How can a steel cylinder be parted in concentric rings with small gaps between them?

Question

For a project, I have to part steel round stock, like the cylinder in Fig. 1, but not perpendicular to the symmetry axis (like a bread in slices) but in concentric rings 3 inch long and having the outer diameter as close as possible to 6 inch, 5 inch, ... , 1 inch.

I mention that I have little experience with machine shops and the capabilities of the tools existent there.

Question: What is the minimum gap I should expect between two consecutive rings, say the one with the ideal diameter of 6 inch and that with the ideal diameter of 5 inch (see Fig. 2)? Can this minimum gap be of the order of 1 mm or it has to be larger like 3 mm, 5 mm, etc. in width?

Fig. 1. 12L14 Steel Round Rod: 6″ Dia, 3″ Long

Fig. 2. Two concentric rings with the outside diameter as close to 6 and 5 inch respectively.

Answer 1

A few points here.

Material

First of all, is it particularly crucial that it be steel? If you can use cast iron instead, the raw material costs around half as much:

https://www.mcmaster.com/products/discs/oversized-easy-to-machine-gray-cast-iron-rods-and-discs/?s=cast+iron+disk

($129.29 for an 8" x 3 1/4 disc)

Whether cast iron works for you depends on what you need from these rings though. Steel is tougher and stronger, were cast iron is fairly brittle. On the other hand, cast iron tends to be more stable, so it may fit better with your concern for maintaining its outer diameter precisely.

Lathe

If you were doing to make these on a lathe, I see only two reasonable choices:

• allow something like 3/8-1/2" clearance between the ID of one ring and the OD of the next smaller, to fit a cutter that you can extend 1.5" out of the tool holder and still have at least semi-decent rigidity.
• make one set of rings from 2 blanks, so one blank produces 1, 3 and 5 inch rings, and the other 2, 4, and 6 inch rings. This gives you 1" for each cut (plus as much more as you want clearance between them).

For what it's worth, the tool you want for this isn't a parting tool, it's a trepanning tool.

Milling Machine

The other possibility would be to use a milling machine. You can use an extra-long end mill with a fairly small diameter (e.g. 1/4^th inch) to make the cuts.

https://www.msdiscounttool.com/catalog/product_info.php?cPath=5692&products_id=102

For this, you'd use either a CNC machine, or a rotary table. Put your blank on the table, cut (just over) 1.5" in from one side, flip it over, and repeat. Since you apparently care about the accuracy of the OD, start by cutting each a little oversize. Then do a quick pass on the lathe to clean up any mismatch between the two cuts and get the OD more accurate. Depending on the accuracy and finish you require, possibly a final pass on a center grinder to get a finer finish and higher accuracy.

Test

Just for grins, I did a quick facing pass on the lathe with a 3/8^ths inch cutter sticking a little more than 1 1/2 inches out of the tool holder. Definitely needed to stick to light cuts, and it still got some chatter, but nothing so terrible it seemed like you'd be breaking bits gratuitously, or anything like that.

That's probably not the best possible test though. I used a carbide cutter, but they really cut their best with heavy cuts (deep cut, fast feed, or both). With it sticking out that far, you're kind of stuck with fairly light cuts. A HSS or Cobalt cutter would probably work a bit better under the circumstances.

Answer 2

Unless you have a specific requirement for the stock to be all of the original cylinder, most machine shops will recommend to cut individual pieces to meet your specifications. A retired machine shop owner presented to me a cylinder with another cylinder within. A tiny vent hole in the outer cylinder could be blocked with a fingertip and the clearance between the two machined cylinders was so tight that the inner cylinder could not be pushed into the opening without releasing the vent.

A machining lathe can provide up to zero clearance between cylinders. Temperature then becomes a factor.

If you require that the stock be comprised entirely of the original (homogeneous?) material, consider to investigate EDM, electrical discharge machining. There are varying methods used, even a couple of DIY solutions that provide for kerfs smaller than a millimeter.

The first suggestion is going to be less expensive than the second.

EDIT: related to new info in comment

A search for 12L14 steel tubing finds a wide range of diameters and wall thicknesses. One such source, Chicago Tube does not include prices and I did not locate a minimum product length, but from a machining standpoint, it becomes much easier.

The stock is placed in the lathe, the end is faced and the ID is cut using a boring tool as noted in other answers. The OD is also cut to specifications with conventional cutting tools.

Each diameter tubing can be found, but without prices, I can't determine viability.