# Fly cutter questions



## Investigator (Apr 8, 2019)

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


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## Investigator (Apr 8, 2019)

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


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## RJSakowski (Apr 8, 2019)

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.


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## RJSakowski (Apr 8, 2019)

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.


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## Investigator (Apr 8, 2019)

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


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## Karl_T (Apr 8, 2019)

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.)


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## ThinWoodsman (Apr 8, 2019)

Investigator said:


> 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?


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## mikey (Apr 8, 2019)

Investigator said:


> 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?
> 
> ...



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.


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## Investigator (Apr 8, 2019)

mikey said:


> 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.


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## mikey (Apr 8, 2019)

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.


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## darkzero (Apr 8, 2019)

If you want to make a disc type fly cutter here are some examples to get ideas from. Link to Clickspring. Myfordboy has an old video showing how he made a disc type fly cutter. I also have a disc type fly cutter but it's big & the cutter hangs out like a conventional fly cutter instead of vertical/pointing down like others have made disc type fly cutters. My disc fly cutter is big though, 5" dia just for the body. I'm sure there are plenty of more examples on the net.









						Large Diameter Flycutter
					

One of the things we all strive for is a decent surface finish on the mill, particularly for larger work-pieces. A brand new end mill can deliver a great finish, but its narrow diameter means that...



					www.clickspringprojects.com
				












						My New (Big) Fly Cutter
					

I do have a 2.5" facemill which I use a lot but I also like fly cutters as they give a better surface finish (IMO). The FM can hog out more material than a fly cutter though.  On my project list was to make a fly cutter that will cover up to 6". I didn't want to make a big conventional styl fly...




					www.hobby-machinist.com


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## Investigator (Apr 8, 2019)

Is there any 'best' way to mount it? R8 arbor, ER collet?


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## darkzero (Apr 8, 2019)

Investigator said:


> Is there any 'best' way to mount it? R8 arbor, ER collet?



Mine have straight shanks like most do. I prefer to run them directly in a R8 collet. Too much stickout from the spindle running in an ER collet chuck IMO (although I run endmills in ER collets).

My big fly cutter that I linked above originally had a straight shank but I converted it to a R8 arbor. That thing is big & got some weight to it so I didn't like the idea of it being a straight shank.


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## Aaron_W (Apr 9, 2019)

mikey said:


> 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.



You mentioned in another post you were using this on your RF-31 in an MT2 adaptor. I was curious about this, I've just got a Clausing 8520 mill which has an MT2 spindle and it would be nice to be able to use the Sherline fly cutter on it. I have an MT2-MT1 sleeve but like all I've found so far it has a flat tang instead of being drilled and threaded for a drawbar. It also seems to just hold the MT1 in by friction which is probably fine on a lathe, but doesn't seem to secure on a vertical mill.


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## mikey (Apr 9, 2019)

I found a 5/8" straight shank adapter with an MT1 female taper at the end. I cut the adapter to length, inserted the Sherline tool into the end and drilled it for a roll pin. After pinning it, it is very secure. I hold the straight shank in an R8 collet and it works quite well. I actually prefer to use this tool whenever I can, and not just for flycutting. It is essentially a single insert face mill that is capable of pretty deep cuts so I often use it that way.


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## Investigator (Apr 9, 2019)

Mikey, do you think a fairly large cutter, similar to the B52 in design, would be too large and or heavy to be held safely with a straight shaft in a collet?


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## Aaron_W (Apr 9, 2019)

mikey said:


> I found a 5/8" straight shank adapter with an MT1 female taper at the end. I cut the adapter to length, inserted the Sherline tool into the end and drilled it for a roll pin. After pinning it, it is very secure. I hold the straight shank in an R8 collet and it works quite well. I actually prefer to use this tool whenever I can, and not just for flycutting. It is essentially a single insert face mill that is capable of pretty deep cuts so I often use it that way.



Ok, so its probably going to be a custom adapter. Now if only I knew someone who had a lathe and mill.


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## mikey (Apr 9, 2019)

Investigator said:


> Mikey, do you think a fairly large cutter, similar to the B52 in design, would be too large and or heavy to be held safely with a straight shaft in a collet?



