Fly Cutter Tool Bits

[Flash319]

I bought some fly cutters for my bridgeport. I was wondering what shape do I grind the tool? Do I grind it like a lathe tool or different? It is the kind of fly cutter that the tool lays on an angle horizontaly not straight vertical.

[Paul Alciatore]

The fly cutters need essentially the same rake and relief angles as a lathe tool. However, they do not necessairly look like a lathe tool. If you have a good text on grinding lathe tools, it should show the various angles and typical values for each. The same ideas apply to almost all metal cutting tools: lathe, drills, saw blades, milling cutters, fly cutters, files, etc. Even the multi toothed tools like saws and mills are just a collection of single point cutters onone tool. And they all cut the same. At least basically they do.

You have to examine the cutter and determine each angle to work with your cutter's geometry. The common, inexpensive sets of fly cutting tool holders use standard lathe tool bits of 3/16", 1/4, and 5/16" size. They hold the tool at a 25 to 35 degree angle to the work's surface. The face or bottom clearance angle viewed from the side needs to be a few degrees greater than the angle the tool is held at - perhaps 30 to 40 degrees if the above numbers match yours. Fron the end view, the face or bottom clearance angle should be about 5 to 10 degrees. The end clearance angle needs to be sufficient to bring the rear edge of the end closer to the axis than the cutting point and the rear of the cutter clear of the circle it sweeps. I like to grind a slight radius on the end as viewed from the top. This allows more clearance while keeping the angle just behind the cutting edge as large as possible.

As for rake angle, things get more complicated. A fly cutter can put the "top" of the tool on a radial line, in front if it or behind it. You need to determine where it is on yours and then see what angle, if any, needs to be ground to product the rake you want. In a lot of cases, you can get away with no change here, just the original flat side of the tool bit. But you may have to experiment for the material you are cutting.

HSS Tool bits are cheap and can be reground many times. Take a good guess at it, grind it, mount it and see how it looks against the work without power. Regrind as necessary. Then take a light cut and see how well it works. Regrind again as necessary.

Also, if it's the same cheap little set like I bought, you will need to replace the set screws. Mine stripped out the second time I tightened them.

Paul A.

[Flash319]

Thanks for your help. I am going to give it a try with your advice. I bought the cheap set so I will keep the screws in mind.