# Best Practice: Roughing Bits



## Metal (Jan 11, 2016)

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.


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## mikey (Jan 11, 2016)

In general, roughing end mills use about a 20% higher speed. Feed varies with depth of cut and end mill diameter. I suggest you go here and look at the speed and feed info: http://www.niagaracutter.com/techinfo/


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## TomS (Jan 11, 2016)

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.



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.


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## mikey (Jan 11, 2016)

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.


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## Metal (Jan 12, 2016)

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.


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## TomS (Jan 12, 2016)

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.



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.


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## Metal (Jan 12, 2016)

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.


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## TomS (Jan 12, 2016)

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!
> 
> ...




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.


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## Metal (Jan 12, 2016)

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.


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## Metal (Jan 12, 2016)

@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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## TomS (Jan 12, 2016)

Metal said:


> 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.



What brand is your mill?  Can you post pictures?  I recommend you find out why your spindle is getting hot and fix it.  It could be just a bearing preload adjustment or worst case the bearings need replacing.  My 2 cents.

Tom S.


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## Metal (Jan 12, 2016)

It is one of these guys:  http://freemansgarage.com/blog/wp-content/uploads/2010/02/mill311.jpg

The motor is not original (the mill is 50+ years old!) and is from a tablesaw apparently, I have found the manual, and the highest rpm setting is 2100, which it performs fine at. (IIRC when I got my tachometer to work I was right around 2k at the pulley setting i'm currently working at)
At the highest setting the tachometer no longer wants to work at all, but it sure sounds quite a bit faster.


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## TomS (Jan 12, 2016)

Metal said:


> It is one of these guys:  http://freemansgarage.com/blog/wp-content/uploads/2010/02/mill311.jpg
> 
> The motor is not original (the mill is 50+ years old!) and is from a tablesaw apparently, I have found the manual, and the highest rpm setting is 2100, which it performs fine at. (IIRC when I got my tachometer to work I was right around 2k at the pulley setting i'm currently working at)
> At the highest setting the tachometer no longer wants to work at all, but it sure sounds quite a bit faster.



Running at the fastest rated speed your bearings do not run hot but when you go above 2100 rpm they get hot.  Am I understanding correctly?  

Getting back to your original post and having a better understanding of your mill you will be limited on the size of cutter you can effectively use.  Again, it's experimenting and practicing that will lead you to a logical conclusion.

Tom S.


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## derf (Jan 13, 2016)

I would go with a 3/8" 4 flute carbide end mill. If you can't use coolant, use a shop vac to pick up chips and create airflow. You could direct the blower end of the shop vac to the spindle to help keep it cool.


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## mikey (Jan 13, 2016)

I would go with a fine pitch HSS roughing end mill as large and as short as is appropriate for your job. It will perform well at the speeds your mill can attain. I suggest the fine pitch tool to keep down chatter in aluminum and WD-40 will do fine for a cutting fluid. Roughers usually create a lot of chips and clearing them periodically helps so the vacuum suggested above works well. 

I normally take a full depth of cut with this tool and slow the speed appropriately. You will have to sort out what feeds to use and listening to and feeling your machine will tell you when its right.


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## Metal (Jan 13, 2016)

Thanks guys, I'm currently using a shopvac periodically whenever the chips build up but directing the blower to the spindle is a great idea!


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## Metal (Jan 13, 2016)

TomS said:


> Running at the fastest rated speed your bearings do not run hot but when you go above 2100 rpm they get hot.  Am I understanding correctly?
> 
> Getting back to your original post and having a better understanding of your mill you will be limited on the size of cutter you can effectively use.  Again, it's experimenting and practicing that will lead you to a logical conclusion.
> 
> Tom S.


Sorry yeah

In the manual it states the highest speed is 2100, obviously the motor I have is faster than that since I clocked it in at ~2000 at about half way through the pulley range, any higher than 2k and it heats up.

when I say the spindle heats up, I mean just the spindle, the bit seems okay, the collet is hot but not burning, the drawbar is much warmer than normal, the area where the collet interacts is too hot to touch after a few minutes, ect.  It appears the heat is coming from the lower spindle bearing just above the collet holder since the upper portion of the spindle is "fine" not really hot at all.  the bearing could be overloaded but I am a little timid in taking a hammer to the spindle to try and get the shaft out of those bearings since when I was refurbing the mill to begin with I wrecked those seals and had to replace them. (its been plenty of hours and they are obviously broken in)

Also one other question: when using cutting oil (I occasionally use some cutting oil if a particular part of a cut causes more vibration than i'd like due to the backlash in the table) is ot normal for it to burn off when cutting?  on youtube I generally dont see slightly smoking chips go flying but it would be hard to detect anyway.  I could also be using inappropriate  cutting fluid.

Also new part!
http://img.photobucket.com/albums/v684/GRSL/2_zpscmbvtaow.jpg


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## TomS (Jan 13, 2016)

Metal said:


> Sorry yeah
> 
> In the manual it states the highest speed is 2100, obviously the motor I have is faster than that since I clocked it in at ~2000 at about half way through the pulley range, any higher than 2k and it heats up.
> 
> ...



I like the new part!  What is it?

The bottom bearing carries the most load so will get hotter than the top bearing.  If your spindle doesn't get too hot at the max rated speed then I wouldn't worry about it.  Unless of course you want to run at a higher speed.

Cutting oil will smoke if the chips are hot.  That's a normal reaction between the oil and induced heat from the chips.  You can try different cutting oils or fluids to see which one gives off less smoke.  Or you can look up "No Fog Mister" on this forum.  I built one for my manual mill drill.  It doesn't cost a lot to make and works great.

Tom S.


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## Metal (Jan 13, 2016)

Its part of the frame of a robot I'm building.  http://img.photobucket.com/albums/v684/GRSL/ruin2cad2_zpsixgboamc.jpg

I might have gotten a little carried away with this whole milling thing


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## derf (Jan 13, 2016)

I have a bench top mill similar to yours, and notice the same problem at top speeds. I replaced  the spindle bearings about 8 years ago and found they  are the angular roller type and are only rated at those lower speeds, so there's not much you can do to them to eliminate the heat. On extended jobs when running at top speed, I directed a jet of compressed air to the spindle to help with the heat. It still got a little warm, but not as much as it did before.
Using a solid carbide end mill will lead to less chatter because of the stiffer shank. Rigging up a solid mount for a shop vac nozzle close to the cutter will create constant airflow and pick up chips. A periodic shot of WD40 will keep the cutter clean and prevent chip weld.


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## Metal (Jan 13, 2016)

Yep you're ahead of me derf, I'm also eying that it looks like where the belt guard attaches, there are threaded througholes that lead directly to the bearing race, If they aren't airtight I could thread npt fittings into them and direct air right against where the heat is generating, I was also considering maybe something like a PC water cooling rig to run coolant through the same path through a radiator and back.

Its all up in the air, let me get this thing dialed in a bit better then i'll worry about higher rpm, I was just looking to cut machining time down to save time.. and reduce the heat buildup since it only starts getting really bad about 15-20m into a run.


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## TomS (Jan 13, 2016)

Metal said:


> Its part of the frame of a robot I'm building.  http://img.photobucket.com/albums/v684/GRSL/ruin2cad2_zpsixgboamc.jpg
> 
> I might have gotten a little carried away with this whole milling thing



Cool!!!


Tom S.


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## Metal (Jan 14, 2016)

Hrm, I have more questions, I'm going to start a general "adventures in milling" thread


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