# Fly cutter for obtuse-angle of lathe inserts



## jwmelvin (Nov 12, 2021)

I always wanted to use the obtuse angle of rhombus lathe inserts (I primarily have C-shape inserts, with an 80° primary corner, leaving 100° corners unused). It seemed to me that the obtuse angle might work well for a fly cutter. I had a hard time finding holders for some reason, ending up with one for the CCxT 09T3_ (ANSI 31.5_) size, rather than the 0602_ (ANSI 21.5_) size inserts I had. The holder I got is a SCKCL1212H09. I've had it sitting there for quite a while. So last night I felt like doing something, and made a fly cutter using a piece of rusty unknown steel (seems like 1018) 3" diameter x 1" long, attached to an R8 arbor with 7/8"-20 threads.

This was my first time cutting internal threads (second time cutting threads at all). It worked out well and was kind of fun. I threaded all the way through so I didn't have to worry about where to stop so much, or chip buildup in the hole. The arbor threads stop short of where the tool holder slot starts. 

cleaning up the second face after threading:



the tool:





I tested it quickly on a piece of unknown steel (also 1018 I'm guessing). I was running at about 1500 rpm, with the insert at a 5" diameter, so pretty fast (1950 sfm); the insert did not show gross damage. There was minimal vibration (I think the fairly heavy holder body helps), but you can see a little bit of ripple on the surface as the tool enters the work. The cut surface was okay but not super smooth; I didn't try playing with parameters and I apparently need to tram the mill again, as it only cut on one side.



I'll have to play with it some more, including using some of the much-sharper aluminum (CCGT) inserts. It may be handy some time to be able to cut a pretty wide surface, as the cutter can extend a good bit more than it did for my test. 

-jason


----------



## MrWhoopee (Nov 12, 2021)

Very nice! Stock removal with a flycutter can be a brutal affair, nice to be able to use the otherwise wasted edges of those inserts. Think I'll be doing something similar.


----------



## woodchucker (Nov 12, 2021)

Next time, put the solid side on the back of the cutter. You want it fully supported in the back not sitting rocking on screws.

It seems to be working regardless of what I am saying, so just make a note of it. Part of the learning process. You want it beating on a fully flat supported piece.  It came out nice otherwise.


----------



## jwmelvin (Nov 12, 2021)

^ I know people say that, but I’m not convinced it’s a real issue. Until the screw preload is overcome, the assembly is the same stiffness either way. And I don’t think this comes anywhere close to overcoming the screw preload; that said I’ve not looked into that further. Just comparing bearing load capacities (about 3000 lbf per bearing) with screw loads (about 1800 lbf per #10-32 screw), I’d be surprised; if anyone knows typical cutting loads, I’m interested. 

Also, reorienting the screws means the tool-holder body determines the cutting-edge location. I wanted the holder to work with various tools without shims, so I put the reference surface on centerline.


----------



## woodchucker (Nov 12, 2021)

jwmelvin said:


> ^ I know people say that, but I’m not convinced it’s a real issue. Until the screw preload is overcome, the assembly is the same stiffness either way. And I don’t think this comes anywhere close to overcoming the screw preload; that said I’ve not looked into that further. Just comparing bearing load capacities (about 3000 lbf per bearing) with screw loads (about 1800 lbf per #10-32 screw), I’d be surprised; if anyone knows typical cutting loads, I’m interested.
> 
> Also, reorienting the screws means the tool-holder body determines the cutting-edge location. I wanted the holder to work with various tools without shims, so I put the reference surface on centerline.


if we had a super high speed camera, I think we would see slight deflections. maybe like a golf club length wise. with a little twisting back and forth.
Maybe I am out of my mind.. who knows..


----------



## aliva (Nov 13, 2021)

I made a similar flycutter, but milled a shallow slot the length of the tool holder, on the set screw side,  this way the holder doesn't fall out during adjustment


----------

