Question about chucking a large part in a lathe

[SPiN Racing]

Howdy gents!

Ok sooo I have been playing with things in the lathe here for a couple years now.. and have learned quite a few little tricks here and there.

My issue I am having today is a methodology one.... I am certain there has to be a better method then I am using.. but I am not sure of it.

I have a 6" diameter disk of 6061-T6, that is 1.5 inches thick.
It was cut on a band saw, on both sides by the metal shop I bought it from.. so both sides are NOT true... but the perimeter is smooth.

SO my issue is.... if I had one side that was true, and faces all perfect.. I could simply chuck it up with that face to the inside.. and then face the back side.. then move to the boring and other procedures I need to do.
As both sides are not smooth.. when I put it in the 4 jaw independent chuck.. I am having a bear of a time getting it centered... and then again just as much of a hard time making sure that its in the chucks properly... so between the two different potential error in alignment issues... Im having a bear of a time.

I have a dial indicator.. but if I get it centered at the chuck location.. its not centered at the far edge. And if I get the 4 jaw centered.. its all crooked.

Is there some trick to this that old school machinists do?

[Tony Wells]

Take a good machinist's square and map out the misalignment. Since both sides are out of square with the axis, you just need to know about how much. Depending on just how much, you can use gage pins, feeler gages...or whatever you can find to ascertain the amount out of square the face is, and where in what we'll call the "B" axis it peaks. In other words, where is the gap between the face and the square the greatest? Use a marker (Sharpie or similar) put a mark on the low spot, where the gap it greatest.

Now to the lathe. Don't try to get it running axially true right off. Just close.......about 0.050 or so is good enough. Now move your indicator to the face, out near the edge, remembering it will be running out the 0.050 or so. With a soft mallet, tap the face towards the headstock until you see the same deviation on the indicator as you measured with the square. It won't have to be perfect, but of course, the closer the better. It all depends on how close it needs to run to the OD. Once you have replicated the out of square face with the mallet, now go back to the OD and indicate it in as normal. If you plan on machining the OD later, likely you ill be machining a feature at this stage that will be true and square to the face......since the first thing to do after you get the OD running true will be so skim cut the face to clean up the sawcut.

At least, that's how I do it. There are other ways, but that always gets me by and isn't difficult now does it take a lot of time.

[Fergus OMore]

Old time machinists didn't always have 4 jaws and three jaw chucks, they used faceplates.

However, you should blue one side, rub it on a marking table of some sort to indicate the high spots, take a file or a scraper and knock the high spots off. Blue and start again-- until you are 'happy' that it will not rock about like a pea on a drum and clamp it to the face plate with the dressed side towards the faceplate. It can now be cleaned up!

[Pete.]

If it's cut straight but not square I would go around the OD with a square and find the 'highest spot' of one face and mark that with a marker pen. Now put it in the 4-jaw with that high spot not quite touching the jaw face, indicate it in on the OD so you have a gentle grip then use a dial indicator on the saddle to clock off the OD as you run the saddle up and down until you get no/little movement on the dial. Now face it off and flip it over.

[SPiN Racing]

Thank you so much gentlemen.

I am going to use a combination of all of the techniques.

As it turns out I cant take anything off the OD.. so I will be breaking out the square and starting with the high spots, and take them down some to get it closer to flat.. so that it is easier with less shims, to get it in the lathe correctly.

Also indicating it with the chuck a measured amount out, that will make it a lot easier.. I hadnt thought to indicate it right at the jaw.. then duplicate it at the furthest distance, to get it square. I had always gone for 0. then re-adjust so the furthest distance is at 0.. then get a center on it to stabilise it. With this part its only 1.5 inches thick.. so its pretty thin, for being 6" diameter.

[Lew_Merrick_PE]

Spin,

Back in the dark ages, we used to set-up a blank on the surface plate such that we could use our (triangular edge) precision square to set the part perpendicular to the surface plate.  We would then cast in some zinc to create a planar face for mounting.  I have also performed this trick using plaster of paris.  The advantage of plaster is that a sheet of waxed paper serves to protect the surface plate.  The disadvantage is that the resulting "base" is quite brittle.  You can do the same thing with epoxy.

Does that help?

[loply]

Can you not make up a suitable arbor and mount it on that?

[Jonny]

This is one thing i do almost daily, well over 700 items.

The o/d of the aluminiunium for starters wont be round!
So you hav eto take some off the o/d after the bore has been put in.

I normally use a 3 jaw chuck with sawn cut blank placed in. True up by eye or use tool tip. If need be get the burr or larger thicker in between two jaws then take a facing cut, thats your working edge.
Remove from chuck and turn around, machined face up against the jaws, but check runout.

Action shot with small 6" chuck 11 year ago

[SPiN Racing]

Quite a lot of methods!

As it turns out I tackled things tonight with my 10" 4 Jaw independent chuck.

I used a machinists SQ and siscovered each one was a tad over .1 out of whack. Soooooo SInce there were saw marks from the bandsaw they used to lop off the slices..in the 4-jaw, with a .1 spacer in the one jaw, between the piece and the jaw.
Then I got the jaws in the ballpark...
THenb got a dial indicator and indicated Each jaw, till the pairs of jaws were symmetrical to each other. Yep Jonny was correct.. its NOT round. LOL
Turns out the opposite sides were off by around .02 SO as long as I got them true to each other, it was square?
Then I went ahad and put the dial indicator onto the piece.. right in front of the jaws.. and then went through more fine tuning of the positioning.. seems my 4 jaws are not exactly the same.
Once it was re-re-re-centered.. I then was able to put the dial indicator adjacent to a jaw.. then move the cross slide.. away from the chuck, and I was getting ZERO movement, letting me know it was square on all 4 jaws.

Once the first side was faced, it was a breeze to flip and re-center it. Standard 4 jaw methodology. (But I did quadruple check the measurements)

Drilled the center hole, and then began boring the center out to the dimentions I need.

I also have a arbor I will be able to mount the chunk on once its center id bored.  THen it will go over to the mill and get the periphery holes banged out of it.

I will take a couple Pics tomorrow.

[Jonny]

Happy your sorted, luckilly most of what i do doesnt 'have' to end up cock on o/d, so can afford to take a skim off o/d when bores in.
There can be swage or draw marks in aluminium round, just had a look at the 120 blanks just collected larger than above piccy dia, they look ok. Whereas the other 100 or so older blanks some have really pronounced draw marks and may need 2mm off o/d. Or even dinks.

Cant you grip through the bore with 3 jaw and use tailstock other end?
Its the only way i can do above quick.