I keep breaking bits!

[mccombbj]

Hello all, this is my first post, but I have been following this forum for some time. I'm fairly new to CNC, and excited to finally have my mill working. Anyway so my question, what does everyone do here about removing material from their workpiece. I'm currently trying to drill through a 0.253 inch piece of alluminum. I'm using a 1/16 inch 4 flute endmill. On each cut I'm only going down 0.2 inches, and a feed rate of 0.3 IPM @ 2000 RPM. So at this rate it takes me over a couple hours to make a complete circle that is 2 inches in diameter. I was hoping to start the mill spread some oil over the piece and come back in a few hours and find it done. Oh I also tied a vacumm to the side of the mill to try and get excess chips out. So my predicament is, when I left the mill and came back my bit was broken. It looked like when I almost got to the bottom of the piece that my vacuum wasn't sucking any of the peices up and it overheated and died. So my question is what do you guys use to stop this problem, should I always just stick around and suck up what I can with a vacuum. Do you think I should switch to a system where I spray vegetable oil on it to push out the pieces?, (can I use vegetable oil?). Any help is greatly appreciated. BTW, I"m using a base sherline mill converted to CNC, and MACH 3.
Thanks for the help guys! I have learned alot from this forum, and if anyone is in the cincinnati area let me know.

~Brian

[patndel]

Hi Brian,

I'm not going to quote you cutting tables or chip loading etc, but I can tell you what I have found works.

I've done some engraving on 6061 aluminium with a 2.0mm endmill with 0.25mm per pass @ 100mm/min (2000 rpm). The problem with aluminium is it tends to weld itself to the cutter, leading to a rough finish and ultimately a broken cutter.

The above is probably conservative, but I only had the one endmill left, and couldn't afford a broken endmill. I've been using a cheap alternative to WD40 to lubricate and blow out the chips after every pass. I've also using kerosene 10:1 with motor oil, and turpentine neat, both are a little hard on the nose after a few hours though.

I do a lot of profiling in aluminium using 3 flute carbide bits, 1.0mm per pass @ 150mm/min (2000 RPM spindle) I can cut all day without a broken bit.

Try taking a shallower cut 0.2" = 5mm you shoud be looking at a cut in the order of 30% of the cutter diameter, so I'd be looking at 0.015" per pass at say 3 IPM @ 2000 RPM as a start. You'll need to get those chips out of the trough left by each pass too, either by blowing them out or washing them out with some form of flooding.

Hope this helps

Pat

[mark c]

Hi Brian,
Are you "milling" a 2" circle thru the alum?
If so .200" is WAY to much of a depth of cut for a 1/16 end mill. I'm not sure what your max rpm is, but dial it up to max, go with a .030" depth of cut (or less) and up the feedrate. At .3 IPM you're only getting the aluminum hot and "welding" it to the cutter. Thats why it broke.
I'd try max RPM (I see on the website that the Sherline is limited to 2800 RPM) If you have more, use it. Try a feed rate of 6 IPM. That will give you a "cut" of .0005" per tooth with a 4 flute end mill. If it "complains" slow it down. Not sure about the rigidity of your setup. You may be able to go faster.
A flood coolant system would be best, a spray-mist coolant good, or just give it a shot of good ole WD-40 once in a while and if you have a supply of compressed air, give that a shot once in a while too.
Aluminum is easy to machine, but it does like to stick to the tools. Anything you can do to get the chips out of the work area and lubricate/cool the work will help.
Hope this helps. Let me know if you need more.

Mark

[ZipSnipe]

Definetly need to keep your bit cooled. Ya can,t just throw oil on it and leave and hope for the best.

[ataxy]

yep .2 is way to deep of a cut as a rule of thumb pass should be half the diameter of the mill so in your case 1/16=0.062 so your pass should be 0.031 deep as for your feed rate at 2000rpm with a 1/16mill 4 flute lets assume a chip load of .0005 per flute wich gives .002/rev so 2000 x .002 = 4"/min feed but make sure you supply it with a constant amount of cutting fluid and make sure that each time you go down .031 you wipeout the chip or else you are going to break the mill also try to use a mill that as flute that are not to long, so .100 to .200 longer then the total depth as the longer it is the more its going to try to flex wich is a nono on a mill that size
also you said drilling some mill, like 2 flute most of the time, can drill but lots of 4 flutes arnt made to take drill cut

[MichaelHenry]

I'm confused on what you are trying to machine:

1) a 2" circular groove partially through the stock?
2) a 2" circular groove through the stock to leave a 2" hole and a 1-7/8" circular cutout?
3) a 2" flat bottomed circular pocket?
4) something else?

As others have mentioned, you usually want to limit depth of cut to 1/2 the cutter diameter or even a bit less for small cutters.

What sort of end mill are you using? HSS or carbide? Centercutting?

