# Flycutter geometry



## mikey (Feb 18, 2022)

In another thread, @Watchwatch queried: *What grind angles do you recommend Mikey?*

Before we can get to grind angles, we need to understand rake angles and why they matter in a fly cutter.

There are essentially two models for cutting tools - Orthogonal and Oblique:




Orthogonal tools were very common back in the day. The distinguishing feature of orthogonal tools is that the cutting edge is perpendicular to the work. It may be canted across the work but the entire edge is still perpendicular. A good example of this type of tool is the common shaper tools. Another is a brazed carbide tool used in a fly cutter. You can imagine that such an approach presents a huge vertical wall at the point of cut and the resulting cutting forces are large. In the case of a shaper that has a lot of power and rigidity, the tool can take a large chip. In the case of a fly cutter, this kind of tool will be very limited in how much it can hog off. Accordingly, very small depths of cut must be taken and feeds tend to be slower or the tool will chatter or lock up. These tools work but are not as efficient as more modern carbide tools or even HSS tools.

Oblique tools, on the other hand, have a cutting edge that angles across the work in at least two planes AND usually have one or more rake angles at the cutting point. Without going to much into it, rake essentially thins the chip and increases chip flow by lengthening a feature called the Shear Plane. For a given depth of cut, the chip is thinner and cutting forces are significantly lower. Some inserts have both side rake and back rake molded into the insert. A good example of this kind of tool is the Tormach Superfly.





As you can see, the insert on this tool has the entire insert canted back, giving it back rake that is additive to the back rake molded into the insert. In this configuration, the back rake of the tool and insert takes on the same force-reducing effect that side rake on a turning tool does. The result is that this tool cuts with very low cutting forces that enable a 3/4HP mill to take stock reduction cuts without even slowing down. I have personally taken 0.070" deep cuts in 6061 without slowing the mill down at all. There is no way I could do this with a brazed carbide tool. So, geometry matters.

Now, HSS tools have an advantage even over these modern inserted carbide tools in that you can use whatever side and back rake you wish. The more side and back rake you use, the lower the cutting forces will be. There is a point where edge retention will become an issue, however, so you have to use some restraint. I will say that a good HSS tool, like a LH square tool, will work really well in a fly cutter PROVIDED you keep the nose radius on the smaller side. By that, I mean maybe 1/32" or so. The reason for keeping the nose radius small is to reduce radial deflection. If you have a bigger mill and have the power, going up to a 1/16" nose radius will work. In times past, lots of guys promoted the use of huge nose radii because it produced a nice finish, and it does, but they won't take a decent depth of cut due to high radial cutting forces.

So, bottom line: a LH square tool will work. 15 degrees of side and end relief and 15 degrees of side and back rake, with a 1/32" nose radius and that tool will cut pretty well in most materials. You can mess with the rake angles to maximize efficiency in materials you most commonly work with. For aluminum, I would boost back rake more, up into the 25-30 degree range or even more. If the edge breaks down too fast, back off. When using HSS, go with M2HSS. It is designed for impact cuts and will hold an edge. Cobalt and Tungsten bits may work but tend to be more brittle and do not tolerate interrupted cuts nearly as well.

Now, with all that said, I no longer use HSS fly cutter tools. The reason is longevity. If you're doing a lot of small projects in soft stuff, like mild steel or brass or aluminum then HSS will work fine. You have to maintain the edge but it will work just fine. My problem is that when used with higher carbon steels like O-1 or even crappy lower carbon steels like structural steels, that HSS tool will dull too fast and it may happen when you're half way through a pass ... not good. A carbide insert will last longer, trust me. Of all the fly cutters out there, the best design I have seen to date for just fly cutting has been the Superfly. The big deficit of this tool is that it will not cut to a shoulder so it isn't perfect.

To take care of the shoulder cutting thing, I use a Sherline inserted carbide straight shank fly cutter that is essentially a tiny 1-1/8" single insert face mill and that has handled all my needs quite well. Don't poopoo this flycutter. It will outperform most flycutters out there but it does so in tiny bites. In steel and aluminum, the finish produced by this thing will frost your eyeballs.

Okay, that's my input. Fly cutters are really useful tools. They are often the first tool we use when prepping our stock for a project so having a good one is important. Hope this gives you some ideas.


