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I get mail with that kind of message every so often.  A machinist uses G-Wizard Calculator and it gives them a higher feedrate, sometimes quite a bit higher, than his CAM software or other feeds and speeds calculators.  The chip load is the same, and the formula to go from chip load to feedrate is simple, so they assume there must be a bug in G-Wizard.  In reality, the formula to go from chip load to feedrate is not so simple if you take radial chip thinning into account.

Radial Chip Thinning is a change in the geometry of the chips being formed as you take lighter and lighter cuts by reducing the Cut Width as a percentage of tool diameter when milling.  This diagram shows how that geometry works:

Radial Chip Thinning results in thinner chips as the cut width is reduced…

It’s pretty obvious from the diagram what’s happening and why the geometry results in thinner chips.  So what should we do about chip thinning?

The answer is simple: we need to stick to our guns and get the chip load we asked for.  To do that means we have to increase the feedrate to achieve the same chip load we would’ve had in a case where there was no chip thinning.  G-Wizard Calculator will do that automatically so you don’t have to worry about it, but most cheaper calculators and built in CAM feeeds and speeds routines do not consider chip thinning or a whole host of other factors that G-Wizard will automatically account for.  To give some more geometry-related examples, consider the diagram above once again and how that geometry works.  In the diagram we’re looking down the spinning axis of the endmill.  But suppose the circle was the tip of a ballnosed endmill or perhaps the circular insert of a button cutter.  What then?

Yup, you guessed it, more chip thinning calculations need to be made for those geometries.  You can chip thinning simultaneously in more than one axis, in other words.  There are other geometries that have similar chip thinning effects.  Consider a face mill with a lead angle.  Instead of a 90 degree shoulder, suppose it has a 45 degree lead angle.  You can visualize that on the diagram and see that it’ll result in a form of chip thinning once again that needs to be accounted for.  G-Wizard handles all of these situations and a whole lot more besides.

What if we just ignore chip thinning and use the “simple” Feeds and Speeds Formulas?

I hear this most often from hobbyists who want to baby their machines and run them as slowly as possible.  They suspect that chip thinning is something that only matters for extracting the last bit of Material Removal Rate in a production scenario, and that’s something many hobby CNC’ers couldn’t care less about.  Instead, they want better Tool Life, and isn’t that what babying the cut will give them?

Well, life’s not always fair and the physics of machining and chip making are not always obvious.  It turns out that the lighter the cut you take, the more pronounced is the chip thinning effect.  You may choose to run a cut width of 10% of the cutter diameter and think you’re really taking it easy on that ole cutter.  In addition, you want to skip the chip thinning, take the conventional feedrate calculation without chip thinning, and maybe cut that in half just to give an even better safety margin.  That’s got to be super easy on tool life, right?  Turns out, maybe not so right.  Welcome to the world of tool rubbing which can radically reduce your poor cutter’s life.

Imagine what a highly magnified view of the cutter edge slicing up a chip looks like:

The top edge is cleanly slicing a chip, the bottom one is ploughing or rubbing its way through the material…

As we reduce the actual chip load, we can reach a point where the chip thickness is less than the radius of the cutting edge.  No matter how sharp your cutter is, that edge has a finite radius and you can program a set of parameters that will cause a cut with chip loads less than that edge radius.  When that happens, the bulky tool edge can’t get under the chip to slice it off cleanly.  All the material sees is the underside of the edge, which burnishes and rubs its way along, generating a lot more heat.  Normally, the sliced off chip carries the heat away, but in a rubbing situation that heat stays concentrated in the surface of the work material and in the cutting edge of the tool.  Once the tool gets hot enough, it softens, looses its sharpness, and succumbs shortly thereafter.  It’s because of all this rubbing that ignoring chip thinning and thinking shallow cuts fed very slowly are conservative is a recipe for sharply reducing your tool life.

Sometimes we have to engage in cuts that are slow and lead to rubbing.  I’ve talked to some machinists who use old cutters and intentionally run them in a rubbing mode to burnish the surface and deliver a finer surface finish.  One fellow even swears by running the cutters backwards to maximize the burnishing.  No harm in that as long as you know when you’re rubbing.  For that reason, G-Wizard gives a rubbing warning any time you’re going slow enough that the rubbing begins to be a risk.  You can read about all this and a lot more feeds and speeds theory in our free Feeds and Speeds Tutorial.  There’s a tremendous amount of data there.  And, if you’d like to try a Feeds and Speeds Calculator that handles all this and more, our G-Wizard Calculator is available on a free 30-day trial.  Give it a shot and see what you think.

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