I disagree with many shallow fast fed passes. Slow the feed a bit to keep the forces acceptable, but put that bit in the stock. Millimeter and sub millineter stepdowns in wood are silly to me. You use the same tiny piece of the endmill to do all the work over and again.
If you do 10mm step downs, then you only use the same 10mm piece of the end mill every time, so what's the difference if you only use 1mm step downs? You're almost never going use the entire end mill except during finishing passes in contour operations. There is practically no difference between doing many passes versus a few as long as you engage the wood efficiently in either case. The main reason for choosing a particular step down is to adjust the amount of force it takes to engage the wood ~~ Less wood takes less force to engage.
By the way, I'm only talking about roughing passes here, and that was my assumption in the previous post. Finishing passes are a different discussion. There are lots of times where you want to use the entire bit to get a super clean edge on a contour to eliminate ridge marks left by multiple passes. But for roughing, and pocketing, or any case where a lot of material needs to be removed, then multiple passes are almost always ideal.
This is the order of logic that i use for just about every job:
1) Select the chip load (mm per flute per revolution)
2) Select the RPM (typically between 16000 and 20000 for me)
3) Calculate your feed rate: (Feed Rate) = (Chip Load) * (RPM) * (Number of Flutes)
4) Adjust your step down to prevent deflection and bit breakage.
I adjust the step down based on my experience with the particular wood that I'm working with. Soft woods like Hemlock cut really easily at 1 or 2mm step down with high feed rates. Harder woods, like a domestic Cherry that I'm using right now, I'm happier with 0.5mm step down at 3500mm/min, and it cuts as clean as butter. You do have to have reasonable expectations for the Shapeoko. It's not a super rigid machine, and the 1/8" bits that we use aren't super strong. You can't put a lot of force on either of them.
I am by no means an expert, but thats my two cents.
Neither am I, and I think the vast majority of us aren't. If you find one, please bring them here!
