Dale, can you tell me which cutter I would need to order from Eddie?
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Also, any more details like thread tap size and such so I can gather materials, along with how long those bars are when you start?
Mac
Capt Eddie offers four basic shapes - round, square, triangles, and 'radiused square' (squares where the face is slightly rounded).
True squares are specialty tools that are great for cutting tenons or for hollowing, but they are also pretty 'catchy' - it takes skill to use them without catching. Likewise, triangles are specialty tools used for detailed work.
That leaves rounds and radiused squares. Rounds are very good for hollowing and shaping coves and beads, while radiused squares are good for straight or tapered work. So if you are focusing on pens, I suggest that you start with the radiused square shape.
The second option you have to work with is size. In my opinion, size itself is not a magic attribute - the reason Capt Eddie offers different sizes is that he is trying to offer replacements for the cutters that come with off-the-shelf carbide tools. If you make your own bar, you just pick a size and plan to stay with it as you eventually purchase replacement cutters.
In Eddie's catalog, he offers both 15mm and 10.7mm radiused squares, but notes that the 15mm is a generic choice, while the 10.7 is intended specifically to replace the cutters on an unidentified manufacturer's tool.
When you buy cutters from Eddie, he provides the matching screws. The key point here is that the head of the screw matches the countersink in the face of the cutter. His 15mm R2 cutters come with 10-24 screws, so you would need to drill and tap your bar for that size screw. He doesn't specify the screw size that comes with the 10.7mm cutters, but you can easily measure the screws once you get them.
As to bars - you can buy round mild steel bar stock at most hardware stores (including Home Despot and Lowes). That's good enough for this purpose. The key point is that the cutter must be larger than the diameter of the bar, and the bar must be large enough to provide the strength required to withstand the forces imposed on the bar during use. A 1/2" bar is strong enough for almost any small turning application, but you can't put a 10.7mm cutter on a 1/2" bar.
In addition to round bars, you can also purchase bars with a square cross section, and some people get away with bars having a hollow square cross section. However, finding those shapes is harder unless you have a convenient industrial metal suppler.
A square bar sits flat on the tool rest. In use, however, you may need to rotate the bar a bit, and that favors the use of a round bar. There is one manufacturer of carbide tools in England who uses square bars, but cuts the flat for mounting the cutter across the diagonal so the cutter naturally sits at 45 deg when the bar is flat on the tool rest.
Most of the stores around here sell bar stock in 3' lengths (and occasional 4' lengths). Sometimes you also see it precut in 1' lengths. I usually start with a bar that is 10-12" long, and plan to embed 3" or so in the handle. Bar stock is relatively inexpensive, so buying a 3' length when you only need 1' today is not a big issue - its something that is convenient to have in the shop. And if you go to a specialty supplier, you can buy whatever length you want.
As others have said, you have to grind a small flat on the bar to receive the cutter. I actually start by making a cut to the desired depth of the flat using a cut-off blade on a Dremel tool, and then grind the flat to that depth - that leaves a clean shoulder. Also, a standard grinder will leave a curved 'flat', but if you are working with mild steel, you can make that truly flat with a file.