Cooking oil is not the best cutting oil for aluminum - try WD40, Tap Magic Aluminum or some other oil designed for aluminum.

Mike

[project5k]

sounds like to me its option #2.. the way i read it he wants to have a 2" hole clear through the 1/4" thick plate... and so yea that would leave a dropout slug.. i dont know the machine that he's using, but it would seem to me that it would be more efficient (timewise) , make sure its end cutting, and feed it faster and less deep, just like everyone else has said... but i was thinking about useing a larger endmill....... change the offset acordingly.. what about going that route.. i just dont understand using such a small bit...

[MichaelHenry]

Yep, if it is option 2, I'd elect to use a larger diameter bit as well unless some unknown spec precludes that option. However a 2" circle has a circumference of 6.2 inches and at 0.3 ipm and 2 passes would only take 45 minutes or so to complete, not the "hours" the OP talks about, so maybe he is making a 2" flat bottom hole.

Maybe he'll pop back and let us know <g>.

Mike

[project5k]

I'm currently trying to drill through a 0.253 inch piece of alluminum. I'm using a 1/16 inch 4 flute endmill. ~Brian

well the way i read it, its all the way through.. tho i was wondering about the math of duration.. maby it just seems to take hours at .3 inches per min...

oh, wait, i got it... run the math on this one... pocket a 2" circle all the way through, instead of just cutting the permiter.. that might account for the time...

thats the only explination i can come up with... ofcourse i love the part about the vac burning up....

[mccombbj]

Hey guys thanks for all your help, it comes painfully obvious that I should have just been going faster with shallower cuts. You guys rock, I hope to come close to the mastery that is shown in this forum, I have a long way to go though.

It would make alot of since to just use a bigger endmill, but I'm actually making a part for my robotic lawnmower. This part will connect my large tires to my keyed dc motor. Basically I have a pipe extruding out the side of the rim, and I'm making a part to press fit into it, and in this 2 inch circle will reside another 0.75 inch hole with a key way. So thats why I need the smaller endmill for the keyway. I guess I could use an 1/8 endmill up to the keyway, then use the 1/16 for just for the keyway, but I'm still learning how to do such things in mach 3/lazycam, and seeing as this is the very last piece I have to make for my lawnmower (all electronics are done and I just need to bolt on the motors and a little more software), I just wanted to get it done.

I appreciate the help again, I have my cnc machine at work right now so I could work on it after work. Hopefully I'll get a chance to try the faster feed rates and lower depth cuts. Should speed up my time to.

Quick question, not to make this too long, but what is the advantage of a 2flute versus a 4 flute endmill. Seems like you would always want to run a 4flute bit becuase you can have faster feed rates. what application would I use a 2flute endmill.
PS. I'm using a carbide tialn coated 4 flute endmill
( www.lakeshorecarbide.com ) < seems to be the best place I found so far.

[mccombbj]

Hey guys thanks for all your help, it comes painfully obvious that I should have just been going faster with shallower cuts. You guys rock, I hope to come close to the mastery that is shown in this forum, I have a long way to go though.

It would make alot of since to just use a bigger endmill, but I'm actually making a part for my robotic lawnmower. This part will connect my large tires to my keyed dc motor. Basically I have a pipe extruding out the side of the rim, and I'm making a part to press fit into it, and in this 2 inch circle will reside another 0.75 inch hole with a key way. So thats why I need the smaller endmill for the keyway. I guess I could use an 1/8 endmill up to the keyway, then use the 1/16 for just for the keyway, but I'm still learning how to do such things in mach 3/lazycam, and seeing as this is the very last piece I have to make for my lawnmower (all electronics are done and I just need to bolt on the motors and a little more software), I just wanted to get it done.


I need to get my terminology right, sorry I meant mill out the circle, not drill out the circle, and I didn't kill my vacuum I just meant the bit, and I'll be using WD40 for now on.

another newbie question, what is the difference between a 2flute vs a 4 flute, why wouldn't everyone just use a 4 flute endmill? what would be a good application for a 2flute.

[under-dog]

.2" is way too deep of a cut.

I have found it better to take shallower passes at a higher feed rate for roughing out. I leave about 5 to 10 thousanths for the finishing pass Finish passes at the same feed rate but because there is not a severe amount of material in the way you can go deeper.

I ran into your situation when I started out.

The fact that aluminum is sort of gummy in nature does not help.

Pweter is even worse gummy wise, although not as hard as alum by any rate.

With deep plunges like that the chips are larger and need to go up the flute to clear. The swarf left in the way to clog becomes as much an issue as the material you are cutting through. The flutes can clog and the cutter will stop cutting and....snap.

I cut some aluminum and alot of pewter and foud that .005 - .010 passes at 10 - 15 ipm work far better. You will need to experiment here.