# Fly cutter fab...before I jump in.



## GunsOfNavarone (Nov 7, 2018)

I haven't started yet, i am posting in this category as I am sure many of you have already made one. I've done a lot of searching in regards to cutter angle on the fly cutter, how big of a circle should i shoot for being able to surface in one pass? Im sure it will come up, this is for a PM 737 (1 hp) seems like blade should be 30 to 60 degrees in relation to material. 
There are so many different designs, I just dont know what is practical with my machine's capabilities (or mine for that matter) I like the style where a bar passes through the flycutter and by adjusting the bar in/out, you greatly change the diameter of the cut.I
Getting into the details about actual cutter and all its angles is a whole other Oprah...I will be going HSS over indexable carbide. 
Anyway, any incite into this project before I jump in would be greatly appreciated.


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## tjb (Nov 7, 2018)

Actually, the timing on your post is interesting.  I recently studied a thread started by Nel957 on building a fly cutter.  Last week, I made one based on a response in that thread - a two-part video posted by jdedmon91.  I had an insert holder that was getting next to no use, so his video was quite helpful.  Here's a link to that thread:

https://www.hobby-machinist.com/threads/one-mow-flycutter.68247/#post-618186

The cutter works very satisfactorily, but there was one feature of the build that I modified this morning.  If all goes well, I plan on posting some photos tomorrow, along with an APB-style question on my modification.  Not exactly sure where/how I'll post that, but I'll try to forward a link to you when I do.

Regards,
Terry


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## GunsOfNavarone (Nov 7, 2018)

Yup, I watched Jim's two part videos. My issues are, i am not nearly as skilled as Jim is. There are a lot of "whoa! Slow down there buddy, you lost me!" Someone in that same thread also suggested it. ..perhaps a bit more cutter angle. I'll take a look at your thread too.  It would be super easy to just buy one, you can get deals that almost equal what it will cost me in materials, but I'll never learn that way.


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## tjb (Nov 8, 2018)

GunsOfNavarone said:


> Yup, I watched Jim's two part videos. My issues are, i am not nearly as skilled as Jim is. There are a lot of "whoa! Slow down there buddy, you lost me!" Someone in that same thread also suggested it. ..perhaps a bit more cutter angle. I'll take a look at your thread too.  It would be super easy to just buy one, you can get deals that almost equal what it will cost me in materials, but I'll never learn that way.


Actually, 'a bit more cutter angle' is the focus of my modification.  The cutter works great, but I could see after the first use a little more angle would have been an advantage.  Gotta run for now, but will post more on how I modified later today (hopefully).

Also, the cutter is not that complicated to make.  The tool holder I used is 5/8" square, so using a 5/8" end mill all but eliminated guessing on milling the slot.  The biggest challenge was turning the shaft.  That had to be fairly precise, so it was slow-going, but it turned out okay.

More later.

Regards,
Terry


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## BaronJ (Nov 8, 2018)

Hi Guys,

Fly cutters are easy to make and don't require any great precision !

I've made many, some for very specific jobs such as cutting dovetails and grooving.
here are some pictures of mine.





This one was a 20 mm diameter shaft, a 75 mm by 20 mm thick disc.  using a piece of 6 mm square HSS M42, held by a single grubscrew.  The shaft is a press fit into the disc.

Less than an hour to make.  



Unfortunately a poor picture of the tool bit cross section.  But it gives an idea of the cutting edge.
This was used to surface a mild steel plate.





On this one, again using a 20 mm shaft, but threaded, because I thought that the side forces with an intermittent interrupted cut might cause the shaft to loosen.

Again using 8 mm square M42 HSS.  I made this to put a slot in the end of a mild steel bar.
The cutter held by two M4 hex grub screws.


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## Meta Key (Nov 8, 2018)

The other day I was scrounging around looking for something and discovered a NOS Jacobs chuck (made in Sheffield England) so I was about to order an MT2 to 1/2" x 20 threaded arbor to match it to my mill.  Now, I think I'll order a few of those arbors so I can throw together a special purpose fly cutter on a whim.

