Turning a large diameter on my ML7

Here's my conundrum.: I have a piece of round 3.5" diameter (88.9mm) aluminum, and I want to turn it down by about .040" to 88mm. My chuck is too small to hold it, but I have 4 jaw that would, if it were longer than 20mm.

I need to drill a hole in the middle anyway, so is my best bet to drill the hole first, tap it for a bolt to hold it, then hold it the 4 jaw and use the dti to centralise it. Then, turn it on the bolt and turn down the diameter this way?

Hope this makes sense.

You should be able to hold 3.5" bar, have you got a set of reverse jaws for your 3 jaw chuck?

Phil.

Kit it is not clear at least to me the length of the material, it appears to be less than 20mm long and 3 1/2" dia ?

How big is the hole you need and will it be plain hole or threaded on finishe component?

Hi Kit,

Yes make up a mandrel if you havent a longer piece of ali. Finish the internal of the ali, than make mandrel to suit and mount on it to turn down the outside diameter. If you plan this properly you will also get perfect concentricity, especially if you turn down a chunk of steel to make the mandrel in the chuck, rather than use a bolt.

The easiest option would be to obtain a longer piece of ali.

Cheers,

Julian

Edited By julian atkins on 04/04/2016 23:12:17

Kit, if you can find the centre of the piece of alloy you have with reasonable accuracy, you could centre-drill it to give you a refererence point then use that to press the piece onto double-sided sticky tape on your chuck nose with a centre in he tailstock.. I got that from the MIT training videos on the web, I wasn't convinced, but it works fine - easier than makng up a mandrel.

Rob.

Edited By Robin Graham on 05/04/2016 01:14:31

If your chosen system allows it, use a centre in the tailstock, it can make a big difference to the stiffness and stability, and use a sharp tool with light cuts

gl

If you go the way Robin suggests and stick/glue onto something you do not have to center it very accurately if you then mount it in a 4 jaw chuck. Then center the job and take light cuts to reduce the plate.

On big,heavy jobs you can weld it to a piece of scrap steel and grind it off after machining.

Paul

If the faces are reasonably flat and parallel mark out the centre position, then centre drill the blank. If you haven't got a face plate remove the jaws from the chuck, place the opposite face against the now flat face of the chuck, then use the revolving centre as a pressure pad locating in the centre drilled cone and with light cuts turn to your desired size.

Glue the aluminium disc to a bit of MDF, with PVA glue, put a sheet of paper between the two. Turn up a disc, in this case aluminium would be best, say up to 50 mm dia x 10 mm thick, with a centre hole on one face. Mount the disc to be turned with it's wooden backing in the four jaw chuck, centre it, take the small disk and sandwitch it with the tailstock centre against the work and get turning, when finished a wood chisel between the metal and the MDF, the paper will split in two, a good wash in water gets rid of the paper, put the small aluminium disc in a safe place for next time (and when that comes, you can make another one, 'cause you can't find the other one until the job is finished).

Ian S C

Hi all, thanks for your input.

I can't weld to it as it is aluminium and I don't want to damage it.

I need to tap a hole in the centre anyway, so a bolt wouldn't be going out my way anyway.

As I see it, your conundrum is that of achieving both a central hole and 88mm diameter starting with only 88.9mm.

I'm not an engineer, but would attack the problem in one of two ways.

1) carefuly centralise the workpiece in the 4 jaw chuck using a dti securely held on the toolpost so that it can be retracted, to avoid the jaws. Then centre drill and drill the hole. I would then make mandrel with a wide face and glue (with cyanoacrylate) and screw the workpiece to that, for skimming off that half mm.

2) make the initial central hole rather smaller than final size, mount, as above, and machine the outside diameter to size before sorting out the central hole position and size. An end mill with collet holder would be the choice, to alter the centre of the hole by a small amount as a drill might tend to follow the previous (depending on finished size, even in aluminium)

Hope you achieve both your requirements. It can be tricky with such small amounts to machine off.

RAB