Help Me To Understand Spindle Speed, Feed And Depth Of Cut.

hoth2o

hoth2o

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I have been doing some studying in both the machinery's Handbook, and the tutorials for my lathe( EMCO Compact 5 PC) I'm trying to understand and get a "feel" for programming the lathe to do what I want. So lets say I want to turn a profile in .75" brass stock. (not sure what exact alloy, but it's brass). I'm going to use a HSS cutter. What RPM do I want the spindle running at? What feed rate? What depth of cut.

While I want the answers for this, I really need someone to walk me through HOW I get these answers. If I can get this example figured out I feel I will be on track to aplly this to other materials and diameters.
 
Feed is easy. You want to take between .0015 to .005 per revolution. More is possible, depending on material, but in brass it will spray tiny bits of brass like a water hose.

Depth of cut. Well it depends on how rigid your lathe is. My 6" Atlas Clausing will take up to .050 on a side in aluminum, but at low RPM, first, lowest speed. Its a pretty light lathe, weigh less than 100 lbs. Cutting steel, I can take up to .020 on a side. A 10 inch South Bend lathe should be able to take .125 on a side if you can duck the chips. I have buried 3/8" of carbide in mild steel on a really big lathe.

RPM is a function of material vs cutting tool. Carbide cutting tools and aluminum, you can't turn your lathe fast enough to fully utilize carbide. The rule of thumb I learned was 90 surface feet per minute for mild steel and High Speed Steel tools. Carbide, four to six times faster. Better grades of steel just slow it down until your chips come off white and turn blue.

Different metals need differing surface feeds, brass is faster than steel, probably 3 to 4 times faster, bronze, until you get familiar with it, use steel speeds.

After 30 years in the trade i go by intuition, not numbers.
 
Bob, that video was very helpful!


T Bredehoft said:
Feed is easy. You want to take between .0015 to .005 per revolution. More is possible, depending on material, but in brass it will spray tiny bits of brass like a water hose.

Depth of cut. Well it depends on how rigid your lathe is. My 6" Atlas Clausing will take up to .050 on a side in aluminum, but at low RPM, first, lowest speed. Its a pretty light lathe, weigh less than 100 lbs. Cutting steel, I can take up to .020 on a side. A 10 inch South Bend lathe should be able to take .125 on a side if you can duck the chips. I have buried 3/8" of carbide in mild steel on a really big lathe.

RPM is a function of material vs cutting tool. Carbide cutting tools and aluminum, you can't turn your lathe fast enough to fully utilize carbide. The rule of thumb I learned was 90 surface feet per minute for mild steel and High Speed Steel tools. Carbide, four to six times faster. Better grades of steel just slow it down until your chips come off white and turn blue.

Different metals need differing surface feeds, brass is faster than steel, probably 3 to 4 times faster, bronze, until you get familiar with it, use steel speeds.

After 30 years in the trade i go by intuition, not numbers.
Click to expand...

Thank you for this info. It helps as I try to get my head around this.
 
Feed is determined by chip load per tooth. There are charts available for that on the web. Once you know your desired chip load for the material, let's say .005" as an example, you select .005"/rev. from the feed choices on your lathe gearing and get busy. If it were a mill you would multiply .005" times the number of cutting edges (four on a four flute end mill), times the rpm. On the mill, turning a four flute cutter at 500 rpm with a desired chip load of .005", becomes .005" times 4 times 500, which equals 10 inches per minute of machine travel.
 
Depends entirely on the machine and the finish required, in 3/4 free machining brass I would turn as fast as the machine will go with carbide insert tooling, depth of cut would depend on the length of the part if unsupported and .150" if supported and more if it is sticking out of the spindle 2 diameters or less, feed at .01 to .015 for roughing, .10/.02 DOC at .004/.006 IPR for finishing depending on diameter and surface finish requirements.

Do not be afraid to turn 360 brass stock from .75" to .437" in 2 passes if the part is rigidly held and short.
Stand back if doing so on an open machine however.

As to your general question about feeds and speeds this is so dependent conditions that many books have been written on the subject over the last 150 years. Begin at Machinery's Handbooks numbers and then adjust everything up or down to suit conditions and effect desired results, this is how machining works in the real world.

Good Luck
 
Last edited:
I just know feed and speed from years of machining. Material, machine condition and cutter sharpness all figure in to the equation.
 
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