A question about turning aluminium

[9W Monighan]

http://www.google.com/url?sa=t&rct=j...IsL7PqOLb5_D-Q

Here is a post from another forum describing it:
Cutting Speed or Surface Speed

Cutting speed and surface speed are terms interchangeable in their usage. These terms and their intent are often misunderstood. To help define cutting speed I will use an illustration.

You and a friend are driving down the road. Your friend reaches into the back seat of the car and picks up a large tool bit. He then takes the tool bit and jams it down through the car’s floor and into the pavement below. As this tool bit is carving up the road like a plow he asks you what the cutting speed is. You look at the speedometer and say "One mile per hour." That is the speed you are moving over the surface and therefore represents the idea behind cutting speed.
So the definition of cutting speed is how fast the tool is traveling through the work while making 'chips'.

Revolutions Per Minute
We need to make the transition from straight line speed to rotational speed because in lathe and mill work we deal with rotation. In the lathe the work rotates; however on the mill the cutter rotates. Diameter and Pi are the factors equating revolutions per minute with straight line Cutting Speed.
Cutting Speed terms are in feet per minute while bar stock and milling cutters are in inches. We can use twelve to convert feet to inches and three to approximate Pi. The formula for RPM is then four multiplied by the Cutting Speed divided by the diameter of the work or cutter being used.

RPM = 4 x CS / Dia.

Look up metal cutting speeds in available reference tables and then experiment to optimize them for your production.


Something Interesting!

One Mile per Hour The speed of an automobile traveling sixty miles per hour equals eighty eight feet per second. A reasonable cutting speed for mild steel using a High Speed Steel tool bit is near eighty-eight feet per minute. Since eighty-eight feet per minute is sixty times slower than eighty-eight feet per second a tool bit cutting mild steel is only traveling one mile per hour.

Cutting Speed is Not Cast in Stone
The actual values for cutting speed are only possible starting points and should be verified by experiment depending on the job. A common consideration for establishing cutting speed is the time it takes to dull a tool. It may be more profitable to run faster for higher production and replace the tools more often. The other way might be to run the tools more slowly for endurance and machine unattended. There is a big variation here so after selecting a starting speed tune it up for the work to be done and the desired productivity.
Increasing the cutting speed from below two hundred surface feet per minute to somewhere above, most materials act and cut differently. A soft aluminum casting run slowly can get gummy and the tool will cut tearing its way along. However if machining at a higher speed, you can't hardly go too fast in aluminum, a cast surface may smooth out and shine like a mirror.
In most applications cutting tools do not have to attain the recommended cutting speed to work properly. A one sixteenth inch end mill would need very high RPM to cut brass or aluminum. It's really the chip load and depth of cut which for the most part makes a tool work effectively.