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## Aukai (Feb 18, 2022)

Does the fly cutter use the same chart for RPM, like the LMS one for milling using the diameter of the cutter?


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## mikey (Feb 18, 2022)

Yes, but like all things, calculations are a suggestion and not written in stone. Studies show that the correct speed is more important to insert life than feed but so much goes into getting speed right - depth of cut, material, rigidity and power of the machine, coolant, etc. I usually just get it close with the usual rpm calculation and focus on getting the feed dialed in. *The tool should move so that the insert is taking a continuous chip. *Most hobby guys feed too slow so the insert is really taking tiny bites and that affects the finish. The same is true when boring. The feed should be fast enough to produce a good chip.


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## tjb (Feb 18, 2022)

I have both the Tormach and the Sherline.  Mikey's right.  They're super tools, and I use both quite regularly.

Regards


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## Batmanacw (Feb 18, 2022)

mikey said:


> In another thread, @Watchwatch queried: *What grind angles do you recommend Mikey?*
> 
> Before we can get to grind angles, we need to understand rake angles and why they matter in a fly cutter.
> 
> ...



Very well laid out. Thank you.

I'm planning on grinding extra rake onto a cemented carbide tool to play with this weekend. They only come with 6° of side rake. I'll try for around 15°. Putting back rake on by hand with my tools might be a bit much but I'll see how it goes.



I wish there was a chart for fly cutting rpm because it seems like fly cutters like to run much higher rpm than you would expect for the diameter.


I have the Superfly and it is extremely impressive..I'm learning more about my other fly cutters because of it.


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## jwmelvin (Feb 18, 2022)

It seems like the Superfly presents its insert very similarly to a modern 45° face mill?


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## mikey (Feb 18, 2022)

Batmanacw said:


> I'm planning on grinding extra rake onto a cemented carbide tool to play with this weekend. They only come with 6° of side rake. I'll try for around 15°. Putting back rake on by hand with my tools might be a bit much but I'll see how it goes.



Most brazed carbide tools come with zero rake so any rake you can put on there, especially side rake (which is essentially going to be back rake in the position the tool is in), will help.



Batmanacw said:


> I wish there was a chart for fly cutting rpm because it seems like fly cutters like to run much higher rpm than you would expect for the diameter.



Most of us use a simple calculation to get our rpm but if you want to nail it down, use the data on the insert packet to get the recommended cutting speed. Typically, it will be higher than an rpm calculation and should satisfy your need for more speed. Just as important is the right feed; kick that up and you'll see an improvement in your finishes.

The Superfly is a good tool, no doubt, but the little Sherline I linked to is better ... except that it only cuts a 1-1/8" swath. That one runs at 2800 rpm in steel and leaves a nice chromatic rainbow pattern. I just love that thing.


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## mikey (Feb 18, 2022)

jwmelvin said:


> It seems like the Superfly presents its insert very similarly to a modern 45° face mill?



Yeah, it does.


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## Batmanacw (Feb 18, 2022)

jwmelvin said:


> It seems like the Superfly presents its insert very similarly to a modern 45° face mill?


Yes


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## Batmanacw (Feb 18, 2022)

mikey said:


> Most brazed carbide tools come with zero rake so any rake you can put on there, especially side rake (which is essentially going to be back rake in the position the tool is in), will help.
> 
> 
> 
> ...



What insert does the Sherline use? Ccmt? Those cutters are $75 on Ebay. What shank diameter?


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## mikey (Feb 18, 2022)

It uses a CPMW insert. Flat top with a 0.015" NR. The one I linked to has a 1/2" straight shank.


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## Aukai (Feb 18, 2022)

Mine has this insert, I don't remember what it is, but I can look it up.


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## mikey (Feb 19, 2022)

Pretty sure that is an SEHT1204AF8N-X45.


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## Aukai (Feb 19, 2022)

Should I get a CPMW too?


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## mikey (Feb 19, 2022)

Nah, the CPMW fits the little Sherline flycutter. Won't fit the Superfly.


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## WCraig (Feb 19, 2022)

mikey said:


> a LH square tool will work. ...  If the edge breaks down too fast, back off. When using HSS, go with M2HSS. It is designed for impact cuts and will hold an edge.


Are you familiar with cast alloy tooling?






