I had a very successful run of chess pieces as more or less a prototype of “could I do this on the Shapeoko”? I was beyond thrilled and am going to try an improved run with the following goals:
- better two-sided flipping
- putting all pieces per color into one piece of stock to optimize cutting vs. tool changes
- this topic: improving yield (reducing defects)
By far my biggest hurdle was transitioning from roughing to finishing on the thin ring-like features on the pieces. It’s hard to find good examples of this as I scrapped them and of course didn’t use my worst results for my build or glamour shots Here’s an example, though (bigger pic in case the upload gets reduced):
My key strategy after observing where and why these chips occurred was this:
- after roughing, my main finishing pass is a parallel path
- if you imagine starting at the base and doing very small step sizes toward the head, the tool will come to the apex of those rings, at which point there is very little material behind the ring since you’ve machined it away already
- on the approach to the apex and subsequent steps toward the back side of the ring, passes can rip out chunks because there’s no “meat” to support the stuff you’ve already finished
Here’s my attempt to put it in pictures.
- on the left you’re seeing that ring from the top (pawn head to the left, base to the right); on the right is a view from the bottom
- the remaining stock after roughing is shown in blue, final part in dotted black, and red is the tool (top down view on left of my 1/8in ball end mill)
- the parallel pass has come down to the bottom of the cut (i.e. the center “parting line” of the piece since I’m using 2-sided machining)
- the bit advances toward the head by it’s step size, and as it takes the next “bite,” it’s removing the green (which includes stock and some chunk of the ring meant to stay)
- my observation has been that since this ring is so delicate, as it takes this “bite” (particularly toward the center line where a) the axial advancement “jolts” forward and b) the flutes are perpendicular to the part shape), it rips out some of the finished ring because there’s no “meat” to support it as it’s already been finished and is by nature so thin
To combat this, I started using a scalloped toolpath machining from inside → out. If you do this within a sketched box in Fusion360, you essentially machine from “highest/furthest from part axis” to “lowest/closest to part center.” In this way, you’re constantly machining stock toward the direction of “more meat.” You finish the most delicate peaks of the mountain when there’s till meat “downhill” on both sides.
With all this backstory, I’m wondering if there are folks doing intricate/small work like this (thinking of 2.5d carvers for example?) who might have additional strategies for me? In particular, I’m wondering about:
- should be using a down cut spiral to further, as the stress on the next-to-be-cut chips is toward the part vs. ripping up/away? I can find plenty of info about clean lines on signs and bottoms of contours between up/down cut, but not on things like carving
- I have flip flopped between feed rates. I’ve tried faster passes, as I’ve felt like more time spent by the bit on each area increase the chances that any given rotation will chip out. I’ve tried slower passes thinking I should baby these areas with tiny bites
- I also don’t have a sense for how much axial/radial stock to leave in my adaptive pass, wondering if too little hurts by not leaving enough “meat” to support the delicate areas, but too much might be hurting because finishing is ripping away bigger chunks, which might in turn be more “rooted” into the surrounding wood and chipping
If you made it through all of that, thanks for coming along for the ride and I appreciate any input!