Critique my fly cutter surface finish

[tjb]

That is very sharp looking. I ended up with something very similar after doing a couple with the angle. It works very well.

As you your question I have found as suggested above that the closer mill is to perfect tram the worse the results are. Knock it .001 out of tram and the marks go away or mostly anyway. .002 and they are all but invisible.

Also using HSS for the cutter give better surface finish that carbide when working with aluminum.

As to the trailing side messing up a good finish the forward side cut it seems to be 2 things with mill being trammed for 1 and needing a deeper DOC for carbide. If it isn't taking a big enough bite with will really never cut a good finish.

Thanks. I've managed to get some very nice results using carbide, but I suspect that may be more luck than technique. I seem to have achieved the best results with a slow feed rate and a relatively high speed - a 4 1/4" diameter at 800 - 1,000 RPM's seems to me to generate a pretty high tip speed.

Sounds like your trial and error is pointing you to the same conclusion that I'm seeing.

Regards,
Terry

 

[tjb]

Terry, I'm no expert but it sounds like the reason you're getting better results when feeding from left to right is because you are climb cutting in that direction. It is not unusual for a fly cutter to leave trace swirls when feeding in the conventional direction if the mill is in tram. My theory as to why this happens is that the insert has taken the meat of the cut on the forward stroke and the insert is deflecting due to radial forces on the back side of the stroke. As you know, very light cuts with an insert will increase radial forces and the insert will deflect so it leaves marks as it skates over the surface. That is how I see it anyway.

The goal with a fly cutter is to obtain a flat surface that is accurately dimensioned. A fly cutter is not a finishing tool. Still, my Sherline inserted carbide flycutter that I use on my Sherline mill and RF-31 will leave a near mirror finish in steel or aluminum. It is a very rigid tool, much like a single insert face mill, so deflection on the back side is minimized. As expected, there is very little back side swirling. The insert I use has a 0.015" nose radius that is about the best compromise I've found for this fly cutter. It will take a 0.005" depth of cut and do so accurately.

In contrast, my Tormach Superfly insert has a huge nose radius and I always have back side swirls. I rough conventionally and do a finish pass in the climb direction to minimize the effect but its still there; you can see it but you can't feel it. I personally do not think it is worth the effort to throw the mill out of tram just to minimize the swirl marks; there are better ways to obtain a nice surface finish.

Bottom line: use the smallest nose radius you can find and experiment with feed direction. Use whichever works best and at least for aluminum, max out your speed and feed fast enough to actually cut. Feeding too slow with an insert increases deflection and will not finish as well.

Thanks, Mike.

I'm no expert either, but I visualize the phenomenon exactly the same way. If you notice in my second photo, there is a set screw on the top of the cutter. Its purpose is to optimize the cut by fine-tuning the cutting angle - which I think you are more properly referring to as the 'nose radius' (?). I basically set it where it looked right and made some test cuts. My results were quite satisfactory, so I took Yogi's advice: "If it ain't broke, don't break it." Any opinion as to whether a slight adjustment to that angle might be an improvement?

Thanks for the response. In my case, the curiosity is more about education than fixing a problem, and it sounds like my mental picture is at least reasonably correct.

Regards,
Terry

 

[RJSakowski]

If you are seeing the same pattern when cutting from left to right as from right to left, you are in tram. An out of tram condition will show a difference with one direction showing a nice finish and the other likely not.

The pattern with vary with depth of cut. A heavy cut will lift the trailing edge due to flexing of the cutter and the marks will be solely from the leading edge, even with an in-tram machine.

One trick that I borrowed from Tormach's SuperFly was to put a small horizontal flat on the tip of the cutter. A significant radius on the tip of the cutter, something like .030" -.050", will also work. Below is an illustration of three different cutter styles and the resultant facing cuts. The unmodified cutter will produce a sawtooth surface while the SuperFly cutter produces a smooth surface as long as the stepover doesn't exceed the width of the flat. The radiused tip produces small scallops, the height of which depends upon the amount of stepover.

 

[tjb]

If you are seeing the same pattern when cutting from left to right as from right to left, you are in tram. An out of tram condition will show a difference with one direction showing a nice finish and the other likely not.

The pattern with vary with depth of cut. A heavy cut will lift the trailing edge due to flexing of the cutter and the marks will be solely from the leading edge, even with an in-tram machine.

One trick that I borrowed from Tormach's SuperFly was to put a small horizontal flat on the tip of the cutter. A significant radius on the tip of the cutter, something like .030" -.050", will also work. Below is an illustration of three different cutter styles and the resultant facing cuts. The unmodified cutter will produce a sawtooth surface while the SuperFly cutter produces a smooth surface as long as the stepover doesn't exceed the width of the flat. The radiused tip produces small scallops, the height of which depends upon the amount of stepover.View attachment 304981

OKay, RJ, I'm probably going to use all the wrong nomenclature, but I have an observation and question. If you look at my first picture, you'll see that the insert I'm using is 'shiny'. I'm not in my shop, so my identification may be wrong, but I believe it's a CCGT 32.51 or 32.52. I also have the 'coated' gold-ish style CCMT 32.51 or 32.52. Just based on looks and feel the CCGT cutting tip seems to be much sharper than the CCMT. Would the difference be essentially equivalent to your graphic of 'Fly Cutter' vis-a-vis 'Radiused Cutter'? All other things being equal, would you anticipate a difference if I changed to the CCMT? I was working with aluminum most recently, so I was using the CCGT insert.

Regards,
Terry

P.S.: I don't claim to be an expert, but I trammed the head using one of those SPI gadgets that has two dial indicators on it, and it registered spot on. Also, all my other operations seem to be fine, and the difference from one direction to the other using the fly cutter is very subtle - barely visible and nothing to the touch. Should I be able to do better?

 

[RJSakowski]

My facing tool uses four 1/2" brazed carbide lathe tools. Following the lead of Tormach's tool, I chamfered the tips and saw a significant improvement on finish. Currently, I have custom ground the ends to a radiused tip.

I don't use much carbide insert tooling but my understanding the CCMT inserts are molded as opposed to the ground CCGT inserts so the latter would be sharper and perhaps better for aluminum. Also, it is my understanding that the .51 and .52 refer to the radius of the tip so I would 3expect the .52 to give a smoother cut.

 

[pontiac428]

I couldn't find a name for the "super fly" grind pattern to use as an example. Thanks, RJ! I think that is the optimal grind for fly cutting. I discovered it by "accident" while considering different radii or even slightly elliptical grinds to ensure some overlap as the tool rotates at faster feed rates.

When considering finish in aluminum, I think the same rules apply as in left-hand lathe bits in regard to steep back and side rake angles. I know little about inserts beyond color, shape, and price, but in HSS there are optimal geometries for different materials. Maybe that's included in the insert holder's angles. Like I said, I don't know, nobody's mentioned it here. Has material and appropriate cutting tool geometry been considered in this troubleshooting process? Is that even important with insert cutters?

 

[ddickey]

I just watched a video and the guy used a 10° negative rake insert. He said it leaves a shinier finish.

Positive vs negative rake: surface finish

I’m happy with both finishes but the finish left by the negative rake fly cutter is fabulous, almost mirror like, the positive rake fly cutter left a more matte finish. I think the lines are …

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