This is a weak post because my father is in the hospital with health issues after having a minor car accident. That, and I spent most of the day working on getting clients back up and running.

So what are “speeds and feeds”? These are the magic numbers to run your machines at to get the “best” results.

Let’s assume you want to make a 3″ diameter hub on a 4″ diameter rod of aluminum.

The starting diameter for your rod is 4.12″.

We look up in a table, the “speed” of Aluminum. We are using 6061 which has a surface speed of 500-800 feet per minute if using high-speed steel (HSS, old school) or 2800 if using carbide.

This means that we want the single point of a lathe cutting tool should be traveling at 500-800 feet per minute over the surface of the aluminum.

With a diameter of 4.12″, the surface speed is:

$$\frac{4.12\mathrm{\pi}}{12}\mathrm{RPM}=500$$

$$\frac{4.12\mathrm{}\mathrm{\pi}}{12x500}=\frac{1}{\mathrm{RPM}}$$

$$\frac{6000}{4.12\mathrm{\pi}}=\mathrm{RPM}$$

$$\mathrm{RPM}=463.6$$

As we remove material, the diameter gets smaller, so our RPM has to increase to maintain the same surface speed. At 4″ the speed should be 477 RPM. At 3″ it is 637 RPM. At 1″ we need a whopping 1900 RPM.

The speeds required are higher if we use 800 instead of 500 SFM and scary fast if we are looking at 2800 SFM.

My lathe doesn’t spin that fast.

But this is only part of the equation, the next part is “Depth of Cut”. This is how deep you are cutting.

I could zip along at 418 RPM at 4″ taking a 0.010 depth of cut. This means it will take 56 passes before I am at a 3″ diameter.

If I take a 0.050 depth of cut, it is only 12 passes.

Besides the depth of cut, there is also the feed rate. This is the distance the tool advances per revolution.

I have a single tumbler Quick Change gearbox. This means that I have three controls to set the speed of rotation of the lead shaft.

The lead screw is threaded 8 TPI, for every eight revolutions, the carriage will advance 1 inch. By setting the gears correctly, I can choose how much the lead screw rotates per rotation of the spindle.

I normally use a feed rate of 0.007.

So, my depth of cut is picked to be either 0.050 or 0.025, taking 0.100 or 0.050 off the diameter per pass. This means that to make a 3 inch boss from a 4.12 inch rod that is 1 inch long, I have to make 11 0.050 passes, taking off 1.100 from the diameter, then I make two finishing passes of around 0.006.

I normally run at the depth of cut I want, merely listening to the lathe to see if it is straining. If it is not straining, the DoC is good.

Removing metal on a milling machine isn’t as easy. First, the tools are more likely to break, which is expensive.

We start with the same calculations for SFM. Instead of using the size of the workpiece, we use the diameter of the cutter.

With an 1/8″ cutter, we should be spinning that thing at 15,000 RPM in aluminum, and 2500 for a 3/4″ cutter.

I’m cutting soft steel, 1018. The SFM for milling is 100 SFM. Using a 3/4 end mill, I should be running at 500RPM. Since I am using a solid carbide end mill, I push this to 660 RPM.

But what about Depth of Cut and Feed Rate?

Well, that is where it gets interesting. According to my tool dealer, the depth of cut should be Diameter * 0.05 to Diameter * 1.5, or 0.38 to 1.125. I picked 0.200.

The same tables show that my tooth load should be 0.005. This means each tooth should be removing 0.005″ of material.

Running at 400RPM with a four flute end mill, this means that the tool should advance 0.005*4 = 0.020 per rev. of the cutter.

That means we should be advancing 400 * 0.020 = 8 inches per minute.

My old Bridgeport isn’t that stout anymore. I’m running at 5 IPM with a 0.200 DoC @ 400ish RPM.

That is smooth and boy does it hog out the metal. I stopped shop time because it was time to do computer work. Not because I was tired or frustrated.

I should have the next toolholders ready for bluing before the weekend.

RPM=(SFM/DIA)*3.82