A shaft of 5/8" or 3/4" would handle it easily. Stick that shaft in an R8 collet and go. That vertical shaft needs to be precisely at 90 degrees to the body of the fly cutter but you can do it, Scott.


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## Creativechipper (Apr 10, 2019)

Can fly cutters be used on a lathe as well as a mill?

 What would the major working differences be?

 What sort of stuff is important to understand about fly cutters to the 1st time user of one?

 Thanks, very interesting thread, love learning a new method technique!!


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## Silverbullet (Apr 10, 2019)

Fly cutters have been used forever, while in vocational high we had projects to do for grades. Well one was design and build a fly cutter. Oh we built many sizes and different angles some even straight up and down with up to 4 tool bits. Everyone worked and gave good finish . Back then we didn't have carbide inserts . Cemented or brazed  yes . But we ground bits has it was a part of the grade also. I've since made many more but still have the few I made in vokie. Good work can be done with them. So go make one or two


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## mikey (Apr 10, 2019)

Creativechipper said:


> Can fly cutters be used on a lathe as well as a mill?
> 
> What would the major working differences be?
> 
> ...



Yes, fly cutters can conceivably be used in a lathe, although that isn't something I've seen done. You would need some way to solidly mount the work piece vertically to the cross slide so you could fly cut it. 

Fly cutting is most commonly used to flatten a surface. Depending on the work and the operator, this can be done with great precision over a rather large surface area, like a cylinder head. Most commonly, fly cutting is used to prepare a work piece on a mill. Most milling projects start out as rough stock that may or may not resemble something square and you need to make it square. To do that, you fly cut it in a specific sequence so that all 6 sides are square to each other. Then you can work that piece however you need to. If the sides are not square then every single operation you do will likewise be out of square so this is an important process. You can also do the same thing with an end mill or face mill. The former is very slow and tedious, the latter is much faster but a fly cutter is usually faster still. 

If you want to use a fly cutter, best get yourself a mill first.


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## Silverbullet (Apr 10, 2019)

mikey said:


> Yes, fly cutters can conceivably be used in a lathe, although that isn't something I've seen done. You would need some way to solidly mount the work piece vertically to the cross slide so you could fly cut it.
> 
> Fly cutting is most commonly used to flatten a surface. Depending on the work and the operator, this can be done with great precision over a rather large surface area, like a cylinder head. Most commonly, fly cutting is used to prepare a work piece on a mill. Most milling projects start out as rough stock that may or may not resemble something square and you need to make it square. To do that, you fly cut it in a specific sequence so that all 6 sides are square to each other. Then you can work that piece however you need to. If the sides are not square then every single operation you do will likewise be out of square so this is an important process. You can also do the same thing with an end mill or face mill. The former is very slow and tedious, the latter is much faster but a fly cutter is usually faster still.
> 
> If you want to use a fly cutter, best get yourself a mill first.


Many use the lathe like a horizontal mill. It takes a sturdy work platform with y , vertical adjustability. Ck Ables shed,,,I think,,, is his title on YouTube , he does some amazing builds with a small China lathe . Milling slotting , fly cutting and all other aspects even boring head.


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## MarkM (Apr 10, 2019)

Flycutters are more for just surface finish.  They can be used to bore holes accurately, use for tools, cut grooves, splines radiuses and probably more.
I made five.  I don t like to stick my tool out too far so I use the next size.  Here is a project I did.  All the radiuses and the 1.25" hole were all done with flycutters.  Those radiuses are exact.  The two pcs. Put together gets you the same 1.500" and 2.500" every spot on the hole.  A groove cut on some jaws I made for a small vise. These for now will be my boring head as well as regular flycutting  duties.
You may want to use brazed carbide at that diameter.   Much better than inserts if you can grind a tool.


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## MarkM (Apr 10, 2019)

One thing that you have to be aware of grinding your tool.  Be very careful adding side relief.  If you take it too far you may take yourself out from having your tool on centre.  Put your screws in the position with the cutting forces. Not going against them.  I don t really like sticking my tool out more than an inch.


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## WCraig (Apr 11, 2019)

Silverbullet said:


> Ck Ables shed,,,I think,,, is his title on YouTube , he does some amazing builds with a small China lathe . Milling slotting , fly cutting and all other aspects even boring head.


I searched for "Ables shed" with no luck.  Can you post a link?

