that "new tool" buzz

I am almost a tool fetishist. To me, tools of all sorts represent capability and therefore power, to a limited degree. Since I was very young I have wanted to make, repair, and modify things, and tools are what enable us to do so. Every time I acquired a new tool, in general, it represented a new capability: I couldn't file things unless I had a file; I couldn't turn screws unless I had a screwdriver.

This passion for increasing my abilities through collecting increasingly powerful tools has continued throughout my life.

This meta-hobby of gathering the resources together to make my actual hobbies either possible or easier took a quantum leap into new territory once I obtained machine tools, since machine tools cannot be used without the tooling and accessories that go with them, and there are nearly infinite vistas of different tools, cutters, and accessories available for each machine (lathe, mill, etc).

So although I scored a cheap SIEG* lathe, it didn't come with a four-jaw chuck or any cutters, centers, dog plate, etc - I have slowly been acquiring those bits over time since I bought it.

And while I was able to pick up an old and well-used Bridgeport mill, it didn't come with ANYTHING; not a vise, not a single cutter of any sort, no collets, and no phase converter with which to feed the motor, so the initial weeks and months after I got it home were occupied with getting a vise, a toe clamp kit, and a rotary phase converter.

(the latter is a story in and of itself which I will tell another time)

 After I first brought home the mill and got it running I fell on hard times financially for a while, and I wasn't able to buy much in the way of cutters.  I had one little box of cheap Enco (RIP) end mills, and a set of fly cutters, because those are also very, very cheap.

 This is a fly cutter (so-called because a single cutting point flies around a central axis of rotation):

 Until now, I've managed to do all my hobby and paying jobs with this very limited set of tooling for the mill.

Now, when you talk to professionals who do this for a living, operating on limited margins, you soon discover they don't have all the tools they'd like to have either, but they will always watch for jobs which require a new tool, as a way of both paying for, and justifying the cost of, new tools.

As one fellow joked, "never accept a job unless it requires you to buy a new tool."

To that end, I have gradually added a small handful of things, including a real Jacob's ball-bearing SuperChuck (which can be used in both the mill and the lathe), a set of basic parallels for the vise, I cheap imitation Indicol dial indicator holder so I can tram the head of the mill, and a free 12" Bridgeport brand rotary table which mostly needed rust removed and oil added to become usable.

Even the table power feed is still awaiting repair.

Yesterday I ordered my first shell mill cutter, because using a fly cutter would have taken bloody forever.  It wasn't terribly expensive (because all I can afford are cheap imports, not name brands) but nevertheless ordering it made me very, very happy.

This here is a shell mill cutter:

The cutting edges are provided by replaceable carbide inserts (the yellow bits), each of which has at least two cutting ends or sides (some designs have four) so each insert can be used 2x, sometimes 4x. You don't sharpen inserts, you just replace them when all of their edges are dull.

The shell mounts on an arbor, which is the bit that goes into your machine; there are several different spindle styles found on machine tools.  This is great for real shops with multiple machines, since one can swap out inexpensive ($20 - $100) arbors letting you use the same (often expensive) cutter on more than one machine.

My mill is a Bridgeport, the spindle of which is intended to take "R8" style collets, collets being a sort of chuck which grips a cutting tool via compression. An arbor for this spindle has the same shape and dimensions as a collet, but instead of a hole in the end to accept an end mill, it has a standardized mounting fixture that accepts cutters like the one above.  Here's a picture of the correct arbor for my mill:

 The arbor was $15, the shell mill cutter was $35, and a box of ten inserts was $5.  I could have spent over $300 on the shell alone, so there must be some tradeoffs involved when buying cheaper versions!

 All of the differences that matter are in the cutting head and the inserts.  The arbor could, in theory, be less precisely made than some name brand equivalent, but I very much doubt it, because it's really easy to get turned and concentrically ground cylindrical parts close to perfect with automatic machinery, and it's really hard to screw them up. So for most hobbyist needs, there is no need to buy name-brand arbors.

 The shell mill itself however, will not be made as precisely as a name brand, meaning that the inserts will not be held quite as precisely in the ideal cutting geometry.  The inserts could therefore have slightly different heights, which reduces surface finish quality and cutting efficiency.

 Also, the steel may not be hardened and tempered as well as a name-brand tool, or the steel itself may not be as high quality.

 The inserts themselves also differ a lot between name-brand and cheap ones: the cutting edges will be smoother and more precise on name-brand inserts, and the effectiveness of the chip breakers will be greater, leaving a nicer surface finish.  I usually don't have to create precision dimensions with excellent finishes - if I need an excellent finish, I usually have the luxury of polishing the part after I've cut it, so I find that, most of the time, cheap inserts do just fine for my purposes.

 Now then, how much difference in execution time will this tool make vs. a cheap fly-cutter?

 The fly cutter uses HSS tools whereas the shell uses carbide inserts.  The surface (cutting) speed for carbide in most materials I will be working with is 3x - 4x higher than HSS.  And of course there are six inserts in this shell vs. only one cutting edge on a fly cutter.  So to make a given cut, this shell mill will have a material removal rate 18x - 20x higher than the fly cutter.

One downside to inserts is that their cutting edge geometry is - technically - shaped for specific materials (usually steel), and there are also different coatings one can get to give better performance with different materials.  So to get optimum performance in aluminum or brass, one should - in theory -  use entirely different inserts - they'd have the same outside dimensions to fit the same cutter/holder, but the tip geometry and coatings will be different. 

But is it absolutely necessary to use "the correct" insert for each material?

Nope!

I use inserts intended for steel for everything - steels hard and soft, cast iron, brass, aluminum, and plastic resins and it works fine. The only people who usually care about getting the insert type exactly right are production manufacturing engineers; you and I usually don't have to sweat it.

Note that fly cutters still have their place: in addition to face milling, they can also be used to bore holes, and unlike a shell mill, the diameter is easily adjusted.  Shell mills are not adjustable.

(arguably, the correct tool for the boring job is an adjustable boring head, but if you want to talk breathtakingly expensive tooling, we could certainly start there; they are spendy!)

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* SIEG - Shanghai Industrial Electric Group. This is the factory in which many different

"discount" brands of machine tools are made, including Jet, Grizzly, Turn-Pro (MSC's

house brand) and the former Enco and Nova.