Hi Steve

The plate with the slit in it is called a catch plate. It 'catches' the driving dog on your part.

The headstock and tailstock are both more taper. I am not sure what taper is in the headstock but I can check when I get home this evening if you like.

Pete.

Steve just so you know a live centre is any centre that turns with the work. A dead centre is any centre that does not. Nowadays the trend is to call solid centres 'dead' and revolving centres 'live' but that's not strictly correct.

With a dead centre the part that is mounted onto it will always turn on axis because the point is 'dead', it does not move. If you put a solid centre into a revolving spindle the point is 'live' in as much as it is rotating and if there is any kind or runout it will be translated into the part. Worse than that if you dismount the part then re-mount it between centres the runout will be translated into the part but possibly in a different orientation. This is why if you ever mount a live centre you should check the end is running true with a DTI before you use it.

Alternatively you can mount an un-hardened live centre into your spindle and turn the point true before mounting the work on it.

Okay so the taper on my Zyto for both the headstock and the tailstock is MT1.

I'm about 10 years into the hobby and have yet to use a Catch Plate and Dogs. It's a traditional method dating back to the time before chucks. I wouldn't bother unless there's a particular reason using it, like the need to squeeze a large object on to the lathe.

Hope others will comment because I'm not god's gift to Model Engineering best practice, but this is my take:

• 3-jaw self centring chuck – quick set up, relatively high run out. Good for most general purpose work unless the work flow requires the job to be removed and replaced in the chuck because their reset accuracy is poor.
• 4-jaw self centring chuck – better grip in some circumstances, rarely useful in metalwork.
• 4-jaw independent chuck – slow set up, but work can be centred or off-set accurately. If no other chuck were allowed, this is the one I would choose. In practice most of my work is done in the 3-jaw because it's convenient, and the 4-jaw comes out only when the 3-jaw isn't good-enough for the task in hand.
• Collet Chuck – quick set-up, low run out, and high reset accuracy without fuss. Much faster set-up than a 4-jaw, but only suitable for work that can be held in a collet. Pricey because of the need to own a collet set but offset if the same collets are used on a milling machine. More popular for Clockmaking and similar than general purpose machining. I occasionally use a collet chuck, usually when work has to be moved repeatedly between mill and lathe
• Backplate and Clamps – very slow set up but can hold oversized and odd shapes with high centre or off-centre accuracy. I've never used mine!
• Mandrel – like a backplate but pre-drilled to hold specific jobs – good for repeat work. I don't know of anyone using them in a home workshop.
• Stub Mandrel – a short stubby rod held in a chuck or collet that's faced-off and either tapped to hold work like a mandrel, or has the job glued to it. I often turn thin blank discs by supergluing them to an aluminium rod.
• Working Between Centres – live or dead centres support the job between short holes drilled in the ends and packed with grease. The job is spun by a dog and catch plate. Takes a fair bit of setting up, but the axis is accurate and resettable from either end. I've never used this method because it's suited to long jobs that go in and out of the lathe, probably also requiring steadies. Something like an axle, which I don't do.

Planning the sequence of operations needed to convert a lump of metal into a finished object is all part of the fun, and there are often several ways of doing it. Not familiar with the Polly, but if the piston is like the ones I have made, I'd probably turn it on the end of a rod held in an ordinary 3-jaw chuck and part or saw off and face to finish. Pistons being short stubby objects, it's probably not necessary to support the tailstock end at all, and no need to mess with centre-drills, centres and dogs. Note when flipping the piston over to finish by facing off, it's unlikely to matter that the chuck reset isn't spot on, and that facing off is impossible when turning between centres. Lesson is work-holding depends on the circumstances: in my experience a catch plate and dog aren't worth the trouble, but it depends on what the lathe is used for.

Recommended read: Sparey's "The Amateur's Lathe". A little out-of-date in that it doesn't cover Carbide Inserts, DROs and such, but still excellent, especially for lathes of Zyto vintage.

Dave

If you are making a Polly you might be interested in the SMEE courses. The main one uses Polly ( albeit brought slightly up to date) as its basis.

SMEE courses

What bits do you have Steve?

Neil