Loosen the bolts that are holding the ply a little and then tap the ply/work to adjust its position until it runs true then tighten up.

Ideally an oversize bit of ply should be mounted on teh faceplate and then turned down until your plastic tube fits snugly. (assuming it has an even wall thickness.

J

Hi Christine –

Another approach – Cut a piece of wood say 2 x 4 x 4 " and glue / screw that to your plywood/work disk assembly, centre of block close to centre of plywood/work disk. Then chuck the 4 x 4 block in your four jaw chuck, and then adjust the chuck to get your work disk running on centre. I've used this plywood-with-back-block method in the 4 jaw many times with success. Main point is – don't limit yourself to just the faceplate for plywood-mounted work. Good luck. JD

Christine

If I understand correctly, you are trying to cut a groove for an 'O' ring in the OD of an existing component?

Ideally you should machine the part OD and the groove at the same time (well, one after the other really). That ensure the groove has a constant depth to ensure the sealing ring is equally compressed.

If, because the OD already exists and is at the finished diameter then you will have to use the methods suggested in the previous replies to adjust the workpiece position. The best (but not the only) way to do this is to use a dial test indicator. If you don't have one let us know and we can suggest alternatives.

I would not recommend using Michael's 'Pusher' method, its a recipe for disaster!

Ian P

Michael

My advice was specific to this particular workpiece. I dont know what optical operatives do but you cannot seriously suggest that this disk (near the limit of her lathes diameter) would succumb to being aligned by being rotated against a fixed stop, even if it was a roller. We dont even know that this workpiece is exactly circular, if its slightly oval or irregular the pusher method would struggle anyway.

As regards lens centering, the only reason I can think of why it would be set up using its existing OD as a reference would be to reduce its diameter, is that the main objective here? (minor pun).

I would have thought the primary goal was to centre on the lens optical axis anyway.

Ian P

why would it need to be accurately centred by its already

Probably not. A photo would help if you have the wherewithal. I'm not familiar with your particular lathe but it looks to be one of the Mini-lathe family. These usually have bolt on chucks rather than a screw-fit type. In the picture below two of the three nuts securing a bolt on chuck can be seen sandwiched between the back of the chuck and the headstock.

Put a book or lump of wood under the chuck to protect the ways if the chuck drops off when you undo the nuts. It's likely to be a tight fit and you may have to gently lever the chuck off.

Next step is to find a 4-jaw chuck that fits the flange on your particular lathe. The flange will have a register and 3 or 4 holes in it. Your looking for a match to the register diameter and the holes. Sometimes a backplate converter is needed. The good news is that ArcEuroTrade and others all carry good ranges of bolt-on chucks. With luck a CJ0623B owner will be along to advise: if not, I'd ring Amadeal and ask if they have a 4jaw chuck to fit the CJ0623b – they used to sell it.

The easiest way to centre work in a 4 jaw chuck is with a Dial Indicator and Stand. I blanched at the cost but it's worth it.

Dave

When a faceplate is attached to a lathe, a one time facing cut is taken to make sure it runs true. Are you sure that he plywood attached to your faceplate also runs true ?

Reading these posts I'm wondering if the "plywood" is the problem ? Unlike metal, if an item is attached to plywood it may compress unevenly causing a small unperceived wobble causing you indicating issues, something to consider.

The details I can find suggest that the lathe may have a 1 1/2" by 8 tpi spindle nose or 39x4mm. It looks like Chester Machine tools did sell them so there is a slight chance that they can help with a 4 jaw. No way does 1 1/2" – 39mm so you may be able to tell which by measuring it. It wouldn't be a bad idea to check the thread pitch with screw gauges as well

The swing over the cross slide seems to be 5 5/16" or 135mm so a 5" or 125mm 4 jaw should be ok but a 100mm or 4" one could be used with the jaws sticking out a bit. I'd tend to go for the larger size for a 4 jaw.

Really for this sort of thing you would have been better off setting up pretty central and then machining both the outside diameter and the groove.

There is one other way of doing this sort of thing if there can be a hole in the middle of the disk. For this size chuck some 1" bar, turn a 1/2 dia spigot leaving a square shoulder. Run a dia part way down the spigot. You then need some large washers to clamp the work between. They could be square pieces of plate of some sort. Maybe even ply but aluminium or steel would be better. Double sided sticky tape could help of ply was used but it can be a pain to part afterwards.

John

–

Hi Christine,
Would it not also possible to drill the 10mm hole in the plywood after it is mounted on the faceplate ? I initially imaginned the workpice to be a ring 190mm OD and 180 mm ID. From what you say now its is a 190mm disk with a 180mm depression in one face and a 10 mm hole in the middle.I am assuming the workpiece is mounted on the plywood AFTER the plywood has been bolted to the faceplate. Some pictures would be helpfull.

Les.

O.K. forgetting the four jaw chuck for now perhaps you could try this:

Get a new piece of plywood or better MDF, cut a 180 mm circle and drill it to bolt to the faceplate. Don't drill the centre hole yet. Bolt it to the faceplate and drill the centre hole from the tialstock using the highest speed and a really sharp drill. The hole will now be truly centred on the lathe spindle.  Clean off all the sawdust!

Remove the faceplate and bolt your workpiece in place firmly but not too tight. Put it back on the lathe and rotate by hand to check that it runs true. If it doesn't a few well judged taps with a soft headed hammer should true it up ( a 10 mm bolt will be slightly loose in a 10 mm hole). Remove from the lathe and fully tighten the central bolt again, replace and recheck. Hopefully it will still run true.

Russell.

Edited By Russell Eberhardt on 06/12/2016 15:27:35