@BaronJ, thanks for the inspiration!


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## BaronJ (Nov 8, 2018)

Hi Meta key,

You might be better off with blank end arbors, if you are going to put a disc fly cutter on one.
The blank ends are soft and turn and thread easily.

I have several MT2 ones that I've made various tools from.


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## Meta Key (Nov 8, 2018)

BaronJ said:


> You might be better off with blank end arbors, if you are going to put a disc fly cutter on one. The blank ends are soft and turn and thread easily.
> I have several MT2 ones that I've made various tools from.



OK, then I think I'll just get a few of each.  They're so inexpensive there's no reason not to have a few laying around!

I really like your fly cutters..  Good thinking.


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## tjb (Nov 8, 2018)

Here are some photos of the fly cutter I made last week.  As mentioned in a previous post on this thread, I based it on a 2-part video by jdedmon91 (Jim) on a thread started by Nel957 (link cited above).  I started with a piece of scrap 3" solid round and milled it down pretty much according to Jim's video.  About 2" of the total length is milled down to 7/8" diameter for the shaft, and the remaining 1" is milled to 2.75" diameter for the cutter.  (The little hole in the shaft was in the scrap I started with.  The shaft is too long, and when I mill it down, that hole will be gone.)

You'll notice there are three set screws to secure the tool holder as described in Jim's video.  The cutter works great, and I'm very pleased with the results.  However, after using it a little, it occurred to me that it probably would have been better for the slot to be set at an angle instead of at parallel to the face of the cutter.  I contemplated making another one, but then I got the idea to put a set screw on the insert end to produce an adjustable angle.  It's a little difficult to tell from the photos, but that's what's on the back face of the cutter.  You can tell (a little) from the last photo that the set screw acts as a stop on the tool holder and is adjusted to  produce a very slight angle.  The beauty of the set screw is that it allows the holder to be set at any desired angle within a given range.  I used it as you see it, and it worked fine like a charm.

Here's a question.  Do any of our seasoned veterans see anything wrong or unsafe with this approach?  Obviously, I wouldn't be inclined to set too steep of an angle, but for a slight amount, it seems to work.

Thanks for any comments.

Regards,
Terry

P.S.:  Guns, it's not that hard to make.  And if you mess up, you're only out a small amount stock.


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## tjb (Nov 8, 2018)

Forgot to mention something:  All four set screws in my post above are 1/4" - 28.

Regards,
Terry


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## BaronJ (Nov 8, 2018)

Hi Terry,

There is nothing fundamentally wrong at all !  I wouldn't bother setting it up to adjust the cutter angle, simply because you would have to rotate the insert much than you are able, in order to exceed the nose radius.

The only other comment I would make is that in order to take advantage of carbide, you would have to run the cutter at a high speed.  This is where the disc type fly cutter wins hands down !
There is almost no out of balance component and the flywheel effect reduces any hammer that occurs with an intermittent cut.

The out of balance issue is one that occurs when using a boring head as a fly cutter.


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## NEL957 (Nov 8, 2018)

Fly cutters are one of the simplest tool to make but in some of the pics I've seen things that are not right. But with that said, there is one cutter that caught my eye as a more than good. I have tried the carbides and I will not waste my time or money on them. I feel the cutter to use is HSS and the best would be a cobalt cutter. The best thing about them is you pull the cutter and stone it a little bit and you are ready with a sharper smoother cut.
I am surprised how long this thread has gone and I love all the input.
Thank all of you.
Nelson


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## BaronJ (Nov 8, 2018)

Hi Nelson,

Come on, don't keep us all in suspense !
I'm here to learn and in the process, hopefully impart some knowledge to others.


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## TomS (Nov 8, 2018)

I know you are looking to make a fly cutter with an angle mounted tool slot.  I made a couple fly cutters without the angle.  Thought you might be interested.

This one is 3" in diameter and can use 2 tool bits.  The shank is 3/4" because that's the largest collet I have.  I run it at about 2600 rpm and 10 IPM.






