# Using a fly cutter



## Investigator (Oct 5, 2017)

I 'inherited' this fly cutter with a lathe I bought.  I have an rf30/31 to use it in.  My questions are on how to use it.  First, how far out can the tool bit extend?  What diameter can I swing with this?  Second, what speeds do I need to use in the mill? say for mild steel and aluminum?

Any thing else I need to know?


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## 4ssss (Oct 5, 2017)

The softer the material, the faster the speed for a decent finish with a fly cutter.  I personally don't use a single point tool in my mill because they are hard on spindle bearings in my opinion.


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## Investigator (Oct 5, 2017)

4ssss said:


> The softer the material, the faster the speed for a decent finish with a fly cutter.  I personally don't use a single point tool in my mill because they are hard on spindle bearings in my opinion.



I've already considered making a fly cutter to do what I need.  Any advice on design or style?  Or is it just easier to buy one off the shelf?


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## richl (Oct 5, 2017)

Lots like you can go around 2.25" to 2.50" with it. I have a bigger mill, but it has step pulleys so I have to work with what I have, for steel crs I use 220 or 330 rpms.
There are things that a fly cutters are nice for, light cuts, wide surface area cuts. Some like them for this, others prefer a face mill. You have this one, might as well use it for now. Do a youtube search for sharpening fly cutters,  you will find several.

Hth
Rich


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## Bob Korves (Oct 5, 2017)

Fly cutters are used with heavier cuts to remove metal and with lighter cuts looking for a nice surface finish.  With light cuts the radius of the cutter can be increased, to a point.  At some point the clamping screws will not contain the loads and will loosen.  You definitely do not want that to happen.  As the cutter is configured in the photos, it would probably be fine with light cuts.  Start very conservative, increase feeds/speeds/diameter gradually, and listen for the mill and/or cutter complaining, and back off if it is not going well.  I cannot accurately see the diameter and shank size of the tool in the photos.  Will it fit in a collet or will it need to be in a end mill holder?  End mill holders are stiff, but the increased stickout still will cause more loading of the spindle bearings and less rigidity overall.  It is impossible to tell you exactly what to do because of all the variables. I have not even mentioned what metal it is you are working with, if there is any looseness in the machine and spindle, and what you are trying to achieve.  Fly cutters are not usually great for hogging metal.

Fly cutters are pretty easy to make, and if you copy a design in common usage it will probably work quite well for you.  It is a good project.  OTOH, when I got into this hobby, I bought a set of five fly cutters made in India for under $30 and they have worked just fine.  I have also picked up at least a half dozen more larger fly cutters in tool lots for dirt cheap, and they all work fine as well.  At some point it is not worth making them at the cost you can buy them for, unless you just want the project.


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## Investigator (Oct 5, 2017)

Bob Korves said:


> ...  At some point it is not worth making them at the cost you can buy them for, unless you just want the project.



My first plan is to make a new QCTP, the plan is to use mild steel.  I will need to clean up about 3" wide just to make to make it pretty and true.  I am beginning to think making a fly cutter for the smaller sizes may be more trouble than it's worth.  Looks like I can buy a set of 3 sizes for around $20.


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## Investigator (Oct 5, 2017)

It is a 3/4" shank.  I have and ER32 holder and collets it will fit, and could get a regular R8 collet for it if needed.  It appears to me the bit in it now extends 1.5" from the center line of the shank.  I assume that means it will cut 3" as it is?

It takes a 1/4" bit.  a 5/16" wont fit.


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## Bob Korves (Oct 5, 2017)

Investigator said:


> It is a 3/4" shank.  I have and ER32 holder and collets it will fit, and could get a regular R8 collet for it if needed.  It appears to me the bit in it now extends 1.5" from the center line of the shank.  I assume that means it will cut 3" as it is?
> 
> It takes a 1/4" bit.  a 5/16" wont fit.


It should be fine for that project.  That is what they are good at.  Light feeds and a high enough speed to get a little color to the chips is a good thing to try to work up to.


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## mikey (Oct 5, 2017)

I use a fly cutter a lot. In fact, almost every milling project that requires squaring a work piece will see the flycutter at least once. Depending on the dimensions, I will often bring the work piece to final outside dimensions with a fly cutter so I will both rough and finish with that tool.

Your fly cutter is intended to work with a left hand HSS or brazed carbide tool bit. Both will work but the carbide tool will require higher speeds to work well and you need to use a grade that can handle interrupted cuts, probably C2. Brazed bits as supplied do not work well; you need to sharpen the edges and put a nose radius on it before it will finish decently. HSS will finish better but cannot handle big cuts.

More modern fly cutters use inserted carbide bars and in my opinion, they are superior to HSS and brazed tooling. They can hog material and finish very nicely but they are more costly to purchase and use. See the Suburban Tool fly cutter and Tormach Superfly for examples. Typically, they require far less horsepower to use vs a face mill because they only use one insert. The Superfly can fly cut a 3" wide swath, while the Suburban tool can go out to 9" or so, I think.

There are other kinds of fly cutters that look like a big round saucer with carbide inserts underneath. These are really expensive.

With regard to speeds, you are essentially using a single point milling tool so the usual formula for speeds will work as a starting point: *RPM = SFM X 3.82 / Diameter of the fly cutter's path*. For example, if you were to try to fly cut a 3" swath in mild steel, you would have 100 X 3.82 / 3" = 127 rpm. This is a starting point and you would try speeds above and below that to optimize the cut. SFM is just the cutting speed of the material you're working with; there are charts all over the net for this.

Feeds on a manual mill are simple, especially if you're cranking on the table feed by hand. You generally want to feel a light resistance to the cut. You will have much better results with power feed and this is also a trial and error thing. On a roughing cut, a good feed will turn the chip a light amber color. On a finish cut you won't see a color change because you will slow the feed to improve the finish.

I use the Tormach Superfly in my RF-31; works good and is worth the cost.

I would just buy a QCTP myself. The imports are relatively cheap and work well. You can also make a fly cutter body and buy an inserted carbide bar from Tormach or Suburban Tools.

EDIT: with regard to speed, carbide tooling generally requires two to three times the speed of HSS so you would change the cutting speed to whatever is appropriate for carbide. For example, the cutting speed for mild steel is 100 SFM for HSS but is 220 or so for carbide.


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