Craig


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## jdedmon91 (Apr 11, 2019)

Investigator said:


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



I made one my self that uses a tool holder that uses the unused corners of a CNMG 432 insert. Now it isn’t 4” in Dia. However you could modify the design

I’m attaching a YouTube video of the build. 







Sent from my iPhone using Tapatalk


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## Investigator (Apr 18, 2019)

OK, I have a plan I'm working on.......
Tell me, how important is it to have the cutting edge precisely on center?  What happens if it isn't on center line?


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## pontiac428 (Apr 18, 2019)

To add to the question, what tool bit grinding profiles produce the best finish and performance (for surface fly cuts)?  Are there any formulae for depth of cut and feeds/speeds?  I am not sure that my books cover that.


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## mikey (Apr 18, 2019)

Investigator said:


> OK, I have a plan I'm working on.......
> Tell me, how important is it to have the cutting edge precisely on center?  What happens if it isn't on center line?



Ideally, the cutting edge of the tool where it contacts the work should be on the centerline of the tool. I would try hard to make it so.



pontiac428 said:


> To add to the question, what tool bit grinding profiles produce the best finish and performance (for surface fly cuts)?  Are there any formulae for depth of cut and feeds/speeds?  I am not sure that my books cover that.



I'm assuming you're referring to a HSS tool for use in a flycutter. You are grinding a LH turning tool, the profile of which is up to you. Keep in mind what the flycutter has to do: Interrupted cuts, stock removal, fine consistent finish.

The interrupted cut part suggests to me that M2 HSS would be the best material to use as it is the most impact resistant HSS available. Impacting cuts require greater edge strength so I would keep the relief angles to a max of about 15 degrees or so.
Stock removal implies deep cuts. If you plan to use it to bring parts to size and rough with this tool then you are going to generate pretty healthy cutting forces. To do that, I would increase side rake to reduce cutting forces instead of increasing the relief angles, for the reason noted in bullet 1. For use in multiple materials, I should think that 12-15 degrees of side rake would work. If cutting soft stuff like aluminum, you can increase side rake and back rake a lot more.
Fine finishes are thought to be brought about with large nose radii. You can do that but it will increase deflection. Personally, I would limit the nose radius to 1/32". I prefer to increase speed and decrease feed to improve finishes. The benefit is that increasing speed will reduce cutting forces all by itself.
I think @ttabbal made a LH Square Tool that he used in a flycutter; perhaps he can give you some input on how that worked.

I no longer use HSS because I got tired of grinding them. Inserts last much longer, work fine and give a decent finish (but not better than HSS will). Hope this helps.


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## ThinWoodsman (Apr 18, 2019)

pontiac428 said:


> To add to the question, what tool bit grinding profiles produce the best finish and performance (for surface fly cuts)?  Are there any formulae for depth of cut and feeds/speeds?  I am not sure that my books cover that.



I have a couple of flycutter bits to grind, and noticed the lack of coverage in the books as well.

So far, Totally Awesome Flycutter Grind seems the best of a sparse lot. Nothing very comprehensive out there. Tom's Techniques has a Cutting Tool Geometry page with an excellent diagram for a fly cutter, though it's just the one. In the video for this he says the diagram shows a right-handed cutter and is wrong, but that seems to have been fixed.


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## ttabbal (Apr 18, 2019)

Since @mikey summoned me....

My HSS flycutter grind is really just a LH square tool with 15 degree angles. I did about 1/32" nose radius, just a little to help smooth the cut. It chews up aluminum, but has to go pretty slow in steel. If you have a power feed it's not so bad. Just let it do its thing. It does leave a really nice surface finish though. If I do a lot of steel, I would like to get a decent carbide tool just for the higher RPM for faster stock removal. 

Here's a steel block I used the flycutter on.


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## mikey (Apr 18, 2019)

Looks pretty good to me!


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## mikey (Apr 19, 2019)

pontiac428 said:


> Are there any formulae for depth of cut and feeds/speeds?  I am not sure that my books cover that.



Totally forgot to address this. There are no formulas for depth of cut, feeds or speeds that I am aware of. Each flycutter has different capabilities and this is especially true of those that use HSS cutters. When I used HSS many years ago, I would try not to exceed 0.010" depths of cut because I wasn't really all that aware of tool geometry back then. So, it took a long time to square the work piece up.