Here's my 1-1/4" fly cutter.  I run this one at 5000 rpm and 22 to 25 IPM.  Shank is 5/8".


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## GunsOfNavarone (Nov 8, 2018)

So tell me, i am seeing mostly (or all) fly cuttters here, very "flat". What are the advantages to say something like these Grizzly fly cutters?
Griz fly cutter


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## rwm (Nov 8, 2018)

I have the Grizzly set as well as a few of the type TJB posted. Practically, they all work the same.
The cutting geometry is pretty similar.



Robert


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## rwm (Nov 8, 2018)

Double post sorry


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## TomS (Nov 8, 2018)

GunsOfNavarone said:


> So tell me, i am seeing mostly (or all) fly cuttters here, very "flat". What are the advantages to say something like these Grizzly fly cutters?
> Griz fly cutter



+1 on what RWM said.  The flat style fly cutters are a bit easier to make as they are mostly simple lathe turning operations, mill a slot for the tool and drill and tap for set screws.  Machining the angle is another step in the process.  Not a big deal, just another step.


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## mikey (Nov 9, 2018)

Just sticking my nose in here to drop an opinion. I like HSS for most things but for fly cutting and boring, I prefer inserted carbide. There is nothing more frustrating than to see the finish change when the HSS tool goes dull part way through the cut and I've seen this enough that I stopped using it. If you must use HSS, I suggest M2 HSS, which is far more impact-resistant than cobalt. I also suggest a nose radius no larger than 1/32" if you want tool life to be reasonable. If you can, increase back rake enough to put the cutting load near the tip and add enough side rake to minimize cutting loads (12-15 degrees is usually enough).

I think a good fly cutter has to do two things: It has to make stuff flat with a fine finish and it should ideally be able to turn a shoulder or ledge.

In the former case, I think a positive rake cutter is best. Yeah, a negative rake cutter will transfer more heat to the chip but it doesn't finish as well as a positive rake cutter and it needs more horsepower and rigidity. Positive rake cutters use less power and finish finer at speeds and feeds most hobby mills can run at.

In order to be able to fly cut a ledge, the tool holder has to have zero rake geometry. That is, the cutting edge that cuts the ledge has to be vertical. One company that makes such a fly cutter is Kristi Tool, the maker of the B-52 fly cutter:




See that vertical face? That is what cuts a ledge, while the nose radius determines the finish. This thing can take a 0.100" deep cut at 6, 000 rpm! There are LH turning and boring tools with this exact geometry; just have to find it.

I don't own the B-52 ... yet. I do own a Tormach Superfly and that thing can take a 0.075" deep cut in aluminum at 2400 rpm without even slowing the mill down and it finishes fairly well. BUT it cannot cut a ledge so it is not ideal for my needs. I have adapted a Sherline inserted carbide fly cutter (it is essentially a single-insert little face mill) for use on my larger mill and it will cut a near mirror finish AND it will cut ledges. The only problem is that it is only 1-1/8" in diameter so I need to make a bigger one sometime soon.

Nothing wrong with angled fly cutters or HSS, and nothing wrong with the flat planer-type fly cutters either, especially if all you want is to make a surface flat. I've just found that inserted carbide will outlast and outperform a HSS tool easily in a fly cutter so that is what I would suggest.

Food for thought.


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## tjb (Nov 9, 2018)

GunsOfNavarone said:


> So tell me, i am seeing mostly (or all) fly cuttters here, very "flat". What are the advantages to say something like these Grizzly fly cutters?
> Griz fly cutter


Guns,

When I did the practice cuts with my fly cutter, I noticed a curious result.  I got a beautiful finish when moving the table from right to left (feeding from the right).  But when feeding from the left, it was obvious there was an ever-so-slight second engagement of the cutting tip when exiting the work piece.  That's when I realized that if the mill is not perfectly indicated, the cutting edge is going to engage the work piece when it 'enters' the cut and also when it 'exits' the cut.  Visualize it this way.  If you're standing in front of the mill, in a perfect world, the head is going to be exactly perpendicular to the table - 12 o'clock straight up-and-down.  But if it's off ever so slightly - say one second BEFORE 12 o-clock - the fly cutter will engage deeper on the left side of the cut than the right.  So if you're feeding right to left, the back of the cut is 'higher' than the front, so you won't notice any distortion.  But if you're feeding left to right, the cutter is going to engage the work piece twice: both when entering and exiting.  (Obviously, that same phenomenon is occurring when using an end mill, but because of the larger sweep, it's only apparent when using a fly cutter.)

