# Horizontal fly cutter



## dewbane (Aug 29, 2022)

I'm going to make wheels for a clock soon, so I turned my attention to how I was going to hold the tool, and what kind of tool I was going to use. Some guys use round stock turned on a lathe. It's easy to get a symmetrical profile on both cutting edges, but you take a lot of meat off the tool, and it's harder to align a round tool. I decided to go with a square tool that's way oversized for the tooth profile I'm going to cut, and I made a big, heavy holder for it. The disadvantage of going so big is the tool will take a long time to go around for another swipe at the part, but I'm hoping the extra mass compared to most designs I've seen will make for a smoother cutting tool. I haven't ground the tool bit yet, so I haven't tested this. I did spin it up to 2400 RPM in the lathe, and I have all my teeth. Well, all but one of my teeth, but that's an unrelated story. 

I started with some rusty 2" steel in the 3-jaw:


I was shooting for 0.750" on the shank, but I mess up the math somewhere and took too much off, so I moved down to 11/16".



It was at this point that I gave up on my rotary table chuck. I indicated the top of the part, and it was way off. I indicated the top of the chuck, and it was also way off. Bad mounting plate, bad mount, not sure. I ditched it all for an ER32 collet holder, but here the part was in the chuck:



I used my new DRO to get back to the center of the rotary table a couple weeks since the last time I used it. Then I indicated the OD of the part. It was off by about 0.005" so I indicated the collet holder, which was only off by about 0.0005". I'd say the 0 point was still the 0 point! It's good to know I can trust my DRO. So then I calculated where to move the Y axis so the edge that would present the cutter would be exactly on the centerline. Not pictured, I proceeded to completely screw up the part, because I didn't lock the rotary table down. Genius! I'm a genius! So I took it to the lathe and trimmed off about 1/4".



For once I did something right. I moved back 0.020" and made another pass. I was aiming for about a piece of paper slop in the slot, to be tightened up with set screws, and I got it.



I mounted the part in a 5C collet block, which reminded me why 5C collet blocks suck. If I had ER collet blocks, I could make all the adjustments on the outside of the vise jaws.



As BlondieHacks says, "Tappy tap tap." I ran the taps in with the spindle set at the lowest possible speed, then hand tapped to finish.



I took the time to stamp my initials and the date. I also hand filed all the sharp edges. This one's going to be a keeper. Even if it doesn't work out for cutting wheels, it will be useful for something. First a dunk in Purple Power.



Next, acetone, which I lit on fire to burn off the residue while heating the part for a dunk in Van's gun blue. I used stainless steel set screws because I wanted this thing to be pretty. Et voilà:


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## homebrewed (Aug 30, 2022)

If that piece of metal held in your fly cutter is representative of what you're going to use as a bit (prior to being ground), I have to confess that I don't understand how this version works.  Is the bar is used to hold a smaller bit that sticks out from the bottom?  Sort of like this?  Essentially, a trepanning tool that is moved across the work rather than cutting a circular channel.

If not, I'd love to be educated.  It's a good day when I learn something new!


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## markba633csi (Aug 30, 2022)

I'm a bit fuzzy also- does a fly cutter for clock-making differ from one used to mill large areas flat?


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## Cadillac (Aug 30, 2022)

Not to come of harsh you did a great job. Might wanna check the center on the tool. If the tool is on center it would be at the high spot which from the pictures it’s .030 or more behind.  A tip have your retaining screws come in from the front of tool so the forces of the cut are bearing on the slot not the screws.


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## jwmelvin (Aug 30, 2022)

I believe this is for gear cutting, using a tool that is the inverse of the tooth profile. In such a case it’s easier to have the tool perpendicular to the rotation axis as opposed to the usual fly cutter orientation.


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## jwmelvin (Aug 30, 2022)

Cadillac said:


> A tip have your retaining screws come in from the front of tool so the forces of the cut are bearing on the slot not the screws.



I know people repeat this a lot but I’m pretty sure it’s misplaced concern. The screws preload against the slot and the tool has the stiffness of the slot unless the preload is overcome. Given how much force a screw can exert, it seems implausible that the screw preload is overcome by cutting forces.


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## homebrewed (Aug 31, 2022)

If it's really meant for cutting gears I think it's a stretch to call it a fly cutter.  Near the end of this blog post there's a photo of a similar-looking gear cutter, so maybe it is.


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## Cadillac (Aug 31, 2022)

Yes it’s for gears after my response I went back and reread his post. Center line will still matter on a direct reading. When he infeeds he will actually be short on his movement. Which might be good so theirs no overshooting your numbers?


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## dewbane (Sep 1, 2022)

homebrewed said:


> If it's really meant for cutting gears I think it's a stretch to call it a fly cutter.  Near the end of this blog post there's a photo of a similar-looking gear cutter, so maybe it is.


