# 3/4" Straight Shaft Fly Cutter



## PurpLev (Aug 9, 2012)

I wanted to have a fly cutter for some time now to get better surface finish than using a 1/2" end mill :thinking:.

I turned this part quite a while ago, just never got to continue work on it and mill it to spec until now:



Its made of 1045 and I'm considering hardening it if I can find a local shop that might do that for me. the knurl could have been better but it does the job of adding holding power to get this in and out of the R8 collet that holds it in place:



The fly cutter has a relief on the top which allows it to clear the R8 collet and make full contact with the mill for extra rigidity. It is utilizing a 1/4"x1/4" HSS cutter that I still need to sharpen, the attack angle is shallower than other fly cutters I've seen which have a very steep angle to the cutter, but I'm planning on sharpening this HSS cutter specifically for this purpose and will accommodate the relief and cutting faces accordingly. since this is for finishing, I didn't see a need to have a steep angle - unless I plan on using off-the-shelf bits which I'm not (at the moment at least :whistle:




All in all, a fun project, and can't wait to get to use it already.


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## pjf134 (Aug 10, 2012)

I made one for the Sherline mill and used 10 degrees and it worked out very well the first time out. I did some fancy grinding on the bit and wanted to see how it would work out and it did just fine so I left it like it was without any changes. I used a course wheel and should use a finer wheel, but did not want to put one on the grinder then.
Paul


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## churchjw (Aug 10, 2012)

Nice job on the cutter.  Post your first cut with it would love to see how it does.  Good idea putting the knurl on it.  You can always turn your first attempt off if you don't like it and try again.  


Jeff


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## November X-ray (Aug 10, 2012)

May I ask what the advantage to hardening it will be?


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## PurpLev (Aug 10, 2012)

pjf134 said:


> I made one for the Sherline mill and used 10 degrees and it worked out very well the first time out.



good to know - that's a very nice finish you got there on the part.



churchjw said:


> Nice job on the cutter.  Post your first cut with it would love to see how it does.  Good idea putting the knurl on it.  You can always turn your first attempt off if you don't like it and try again.


Thanks - I will post updates after I grind the toolbit and take some cuts with it - I am working on a tangential tool holder for the lathe and this will be used to finish off the faces... hopefully soon



November X-ray said:


> May I ask what the advantage to hardening it will be?


to resist any long term deflection/bending/malforming of the fly cutter over time from having to resist the cutting forces and provide more stiffness that is transferred to the cutter during the cuts.


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## November X-ray (Aug 10, 2012)

Thanks for the pictures, please let us know how it works when you get the chance to use it!


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## PurpLev (Aug 15, 2012)

I got a chance to sharpen the bit and took a test cut:




you can see the difference between the previous end mill cutter finish and the fly cutter - definitely an improvement.

here is another shot of the final face finished with the fly cutter compared to the adjacent face finished with the end mill:



having the fly cutter face cut at a shallow angle brings it very close to the part to be finished, but for a finishing cut that should be OK most of the times. another thing is that the cutter needs to be sharpened accordingly as regular cutters with a 10degree offset on the cutting edge won't be able to reach the surface of the part to be finished, but with tooling sharpened appropriately it seems to be performing very well - I am very pleased with the new finishing possibilities.


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## November X-ray (Aug 15, 2012)

The finish looks great, care to post a few pictures of the tools special grind?


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## PurpLev (Aug 15, 2012)

November X-ray said:


> The finish looks great, care to post a few pictures of the tools special grind?



:thinking: I guess that would have been a good picture to take as well huh?

I'll snap a pic when I get home later and post it. it's ground like any other lathe bits with the exception that the front of the leading edge is kept straight and not angled in (similar to a straight bit on the leading edge).


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## Rbeckett (Aug 15, 2012)

Yes please, a picture would really help of the cut your using on the tool.  The shallow angle looks like it does a great job too.  It's funny becaues LMS just put their three piece flycutter set on sale again.  Build or Buy?...... eh what the heck, BOTH!!!!
Thanks,
Bob


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## 8ntsane (Aug 15, 2012)

Yep, Home built tooling is allways a good thing to see. Nice work Sharon


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## jgedde (Aug 15, 2012)

Sharon,

The stiffness won't increase with hardening (I know it's counterintuative, right?).  This is because Young's modulus is fixed.  Wear and tear will be reduced however.  

If it were my project, I would just leave it annealled.

John


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## Charley Davidson (Aug 15, 2012)

Very nice job, I thought a guy was gonna give me one he made in trade school that was made with R8 taper but I guess he changed his mind:slapping:


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## jgedde (Aug 15, 2012)

Yes.  Very nice job indeed.  Forgot to say that in my last post.

Is that just a long bit in there or does the bit protruding from both sides have a use?  Do I need one too?  :thinking:

Also, my fly cutter set (Chinese) has a much steeper bit angle.  I've been told before that many machinists prefer a shallower angle like yours.  What're the advantages/disadvantages of the different angles?

