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Chief bozo

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I am confused! an easy thing to happen. The “feeds and speeds” numbers that I am used to working with are in the neighborhood  of 400 SFM for aluminum. Yet the Scotsman 350 NF coldsaw with a 13.75″ blade turning 3000 rpm is more than 10,000 SFM. The Feeds and Speeds calculator won’t even calculate the real number, it maxs out at 10,000. So how can we get away with such high velocities with a cold saw and not with a cutting tool in a mill? Is it chip removal?

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Lynn, Surface Speed is all about temperatures.  The goal is to keep the temp from getting high enough to soften the cutting edge.  Once it softens, it becomes dull in a hurry and tool life suffers.

The saw blade has the luxury of being both beefy to hold a lot of heat and that it spends a fair amount of time in air cooling during each rotation.  In this respect, it’s kind of like what happens with a very small width of cut and a high speed machining toolpath for an endmill.  I plugged in the figures (their catalog said its a 12.5″ saw diameter, so I used that with 3000 rpm) and I get 9817 SFM for the surface speed.  

Their catalog says to cut no more than 3″ diameter solid.  This will be like limiting the width of cut on an endmill–3″ is 24% of diameter.

So just for grins, let’s see how an endmill does.  I’m going to do a 1″ diameter TiAlN Carbide Endmill.  Cut Depth will be the saw blade’s thickness of 0.125 and Cut Width will be 0.24″–24% of diameter.  

Now just for starters G-Wizard is offering this as a 1725 SFM cut.  If we switch on the HSM, the recommended RPM’s jump to 12548–pretty close to the saw.  The Surface Speed is now 3285.

We are still not getting the 9800 SFM the saw is at, but the geometry is totally different on the endmill and inferior for cooling because it has to work harder at chip evacuation.  

I don’t find these two numbers to be that incompatible in light of all that.

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