# A Tale of Two Flycutters



## BobWarfield (Jan 22, 2008)

Fly cutters are great tools for the mill. They're relatively cheap, and they cut a nice wide swath. In fact, while I have a multiple insert face mill that I use when I need to remove a lot of material, I still return to the flycutter for some operations. Namely, when I have to face a big plate, I want the widest cutter possible, because that leaves me the best finish.

Recently, I was working on the table for my disc grinder. I'll publish pix when the grinder is done, or you can follow along on my site: http://www.cnccookbook.com/CCDiscSander.htm

What I wanted to pass along here is a little comparison work on two popular fly cutters:







I know you must have seen the one on the right, or its like. They seem to be the most common. I got the one on the left off eBay when I went looking for a really big flycutter for facing larger plates. You can make one two. I believe Widigitmaster has plans for one over on CNCZone somewhere. 

Here is a sample cut with the smaller flycutter for my grinder table:






It's not bad, but I hate that ugly line making two passes leaves.

Now here is the result from the larger flycutter:






That's a much happier result. It isn't as good as ground, and I won't claim a mirror finish, but it is flat, which is what I was looking for.

One more thing to notice for newcomers:






That's a closeup from the first flycutter. Note how you can see a Moire sort of pattern? The circular tool marks go both ways, indicating both sides of the plane of the cut were engaging. They're not quite even though, indicating the head of my mill was slightly out of tram (i.e. not square). The bigger the flycutter, the more sensitive it is to tram. Note on the pic from the big cutter (the one with the 1-2-3 block on the surface) you only see one set of marks? With the arm so long, the difference in cutting depths at the two sides of the cutting plane is now enough that you can clearly see the out of tram condition.

The tram hardly matters for this operation, but if I need to cut something accurately, I'll have to get out my Indicol and tram that head!

You'll also note some irregularity around the edges. That indicates flexure. Too much overhang from that vise. Ideally, I would have supported the plate at the ends with some machinist's jacks to help that situation. Again, it hardly matters for my grinder table, but I want to pass along such tips if you need more precision.

Cheers,

BW


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## BobWarfield (Jan 22, 2008)

One more pic. I like to run these cutters fast enough to generate "blue" chips:






That's too much for HSS steel cutters, but my flycutters use brazed carbide lathe tools. Blue is okay with carbide. For HSS, brown is about as much as you want to see.

Cheers,

BW


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## rake60 (Jan 22, 2008)

Nice Pics Bob!

Would you please add a little information as to the depth of cut and feeds you use 
when running your cutters.

A flycutter is great for covering a big surface.
For most machines they are not a tool you would want to subject to a heavy cut.
For my own homemade mini flycutter in my .030 depth of cut is a maximum.

The chips you have there look to be at least double that.
Of course that is not on a mini mill either.

What depth of cut can you get with those cutters without horsing your mill?

Great information!
Thanks for posting it here!!

Rick


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## BobWarfield (Jan 22, 2008)

Rick, there's a lot of magnification in the chip picture. In reality, I was cutting about 15 thousandths (0.015") max depth of cut as measured on my quill DRO. I drop back to 5-10 thousandths on the last pass in hopes of a little nicer finish. 

Spindle rpm for this job was 910 rpm. It ran fine at 1600 rpm, but was much noisier. Since I already had chips I liked, I backed off. Feedrate was about 1 turn a second while roughing and 1 turn every 1.5 seconds for finish. Given 100/1000's on the dial, that works out to a feedrate of 0.1"/sec = 6"/minute down to 0.067"/sec = 4"/minute.

You're quite correct on not hogging too much with a fly cutter. I can take down 0.040 to 0.080 with my multiple insert carbide face mill, no problems.

Best,

BW


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## mklotz (Jan 22, 2008)

When I made my mondo flycutter, I provided for two HSS tool bits 180 degrees apart. One of the bits is set 0.005" lower than the other and on a radius about 1/8" less than the other bit. That way I get a roughing and finish cut in a single pass.


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## Spin Doctor (Jan 23, 2008)

If I were the guy in charge every single flycutter on the face of the Earth would be parked right next to the Titanic. They are hard on mills due to the interupted cuts. Sure for aluminum and plastics they are not too much problem but for steels and bronzes they generate too much shock loads on the mill. They are hell on bearings, keys and keyways on shafts and motors. I dont know how many times when I was working in my old job I would go over to use a BP right after some moron was using a flycutter and sure as God made little green apples the head would be out of tram.

PS IIHMW most of the three jaw chucks would be in a pile right next to the fly cutters


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## mklotz (Jan 23, 2008)

Spin Doctor

I gather from your post (and others on the HSM board) that you are a professional machinist.

Please try to remember that many of the people on this board are amateurs who don't necessarily have the luxury of a fully equipped shop containing all the tools needed to avoid using fly cutters or 3jaw chucks, for that matter.

Rather than characterizing fly cutter users as "morons", perhaps you could suggest some alternate methods of putting a dead flat surface on a large workpiece in a shop where the only powered machine tools might be a small, light lathe and a mill/drill, which are the tools available in my shop.


