I broke an end mill

Machine Shop
#1

I broke a 1/4" 2 flute end mill today. I’ll be glad to get a new one. It brings up a couple of questions I have.

I was milling (2) 3" slots (or pockets) in a piece of aluminum. 1/4" for a screw and a 1/2" for the screw head. I cut the 1/2" one first (.2" deep) in one pass successfully. Then I cut the 1/4" one through the bottom (.2" to the bottom but I cut down to .22 to cut cleanly through). It snapped about half way through the 3" length). My method was to bore down to depth and then turn on the auto feed. The mill speed was was 1200. Is this the correct way to do it? I finished the 1/4" slot by taking .05" cuts.

I looked at the MIT video on cutting slots and he gave a rule of thumb for aluminum that you can cut the same depth as the diameter of the bit. Does this sound right? I have been cutting at less than a quarter of this depth which explains why it took so long.

I did not hear any funny sounds before it broke. I really don’t have a handle on the feed/speed for different materials and different size mills. Yes I can look at a calculator on line for feeds/speeds but the mill doesn’t have a speed indicator so that doesn’t help much. I don’t know anywhere to get a good guide on depth of cuts. I sure that “it depends” but is there a good source for a safe range of depths to start and a speed guide for 2, 3 and 4 flute bits for common material?

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#2

Did you put any cutting fluid, ie WD-40 (great for aluminum, not for steel)? Also, were you constantly clearing chips from the cutter?

#3

The Bridgeport most definitely has speed indicator. It’s just about eye level for me (6’ tall) it shows both the HIGH speed range - the RABBIT and the LOW speed range - the TURTLE. I know I point this out in my classes because it is humorous that the gear selector reads high and low and the speed adjustment uses pictures. How long since you took the Bridgeport class?

You don’t mention how much lubricant if any were used or if cutter was carbide or HSS, big difference in RPM. When you do a plunge cut 180 degrees of the blade is engaged as it cuts.

If it snapped, it could be the cutter was dull or the feed rate too high, chips weren’t being removed adequately, etc.

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#4

I think @mikeglass means there is no accurate feed display for the powerfeed on the table x-axis.

Spindle speed he noted at 1200 [RPM].

#5

Maybe someday when really bored, I’ll get a stop watch and time the speed under no-load over 20 inches for each number on the adjustment knob. Then maybe do some cuts under load for Aluminum to see what the drop is.

Folks would still have to do their chip load calculation.

If there are some folks interested, we could spend a day doing some cuts with different size cutters and number of flutes and create a quick reference chart. I know, I know, there will probably have to be a lottery to see which of hordes of volunteers for this exciting task get to take part. And howls of disenfranchisement, bias, bribery, and who knows what else for those that don’t get to take part in the excitement.

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#6

my personal take is that 0.2" is a bit agressive for a 1/4" bit. half that would have been my max.

#7

I was using 3-n-1 and least that’s what the can said.

1 Like
#8

Yes, that’s what I meant.

#9

Key thing is you were using a lubricant - which is good. Unless it was clear liquid, it probably wasn’t 3-in-1. But it was still doing what you want it to do.

Hey, cutters break. 1st - Thanks for informing us and 2nd - trying to get help figuring things out. As a general rule, for cutters under .500" I assume that the effective cross section through the center is 50% of the diameter. So on a 1/4"/.250" cutter there is only 1/8" going down the center and this thin piece is experiencing twisting rotational forces and being bent as it is being pushed against the material.

If it was a carbide cutter, they don’t bend much and just catastrophically. They are strong but really don’t give much warning. Big advantage of Haas nad CNC’s is the library will compute the speeds and feeds - the disadvantage is - steeper learning curve.

If you measure the depth from the edge of the flute to the solid center X 2, subtract that from diameter you’ll get cross section size (or a fair approximation of it)

Isn’t machining fun!:innocent:

#10

@Photomancer If there are some folks interested, we could spend a day doing some cuts with different size cutters and number of flutes and create a quick reference chart.

I’m in. It wouldn’t take that many samples to get some reasonable values. If I know values for 1/4" and 3/4" mill then I can make a good guess and what a 1/2" mill would do. Three metals would probably be enough aluminum, brass and mild steel. Keeping the values somewhat low would help account for dull tools.

Some numbers for the Sherline would be good too. It feels so delicate next to the Bridgeport.

“[quote=“Photomancer, post:3, topic:24145”]
If it snapped, it could be the cutter was dull or the feed rate too high, chips weren’t being removed adequately, etc.
[/quote]
I noticed when looking for another 1/4” mill that some had chips off of the cutting tips.

@Photomancer Hey, cutters break. 1st - Thanks for informing us and 2nd - trying to get help figuring things out. As a general rule, for cutters under .500" I assume that the effective cross section through the center is 50% of the diameter. So on a 1/4"/.250" cutter there is only 1/8" going down the center and this thin piece is experiencing twisting rotational forces and being bent as it is being pushed against the material.

Despite the breakage, today was a learning experience. A cut with a 1/2" mill I took in 1 pass today but I did it in 4 or 5 passes early this week. I’m becoming more comfortable with the machines so I’m will to push them a bit further. I’m not sue how to tell if it was a carbide bit that broke, it says “1/4” 2FL SQ SE".

#11

EXCELLENT! On the learning and excellent on helping oput. It will probably be the week aftaer next as I will be in OK next week - brother-in-law having surgery and I’ll be there so sister doesn’t have to take off work.

Look forward to the help.

#13

Good, I suspect that your spindle speed and feed were a bit slow which led to heating up the material, which gummed up your endmill. Also your chips probably were not clearing (i am constantly brushing chips out when I cut pockets on the bridgeport)
I’ve definitely cut as aggressive or more aggressive than your depth of cut and stepover with the same size cutter with no issues, so its probably your speeds (table feed and spindle) and chip ejection.

Also one thing to note, HSS (high speed steel) is flexible when using cutting fluid.
Carbide however is NOT. You either cut your part dry, or you flood the damn thing with coolant. Additionally, Carbide can be run fast, but does not handle shocks well, so don’t plunge into your work piece like you would with a drill, ease into it.
I’m not sure we have carbide endmills (definitely inserts) but definitely something to keep in mind.

#14

Chip clearing and the type of cutting fluid may have been a factor. The cutting fluid looked almost like motor oil and although I tried to clean the chips as best as I could, the cutting fluid made thing sticky and the chips were hard to clear away.

I may also order a couple of endmills for myself so I’ll know the condition they are in when I start.

#15

For aluminum, I highly recommend WD-40.

Cool, I would buy some endmills that we don’t have for yourself. We have a TON of 1/4" and up endmills but I’ve yet to find a 1/8" or smaller. Double ended endmills are also pretty useful too. Costs more, but usually cheaper than buying two individual endmills.

#16

Not that we want to waste them but we do have a very large number of cutters from Freds on hand.

#18

they are mostly large. You need to check the other slots when looking for a particular end mill. they don’t seem to end up in the correct slots. I know, right?
If we’re truely out, we will get more.