Suggestions for better finish?

[cnczoner]

New to milling and I've been searching/reading a lot on this and other forums, but my results tell me I'm missing some knowledge still. I'm trying to machine alum. Current grade is unknown, but I do have some 6061 now that I'll experiment with later today. My mill is a converted HF (geared head mill) running at 1500 rpm. The mill came with this set of (used) TiN-coated bits, which I know will not be the best things out there, and also one UNcoated 3/16" HSS bit. I tried a couple 4-flute TiN bits and the HSS (un-coated) bit, and no-coolant, WD-40, and 3-in-1 oil. Also did some chip-clearing with an air compressor and nozzle.

Some photos will make it easier to describe what I've got so far...

(1a) TIn coated 0.25" bit, 4-flute, dry, 10ipm, on the right. This is one 0.125" deep pass.
(1b) Same as above, but WD-40 and air blasts on the left.
Yes, the bits are all gummed with the aluminum now.
In this and other tests, I really don't see any difference between WD-40 and 3-in-1 oil.
I did not try the 2-flute TiN bits as they seemed to be used/chipped.






(2a) HSS 3/16" bit, 2-flute, WD-40 and air blasts, 0.125" DOC passes, 5 IPM.
(2b) (not pictured): same as 2a but at 10 IPM. I saw no difference in the finish, so I stuck with 10IPM from this point on.





(3a) (not pictured): HSS 3/16" bit, 2-flute, WD-40 and air blasts, 0.025" DOC passes. 10 IPM. This gave me a very slightly better finish to the horizontal face (not the sides of the cut).
(3b) Same as 3a, but this time, I tried added a 0.025" overlap (outwards from center of circle) finishing pass with the same bit. The finish is noticeably better!
(3c) (not pictured): Same as 3b, but I tried a shallower finishing pass of 0.010" and it was worse. Perhaps that's less than the backlash on the machine?





So far, in my research, it seems that for the best finish, I need 2-flute, UN-coated HSS, and a larger diameter (though I'd probably not go more then 1/2" to 3/8" due to the cuts I need to make). Also, known-quality bits will probably make a difference. Is 1500rpm good though? I can't get more, but should I go for less? Is 10ipm good? I need to make changes before I can get more speed, so hopefully you won't say I need more.

Speaking of backlash, this machine has the stock lead-screws, and I will upgrade to ball-screws some day. If any of these finish issues can really be traced to a backlash problem, I don't mind changing to ballscrews now.

Thoughts/suggestions much appreciated. Thanks!
-Neil.

[cnczoner]

I should also mention that the finishing pass was not cut the full depth all at once (IE: sink the bit 0.25" down into the open-space in the hole, then move outwards until it contacted the inside wall of the hole and run the finishing pass).

Should I slow down the finishing pass to 5IPM or even 1IPM?

[Geof]

It is quite likely the cutter not sharp. The big burr along the edge of the cut indicates a dull tool. Although you can get this with some of the very soft aluminum alloys even with a sharp tool.

The reason I pick on the tool is because I have bought those cheap sets and sometimes they are simply ground incorrectly and the cutter has no edge or end clearance.

Your 1500 rpm is a bit slow and if you could at least double this that would help; failing that slow your feed way down.

For a cutting fluid that you just brush on try automatic transmission fluid it works better than oil or WD40.

I suggest to get hold of a good cutter, two flute ground for aluminum with a high helix 3/8" dia.; then do your experimenting. This way you know the cutter is good.

You will find big differences between aluminum alloys. 6061 machines not bad, 7075 also is not bad. The low temper alloys used for fabrication are gummy it is possible this is what you have

[MrWild]

It isn't just the right cutter, or a sharp cutter, but the use of the right coolant/lubricant, and the TYPE of aluminum. Saying you are cutting aluminum is a huge field. Some aluminums cut really poorly and are gummy and nasty, while others cut clean with no lube at all. Do you know what kind oiof aluminum you are playing with?

[mrk]

For aluminum on you machine you will want to run every tool at 1500 rpm. You should adjust feed rate based on the tool size. Make sure you keep the chip load in range or you will clog and/or snap tools.

I attached an Excel file with formulas to calculate based on inch per tooth. You can enter your tool size and then the formulas will calculate your feed based on your max RPM.

-Mark

[tenaja]

Cutting 2024 aluminum is awesome; it cuts smooth & easy. Compared to it, 6061 is gummy and leaves a terrible finish unless you have perfectly new and sharp tools.