I have since switched over to carbide and usually use the common formula for speed (CS X 3.82 / D) and feed so the tool takes a continuous chip (it is faster than you think). I try to listen to and feel the machine. If I think the machine can handle it, I up my speed to the max it can handle without vibrating. This is to lower cutting forces. If I feel the mill is loading up too much, I back off the speed. Sorry, I know this doesn't help much but that's how I do it - listen to and feel the machine.

My Sherline flycutter is a single insert tool that easily handles up to 0.020" to 0.050" roughing depths of cut in mild steel. My Tormach Superfly can handle the same in steel and up to 0.070" in aluminum. For taking fine cuts, I limit my cuts to 1/2 the nose radius or deeper; I do not go shallower than that to limit deflection. When I do fine cuts, cutting forces tend to be higher because of tool deflection so, again, I increase my speed to reduce those forces and tend to feed a little slower; this produces a very fine finish even with carbide tooling and the cut is very accurate.

I always use cutting oil with a flycutter. I prefer A-9 for aluminum. For steels, any cutting oil I have handy seems to work okay.

Much depends on the geometry of the cutter, the rigidity and power of the mill and experience. The more you screw up, the faster you learn!


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## MarkM (Apr 19, 2019)

Having that cutting edge on centre is everything when it comes to an efficient cut.  Being above or below changes the cutting forces and your tool geometry goes out the window with the radius of the hole.  
One thing you do need to have in your grind for flycutting is the appropriate relief on your trailing edge.  Much more so than a lathe tool.  
I keep my stickout to one inch or less to avoid the deflection Mikey is talking about.  I See people sticking out there tool ridiculous amounts.  I am sure Mikey does it right!
I like hss with cobalt unless I need the speed from the carbide.


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## Investigator (Apr 19, 2019)

somewhere I lost a reply I made.    My plan involves cutting a pocket for carbide inserts on the bar.  I stumbled on the answer to my question about being on center line.  Truth is, the tip of the cutter will always be on center line as it rotates, what changes is the angle of the cutter in relation to the work as it moves across the surface.


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## ddickey (Apr 19, 2019)




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## mikey (Apr 19, 2019)

Gotta' say, Joe sure has a way of proving a point!


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## Aaron_W (Sep 3, 2019)

mikey said:


> I found a 5/8" straight shank adapter with an MT1 female taper at the end. I cut the adapter to length, inserted the Sherline tool into the end and drilled it for a roll pin. After pinning it, it is very secure. I hold the straight shank in an R8 collet and it works quite well. I actually prefer to use this tool whenever I can, and not just for flycutting. It is essentially a single insert face mill that is capable of pretty deep cuts so I often use it that way.



Just a follow up. I started to look at my options, and contacted Sherline to see if it was possible to get an unfinished flycutter that could be finished with a different taper or straight shank. It isn't as step one is forming the shank, but they have just added a version of their insert flycutter with a 1/2" straight shank instead of the MT1. It wasn't even up on their website yet, but is now. I have one on its way to me.

Sherline Insert Fly Cutter / Straight shank


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## mikey (Sep 3, 2019)

That might be useful. Let us know how it works out for you.


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## Aaron_W (Sep 4, 2019)

mikey said:


> That might be useful. Let us know how it works out for you.



It seemed like it would be easier to either use directly with an MT2 collet, or if I run into slippage buy an extra 1/2"-MT2 collet and fasten the two together vs playing around with trying to make an MT2 to MT1 adapter. I can't share the flycutter between the two machines, but it will still provide the benefit of using the same inserts.


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## mikey (Sep 4, 2019)

The Sherline flycutter is actually a really nice single-insert face mill that cuts to a shoulder. At high speed, it puts a really good finish on a part and I actually use it a lot on my RF-31 when I need to cut a ledge into a part. It is more than capable of taking really heavy cuts if you have the power and rigidity, albeit in 1-1/8" bites. With a 1/2" shank, you can use it in an ER collet (or an R8 collet) and have a rigid set up that will work well for most machinable materials. Plus, you have the added vibration damping of the ER collet chuck and that will make the finish even better. I might just buy one myself. Thanks for the heads up.


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