First attempt at making a fly cutter from scratch

My only fly cutters are cute little things that I bought for my mini mill and which take miniature ¼" tool blanks.

I’m nowhere near convinced that this is a good idea, but I have some ½" tool blanks, a ½" left-hand turning tool and ground inserts for it, and a chunk of 1⅞" 1018 cold-rolled steel rod…


No plan of mine survives first contact with the machine shop.

I added dimensions for all the holes indexed to the tip of the tool where I can pick up a reference.

While I was at it, I made the set screw holes not line up back to front so that I can add more in the long face, which gives me more clamping options. The screws in the back are primarily for if I use this with a bar that is less than ½" thick. It turns out that my left hand indexed lathe tool with CCMT inserts is slightly thinner than the ½" tool blanks; to fit the tool blanks I need the slot thick enough that I’ll want to pin it to the center reference surface from the other side. But four holes in the primary surface mean that I can reasonably pin a tool in just three of them for extra reach if I want to.

Drawing.pdf (40.0 KB)


When I created the sketch for the tool groove, I didn’t notice that I was sketching on the wrong side of the face. Then I quit worrying about handedness because I had solved that problem; think about one thing at a time!

I’m impressed that I completely finished it before mounting a lathe tool in it and noticing that I had made a backwards fly cutter. That is, I’ll have to run the mill in reverse. I feel dumb. On the other hand, that lets me use all my right-handed lathe tools, I have a lot more of those than I have left-handed ones, and my mill doesn’t really care what direction it is running. So maybe it’s actually a fortunate error?

Other than that minor problem, it looks quite nice.


Very nice and professional looking. I am jealous!



Professional-looking in a mirror universe anyway? :rofl:

Other mistakes:

  • Forgot to switch to a polished insert for my finishing pass on the shaft, so the finish is rough, but it’s exactly at dimension so I don’t want to polish it further
  • Should have done a cleanup pass on the body on the lathe, to ensure that it is exactly concentric with the shaft. I haven’t tested its runout.
  • Cut too long in the band saw, and removing stock with the face mill is excruciatingly slow in my bench mill, so the body is taller than I designed for.
  • Accidentally used a bad face on an insert in the face mill, leaving tool marks on the bottom face.
  • Ran the face mill too fast.

Things I did right:

  • Did all the rest of the mill work at appropriate feeds and speeds, using plenty of oil and slowing down whenever it smoked to stay cool.
  • Cut the ½" slot with a ⅜" mill in the middle of the gap (sooo slowly; ⅓mm DOC, very slow feed), then opened out the sides to ½". Conventional milled to 0.1mm remaining, then cleaned and climb-milled for a smooth face.
  • Made the slot 12.5mm deep instead of 12.7mm so that the tool stands slightly proud of the slot; easier to feel that it’s fully engaged that way.
  • Drilled the holes with a slightly oversized drill bit made power tapping with a spiral flute tap easier. Didn’t break the tap.
  • Used an end mill to cut flat spots for the holes on the curved surface.
  • Chamfered holes before tapping.
  • Locations of the set screws worked well.

Surprising win: That slot is a tight fit for ½" tool blank. I cleaned the bottoms of the holes by pounding a tool blank through the slot and it sheared off all the swarf from the holes.


Drawings for a right-handed fly cutter this time! May or may not make this one.

Drawing.pdf (33.7 KB)


I just put a ½" x 2" bar in it long enough to tram my head. It reached near both ends of the table when I turned it, and I was able to get the head within a thou over the entire reach of the table. The apparently out-of-square indicator stem is mostly illusion from the camera angle.

This shot also shows how it is backwards.


I wish I had room for full size machine shop tools. But if I did, I would not have a life or real job! :grin:


The pressure clamping the tool into the flycutter should push the leading edge of the bit to centerline of the spindle, right? I was thinking the screws in the center rib shouldn’t be used as they’ll push the tool away from cl.


@NewbieLoren That’s quite right. However, the slot is a very close slip fit for my larger tool blanks, so if I’m using those I can get a little extra grip for sketchy setups with more than reasonable stick-out. I probably shouldn’t actually ever do that, but I drilled the holes as long as I had the setup in place.

(I was originally expecting to use this to do very very light surfacing of a plate that exceeded the Y travel of my mill, before I ended up not needing to do that after all.)

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Yeah, stay away from sketchy. lol