John Wilding calls it a fly cutter. It seems to be a well-established name for a single point tooth cutter.



Cadillac said:


> Not to come of harsh you did a great job. Might wanna check the center on the tool. If the tool is on center it would be at the high spot which from the pictures it’s .030 or more behind.  A tip have your retaining screws come in from the front of tool so the forces of the cut are bearing on the slot not the screws.


I dialed the DRO to the center of the table, plus half the tool diameter. By eyeball it "looks about right" and I don't see the 0.030" error you're talking about. It's entirely within the realm of possibility that I'm stupid, and I missed something. I copied the design of one of the usual angled fly cutters for surface finishing, so I didn't give any thought to which side to put the screws on. I just made the same kind of thing in a flat version, sized to accept my tool bit of choice (1/4" square O1). The tool bit is massively overbuilt relative to the profile I'll be cutting when I get it shaped. It ends up being only something like 0.051" across at the tip. I was trying to have a massively well-supported edge since it has to make about 60 million passes.

This cutter is probably eight to ten times bigger than the ones clockmakers normally use, and it might turn to be that making it so large was stupid. It will be massively well-supported, and have a lot of momentum for smooth running, but it will take an eternity between those 60 million nibbles. It's going to be excruciatingly slow, which I can tolerate if it cuts well-formed teeth and doesn't break halfway through making a gear.


Cadillac said:


> Yes it’s for gears after my response I went back and reread his post. Center line will still matter on a direct reading. When he infeeds he will actually be short on his movement. Which might be good so theirs no overshooting your numbers?


What am I not seeing? Why do you guys think my center line is off so much?

I sanity checked a few things. Drill bits in a chuck in the spindle spin clockwise, viewed from the top, in forward. The end of my bit that's sticking out the most is indeed supported by screws, not slot. That's just the design I copied. I measured the diameter at the slot at 1.923" and roughly perpendicular to that axis I got 1.928", so I guess 0.005" of the high spot got shaved off, and the bearing surface of the slot could be that far off center. If that's the case, I can shim that out easily enough. I'm currently chasing a 0.005" run-out problem with my lathe that could also explain this discrepancy.

Or maybe I'm missing your point entirely. Okay, time to put on my big boy pants. I made myself late to work messing around with this when I did. Should have saved the forum for after work.


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## Marquez (Sep 1, 2022)

dewbane
Quick question... I'm new here...​What is Van's Purple Power and Van's Gun Blue?

I am appreciative of you showing your craft and the steps on getting there. I joined this site to learn more about metal finishing, especially with regards to what can be accomplished at the home.

Thank you


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## homebrewed (Sep 2, 2022)

dewbane said:


> John Wilding calls it a fly cutter. It seems to be a well-established name for a single point tooth cutter.


There you go -- I did learn something new!


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## Dabbler (Sep 2, 2022)

@dewbane Your cutter holder looks great to me!  No it is not off centre in my viewing.


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## dewbane (Sep 3, 2022)

homebrewed said:


> There you go -- I did learn something new!


It seems like "fly cutter" is just one of those annoyingly vague terms. Another one that comes to mind is "bill." A bill can be a piece of paper telling you how much you owe, it can be a proposed piece of legislation, it can be the brim of a baseball cap, it can be a duck's beak, it can be a piece of paper money, and a bunch of other stuff.

So at least I didn't make a bill for my lathe. Then nobody would possibly be able to guess what it was for!


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## Flyinfool (Sep 3, 2022)

Int the one photo before bluing there is a drop of water near center that makes it look like the slot is way off center. If you zoom way in you can see that the slot is only a couple thou off center and it is off on the good side. I would much rather be shy of center than over center. 

If you think about the forces during cutting it does not matter which side the set screws are on. The commercial fly cutters all have the set screws on the side with less material to drill through. It saves time and money in the drilling and tapping operations. In use there will be near equal force on the screw at one end of the cutter and the other end will be forced against the  tool body.

If you are having a hard time visualizing this make up something with a slot that is twice as wide as the cutter and apply pressure in the direction of cut to see where the tool moves in the slot. it will turn until it is touching the opposite corners at each end of the slot. now if you put in a set screw to hold down the one end the force will be on the setscrew and the far side of the slot. So it does not really matter which side the screws are in, the thin side is just easier to do.

I know you want pretty, but Stainless hardware is pretty soft and may not hold as well as a steel setscrew. The SS screw tip will flatten under the tightening load. Stainless fasteners also have the nasty habit of thread galling where the screw threads literally weld themselvs to the hole threads. I only use SS fasteners when absolutly needed for some special situation like a very corrosive environment.


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