Fly cutter questions

[Investigator]

I have a project that I would like to have a fly cutter to use on. I haven't got one at the moment, and am debating about building my own. I have found several styles. I wonder what the pros and cons for the various styles are? What size should I make or buy? I see the import sets of 3 cutters of different sizes. Is it important to have various sizes? Could I do smaller work with a larger cutter?

As far as styles it seems that some type of counter balance opposite the cutting edge, or a larger diameter holder with a minimal amount of cutter extended allowing it to act like a fly wheel might be good?

I'm open to suggestions and hope to hear many of them.

Scott

 

[Investigator]

Forgot to say that the project I have coming up will be a 4" wide piece of steel.

 

[RJSakowski]

The minimum diameter that a fly cutter can cut is limited by the diameter of the cutter. The maximum is limited by the amount of stick out of the tool bit. The smallest in my set can cut a 3/4" diameter at the smallest while the largest can cut a 1-3/8" diameter.

 

[RJSakowski]

You are going to be running at fairly low rpm so imbalance isn't much of an issue. More to the point, the single cutter generates unbalanced forces which can lead to vibration when dealing with large cutting diameters, particularly in less rigid mills.

 

[Investigator]

I was thinking of using carbide insert tooling bits if that makes a difference

 

[Karl_T]

IMHO, HSS makes a better fly cutter. I use them to just skin the face of a part, often only taking 5 thou. A finely sharpened and honed HSS bit with a nice radius at the end will leave a finish smooth as a baby's behind. Carbide don't like these light cuts and will chip if honed to a super shard edge. I run a five inch diameter cutter on my Supermax (5hp mill that weighs about 6000 lbs.)

 

[ThinWoodsman]

Is it important to have various sizes? Could I do smaller work with a larger cutter?

This is something I have wondered as well. Obviously a smaller-diameter cutter will run slower (in terms of sfm), but given the nature of the fly cutter's cuts it seems overkill to provide a dedicated cutter for those scenarios.

Is there a problem with interrupted cuts, such that a smaller-diameter fly cutter would be preferable to a large-diameter fly cutter on narrow material?

 

[mikey]

I have a project that I would like to have a fly cutter to use on. I haven't got one at the moment, and am debating about building my own. I have found several styles. I wonder what the pros and cons for the various styles are? What size should I make or buy? I see the import sets of 3 cutters of different sizes. Is it important to have various sizes? Could I do smaller work with a larger cutter?

As far as styles it seems that some type of counter balance opposite the cutting edge, or a larger diameter holder with a minimal amount of cutter extended allowing it to act like a fly wheel might be good?

I'm open to suggestions and hope to hear many of them.

Scott

Hey, Scott. Fly cutters are a really important tool and I hope this turns into a good discussion. I have some comments:

• There are many versions but really only a few basic styles - the ubiquitous single arm cutter and the disc-type with a HSS or carbide insert. You can also use a face mill with one or two cutters in play. The single arm is simple, usually cheaper and it works. The disc type is more costly but balance is less of an issue; depending on the disk diameter, it can cut a huge swath while producing a very nice and accurate finish.
• The cheap sets with three sizes are copy cat versions from Chinese makers. The one you use is supposed to suit the size of the part you're cutting. Given that the key force you're dealing with is Centripetal force and further given that this force actually decreases as the radius of the cutter increases, this makes little sense. Best idea is to get or make one that is as large as you think you'll need and if it proves too small, make multiple passes. All of us run into projects that are too big for our fly cutter and we all make multiple passes. It doesn't look as nice but it it is still flat, which is the main goal.
• The stick out of the cutter will be the radius. If the cutter tip sticks out 3" from the spindle centerline then you can cut up to a 6" wide work piece.
• HSS can potentially produce a finer finish and take finer cuts but they don't last as long; on harder materials, you can sometimes see the finish change as the cutter wears. This is especially true when the nose radius of the cutter is large.
• Carbide tends to last much longer but requires a minimum depth of cut. This minimum cut depends on the nose radius of the insert so if you use one, pay attention to the nose radius. Most carbide insert fly cutters are much more capable of stock reduction vs HSS. You can really hog off material with some of these contraptions. The Tormach Superfly can hog aluminum to a depth of about 0.070" per pass on my RF-31 - that is a big cut for a small mill.
• If you do go with a purchased carbide fly cutter, look for one that holds the cutting edge of the insert at a right angle. This will allow you to cut to a shoulder to make ledges. This is a very nice capability to have and you'll wish you had one one day. Not a deal breaker if all you want is to flatten a part but think about it.
• None of the fly cutters I've used have been counterbalanced. It would be nice but since the cutting forces produced are vertical and since I cannot run all that fast anyway (2200 rpm max), this hasn't been a major deal for me. If I was cutting big pieces, like a cylinder head for a car, then I would be more concerned about it and I would go for a disc-type fly cutter. For most smaller stuff I work with, vibration has not been an issue for me.
• Finally, smaller and more solid can be better. My best fly cutter is a little one from Sherline that is only 1-1/8" in diameter. It is essentially a single insert face mill that cuts better, finishes finer, can hog really well and can cut a shoulder. I also use a Tormach Superfly and I like it except that it won't cut to a shoulder.

All of the current designs have pros and cons. Good luck with this and I'll be interested to see what the other guys have to say.

 

[Investigator]

Hey, Scott. Fly cutters are a really important tool and I hope this turns into a good discussion. I have some comments:

All of the current designs have pros and cons. Good luck with this and I'll be interested to see what the other guys have to say.

Mikey, I was hoping you would show up. obviously you have experience with the RF30 mill, I'm leaning toward making a disc type for use with carbide insert tooling. I'm thinking about 3.5-4" diameter. is that something reasonable for our mills? The material I need it for now is a 4x4 piece of either a36 or 1018 type plate, just to true it up.

 

[mikey]

I'll be honest. I like the looks of a disc-type fly cutter but I don't see a major advantage to them. Yeah, you can balance them more easily but since balance is not a really big problem at mortal speeds I don't feel they are worth the cost. They are also large and take up a lot of storage space.

If I was going to fabricate one today, I would copy the B-52. Simple, balanced and can cut to a shoulder. The B-52 is hardened but
since the Modulus of Elasticity is what we're really concerned about, hardness has nothing to do with it. I would focus on getting the insert seat accurate, then I would be very careful about getting the vertical shaft placed dead center. TIG weld it and then grind the non-cutting arm to balance. It is probably the simplest design I've seen and it has all the advantages a fly cutter needs.

EDIT: I would suggest you make yours with at least a 3" arm (center to tip of cutter) so you can at least do pieces up to 6" wide. This gives you a lot of flexibility and it will easily cut smaller pieces; you basically use it like a big end mill and conventional cut with it.