# Fly Cutter RPMs, etc.



## devils4ever

I see there are a lot of other fly cutter threads going on at the moment, but I don't want to hijack them. So, I'll post a new one here. (Hope this it the right thing to do!)

In any case, I bought a fly cutter (with HSS bit) with a R8 shank from Micro-Mark that I have just started to use on my PM-25MV mill. I'm trying to to determine the proper RPM for this. I know I don't want to go too high since it's not balanced like an end mill.

I see a lot of equations for determining RPMs based on SFM and diameter such as: RPM = SFM X 3.82 / D. But, on a manual mill, how do I determine SFM?

Thanks.


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## John TV

So I am one of the fellows asking the fly cutter questions but I think this is the formula you start with ( see pic) then adjust from there. So if calculated rpm is say 200 then you multiply that times chip load per tooth, say .002 and multiply that times number of flutes in your cutter, say 4. 

So 200 x .002 x 4 = 1.6 inches per minute feed rate. Then adjust to machine, sound, experience ( which I don't have much of). If I'm wrong one of the folks with much more experience can correct me. Good luck. 
	

		
			
		

		
	




Then for manual feed, practice hand revolutions say one per second and see how far table travels in one minute. Adjust from there, faster/ slower to approximate speed. Just a thought
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## benmychree

On ordinary steel, cutting speed in FPM should be about 100 for HSS tools.  In high school shop, we were taught to round off the 3.82 to 4 to simplify the math and be able to do it in your head, so take the cutting speed in FPM, multiply it by 4, and divide that product by the diameter of the moving part.


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## devils4ever

Okay, so for steel, I would use: RPM = SFPM X 4 / Dflycutter = (100 ft/min) * 4 / 4" = 100 rpm

I'm testing with 6061 Aluminum which I think is about 1000 ft/min? This gives (1000 ft/min) * 4 /4" = 1000 rpm.

For a fly cutter with one cutter edge: Feed = 1000 rpm * 0.002" * 1 = 2"/min

My mill advances 0.100" per revolution. So, (2"/min) / (0.100"/rev) = 20 rev/min.
(20 rev/min) * (1 min/60 sec) = 0.333 rev/sec. Or, 3 sec/rev.

Sound correct?


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## mikey

The cutting speed varies with the tool type. It will be 2-3 times higher if using carbide. For aluminum when cutting with carbide you will almost always be running at max speed unless your tool is a foot long.


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## benmychree

So far as feed is concerned, I pretty much go by feel and intuition; I do not use fly cutters, but rather face mills and shell end mills.  The best thing to do is get a cutting speed/feed slide rule calculator, cutting tool companies give them away.


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## MrWhoopee

1000 sfm is appropriate for carbide in aluminum. 200-300 is much closer for HSS. Bookmark this table for a quick reference.





						Cutting Speeds for Turning and Threading with HSS Tool Bits | Smithy - Detroit Machine Tools
					

MATERIAL STRAIGHT TURNING SPEED THREADING SPEED Feet Per




					smithy.com


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## benmychree

MrWhoopee said:


> 1000 sfm is appropriate for carbide in aluminum. 200-300 is much closer for HSS. Bookmark this table for a quick reference.
> 
> 
> 
> 
> 
> Cutting Speeds for Turning and Threading with HSS Tool Bits | Smithy - Detroit Machine Tools
> 
> 
> MATERIAL STRAIGHT TURNING SPEED THREADING SPEED Feet Per
> 
> 
> 
> 
> smithy.com


Good practical data!  That and a slide rule calculator, and you have a easy to use practical method of determining cutting speeds. and RPMs.


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## higgite

MrWhoopee said:


> 1000 sfm is appropriate for carbide in aluminum. 200-300 is much closer for HSS. Bookmark this table for a quick reference.
> 
> 
> 
> 
> 
> Cutting Speeds for Turning and Threading with HSS Tool Bits | Smithy - Detroit Machine Tools
> 
> 
> MATERIAL STRAIGHT TURNING SPEED THREADING SPEED Feet Per
> 
> 
> 
> 
> smithy.com


The Smithy tables give vastly different SFMs for milling aluminum vs turning aluminum with HSS cutters. For turning they say 200-300 SFM, but for milling they say 400-1000 SFM. Shouldn’t they be the same? Or at least close? For other metals they list, the SFMs aren’t _exactly_ the same for turning vs milling, but they are in the same ballpark. Think Smithy made a typo or what?