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## Spin Doctor (Jan 23, 2008)

Fly cutters have their place. Unfortunately far too few machinists professional or otherwise keep them there. Light cuts are fine. Trying to take too much for the machine is not. And it doesn't matter if the mill is a Seig X1 or a Bridgeport. Remember common sense is usually a good way to go. IMO a far better solution is would be to either pick up one of the low buck insert face mills or build one. On low power machines the insert should have a fair amount of positive rake on the cutting edge. A flycutter can build up some pretty high SFM at the speeds most of the hobbiest mills will run at. A Grizzly 3103 at its lowest speed is still 240RPM or so. A 4" fly cutter is moving at 250SFM. Way too fast for most stuff besides Aluminum and plastics (and even the plastics can have problems if the chips don't get out of the way or there is not enough feed per revolution. What makes a multi insert cutter better is the fact that once you get deep enough into the cutting pass you always have at least one edge engaged in the cut putting a load on the spindle. This reduces the shock on the machine a lot. And right now I don't have a fully equiped shop at my disposal. I am putting together a home shop a little at a time and fully well realize the limitations that can place on the way you get things done. Now as to why I dislike 3 jaw chucks. Once they have been around for a while they will have been involved in a few upgehfuchs. The jaws will be slightly sprung. The work piece will not be held as securely. If your lathe has a 4 jaw don't be afraid of it. Indicating a piece in is not rocket science. It is simply a matter of practice. And even with rough stock if you have a little on the OD to play with using a 4 jaw to do work in the lathe will make things easier in that it is a lot harder to get the piece to move from machining pressures. Especially in small machines.


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## BobWarfield (Jan 23, 2008)

Spin Doctor  said:
			
		

> If I were the guy in charge every single flycutter on the face of the Earth would be parked right next to the Titanic. They are hard on mills due to the interupted cuts. Sure for aluminum and plastics they are not too much problem but for steels and bronzes they generate too much shock loads on the mill. They are hell on bearings, keys and keyways on shafts and motors. I dont know how many times when I was working in my old job I would go over to use a BP right after some moron was using a flycutter and sure as God made little green apples the head would be out of tram.
> 
> PS IIHMW most of the three jaw chucks would be in a pile right next to the fly cutters



LOL, you're funny Spin, but you're not going to get you way!

Both these tools (fly cutters and 3-jaw chucks) have their place, especially in the home shop. Fly cutters are the best way to get a decent finish on a big wide plate in the home shop. The multi-insert cutters (I have a couple and will write about them here too before long) are way better if you have much material to remove, but they just don't deliver the surface finish for areas wider than they cut in a single pass. Fly cutters are also very versatile. Boring heads are basically fly cutters, and form tools are another common type of fly cutter. 

You bring up a good thought on the SFM. Let's consider the case I was working. I was flycutting a surface 6" wide in a single pass. As I mention, I ran at 910 rpm. So, what was my SFM (Surface Feet per Minute)? It was about 1500. Whoa! That was too fast.

What should it have been? This was very mild steel, so HSS wants maybe 150 or 200 max SFM. My flycutter used brazed carbide, so figure max 3x that or maybe 600 CFM. The main impact of running to fast was more tool wear and poorer chip control. For optimal tool wear, I should have been running at more like 1/3 of what I was or 300 rpm. OTOH, the carbide tool was dirt cheap, and I was able to feed faster and still get the same finish. 

I used to have this fantastic feed and speed software that Michael Rainey puts out. Unfortunately, I lost it in a hard disc crash. I can get the software, but I left my password to unlock it in an Outlook archive that didn't get backed up. Doh! I back it up now, but I need to contact Michael and get another copy. It's really handy to have that sort of thing in the shop.

Cheers,

BW


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## shred (Jan 23, 2008)

BobWarfield  said:
			
		

> I used to have this fantastic feed and speed software that Michael Rainey puts out. Unfortunately, I lost it in a hard disc crash. I can get the software, but I left my password to unlock it in an Outlook archive that didn't get backed up. Doh! I back it up now, but I need to contact Michael and get another copy. It's really handy to have that sort of thing in the shop.


I use the AutoEditNC (free download) feeds-and-speeds calculator all the time. I derate a little for my machine, and would prefer it to have more materials, but it's always in the ballpark. What does the Rainey software do?


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## BobWarfield (Jan 23, 2008)

shred  said:
			
		

> I use the AutoEditNC (free download) feeds-and-speeds calculator all the time. I derate a little for my machine, and would prefer it to have more materials, but it's always in the ballpark. What does the Rainey software do?



Lots of stuff: http://www.cncci.com/products/mepro.htm

It has a nice materials table, but everything is table driven so you can customize it. He is a regular over on the PM boards and a lot of guys swore buy it so I got a copy. It was worth the price of admission!

Best,

BW


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## macona (Jan 25, 2008)

I have to agree with spin doctor on this one. Find a small (2-3") indexable cutter that uses the ADKT type insets. They can be found on ebay real cheap quite often. I have seen cutting heads go for about $40 and I picked up a whole bunch of inserts for something like $35. ADKT style inserts are great for low power machines and leave a darned near mirror finish.

You can also make your own from a circular chunk of aluminum. Make it the diameter you want and drill arount the perimeter and tap to hold in either regular HSS or carbide cutters. Use a granite slab to set the cutter height consistent between all the cutters.


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