Aluminum: Good Finish w/End Mills & Fly Cutters

[Phil3]

I am a novice and trying to get a good finish on 6061 aluminum flat surfaces. Not going well. The end mill 3/8" 4 flute leaves a slight pattern of semi circles if I use the end of end mill. I am using a spindle speed of 1200 rpm. I can't quantify the feed rate, but it is slow, and the depth of cut may be just a few thousandths. I would like a smooth finish devoid of tool marks. Maybe I am asking too much. The semi-circles are not enough to catch a fingernail on, but are plainly visible.

Side cutting with the end mill on an edge of aluminum bar stock leaves a series of vertical ripples no matter what I do with speed, depth of cut, or feed rate. ???

I then tried a fly cutter, but this was disastrous. Not really knowing how best to use these, I used one that had would best fit the piece of aluminum I had, even though the cutter was coming off the edge every revolution. To minimize impact, the cut was very shallow. But the finish was awful. A kind of smeared mess. I am sure my cutting tool is all wrong, but just can't get my head around how the cutting tool bit should be ground. I feel like I am grinding and grinding until I have no tool left. Clearly, I need a better understanding, like holding one in my hand.

Any advice would be helpful. I am using a Rotex vertical/horizontal (2 motors) knee mill with a 5" x 20" table.

Thank you.

Phil

 

[markknx]

Phil,
Some photos always help.
Are you sure your set up is solid? spindle locked? any axis not used in the cut locked? and a good sharp end mill? Then this may be as good as it gets, these swirls are a trademark of the horizontal mill.
you may be able to get them better than what you have I don't know but if you can't feel them with a finger nail you are off to a good start. Have you tried a deeper cut? what are you using for coolant/lube? wd 40 is good so is keroseen. (make sure you have ventalation)
As far as fly cutters go make sure you have good quality ones. I'm not real good on those but I do know just the tip should be on the part and it needs to be very sharp for a smooth finish. If all else fails and the part is not real close tol. the good news is AL polishes real easy just hit it with some 600 wet, 1000 wet and buff. you can go 1500 and 2000 if you want it like chrome. But that is a new topic .
Mark

 

[DMS]

The finish you are talking about is not uncommon for surfacing with an endmill (unless it's worse than I am imagining, pictures would help here). The thing is that our eyes are very sensitive, and any surface variations are visible, even though the actual variation in the surface will be immeasurable by any tool you have in your shop (other than your eyes of course). Asking for a finish with _no_ tooling marks is asking a bit much. The good news is they are usually easy to get rid of. For aluminum, if you want a matte finish, scotch brite pads work great (the kind you get at the grocery store work fine).

A fly-cutter is capable of much better finishes. The fact that you describe your results as "a smeared mess" make me think you are running too fast (too much RPM). When cutting metal, you want to keep in mind that you don't really care about RPM, what you care about is surface speed (how fast the cutting edge is moving across the material. On a mill, that is directly related to the diameter of the end mill. There are tables that you can look things up in, and also software to calculate this for you. You can also do a quick calculation. You need to know a few things to start with.

1) what material are you cutting (aluminum here)
2) What is your cutter made out of (I'll assume high speed steel here)
3) What diameter is your cutter (I'll assume your fly cutter is around 2" od)


Recommended surface speed for aluminum being cut by HSS is around 200 feet per minute (this is on the low side). The formula for calculating spindle rpm is 4*SFM/CUTTER_DIAMETER. Here, we get 4*200/2=400RPM.

As far as grinding the tool bit, take a look at this video

[video=youtube_share;tCrJVYF95aE]http://youtu.be/tCrJVYF95aE[/video]

Also keep in mind that fly cutters are not meant for deep cuts, you probably want to stay below 0.040" if your machine is stout, less than that on a lighter machine.

 

[GK1918]

Excellent contribution, especialily the honning part, there is not a whole lot out there on honning. I think the origional question with
alum, should be using a 2 flute rather than four, a four flute tends to push chips into cuts making a mess. One will see when you have
to clean the end mill with an ice pick.

 

[Phil3]

Re: Aluminum: Good Finish w/End Mills & Fly Cutters

I posted photos of aluminum that I tried to finish this morning with a 3/8" 4 flute end mill and also with a fly cutter. After reading some more, and using WD-40, things came out better. The photos are as good as I was able to get. The end-mill photos probably show little problem, and the finish is much better with a higher speed and WD-40, as well as a slow feed rate. The one "gold" pic was taken under incandescent light, hoping it would show the finish more.

