Two Bit Fly Cutter

[jjr2001]

Here is the latest project for the shop. I have a good single bit fly cutter with an r8 shank that works great
in my mini mill. It was built using Rossbotics plans:http://www.homemadetools.net/forum/f...585#post124734
I wanted to make one a bit larger and with two bits to see if it would provide an even smoother cut.
Well this is the result and it provides a glass smooth finish, at least by touch. You can still see tool marks but you
can not feel them. This may be my best fly cutter for a smooth finish.

It is 2.5" in diameter with a 5/8" shank. The shank is drill rod and the fit is interference by about .002".
I wanted to make sure it would not turn on the shank so I added two set screw cotters. The cotters
are 1/4-20 and secured with blue loctite. As Tom Lipton says, "nothing built too strong ever broke".
https://www.youtube.com/channel/UCZC...yjrKT4OZne-JNw

The cutter disc was placed on the shank prior to final finishing on the lathe. Lathe bits are 1/4" and are a slip fit in the grooves.
10-32 allen cap screws secure the bits. The cutter disc is .750" thick and made from a chunk of hot rolled steel. Having a square 5C collet block really helped to get all of the milling and drilling lined up.

I think it should out last me!

Cheers, JR

[milomilo]

How do you get both cutters to the exact same depth?

[jjr2001]

Thanks for your question Chris.

I set a parallel in the vise or on the bed if not using a vise and set them by touch.
Sometimes it takes a couple of tries to get it within .001".
I also have a tool setter that I have not yet tried but that should work also.

With a .002" or .003" difference in tool height I don't see any difference in finish.
The fact that the fly cutter is balanced is worth the effort even if the exact tool height is not set.
Runs very smooth when compared to my original "slanted tool" fly cutter. With a 2.5" diameter it is still
a very good tool for my mini mill or any mill. Here is a link to my tool height setter.
http://www.homemadetools.net/forum/c...953#post124982

I also have a 1 3/8" diameter single bit fly cutter that is quite good and runs very smooth. It is of a different
design and is very close to balanced even with the cutter on one side.
http://www.homemadetools.net/forum/f...585#post124734

The tool can be setup with the cutters ground dissimilar. The two I am using are ground very close to the same but not exact.
If one tool is ground with a larger round nose it should function as a "finish" cut while the pointer one is the first or rough cutter.
There is enough room in the tool bit slot to have them each scribe their own diameter circle providing a coarse cut and then a fine cut
at the same time..

Cheers, JR

[DIYer]

Thanks jjr2001! We've added your Fly Cutter to our Fly Cutters category,
as well as to your builder page: jjr2001's Homemade Tools. Your receipt:

Fly Cutter by jjr2001

tags: fly cutter

[Toolmaker51]

Thanks for your question Chris.

I set a parallel in the vise or on the bed if not using a vise and set them by touch.
Sometimes it takes a couple of tries to get it within .001".
I also have a tool setter that I have not yet tried but that should work also.

With a .002" or .003" difference in tool height I don't see any difference in finish.
The fact that the fly cutter is balanced is worth the effort even if the exact tool height is not set.
Runs very smooth when compared to my original "slanted tool" fly cutter. With a 2.5" diameter it is still
a very good tool for my mini mill or any mill. Here is a link to my tool height setter.
http://www.homemadetools.net/forum/c...953#post124982

I also have a 1 3/8" diameter single bit fly cutter that is quite good and runs very smooth. It is of a different
design and is very close to balanced even with the cutter on one side.
http://www.homemadetools.net/forum/f...585#post124734
Cheers, JR

A number of methods work for multiple bit flycutters. Compared to facemills, flycutting can work with staggered bits; essentially single point not unike boring, while a facemill may tend to drag the back side of insert. Facemills work best vertically to dispense chips, improving finish.

Two identical leading edges will be difficult getting same plane and periphery. With a tool grinder that lessens, but then accentuates [u]any[u] amount spindle out of perpendicular.
A good combination: A sharp cornered bit a couple thou higher and further from center line of holder, with other bit carrying a small radius used to attain finish height. HSS is easily the best choice finishing most materials, and minimal removal of stock. Cemented carbide works heavier cuts, mill a 1° to 3° seat cut lengthwise to impart a little shearing action which is foreign to carbide usage.

The improvement of vertical bits versus extended is physics, lessened harmonics, plus acute angle tends driving cutter downward into material.
I grind the equivalent of a small chamfer on my extended variety. Accomplishes roughing action, where the lowest corner has near zero contact with material, even radiused, protected by the leading edge.

The tool can be setup with the cutters ground dissimilar. The two I am using are ground very close to same but not exact.
One tool is ground with a larger round nose to [should] function as a "finish" cut while the pointed one is the first or rough cutter.
There is enough room in the tool bit slot to have them each scribe their own diameter circle providing a coarse cut and then a fine cut at the same time...

[jjr2001]

Thanks TM for the good info. I did not try to grind both bits exactly equal so I am sure they do track in slightly different orbits.
One was honed with a larger radius than the other and I think I will just test it to see if it is the larger orbit. I like the finish as is
but there is always room to make it better.

Cheers, JR

[tonyfoale]

How do you get both cutters to the exact same depth?

You do not have to. There can be benefits in having them at different heights and radii. See A double edged fly cutter.

[Toolmaker51]

I've been reminded of making rifle scope mounts, radius to match that of receiver (Remington 700). No, I can't verbalize formula used, it's in Machinery's Handbook, but the geometry to visualize tilts the head for a flycutter set to proper ''radius'' (in reality, slightly elongated), traversing length of stock, spread across two vises. The ideal result generates roughly 2/3rds contact between mating parts, thick enough at apex to not flatten under tension of mounting screws. I made Picatinny rails, common integral mounts would vary in difficulty.