Stepover tips for smooth surface?

[Emmess (Matthias Solga)]

I plan to cut some cork coasters, would like to use a bowl cutter Amana 45982 Carbide Tipped Bowl & Tray 1/4 Radius x 3/4 Dia .

I try to get the smoothest surface possible and try not to waste too much trying. So what are your recommendations in regards to stepover?

[ksymons (Ken Symons)]

Not sure if it’s the exact same application but I use that bowl bit to carve some holiday trays. I usually do a rough pass then a finishing pass. You can see the depth of cut and step-over settings in the attached file. It is nice not to have to do much sanding if any at all.
ChristmasTreeTray.c2d (1.5 MB)

Also, my machine is a SO 5 Pro with the 65mm VFD spindle.

[QCDvacuum (Jeff Cable)]

This may help. Milling Step-over Distance Calculator

[mhotchin (Michael Hotchin)]

This bit has a 1/4" flat section on the bottom, so for a smooth surface you need to have a stepover of <1/4". I would choose my depth / DOC so there there is only a small amount removed on the final pass, so the bit is under less forces for that pass.

HOWEVER, there is a bug on pocket operations where the first stepover can be up to twice the value that you specify. If that happens with this kind of bit, you’ll be left with a ridge down the center.

Bowl bit leaving material behind in the middle

I have noticed that sometimes CC uses the wrong step-over (too large) for the second pass when doing a pocket. All the other passes are right. I haven’t spent the time to chase it down, but it’s clear even on the preview that the first step-over is larger than the rest. EDIT - yes, CC is using the wrong step-over for the first step-over. This is a 3/4" bit, 8mm stepover. Grid is 10mm. [image]

It depends heavily on the size of the pocket, the bit, AND the stepover, so the only practical way to see if it affects you is to look at the toolpath and decide if the first stepover is too big. If it is, you would have to lower the stepover. Worst case is you would need a stepover < 1/8" to work properly.

Bug - Pocket toolpaths using wrong stepover

As a new discovery, this high initial step-over can cause parts of the pocket to not get machined. If the stepover is more than half the width of the bit (perhaps not the best practice, but still a valid thing to do), you can have material left behind. Since the initial stepover can be double the requested stepover, and the stepover is more than 1/2 the bit diameter, as a worst case scenario the initial stepover is more than the width of the bit. An exaggerated example - 3/4" bit, 14mm stepov…

[Emmess (Matthias Solga)]

Thanks a lot! I think it is a similar concept
thin coaster.c2d (276 KB)
, so I can use your recommendations and ideas. The least: I learned that the different cuts even can get names!

[Emmess (Matthias Solga)]

Jeff, thanks, great website! And the specific chart helps a lot! Reduces guessing and trials to a minimum.

[Emmess (Matthias Solga)]

Thanks for the heads up. I think CCV8 is a big step forward due to the great preview feature. And yes, the warning to use the preview as an important tool for checking will save tons of trees…

[QCDvacuum (Jeff Cable)]

No worries, Matthias. While I always appreciate the practical aspects of learning practical and physical skills, sometimes knowing where and what to look at is very valuable.

[SDGuy (Anthony)]

Here’s how I do it.
It’s the same bit that you have .75 with a .25 Rounded Edge.
My first pass is a roughing pass.
.375 stepover, .05 DOC
20/40
Total Desired Depth is .5, Roughing pass goes from 00 to .48

2nd Pass is Finish Pass
.0625 stepover .05 DOC
20/40.
Finish Path Starts at .48 and finishes at .5"
This saves A TON of sanding!

Bowl Bit Catchall.c2d (80 KB)