Best Practice: Roughing Bits

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Metal

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I pretty much only cut aluminum.

That being said, I have a benchtop CNC mill, and am looking into speeding up my cutting time, for roughing out the parts at least. I've ordered a 3flute corncob rougher to start, and try and figure this out before I pony up for a "good" one.

From what i've read/figured, I"m supposed to be roughing at about the same rpm, with a slightly slower feed rate, but much higher depths so that the small cutting surfaces all along the rougher can do their job, is that accurate? As my mill has some backlash, I want to err on the side of caution.
 
Metal said:
I pretty much only cut aluminum.

That being said, I have a benchtop CNC mill, and am looking into speeding up my cutting time, for roughing out the parts at least. I've ordered a 3flute corncob rougher to start, and try and figure this out before I pony up for a "good" one.

From what i've read/figured, I"m supposed to be roughing at about the same rpm, with a slightly slower feed rate, but much higher depths so that the small cutting surfaces all along the rougher can do their job, is that accurate? As my mill has some backlash, I want to err on the side of caution.
Click to expand...

You didn't say if you are using a HSS or carbide rougher. The link in Mikey's post is for HSS steel cutters. If you go with carbide you can increase SFM and IPM significantly which will reduce your cycle time providing your mill is capable. The downside is carbide roughers are not cheap. It's a balancing act.

Tom S.
 
It might help to clarify what you're looking to do. Are you trying to rough out a profile, slot or bring a part to rough size externally? You mentioned a corncob rougher but I assumed you were talking about a roughing end mill - my bad. Tom is right - carbide cutters run at much higher speeds provided your mill can get into the SFM range they require.
 
My mill uses a belt drive, and i'm pretty much maxed out @ ~3000rpm (any higher and the spindle seems to heat up too quickly) My question was based around depth of cut which I couldn't find much info on and SFM is a 2 dimensional value.
 
Metal said:
My mill uses a belt drive, and i'm pretty much maxed out @ ~3000rpm (any higher and the spindle seems to heat up too quickly) My question was based around depth of cut which I couldn't find much info on and SFM is a 2 dimensional value.
Click to expand...

For the most part published information by cutting tool suppliers is based on cutter capabilities only. Understandably cutter suppliers do not know anything about the machine their cutters will be used in so that is not part of their equation. This is where practice and experience come in. You are doing the right thing by asking questions and experimenting with different style cutters. Keep this up and you will become an expert.

Tom S.
 
I appreciate that, On my facebook an old "sort of a machinist" always is impressed with the stuff I come out with with the minimal experience and equipment that I have.

I've up until now gone by sound, if I hear a louder "ba da da da da da" of the cutter sounding like its impacting the metal, i'm going too fast, if its squeaky, its going too slow, using speeds and feeds as a rough guideline. Before the mill was CNC I could feel the difference in the handles, now that its cnc I've lost soft skills like feeling how the gibs are adjusted so its been a new learning experience, ive only had a mill for ~4 months!

My guess is that speeds and feeds are based in a fantasy world where putting load on a motor doesn't slow it down (or you're running huge machinery that just doesn't care!" My main hope here is that I can rough out a part much faster than I can with just using endmills.

Here's an example:

http://img.photobucket.com/albums/v684/GRSL/1_zpsggyag0w1.jpg

Now thats a pneumatic cylinder head that is mostly done, this is like my 3rd part ive made since CNC.
The part is machined out of a 1" slab of 6061, the outcropping on top is around 1/2" deep, maybe a little more, I forget off the top of my head.
There is a ton of material to remove around the outcropping, this took about 2 hours with a 3/8" 2 flute bit @ ~12ipm with a .06" depth of cut per pass. the speeds and feeds that im seeing are way, way, way higher, for more legit machines i'm sure (I am on a 500lb benchmaster mill) The cutting pattern was generated by hsmxpress (a free addon for solidworks)

The bit hasn't arrived yet, but I picked up a 1/4" 3 flute HSS rougher from niagara cutter. I'm hoping to do about 30% faster with it (3 teeth instead of 2, deeper depth of cut because rougher) but am not sure how far to go to start and "try" I want to avoid breaking this bit.

I might be stuck as-is until I end up finding a millrite or another mill of about that size locally.
 
Metal said:
I appreciate that, On my facebook an old "sort of a machinist" always is impressed with the stuff I come out with with the minimal experience and equipment that I have.

I've up until now gone by sound, if I hear a louder "ba da da da da da" of the cutter sounding like its impacting the metal, i'm going too fast, if its squeaky, its going too slow, using speeds and feeds as a rough guideline. Before the mill was CNC I could feel the difference in the handles, now that its cnc I've lost soft skills like feeling how the gibs are adjusted so its been a new learning experience, ive only had a mill for ~4 months!

My guess is that speeds and feeds are based in a fantasy world where putting load on a motor doesn't slow it down (or you're running huge machinery that just doesn't care!" My main hope here is that I can rough out a part much faster than I can with just using endmills.

Here's an example:

http://img.photobucket.com/albums/v684/GRSL/1_zpsggyag0w1.jpg

Now thats a pneumatic cylinder head that is mostly done, this is like my 3rd part ive made since CNC.
The part is machined out of a 1" slab of 6061, the outcropping on top is around 1/2" deep, maybe a little more, I forget off the top of my head.
There is a ton of material to remove around the outcropping, this took about 2 hours with a 3/8" 2 flute bit @ ~12ipm with a .06" depth of cut per pass. the speeds and feeds that im seeing are way, way, way higher, for more legit machines i'm sure (I am on a 500lb benchmaster mill) The cutting pattern was generated by hsmxpress (a free addon for solidworks)

The bit hasn't arrived yet, but I picked up a 1/4" 3 flute HSS rougher from niagara cutter. I'm hoping to do about 30% faster with it (3 teeth instead of 2, deeper depth of cut because rougher) but am not sure how far to go to start and "try" I want to avoid breaking this bit.

I might be stuck as-is until I end up finding a millrite or another mill of about that size locally.
Click to expand...


Nice looking part! I too was limited on metal removal rates not because of cutter limitations but because my max spindle RPM is 1970. Because of the RPM limitation my feed rates were in the single digit range. It took forever to run a program. My solution was to add a high speed auxiliary spindle (5000 to 28000 rpm). It's amazing to see a 1/4" 4 flute carbide end mill at 15300 rpm and 79 IPM. RPM is your friend providing your machine has the speed and is rigid enough. Flood coolant is also helpful as it flushes out the chips and extends tool life.

Good luck with your experimentation.

Tom S.
 
I do have the option to increase rpm, the issue i'm seeing is the spindle gets too hot to touch and i'm not sure where the danger zone is, I also noticed some grease escaping from the bearing around the pulley at that rpm, but its not clear if that was happening or it was just hanging out to begin with and the centrifugal force was dragging it out. Since I change rpm with gearing the extra rpm comes at the cost of torque and I only have ~3/4hp to hand out. I "could" run some coolant through sealed tubing around the spindle to try and keep the spindle a little cooler or make some kind of heatsink.
 
@also, flood coolant is a no go, just too much mess for my current setup, I spray some coolant on the bit every once in a while manually and use a shopvac to suck up chips when they build up (as well as wax the bit)
 
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