That's when it occurred to me, fly cutters seem to be fabricated according to one of two designs:
1.  There is a single 'tooth' that lays PERPENDICULAR to the body of the fly cutter, and it is the only point which engages the work surface.
2.  There is a tool 'bar' of some sort - a tool holder with an insert, or a HSS blank - either of which is laid PARALLEL to the body of the fly cutter.
Peruse through the pictures on this thread, and you'll see several examples of both.

In Type 1 fly cutters, there is no possibility for more than a single point of engagement between the cutting tool and the work piece.  So even if the mill is not dead-perfectly indicated, the tool will do its job.  (Obviously, in theory there will be some distortion, but unless the mill is way out of alignment, I would think it would be negligible.  Thoughts from our experts?)

As noted above, however, Type 2 fly cutters have the potential to engage the work piece twice IF a. the fly cutter is milled exactly flat, AND b. the mill is even microscopically out of alignment.  The solution to this potential problem seems to be to fabricate the cutter in such a way that the cutting tip enters the work at an exaggerated angle.  This can be accomplished in one of two ways:  Either by milling the 'face' of the cutter at an angle (which appears to be the way most commercially available cutters using a HSS blank are offered, like the Grizzly set you mentioned), or milling the 'slot' at an angle.  Since I milled mine with both a flat face and slot, and the mill is evidently not dead-on indicated, an adjustment needed to be made.  Hence, my solution of adding the set screw.  (Just for the record, my mill is indicated but obviously not quite as 'true' as I thought.  My guess is that may be the case for most hobby applications.  Experts' thoughts?)

Bottom line is that regardless of design, ANY fly cutter should be constructed in such a way that the cutting tip only engages the work piece once per revolution.  A properly constructed Type 1 or Type 2 will accomplish that.

Comments experts?

Regards,
Terry


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## BaronJ (Nov 9, 2018)

Hi Terry,



tjb said:


> Bottom line is that regardless of design, ANY fly cutter should be constructed in such a way that the cutting tip only engages the work piece once per revolution. A properly constructed Type 1 or Type 2 will accomplish that.



If you think about it, The single cutting edge of any rotating tool has to be absolutely perfectly parallel to the work surface in order not to make a second cut.  Since this is almost impossible to achieve, particularly on a hobby machine, there will always be two cuts 180 degrees apart unless the machine head is canted, ie out of tram.



If you look at the cutter path at the bottom, the red line shows the path the cutter takes if the mill head is out of tram.  The cutter will clear the work when moving one way and take a second cut when moving the other way.

This applies to any rotating tool.


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## NEL957 (Nov 9, 2018)

Baron
You know it is your tool. This tool is shaped like a thumb nail and is the best finish possible. There is one advantage to a machine trammed off center will not produce the crosshatch marking.
Very nice cutters
Nelson

View attachment 279324
View attachment 279325
View attachment 279326

This one was a 20 mm diameter shaft, a 75 mm by 20 mm thick disc.  using a piece of 6 mm square HSS M42, held by a single grubscrew.  The shaft is a press fit into the disc.

Less than an hour to make. 

View attachment 279327

[/QUOTE]


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## GunsOfNavarone (Nov 9, 2018)

Well, I'm glad I asked. There is a wealth of good information here. Sometimes its better to just jump in as i would have knocked SOMETHING out, but probably learned quickly what i need to do differently. At least I have some ideas now of where to start.
Cheers!


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## BaronJ (Nov 10, 2018)

Hi Nelson,

Thank you very much for your kind words.  
I've learned a lot from the members of this and other forums !  I hope that I can give a little back.


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