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radkins
11-08-2015, 04:53 PM
I didn't want to highjack the other thread about the flycutter vs mill surface finish however it brings up something I have been wondering about. Having the mill head in perfect tram was mentioned and I suppose that's fairly obvious but the best I have been able to do does not result in enough "perfection" to get a perfect finish. When entering the cut the surface is almost perfect, nearly to the point of a ground finish appearance, but then despite anything I have tried the trailing cut ruins this finish. I would "assume" that the trailing cut is not supposed to cut at all and indeed there seems to be no measurable cut but still it's enough to mess up that shinny surface and I was wondering if there is a way to avoid this or is that just the nature of the beast?

I have noticed that with very minor tram errors the finish is better or worse depending on the direction of travel but I usually can get the tram to the point that travel direction makes little or no difference.

dalee100
11-08-2015, 05:26 PM
Hi,

It is the nature of the beast. Even if you somehow managed to achieve perfect tram, tool flex will cause just a bit of drag on the backside. I find I get the best finish when running at high rpms and low feeds. I have no problems running a HSS tool bit at 1000rpms or a bit better in steel with .010"-.015" DOC.

Dalee

Mcgyver
11-08-2015, 05:52 PM
If the heads in perfect tram, you should get a cross hatch pattern. If you want to avoid the second cut, although it is indicative of a properly set up mill, guys will sometimes put it out a tram a thou and take a super light cut. Personally I wouldn't bother; its milling and there are going to be tool marks. Grind lap polish scrape tumble bead blast file abrade or buff if there aren't supposed to tool marks.

DATo
11-08-2015, 09:10 PM
As dalee100 said, speed and feed figure significantly in getting a good flycut finish, but so does making sure you are using some lubricant to keep the cutter edge "clean". Material can build up on the cutting edge if it isn't lubed and then you have material trying to cut material rather than the cutting tool trying to cut material.

Also, the manner in which your cutter is ground can also affect the resulting finish as well. If you are using a cutter which is ground much like a lathe cornering tool you might try breaking the edge with a flat right at the point, or grinding a slight radius on the point. Imagine you are looking through a microscope at the tool along the surface of the Y axis as it is cutting ... right down at the cut, with a pointed tool, you will see vertical points sticking up and valleys pointing down. The distance between them will be equal to the feed rate you are using (higher feed = greater spacing) so if you decrease the spacing by going to a lower feed rate the points will be very close together and be much smaller in height, thus the finish will look better. Now, if your tool's "point" is a radius you can imagine a round shape doing the cutting which would lead to an even "flatter" tool mark footprint.

Toolguy
11-08-2015, 11:21 PM
As DATo says, the secret to keep the trailing edge from messing up the cut is lubrication. You want the head in perfect tram and have lube on both side of the cut. One easy way is to run coolant, but most HSM mills will make a big mess out of that. You could use a squirt bottle with the appropriate thin viscosity cutting fluid. The name of the game is to keep any material from sticking to the cutting edge and getting deposited on the back side.

Sunset Machine
11-09-2015, 11:03 AM
I run a flycutter with the leading side .001" deeper than the trailing. It'll cut pretty deep, 3/8" in steel if I recall. Think about the toolbit on the trailing side, cutting inside out like that. Would a lathe do any better when using the wrong handed tools? Grind accordingly (no side rake, light cuts), or tilt the cutter.