The flycutter photos were taken at different angles to illustrate the finish. While the photos really don't show it, there appears to be very minor chatter marks. A magnifying glass can show it.

The end mill side cut finish is the worst, and was also done with WD-40. All cuts taken with a depth of around .005". RPM was 1200. I am new at this so was not taking any chances on cutting too fast, too deep, etc. The flycutter uses a 1/4" HSS bit and the cutting edge is 1" out from the centerline of the flycutter shaft.

The side cut with the end mill is indifferent to climb milling vs conventional. The flycutter does seem to show a better finish in one direction vs another as does the end mill. Not so much on the end mill, but more so on the flycutter.

The assumptions by DMS on my setup are exactly correct and the calculated speed of 4000 rpm are far below my 1200. My mill goes to 5200 rpm, but the next speed down is 2500. But clearly, I need to speed things up. Makes me nervous that flycutter whizzing around that fast, the cutter held in my two screws!

I hope the pics show up. If not, I'll keep working on that.

Thanks.

Phil

 

[Chuck K]

It appears to me that the cuts were made with a dull tool. That's just a guess because the pieces might look completely different if I was holding them in my hand. Could just be a feed problem too. Like the others have said there are a lot of variables involved. You should be able to get a near mirror finish with the fly cutter if your tool is sharp and the geometry is right. You just need to experiment until you find what works best. Good luck.

Chuck

 

[xalky]

The video above is very good. He's a bit chatty, but definitely worth the watch, there's a lot of great info in there.

 

[markknx]

Phil,
Are you sure the mill is trammed good. also try a little deeper cut. and check the cutter shape and sharpness. be sure it has a good relief, and enough radius. 1" stick out on the 1/4 fly cutter bit might be to much, can you shorten it to about 1/2.
Mark

 

[Rick Leslie]

I would also add that 6061 tends to be "gummy", for lack of a better word. The chips tend to stick to the work surface making an otherwise smooth finish turn out splotchy or rough. I have noted that this can be reduced somewhat by the correct side milling method (climb/conventional?). I've never been one to concern myself with the technical aspect of feeds and speeds so I can't speak to the correct formula, but I can stumble around and hit on an acceptable surface finish from time to time. If the finished dimensions aren't super critical, do like was suggested above and polish the final cut.

Starting with the basics always helps: tramming, rigid set-up, etc. Good luck and keep us posted.

 

[Bill C.]

I am a novice and trying to get a good finish on 6061 aluminum flat surfaces. Not going well. The end mill 3/8" 4 flute leaves a slight pattern of semi circles if I use the end of end mill. I am using a spindle speed of 1200 rpm. I can't quantify the feed rate, but it is slow, and the depth of cut may be just a few thousandths. I would like a smooth finish devoid of tool marks. Maybe I am asking too much. The semi-circles are not enough to catch a fingernail on, but are plainly visible.

Side cutting with the end mill on an edge of aluminum bar stock leaves a series of vertical ripples no matter what I do with speed, depth of cut, or feed rate. ???

I then tried a fly cutter, but this was disastrous. Not really knowing how best to use these, I used one that had would best fit the piece of aluminum I had, even though the cutter was coming off the edge every revolution. To minimize impact, the cut was very shallow. But the finish was awful. A kind of smeared mess. I am sure my cutting tool is all wrong, but just can't get my head around how the cutting tool bit should be ground. I feel like I am grinding and grinding until I have no tool left. Clearly, I need a better understanding, like holding one in my hand.

Any advice would be helpful. I am using a Rotex vertical/horizontal (2 motors) knee mill with a 5" x 20" table.

Thank you.

Phil

Semi-circles means you need to check or recheck the milling machine head for alignment. Its got to dead on zero. I use a indicator mounted on a arm with a six inch swing to sweep the table. I use a Starret indicator I bought back when I was a teenager. It came with several rods and clamping devices. You need to setup something like it or use a 3/8" rod bent 90 degrees to mount one end in a chuck or collett and the indicator on the other end. Milling machines can slip a thousandth or two just from heavy use. It happens all the time. Aluminum just shows more marks. Good luck