I needed to make a 0.75 inch diamter disk with two small holes in it for 3-48 mounting screws.
I drilled the holes in the mill and then parted the disk off about .070 thick on the lathe. The parting tool when quite a bit off when it encounter the holes leaving the back of the disk very irregular. I managed to grind the disk flat, but I want to learn how to do this better next time. Obviously parting an irregular cut is not desirable and I got away with it because the holes are tiny. Bigger holes probably would have broken the parting tool.
If I use the banksaw to slice the disk off, I suspect it will be just as irregular as using the parting tool.
If I part the disk off before drilling the holes, I think it would be difficult to mount the disk and indicate it to accurately locate the holes. The only idea I can come up with it to superglue it to an aluminum stub shaft and drill.
It sounds like you have problems with the parting, or cutoff operation. This is common to all small lathes. The best solution I've found so far is to mount a toolpost on the rear of my cross-slide table and in that rear-mounted toolpost, to mount a parting tool upside down. I used the cross-slide table and rear toolpost kit from MLA.
To do this without any parting whatever:
turn the bar to 0.75" round, face smooth
cut off with hacksaw or bandsaw, leaving excess
mount disc faced side down in 5C step collet held in collet fixture in milling vise
face off to thickness and mill holes as desired
Grant Erw> I needed to make a 0.75 inch diamter disk with two small holes in it
If the disk you're making is .070" thick you could shallow drill or end mill the holes so they stop just shy of the cutoff plane, then finish drilling them after parting the piece off.
You should be able to figure out a way to hold the disk well enough to let you hand position it and drill through the already located holes in a drill press. You could use a shallow 3/4" diameter recess in a piece of hardwood or aluminum with a little homemade hold down clamp if you want to make sure the piece doesn't fly away.
Might be a good idea to look into the techniques used by the watch and clockmaking crowd to work on thin, flat stock. I seem to remember things like hardwood facings on lathe faceplates, and double-backed tape.
Holes are best done last, regardless of the size of your machine.
Do you have soft jaw capabilities? That's the slickest way to do a thin item. Once you've turned and cut off the piece, the bored soft jaws will hold it dead perpendicular and concentric within a half thou easily. Face to length, deburr, then drill your holes as required.
You can also turn it from sheet metal of the proper thickness after cutting it out oversized, then pressing between a piece of stock held in a chuck and a plug that is center drilled to accept a live center in the tailstock. Light cuts are required, but it works fine. I use both methods, depending on the job at hand.
The sole negative of soft collets or step chucks is their inability to hold lengths reliably. Soft jaws are an absolute stop, whereas collets of any style are subject to length variations due to variations in diameter and tightening of the drawbar. It can be an issue when you're trying to hold a length closely. Otherwise they work great. Holding thin items by either of the methods is far superior to any other method. The support added by backing the piece prevents it from moving about under cut. Soft jaws hold parts without distorting or marking, due to the identical radius one machines in the jaws. Same applies to machined collets.
It's far easier to machine blanks, then drill your holes. You can stack multiple pieces, then turn them all at once. If you rely on pins to locate, unless you use a diamond pin for one side, the holes must be perfectly located. I guess it all depends on the job at hand, and circumstances particular to the specific application, for both methods work.
Ok. I have a buck 6 jaw with 2 piece jaws. Seems like I can make some aluminum blocks for the top jaws and bore them for the job at hand. Buck soft jaws seem outragously expensive. Not sure how much soft or brass 5C collets cost but it strikes me as wasteful to use one for a a "one off" project.
Y'sure they're all copper and some aren't copper clad zinc?
You just jogged my memory...I still remember the time my chops were saved right on a Philadelphia street when I discovered that a dime was exactly the right diameter to replace the little dished plug which somehow fell out of the back end of my '57 Chevy's 6 cylinder engine's die cast choke vacuum pulloff cylinder. I pushed it in with ball point pen and it jammed there well enough to let the engine run fine. Talk about falling into s**te and coming out smelling like a rose. It was one of those fortuitous events I'll never completely forget.
You don't specify what kind of finish you want, or how dished the back surface of your part can be (and don't say *perfect*).
While I appreciate others' suggestions on extra fixturing, this road will at least double the length of your work (especially frivolous on a single piece).
You should try a larger parting tool. I've had good success with a brazed carbide parting tool at work. Just neutral rake with clearance on the sides to prevent rubbing. I believe it was at least 1/8" wide. If you want to go nuts, the indexable carbide parting/grooving tools are nice and you'll have invested in a usefull tool.
I would try to drill the holes before parting as you tried originally. Being so thin, you're asking for deformation due to clamping and drilling.
Probably not -- but interrupted cuts do spoil the finish. The more rigid your lathe, the less it will impact the finish, but there will always be *some* effect.
Maybe not as bad -- especially if you cut through both holes at once. But you will probably want to turn or grind the side which has been bandsawn anyway.
How about taking a 5C soft collet, and making a shallow bore in it to receive the disk, then putting that collet in one of those square or hex collet holders in the mill to allow you to hold the disc steady and index it properly.
Out of curiosity -- what was the material being cut?
You can locate the collet block with mill stops then sweep in the bore before installing the 3/4 collet.
If you want to go this route, might I suggest that rather than buying and machining a soft collet, do the following:
Buy a collet stop. Get the one that is made of steel and threads into the back of a 5C collet (internal thread on collet). It has a 1/2 through hole and two set screws into that hole. It comes with a variety of pre-made stops and you can make many more.
Get a piece of Aluminum rod turned to .73-.745" for about 1 inch. It needs to be long enough to reach the bottom of the collet to the stop you bought. The stop end of the Aluminum has a 1/2-13 threaded hole about 1/2" deep or so. Bottom a stud into the hole and locate into the 1/2 thru hole in the stop. Tighten set screws. Your collet should now have a piece of AL barely sticking out of the collet (or flush) and tightend into the stop. It should be a reasonably rigid setup.
Now, install the 5C collet in your lathe. Face the piece of aluminum so that it finishes about .070" below (into) the flush surface of the collet. This is where your decision to turn the OD anywhere from .73-.745" may make life more difficult or not. You should now have a positive stop .070" in the collet that will hold a part flat. You will also be able to drill thru your disc and into the Al stop.
As with the machinable collet, you can easily drill multiple parts that will have holes perpendicular to the disc face. But this method is cheaper, takes about the same amount of time to setup and you'll still have a stop at the end of the day. With the machinable collet, you'll eventually "use up" the collet.
I've done the above method and it works well for thin disc.
And, you'll have a pair of soft jaws for your mill vise!
Well, only you know your part tolerances and abilities so if you think that'll work, then it certainly is quick and dirty.