there's no shame in having to do an overhaul now and again :-)
Mark Rand RTFM

Well, yes, but not in this axis !
Thanks, I've had a quick Google and there certainly seem to be lots of ideas out there.
A little scary to be drilling into this lathe though :-O
Adrian.

Thanks. An interesting read.
I think the play is lateral, rather than vertical, but have book-marked that page for investigation tomorrow.
Adrian.

From my interpretation of the C3 construction, vertical play will affect lateral play. If it doesn't, the saddle lower surface would need re-scraping/re-machining. I'm assuming that the saddle actually locates on the dovetails, not on the top, flat, surface of the bed.
I'll have to take my magnifying glass to the leamington show and crawl all over one of Ketan's examples:-)
regards Mark Rand RTFM

Some mini lathes have a tapped hole in the front end of the saddle, at the middle. That's what I first used in my Conquest. It's meant for a travelling steady. Perhaps not as good as a hole at the back, eg
but it can be used without any modification to the lathe.
BTW, that's the best type of carriage lock, by *far*. Don't bother with any type which doesn't jam the carriage down on the ways, and which doesn't do the jamming from in the middle of the ways.
If you have a H-shaped carriage (I think these are pretty rare in the UK tho') you may need to add a bit of metal in the gap of the H, as in
If you decide to drill and tap, it's not hard to do - the cast iron is nice and easy to work with. Just use some method to get the hole and the tap vertical. M6 is about right.
-- Peter Fairbrother

Yes, the carriage locates on the V at the front and the flat bit at the back. The rest - eg the flat bit at the front - shouldn't touch the carriage.
The OP's problem however is that he is using the half nuts and the leadscrew to lock the carriage, and there is just too much lateral backlash and general slop for that to ever work well.
It's a design lack of the minilathe, and probably the first and/or most important mod most people do is to fit a carriage lock (the next most important one is a lever-action tailstock).
IMO you cannot face properly on the minilathe unless you fit a carriage lock.
There will be some vertical play in the carriage, and it may well affect lateral play, and also twist - but there won't be nearly as much play from that as from the slop in the halfnuts/leadscrew, and a good carriage lock which will jam the carriage onto the ways will stop both (the play and the slop) dead.
You can get the vertical play down to almost zero for when the carriage lock isn't in use, but it's quite a job - the bits which hold the carriage down need to be shimmed (from the factory they have pushme-pullyou bolt systems which do not work well and need to be replaced by shims) and while you are at it you need to work on the bottom outside edges of the ways - scraping or the like - as well.
As I say, it's quite a big job. Good for normal use, but not absolutely necessary if you have a good carriage lock (unless you are doing a CNC conversion).
A good carriage lock, on the other hand, is IMO essential.
-- Peter Fairbrother

Whilst this will make it more rigid, there is play in the lead screw so this will not stop some sideways movement as you observe. You also need to lock the saddle onto the ways. The simplest is to just use an engineers clamp with the edge of the lower jaw under the ways and the top jaw clamping onto the saddle. If like me you have the older H shaped type of saddle with a cut out you will need to span the cut out with a piece of metal - I've even used an Allen key which was to hand. In the long run the best thing is to make a simple clamp that fits onto your saddle, I designed my own although there are various designs available eg on the Yahoogroups for the minilathe or 12x7 and 14x7 lathe.
Originally I was going to drill and tap an M6 hole near the back of the saddle and use an Allen screw tipped with a piece of brass so as not to mark the ways srewed down onto the ways. Although this is the method that is found on the college lathes which are much more rugged on a lighter lathe it will raise the saddle so is not a sensible thing to do.
Alan

As others have said, you can add a saddle lock but in the mean time you can improve things a bit by applying some pressure to the saddle hand wheel with the half nuts closed while making your cut. It is not ideal but better than nothing.
Archie