						Cast Alloy Lathe Tools
					

overview of new and vintage cast alloy lathe tools



					www.conradhoffman.com
				




I don't have practical experience but from reading about them, it seems like their properties are very well suited to use in a fly cutter.  I've picked up a fair number of pieces of Stellite in auction box lots--very low cost--and I intend to give this a go at some point.

Craig


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## Watchwatch (Feb 19, 2022)

Thank you Mikey


Sent from my iPhone using Tapatalk


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## mikey (Feb 19, 2022)

Watchwatch said:


> Thank you Mikey
> 
> 
> Sent from my iPhone using Tapatalk


Yup, you're welcome.


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## mikey (Feb 19, 2022)

WCraig said:


> Are you familiar with cast alloy tooling?
> 
> 
> 
> ...


Sorry, Craig, I have not. Stellite is hard to grind and I don't turn the kind of stuff that requires it. Tough stuff, though.


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## Batmanacw (Feb 19, 2022)

These are the cemented carbide tools I'm using most on my old style fly cutters.










Just to add to the discussion.


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## Alcap (Feb 20, 2022)

I’ve used some fly cutters on my small Clausing mill with limited success. The last try the HSS bit cut it seemed to almost rub the surface . I believe I had plenty of clearance . I’m hoping this thread will help me out . A question I have on the geometry, does the cutting tip need to be on center ? I’m thinking the screw side would be on center so once you start grinding a blank it will go off center . Is that an issue ?


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## jwmelvin (Feb 20, 2022)

Batmanacw said:


> These are the cemented carbide tools I'm using most on my old style fly cutters.



Looks like you have a little back rake on there? Seems like that could make a difference. I’ve also used the same type of tools in my fly cutter, though also have a blunt-corner ccgt holder that does okay (and aluminum-cutting inserts should have some back rake).


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## mikey (Feb 21, 2022)

Alcap said:


> I’ve used some fly cutters on my small Clausing mill with limited success. The last try the HSS bit cut it seemed to almost rub the surface . I believe I had plenty of clearance . I’m hoping this thread will help me out . A question I have on the geometry, does the cutting tip need to be on center ? I’m thinking the screw side would be on center so once you start grinding a blank it will go off center . Is that an issue ?



Generally speaking, the tip of the tool should be on center. This requires that you be able to grind an accurate tool.

If you grind a good LH tool then it should not rub. The point of contact on a LH HSS tool is very small and should be up very near the nose radius. If you get the relief and rake angles right there should be no way anything can rub. If you're rubbing, post a pic of your set up so we can help troubleshoot.


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## Alcap (Feb 21, 2022)

I sharpened the tip again , 3rd picture i wanted to see what it looked like against a known center line . I set the vice jaw on center . I dont think it looks too bad. I found some scrap material and took a few passes . RPM 1000 , hand turned , about .020” depth . It doesnt seem to be rubbing like the metal the outer day , maybe hard scale and too light of a cut ?


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## mikey (Feb 22, 2022)

It looks like you have enough relief and rake to cut decently and the point of contact is fine so this tool should not rub. You will find that with rough steels like in Pic 3, you need to go deep enough to get under that scale or the tool will dull quickly. HSS also does not like excessive speeds and I would slow it down to maybe 600 rpm or so. I would conventional cut the surface and then maybe take a 0.005" deep climb cut at higher speed for a nice finish.


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## Parlo (Feb 28, 2022)

Not a big fan of flycutters, it's the invisible spinning bit that's scary! Having said that, if the radius is set to just larger than the part width there is no danger past the part edge. I see most people try to achieve an almost 90 degree tool mark to the edge which unfortunately means a much larger cutting radius than required. There are drawbacks with a large cutting radius such as a slower rpm = longer machining time, the invisible danger previously mentioned and the impacts on the spindle bearings.

An important consideration most often overlooked is a tangential approach to the workpiece. If the radius is set just past the part width then the tip engages slowly into the workpiece, then fully engaged half way & then gradually decreases on exit. This results in a much smoother cut, when a large radius is used the tip impacts the part edge at full load. This leads to a risk of chipping the tool and a poorer finish. When this clattering occurs a sharper rake angle is often added to the tool which will wear quickly. A tangential engagement also protects the spindle bearings from the shock loads associated with flycutting.

