[autobot]

I am new to machining, what is the rule on feed-rate and spindle RPM for metals (as well as plastics).

Is it just always better to go slow feed-rate (as long as time to machine a part is not a factor at all)?

As for RPM, is higher RPM better (other than when it generates too much heat to melt or weld the milled material to the tool)?

[fizzissist]

The rules are very simple, and basically fit into 4 primary categories, each dependent upon the material being machined, and the material being used as a cutting tool.

1. Feed. Feed the tool just the right amount, usually measured in feed/rev and depth of cut

2. Speed. Rotate the tool or the workpiece at just the right rpm.

3. Rigidity. Have everything clamped as close to the cutting region as possible, and as tightly as possible.

4. Geometry. Make sure that you're using tools with the correct cutting geometry for the material, and that they're on center.

Ta-Da!! Welcome to the club, you're now a machinist. Wasn't that easy?

....For some nice bedtime reading, pick up a copy of Machinery's Handbook. You don't need a brand new one, a 25th edition or later will do nicely. They're on Ebay. Charts, graphs, tables, discussion and overview...all manner of book learnin' to answer a myriad of questions.

[Wolfesmetalfab]

look up SFM for the types of cutters you will be using, HSS or Carbide. That will give you a number to start with, then you need to calculate the rest.

Here is a calculator that I use to help me quickly figure things out.
Milling Speed and Feed Calculator

Lets say you are using a HSS tool on Aluminum. An acceptable SFM would be 200 to 600 SFM. An acceptable feedrate would be .0005 to .002 per tooth per revolution.

This is why we have calculators!

Lets use kind of mid range values. So like 400 SFM with .001 feed, and lets use a 1/4" 4 flute endmill.

The calculator tells me to use 6112 rpm at 24ipm (actually calculates to 24.446) from the info I input.

Lets break it down.

To get the rpms, you take SFM X (12/Pi) / Diameter=RPM (or simplified... SFM x 3.8197 / D=RPM) but many people make an even easier formula of SFM x 4 / D=RPM that can be done easier in your head.

For the sake of calculating with the above calc program, we will use 400 x 3.8197 / .25 = 6111.52 RPMs

If we use the easier forumla we get 400 x 4 = 1600 / .25 = 6400rpms
That was much easier and you can do it in your head once you remember it. Also, it is close enough for many people, especially small shops or hobbiests.

To get the feedrate, we are going to feed .001 IPT (inches per tooth), we have 4 teeth, so we would have .004 IPR (inches per revolution)

Now Combine the 2, and you get 6112rpm x .004 IPR = 24.446 IPM (inches per minute) which is your feedrate.

Hope I didn't overwhelm you, its not too bad once you absorb it all, and once you start using it awhile, it will just sink in.

Brian =)