John


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## pjf134 (Aug 16, 2012)

John,
  I used 10 deg. because it was stuck in my mind for some reason, maybe when I worked in a machine shop. I figure if I did not like it  I could increase the angle. The cutter bit I was just playing around in my head with a grind and thought I would try it out and after I put it in the holder it looked like I made a mistake and decided to try it anyway the way it was and it worked better than I thought it would so I left it like that. I tried to take a pic of the cutter to see how I cut it and this pic was the best.
 Paul


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## PurpLev (Aug 16, 2012)

Thanks for the replies everyone.



jgedde said:


> Is that just a long bit in there or does the bit protruding from both sides have a use?  Do I need one too?  :thinking:
> 
> Also, my fly cutter set (Chinese) has a much steeper bit angle.  I've been told before that many machinists prefer a shallower angle like yours.  What're the advantages/disadvantages of the different angles?
> 
> John



*John* - that IS a long bit (4" I believe) - I got a few as they were darn cheap and I planned to split them down to shorter bits for use in boring bars and other applications. As for the angle of attack, as I mentioned- mine is a shallow angle of about 12 degrees. I chose the angle from how the edges on the cutting tools are cut (12-15 degrees). I'm not sure if there really is any advantage to this other than the tool having less variation in thickness all around in terms of centrifugal forces and more stable operation. the disadvantage is that the tool is REALLY close to the work piece. I'm planning on doing a dry run over the part before finishing to make sure nothing will snag/hit the cutter more than it should. all in all, It's a finishing tool - as long as it does what it needs to do, I wouldn't over think it. the decision here for the angle was more of a choice than a necessity.

And...

as promised, here are pictures of the ground tool bit:

front view showing the rounding of the cutting edge



side view showing relief cuts (face and back):


And one more for good measure:


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## Turbinedoctor (Sep 3, 2012)

I have been asking myself if I needed a fly cutter, after seeing the finish over the end mill I may have decided that I do need one. Next question is to build or buy. I am thinking build now afer reading this. 

Job well done!!

Turbo


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## PurpLev (Sep 3, 2012)

Turbinedoctor said:


> I have been asking myself if I needed a fly cutter, after seeing the finish over the end mill I may have decided that I do need one. Next question is to build or buy. I am thinking build now afer reading this.
> 
> Job well done!!
> 
> Turbo



Thanks.

I must say that the finish is by far superior to facing with end mills, you should definitely have one, build or bought.


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## Hawkeye (Sep 3, 2012)

I got a 4" shop-made fly-cutter with my Victoria mill. One piece heavy body with an MT3 shank turned on it. Your results look so good I just made a note to pick up a piece of 5/8" HSS to go in it.

Nice tool and nice results, Sharon.


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## PurpLev (Sep 3, 2012)

Hawkeye said:


> I got a 4" shop-made fly-cutter with my Victoria mill. One piece heavy body with an MT3 shank turned on it. Your results look so good I just made a note to pick up a piece of 5/8" HSS to go in it.
> 
> Nice tool and nice results, Sharon.



Thanks Mike, curious to see what the 4" FC looks like. got pics?


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## Hawkeye (Sep 3, 2012)

As you wish, my friend.

The body is about 1 1/2" thick, so I expect it to be pretty stable. As mentioned, the slot is 5/8". I'm planning on adding a setscrew at the cutting end to set the angle of the bit. He cut the slot with no angle.

The parallax seems to take the taper out and make the shaft look straight. It is actually an MT3.

I sure miss the ability to put the photos where I want them. It was so nice to have the caption above the referenced picture. Can't even see them in the post as you write it any more.


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## PurpLev (Sep 3, 2012)

Thanks! thats a big one alright. surprised the slot was milled parallel to the top and not angled. was it ever used this way?


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## Hawkeye (Sep 3, 2012)

Two of the setscrews show a bit of use, but the one closest to the cutting end of the slot doesn't look like it had been tightened, so I can't say if it ever saw use. The machine shop hadn't been used for about 12 years and the owner died a couple of years ago. There was no one around who knew much about the place.

Taking a good look at the fly-cutter, he put the setscrews on the 'short' side of the disk, meaning that they take all the strain on the bit. I think I'd have drilled and tapped the 'long' side, so that the side of the groove was taking the strain on the bit. I might do that anyway.


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## cathead (Jan 30, 2016)

Good Job!   One thing you could do to improve the knurl would be to make a fine cut on the knurl and knurl it again.  
The diameter of the material has to match the knurl in order to get a definite clear knurl.  I suppose one could do the
math and figure out exactly what diameter is the best.  If I get a marginal knurl, I just face it off lightly and usually the second time
around, the knurl is much better.  I will have to make a shallow angle knurl and see how it performs as your surfaces
look really exceptional!


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