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GWizard question regarding finish versus roughing feeds and speeds


I’m curious about the result I get from GWizard, for an 1/4 inch 1 flute carbide flat endmill, DOC 0.25 WOC 0.25 I get range from 16000 rpm and 4.5 IPM for a “fine cut” to a 19000 rpm and 133 IPM for a “rough cut” in hard wood. You just posted an article  11 Feeds and Speeds Considerations for Lightweight CNC Routers  and if I refer to the “sweet spot” diagram, it clearly shows you’re in the green and get a fine surface finish at higher spindle speed and at moderate feed rate. It seems contradicting and the article mention the problems of being to conservative.

I though the idea of using GWizard was to forget all the guess work.


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Best Answer

Yves, one thing that’s not immediately obvious from the Sweet Spot graphic (but if you think about it, the graphic does reflect this) is that it isn’t the absolute values but rather the ratios of rpm to feedrate that govern whether you’ll get more of a roughing or a finish cut.

Let’s look at your two values from G-Wizard:

Roughing:  19000 rpm and 133 IPM.  So we will have a little under 143 revolutions per inch of feed motion.

Finish:  16000 rpm and 4.5 IPM.  Here we have 3556 revolutions per inch of motion–almost 25x more.

To put it another way, that finish pass is making cuts that are 25x finer than the roughing pass.  Any tool marks will be 25x finer and therefore much less distinct and much closer together.

G-Wizard made it possible not to have to worry about any of that–you just move the Tortoise-Hare slider to reflect whether you’re doing Roughing or Finishing and away you go.

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