End mills break when making a dog clutch

Ignoramus17758 · 2011-02-21T05:28:01+00:00

I am trying to make a dog clutch that mates with the knee crank andreplaces the crank handle: wrote a G code program that makes such a clutch from a flat block ofmetal, with a 1/8" end mill. In machinable wax, it does a great job, zips through it, and makes asurface that mates with the crank handle perfectly, I was veryimpressed. In steel, however, this little 1/8" endmill works for a while, thenbreaks after 1/2 hour.1/8" end mill4 flute1 IPM feedrate2,000 RPMplenty of coolantunknown steel, but feels like 1018. I am sure that something simple and stupid is wrong. I am not reallysure how to approach this. I will, first, get a known best quality endmill, they are stillsomewhat affordable in 1/8". I am thinking, get a 2 flute, 1/8"endmill, 1/2" DOC. I am thinking TiN coating?In addition I will get a known 1018 block, I only need a small piece.I do not want to up the RPM beyond 2,000: at 2k RPM, the spindle isnice and barely warm to the touch, and at higher RPMs it getswarmer. Since this is a job that takes forever to complete, I wouldprefer to keep the sipndle cool. Any ideas?I know that I could make the program run faster with tool changes anda larger end mill doing boring and such. But I wanted to learn moreabout small work. i

B

Bob La Londe 15 years ago

formatting link

>

That's what wax is for. To show the product (and to make lost wax molds).

I don't do much with steel, but end mills definitely have a fixed life. I use a lot of those smaller ones in aluminum and the life has become pretty measurable. Since I do aluminum I use the solid carbide. I've tried a number of different brands and types, and I've found the Kyocera ones work good for me. I don't have any HSS or Cobalt so I have no personal experience to draw on there.

Plug that into ME Consultant and see what kind of numbers you get back. Also, depth of cut could be an issue. With that small stuff I have to compromise depth of cut to about half what they tell me is possible, Sometimes a lot less. Steel is nice in that it doesn't build up on the cutter like aluminum does, but it also takes a lot more force to cut.

Nah, you are probably pretty close. Just may have to tweak things a bit.

1/2" DOC? Wow, that is aggressive unless you are taking a very small stepover. I was thinking a LOT less. Then I just go seat of the pants. Most of my 1/8 end mills are not even fluted that deep. I like 2 flute in aluminum, but doesn't more flutes usually mean a better cut and faster feed rate possible in steel?

I AM SO CONFUSED.

Like I said. Plug your numbers into ME Consultant and then adjust from there.

The first thing I learned about small work is remove as much as possible with a larger cutter first and then ONLY use the smaller cutter when necessary to finish. I am a lot happier with that. When I talk about jobs taking 6-30 hours Its usually detail work that I just can't do with a bigger cutter. Also, remember that even if your cutter can handle the depth of cut you get friction from the next pass if the sides of a pocket or feature are above the flutes.

Lets us know how it works.

Vote

S

SSM 15 years ago

.020" DOC, 2500RPM, 30IPM. Use carbide endmill, .125" stepover. Run like this until you get to .005" above your desired depth. I do it all the time, every day.

Have fun.

Vote

K

Karl Townsend 15 years ago

that's the coupling you normally see between the servo and ball screw.

0 backlash.

Vote

N

Ned Simmons 15 years ago

Oops, I meant feed per tooth...

Vote

N

Ned Simmons 15 years ago

Unlike steppers, servo tuning is compromised by lack of angular stiifness in the loop. Lovejoy couplings are quite soft in that regard, and I think that's the sense Pete was using when recommending a "hard" coupling. Oldham couplings are often a good compromise when highest performance is not required.

Vote

P

Pete C. 15 years ago

Yep.

The key thing we need to get across to Iggy is that in machining the feed rate is not a case where you can go from zero up to the maximum permissible feed before having problems, rather there are both maximum and minimum feed rates and exceeding the limits in either direction will cause problems. A minimum feed rate is required to ensure that there is cutting action vs. rubbing action, and allowing rubbing from too low a feed rate will cause a failure just as surely as too high a feed rate will.