I, too, have used cheap tools, and they leave a terrible finish on everything they touch. A high quality cutting tool is worth its price after you're frustrated with crap.

[cnczoner]

Thanks for the replies. Good to know that's it's most likely the endmill as those are consumables anyway. But that means I won't be getting a good finish today.

I had researched aluminum grades also, but 2024 or 7075 is much more expensive. Can't justify it for my apps. I outsource my production stuff to other shops and their 6061 finish is awesome! So I know it can be done with 6061... it's just a matter of how do *I* do it now.

Most everything I will be doing with this mill is prototypes and one-offs with features cut into alum plate or flat bar about 1/4" to 3/8" thick. Finish is important, speed is not. And I fear the cost of a larger vibratory tumbler that would fit a 15" x 6" plate!

The other question now is which endmills to choose. So far I've learned that I need plain HSS, high-helix (60-deg), and larger diameter (for max SFM). But wouldn't high SFM mean more heat? Or do I then slow down the feed speed or depth-of-cut? I don't need sharp corners, but so far it seems that rounded corner bits cost a lot more. Once I determine the specs of the endmills I need, how do I do which brand(s) I should choose? It's tempting to choose Discount-Tools, MSC or Enco on-sale endmills (that meet my specs), but is that as good as something from one of the industrial suppliers? I suspect this may be a separate topic in itself though.

[Geof]

Originally Posted by cnczoner ....I outsource my production stuff to other shops and their 6061 finish is awesome! So I know it can be done with 6061... it's just a matter of how do *I* do it now. .....

It is a combination of speed with flood coolant; ask your shop what rpm/sfm they run at. Carbide cutters run at 1500 to 2000 fpm with copious coolant to keep things cool will give a very nice finish, this is what the parts in the picture where run at. Unfortunately this is more or less impossible with small hobby machines.

[cnczoner]

Mark, thanks for the XL file. The obvious question now is how to I know what that range is? Seems like it should be specified by the endmill mfgr, but looking at Enco etc, I don't see any data on the chip load that I should be targeting. Some googling gives me widely ranging numbers, and does not specify the material, endmill material, nor conditions (cooling etc). Any leads on this?
Thanks,
-Neil.

Originally Posted by mrk ... Make sure you keep the chip load in range or you will clog and/or snap tools....

[cnczoner]

Nice! I've seen photos (on some of the forums/mailing lists) in the past of very nice finishes achieved on alum with benchtop mills, including the smaller micro and mini mills. And those were as it came out of the mill. So I'm sure my mill is capable. Question is... am *I* capable?

I was expecting that I'd find/use an endmill which would give me a good finish and I'd use it to mill all the rest of the material but run slower. This was to avoid tool changes. But now I think I'll go with a rougher and then a finishing mill.

Originally Posted by Geof ... this is what the parts in the picture where run at. Unfortunately this is more or less impossible with small hobby machines.

[mrk]

I am still fairly new with only about six months actual experience and for me that means hobby time in my garage (my real job has nothing to do with cnc).

My advice is that if you are getting started by the cheap end mills since you are going to clog/snap/chip and otherwise abuse them a bit with your learning mistakes. I have quite a few broken end mill from my early unstable steps into teaching myself the basics.

I get end mills from JL industrial and they have a very nice catalog both real and online.

The catalog lists a general suggested speeds/feed tables in the back and some of the manufactures list speed/feed recommendations. The following link is the table that I started with:
http://tinyurl.com/2b83nr

The table lists aluminum sfpm range of 250-280 for slotting and 600-800 for peripheral milling. The max rpm on my mill is 3,000 so for me that usually means i am running at max. Once my speed is known the spread sheet helps me calculate feed based on feed per tooth. I generally use .5% of the tool diameter. I probably should increase my fpt but I get nervous about snapping or clogging tools and my machine is not very rigid so generally .5% works fine as a starting point.

Where I get into trouble, is full width cuts using small tools, deep pockets, and tight radius arcs. If my feed per tooth is too high, the tools tend to clog and can snap.

[cnczoner]

I actually want to go the other way for a short time -- ie: to get some good bits to learn and verify I know what to do for a good finish, then I'll step back to some cheaper practice stuff and mostly mill wood to learn. So far though, I'm writing g-code manually, and it's quite easy for what I want to do. Later I'll think about CAD/CAM etc.