# Home Made Fly Cutter



## AmericanMachinist (Sep 4, 2020)

I made this home made fly cutter from 1.5" 1018 for 1/4" tool bits.   If the 2-1/2" bit was fully extended it would cut 3-1/2".

I need to do some reading and figure out why my chips were so stringy.   Probably didn't grind the tool bit properly. 





Cutting the bottom face.  The Mini Mill didn't like to do more than about 0.020" cuts about 1/2" deep.   I elected to go with approximately a 10-degree down angle after seeing This Old Tony set his there and it looked functional and appropriate. 






Setting up for the 10-32 set screws.   Drilling, chamferring, tapping... 






Indicating in the angled face to flat with the mill table.






The finished tool.  That's not the appropriate cutter installed, just in place for test-fitting.  






Another shot of the tool






First test cuts on a 1" x 2" piece of unknown alloy, likely A36, etc..  The final cut was a few thousandths deep.  Hand fed a couple though per rev on the cutter.  Hard to gauge RPM on the mini-mill but based on the frequency of interrupted cuts, probably 200-300 RPM = 100-150 SFM.  I need to experiment some more and see if I can improve my grind and overall get a better surface finish.


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## benmychree (Sep 5, 2020)

spiral chips are not stringy, by definition, stringy is straight, aimed at your person, generally.


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## mikey (Sep 5, 2020)

Not sure which lathe you have but you can generally increase your depth of cut and kick up your feed rate to reduce stringing.


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## AmericanMachinist (Sep 5, 2020)

mikey said:


> Not sure which lathe you have but you can generally increase your depth of cut and kick up your feed rate to reduce stringing.



Thanks - It's a Logan 200, 10" lathe with Craftsman 1/2 HP motor.   If I cut any deeper than about 0.025" and take more than 0.004" it starts to stall the spindle and then the belt instantly walks off.  I've heard folks say a Logan 200 should be able to take more.  I wonder if I have an issue with the motor.   On my short list is to make sure the pulley shafts are aligned, and try washing the belt to see if I can gain traction.


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## mikey (Sep 5, 2020)

It might be your tool. I have a Sherline lathe that will double that depth of cut so it probably isn't the lathe. Well, it might be something in the pulley/belt set up but I am guessing your tool might also be an issue.


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## AmericanMachinist (Sep 7, 2020)

mikey said:


> It might be your tool. I have a Sherline lathe that will double that depth of cut so it probably isn't the lathe. Well, it might be something in the pulley/belt set up but I am guessing your tool might also be an issue.


Tonight I upped the feed per rev from 0.004 to 0.009, and could only remove about the same volume of metal in a pass.  Beyond 0.010 to 0.012" depth of cut, things came to a halt.   

I moved the lathe belt one speed slower, to a pulley position where the belt couldn't walk off.  Now instead of walking off, the belt just slipped when the spindle stopped spinning.  

I increased the belt tension, and it now took more force to slip the belt.   But, it would still slip if I went much above 0.010" depth of cut at 0.009" feed per rev.  

I may clean the belt and pulleys well and see if there's any noticeable difference.  

I think my tool bit was reasonably sharp, honed with a ~1200 grit diamond grit hone.   I'm sure it could have been done better though, as well.


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## mikey (Sep 7, 2020)

It may not be about sharp. Tip geometry makes a huge difference in how a HSS tool performs and I am willing to bet that we can dramatically improve your lathe's performance with a little bit of modification to the geometry. Can you please tell me what your side relief, end relief, side rake and back rake angles are? Also, what shape are you using? Here I'm talking about your side and end cutting edge angles. 

If all of this sounds like Greek to you, its okay. It did to me, too, at least for a time. Show us your tool and I think we can help you make it better. There is just no way a little Sherline should out-cut your Logan 200, no way.


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## AmericanMachinist (Sep 7, 2020)

The cutter is 1/4" Rex aaa.   These pictures are after tonight's turning.  

I have the tool holder sitting flat on the compound, which puts the tool edge right at the centerline (matched to a live-center).

From above (sorry didn't realize how dirty it still was when I snapped these, until I saw them blown up...).





The trailing edge (right side of tool).  





From the front.












Thanks!


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## mikey (Sep 7, 2020)

Okay, that helps a lot. You have a grasp of the basic tool angles so that's good. Here is what I see that might help.

Your side cutting edge angle is almost zero. This is the left side of the tool at the cutting edge. While this is not going to keep the tool from cutting, it does present a larger contact surface and that increases cutting forces. It would work better if the side edge was angled more. See this article to see what a more conventional shape looks like.
You have side rake but no back rake. Some folks grind tools like this and seem to like it. I prefer to have both rake angles ground into the tool. Both side and back rake reduce cutting forces and they have the greatest impact on cutting forces of all the angles on a turning tool. Having side rake helps but having side and back rake helps more and I would include the back rake angle.
The cutting with all right hand turning tools occurs at a very sharp and distinct intersection between two planes, the side rake and side relief angles on the left side of the tool. This can happen only when the side relief angle and the top rake angles are dead flat. Your angles are not flat so the intersection is not sharp. Gotta fix that.
Here is what I suggest. Read that article I linked to and see if you can reproduce the tool I ground there. That exact tool will take a 0.050" deep cut (0.100" reduction in diameter) in mild steel on a Sherline lathe and should work for your Logan. You can also join us on the model tools thread where the other guys are likely to see your posts and can jump in to help, too.

Give that grind a try and report back with pics and we can take it from there. I'll stay with you on this.

Oh, by the way, try to reduce your overhang from your tool holder by about half. The tool will cut better.


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## WCraig (Sep 7, 2020)

mikey said:


> You have side rake but no back rake. Some folks grind tools like this and seem to like it. I prefer to have both rake angles ground into the tool. Both side and back rake reduce cutting forces and they have the greatest impact on cutting forces of all the angles on a turning tool. Having side rake helps but having side and back rake helps more and I would include the back rake angle.


Since the OP is using an Armstrong-style tool bit holder, the back rake is automatically provided, no?  The second picture above shows this pretty clearly and that the end of the tool is ground to a relatively acute angle to be able to still provide relief.

Otherwise, I think you are spot on.  Particularly that the planes don't all come together at the cutting tip.

Craig


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## mikey (Sep 7, 2020)

You're right, Craig. I forgot about the Armstrong tool holder angle. Is there such a thing as tool fixation?


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