Vote

C

Cydrome Leader 15 years ago

this type of coupling can still act rather springy, even though there isn't really and backlash.

Vote

W

Wes 15 years ago

Why not use carbide? If you push it too hard it will break quickly. If HSS is breaking after 30 minutes on a cnc, I suspect you got the life time out of it.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

Vote

P

Pete C. 15 years ago

Look at his cut parameters, as Dan noted, his chip load is 0.000125 per tooth. His feed is too low and the end mill is rubbing and dulling.

Vote

R

Rich Grise 15 years ago

formatting link

>>

So, say I'm making a rectangular hole. Would I cut it with, say, a 1/4" side mill, and finish up with a broach?

Thanks, Rich

Vote

P

Pete C. 15 years ago

formatting link

> >>

You would certainly need to finish with a broach if you need square inside corners. If you can live with rounded inside corners of a small diameter end mill you can certainly do a finish pass with a small end mill, but it would be insane to try to mill say a 2"x3" pocket with an

1/8" end mill just because you need 1/8" for the finish pass.

At any rate, that is not relevant to Iggy's machining issue, since his part does not have any inside corners. For his part he should be using a

3/16" end mill or slightly larger, milling the outside diameter, the inside circular pocket, the radial bulk material removal cuts between each section of the "crown", and then finishing by milling the exterior of each "crown" island wedge. The limitation on the end mill diameter is the spacing between the inner points of the "crown" islands which is around .2".

Vote

I

Ignoramus562 15 years ago

is breaking

I think that you have an extra zero in your calculation.

A little update: I am now using a 12L14 bar that is exactly the right diameter. I drilled a hole in the middle with a drill bit and am machining the teeth right now. I found a couple of bugs in the code, that I fixed, and so far, it is running OK.

If the teeth do not exactly mate, I can file them down with a regular file. I will see how it goes, I have a little extra of this 12L14 bar stock.

i

Vote

I

Ignoramus562 15 years ago

I am not totally sure, but I think that I can handle that sort of thing with proper choice of servo loop parameters P, I and D.

i

Vote

I

Ignoramus562 15 years ago

My servo motor driven rotary table (4th axis) is based on this kind of coupling.

As for the knee, if, say, I have one degree of "slop", which is a lot, this amounts to roughly 1/5"*1/360 = 0.0006" of accuracy. Remember that 360 degree turn is only 1/5 of an inch or something like that.

I am not building a space station here, so 0.0006" extra inaccuracy, I can live with. (and deal with programmatically)

i

Vote

C

Cydrome Leader 15 years ago

maybe, I don't know- but I was surprised as how much those things can bend and twist. Keep them as short as possible to minimize this.

Vote

R

Rich Grise 15 years ago

Thanks! :-) Rich

Vote

P

Pete C. 15 years ago

HSS is breaking

No, the calculation Dan posted is correct:

"So your chip load (per rev) is 1"/2000rpm = .0005"per rev. /4(flutes)=0.000125."

1 IPM / 2000 RPM = 0.0005" of feed per revolution of the end mill, divided by the 4 flutes = 0.000125" chip load per tooth, which is too low by a factor of probably 10X.

Bugs in the code can certainly get you, and they can be subtle enough to no be noticed on more forgiving material like wax. Have you got any CAD / CAM software yet, or are you still hand coding with your modular routines?

Only if they're too tight. If they are too loose filing won't help.

Vote

I

Ignoramus19981 15 years ago

HSS is breaking

OK, great, sorry.

I am hand coding with modular routines.

I have not even looked at CAD/CAM.

I have to admit that I kind of like that, I am in my natural programming mode of thinking.

Yep.

So, in the end, they were just a bit too large to mate, I filed them down slightly with a file, and now they seem to mate, though tightly

-- which is great.

i

Vote

P

Pete C. 15 years ago

HSS is breaking

CAD/CAM has some advantages in visualizing the part, cutter clearances and the like, and of course a huge advantage in dealing with machining operations on complex contours.

The end result is what matters, at least for one off parts.

Vote