Speeds & Feeds

[Bob La Londe]

Ok, I do not get it. How the heck do you calculate how fast to cut, turn
feed, etc. I've been trying tables and formulas and basically just
destroying stuff before tackling each project.

I've read a dozen pages on-line and read some of the responses here in the
archives and I think there is a fundamental concept or group of constants
that every person who knows how to do it assumes everybody else already
knows when they explain it.

How do you calculate how fast to feed for a particular end mill?

Example, (CNC MILL)
Material = aluminum
Mill = .0625 (2 teeth carbide / could use tin coated or hss if it would work
better)
Radial depth of cut = ????
Axial Depth of cut = ????
Spindle Speed = ????
Feed Rate = ????

Limitations of the mill.
Max RPM = 10,000 (Five speeds. More if I do not mind hanging the pulleys
part way off the shaft)
Max Plunge Rate = 10 (Z) ((Axis hangs if you try to go any faster)
Max Feed Rate = 20 (X,Y) (Could probably go 30 as long as load was minimal)
Holding Force of Stepper Motors = 280 ozs.

The idea is to find the fastest way to cut a pocket or du carving without
breaking end mills, dulling them before the job is done, or having the
machine bog down against axial forces. How can I plug in data like that to
figure out what is going to work best. I'm tired of ruining work pieces and
breaking mills to find out.

Then how do I plug in that data to calculate the same for a smaller or
larger end mill?

With coolant?

Without coolant?

[sambo67]

For aluminum using a carbide endmill start with 900 - 1500 sfm with 40% radial depth of cutter and no more than 1 1/2 times the diameter of your cutter on axial depth . FPT depends on cutter diameter. The bigger the endmill the more chip clearance you have .Be sure and use coolant.

[Bob La Londe]

Originally Posted by sambo67 For aluminum using a carbide endmill start with 900 - 1500 sfm with 40% radial depth of cutter and no more than 1 1/2 times the diameter of your cutter on axial depth . FPT depends on cutter diameter. The bigger the endmill the more chip clearance you have .Be sure and use coolant.

Ok... that sounds just like some of the data from tables I already got. How do I use that to calculate the spindle speed, feedrate and depth of cut that will work best. At almost any speed you can see moving a 40% radial cut and 150% axial cut is going to snap a .0625 cutter faster than a toothpick in a wood chipper. Slower feed rates work harden the aluminum and it just stops cutting. Still breaks the mill like the proverbial toothpick, but slower.

Like I said, there is some fundamental concept here I must be missing.

[sambo67]

Spindle speed is calculated using SFM x 3.82 divided by cutter diameter.It will probably give you a higher RPM than your machine has . You might try for an e/m that small, using the higher end of your rpm max with about .00025-.0005 fpt.Also slow feed rate on lead in's and lead outs where this is the hardest on your cutters.Hope this helps.This is with using carbide only. The 1 1/2 times is the maximum.For smaller e/m's decrease as needed.keeping e/m stickout as short as possible

[Bob La Londe]

Originally Posted by sambo67 Spindle speed is calculated using SFM x 3.82 divided by cutter diameter.It will probably give you a higher RPM than your machine has . You might try for an e/m that small, using the higher end of your rpm max with about .00025-.0005 fpt.Also slow feed rate on lead in's and lead outs where this is the hardest on your cutters.Hope this helps.This is with using carbide only

Aluminum is 600-1400 if I recall. If we pick a mid range number of say 900.

(900 X 3.82) / .0625 = 55008. Yeah just a bit over the 10,000 I can turn. There are not too many machines out there without space (null G) formed bearings that will turn that fast.

.00025 (fpt = feed per tooth ?)

Ok, so that would mean for a two tooth .0625 mill I could remove aproximately 2.5 cubic inches per minute total material at 10,000 rpm? That seems like a lot.

How do I get a feed rate (IPM) and material cut (axial and radial plunge) from that?

Say at 10 IPM that would mean something like a pass of .25 total cutting cross section. With a .0625 taking a pass at 40% radial depth (.025) the math would indicate an axial depth of 100. LOL. Obviously something doesn't add up. Like I said before even, 40% & 150% is too much for the cutter. It will snap or crash the machine at almost any feed rate.

Aluminum I have found is touchy. Too fast and it gums up and sticks to your cutter leading to failure if it doesn't just break from the load. Too slow and the work piece hardens making the next pass impossible at the same speed and feed.

I know my work will sound stupid, but I want to be able to figure this out for a good starting point for a project. Not just chuck up a cutter and push it until it breaks or damages the work piece 20 times before I find something that works. If I could make three passes on a scrap test piece maybe breaking one cutter to fine tune my speed and feed near max I would be thrilled.

[sambo67]

At 10000 rpm using .00025 fpt it will be 10000 x .00025 x 2 for the number of flutes you are using.Works great for me

[Bob La Londe]

Originally Posted by sambo67 At 10000 rpm using .00025 fpt it will be 10000 x .00025 x 2 for the number of flutes you are using.Works great for me

I guess I'm lost. That gives 5. 5 what? IPM? At what axial and radial plunge?

[sambo67]

5.5 is IPM and if you are side milling use light step overs (radial depth).If you are slot milling use lighter depth of cuts ( axial depth) be sure to flood coolant because a small e/m like this has very little chip clearance and will load up and break as you have noticed.Also if you are pocket milling you want the chips out of your way as not to recut chips which will also cause tool to break

[FandZ]

Originally Posted by sambo67 start with 900 - 1500 sfm

I hate to show my ignorance here, but what does SFM mean?

[Fegenbush]

Ok, seriously, you need to get the book:

Amazon.com: Machinery's Handbook, 27th Edition (Toolbox Edition) (9780831127008): Franklin D Jones, Henry H Ryffel, Erik Oberg, Christopher J McCauley, Ricardo M Heald: Books

Amazon.com: Machinery's Handbook, 27th Edition (Toolbox Edition) (9780831127008): Franklin D Jones, Henry H Ryffel, Erik Oberg, Christopher J McCauley, Ricardo M Heald: Books

There is really no substitute, it has all this information in it. Pick up a used copy if you can.

Fegenbush

/SFM = Surface Feet per Minute

[FandZ]

Thank you, Fegenbush. Yes, your probably right. There are some good reviews on that book. I think I will have to get it. I have tried googleing SFM before, never found an answer though. So thanks

[BobWarfield]

If you get tired of the math, you could try G-Wizard:



Best,

BW