Here is a video demonstrating the advantages of a tangential approach on the edge of a this piece protruding high out of a vice. Turn subtitles on for a clearer explanation.


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## mikey (Feb 28, 2022)

Parlo said:


> I see most people try to achieve an almost 90 degree tool mark to the edge which unfortunately means a much larger cutting radius than required.



Not sure what you mean by a "90 degree tool mark to the edge" but I've been using a fly cutter for quite some time and rough in a conventional direction with the tip of the tool just past the edge of the work nearest me. I suppose this amounts to a "tangential" approach as you illustrated. On this side of the pond, that's called the proper use of a fly cutter while feeding conventionally. I admit to some surprise that this common technique is not as widely used as I thought.


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## Parlo (Mar 1, 2022)

mikey said:


> Not sure what you mean by a "90 degree tool mark to the edge" but I've been using a fly cutter for quite some time and rough in a conventional direction with the tip of the tool just past the edge of the work nearest me. I suppose this amounts to a "tangential" approach as you illustrated. On this side of the pond, that's called the proper use of a fly cutter while feeding conventionally. I admit to some surprise that this common technique is not as widely used as I thought.


It is surprising that the technique is not widely used considering its advantages. All references I've see on YouTube have the cutter centre over the part centre. I suggested that the cutter centre could still be over the part centre if the cutting diameter is set to just over the part width. My reference to a 90 degree tool mark is what would be achieved with a huge cutting radius ie. cutting across the part at right angles to the feed.
Thanks for the reply.


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## mikey (Mar 1, 2022)

Parlo said:


> It is surprising that the technique is not widely used considering its advantages. All references I've see on YouTube have the cutter centre over the part centre. I suggested that the cutter centre could still be over the part centre if the cutting diameter is set to just over the part width. My reference to a 90 degree tool mark is what would be achieved with a huge cutting radius ie. cutting across the part at right angles to the feed.
> Thanks for the reply.



On the contrary, I think it is very widely used, YouTube notwithstanding. Not everything is on YouTube and shockingly, not everything on YouTube is actually correct.

Thanks for clarifying the 90 degree thing.


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## Parlo (Mar 2, 2022)

Thanks for your reply, I was a little surprised that a Moderator would be so condescending to suggest I believe everthing I see on YouTube.

Just to be clear, are you specifically refering to tangential approach milling being commonplace? This is where the tool is deliberately set off centre to the part to allow the teeth to engage at almost the same direction as the feed.

Or, are you referring to machining from right to left where the chips fly away from the operator. Which is almost universal.


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## mikey (Mar 2, 2022)

Sorry, didn't mean to sound condescending in the least. My apologies if I came across that way. 

I'm referring to conventional milling with a fly cutter in the same manner as used with an end mill. This is common practice, at least to my knowledge.


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## Alcap (Mar 2, 2022)

If I’m understanding ,it’s best to set the the diameter of the fly cutter to just go past each side ? On this picture I should have used a bigger one to be able to make just one pass ?


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## Parlo (Mar 2, 2022)

Alcap said:


> If I’m understanding ,it’s best to set the the diameter of the fly cutter to just go past each side ? On this picture I should have used a bigger one to be able to make just one pass ?


If you have to take a full width cut then yes, I would set the diameter of the cutter to just past the width of the part.
I'm not a fan of huge diameter flycutting especially if the tool extends well past the cutter body. It's prudent to use the largest holder you can, even if the tool is within the holder diameter. This prevents an invisible spinning toolbit and is far sturdier.


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## Jim F (Apr 13, 2022)

sorry if this sounds ignorant, but. the way I see a flycutter working, the tool is against the center shoulder, correct ?
My flycutters are pushing the tool away from center.
Am I wrong in my thinking ?


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## Parlo (Apr 14, 2022)

Jim F said:


> sorry if this sounds ignorant, but. the way I see a flycutter working, the tool is against the center shoulder, correct ?
> My flycutters are pushing the tool away from center.
> Am I wrong in my thinking ?


If your cutter body has a slot with one side on the centreline, this is the datum and the screws should push the tool against it. Tools with an insert usually have the insert tip at the same height as the top of the tool. So locating the top of the tool against the slot will bring the tip on the centre of the body.
Hope this helps.


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