Turning: https://smithy.com/machining-reference/lathe-turning/page/2
Milling: https://smithy.com/machining-reference/milling/page/12

Tom


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## MrWhoopee

higgite said:


> The Smithy tables give vastly different SFMs for milling aluminum vs turning aluminum with HSS cutters. For turning they say 200-300 SFM, but for milling they say 400-1000 SFM. Shouldn’t they be the same? Or at least close? For other metals they list, the SFMs aren’t _exactly_ the same for turning vs milling, but they are in the same ballpark. Think Smithy made a typo or what?
> 
> Turning: https://smithy.com/machining-reference/lathe-turning/page/2
> Milling: https://smithy.com/machining-reference/milling/page/12
> 
> Tom



Yes, the speeds should be the same regardless. I have no idea why they even have two different charts. It's all about the the relative speed between metal and cutting tool. That speed for milling aluminum is too high.


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## John TV

So your calculations sound correct but 4" seems like a fairly large fly cutter, do you need it that large, is there a lot of stick out? I would think your machine will tell you a lot after just a few seconds of cutting. I think if it were me I would start at about half that speed and experiment a bit. Also would try to keep stick out and depth of cut reasonable. Again, I'm so new to this I'm not sure that I am adding anything to this thread but it's still fun learning from all these members. 


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## devils4ever

Depending where I look, I see vastly different values for SFPM. 

I'm using a HSS cutter with minimum overhang. I'm guessing the diameter to be about 4", it might be a little less. 

I'll see if I can get a slide rule calculator. That might help a lot. Plus, I'll try different RPMs and see how it works out. I'm assuming I don't want to go above 1500 or so RPMs? Is it safe to go higher on my PM-25MV mill?


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## MrWhoopee

I tend to be a little more conservative than John (at least with HSS), but still in the same range. Using 80 SFM for mild steel, (80x4)/4= 80 rpm. In aluminum, (200x4)/4=200 rpm. In no case will you even get close to 1500 rpm with HSS and a 4 in. flycutter. Unless the cutter is balanced, I suspect that the vibration would become unnerving before you hit 1500 regardless. Cutting speed is not the only limiting factor, it's just the last one when vibration and rigidity permit higher speeds.

Once you start to get the feel for this stuff, you will do these calculations very quickly in you head. Cutting speeds are not hard & fast numbers, they just get you into  the right range to prevent destroying the cutting edge on the initial cut. Start low and work your way up. If you're running production on a CNC, you want to minimize cycle time which requires maximizing stock removal. As a hobbyist who is paying for his own tooling and not charging by the hour, other factors are more important.


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## Driveslayer45

I have recently been using a fly cutter for various projects. the most recent flattening 1.75" wide stainless, 304 maybe. i am using a soldered carbide cutter about 1.5" diameter. i found i had to take deep cuts or my carbide would get dull quickly.  running at ~700 rpm, not sure of the feed rate, but cuts at .015 would dull cutter, cuts .040-.050 worked great with beautiful curled chips. i am running right down the center and my cutter is taking a full width cut.

mill is a  pm727v


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## MrWhoopee

Driveslayer45 said:


> I have recently been using a fly cutter for various projects. the most recent flattening 1.75" wide stainless, 304 maybe. i am using a soldered carbide cutter about 1.5" diameter. i found i had to take deep cuts or my carbide would get dull quickly.  running at ~700 rpm, not sure of the feed rate, but cuts at .015 would dull cutter, cuts .040-.050 worked great with beautiful curled chips. i am running right down the center and my cutter is taking a full width cut.
> 
> mill is a  pm727v



I would say that 700 rpm is at the very high end of what you can expect from a brazed carbide (USA or China?) in 304, but since it holds up with a heavier depth of cut, that's not the problem. My guess would be that at .015 DOC the cutter is doing more rubbing than cutting and the material is work hardening.


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