Chip characteristics as a sign of good/bad cutting parameters

Being the naturally cautious type, I have yet to try my Nomad with anything other than wood. Lots and lots of wood, while I try to figure out how to use this wonderful tool properly. :smile:
But one thing I’ve noticed is that the chips being thrown off by my cuts are a bit odd. I’m ending up with a mix of sawdust that’s nearly the consistency of talcum powder, and some long soft “strings” of wood that tend to wrap around the cutter, cling to cut edges, and jam up my vacuum cleaner (ShopVac Micro). I get this in both “raw” wood and plywood. Most of the wood I’ve tried to far has been craft-supply Basswood.
Now, this doesn’t seem to cause any actual problems with my cuts, but it go me to thinking (bad habit I have), could this be a sign that my cutting parameters aren’t optimal? And, thinking more generally, are there rules of thumb out there for using your chip characteristics to optimize your cutting parameters? A lot of this probably falls into the category of “the black art of experience,” but given how the Nomad is aimed in part at complete amateurs (like yours truly), I thought it might be worth throwing out there to see what the experienced woodworkers and machinists might have to say.

Hi @SkyeFire,

Great question—[PreciseBits][1] has a lot to say about how to optimize your feeds & speeds with micro-tools, and more generally with cutting wood and other non-metallic materials. You can find a lot of advice out there for feeds and speeds, but generally speaking it can be boiled down to:

When roughing, load the bit with as much materials as you can—without breaking it or bogging it down/losing steps. When finishing, it depends on the desired surface finish quality desired.

For “chip characteristics” it varies depending on the material, but generally speaking you’ll want actual chips rather than a powder. Long, stringy pieces is just typical of softer woods like basswood due to the flexibility of the fibers. You can reduce the stringiness by using “climb only” roughing passes in such woods. If you’re getting a powder, your chip-load is probably not high enough.

The idea is that if you’re not cutting with sufficient chip-load, you’re wasting time and you’re wearing out your tools faster because you’re burnishing the work-piece (rubbing it) instead of cutting it. This rubbing is just causing frictional heating, which is bad for the work and the cutter edge, unless it’s designed for that and that’s the intent (there are some engraving burnishing tools out there).

You’ll start to figure out what the mill sounds like when it’s running and it’s “happy” as opposed to when it’s not. Generally speaking, squealing is not good, and excessive vibration is not good. You want a consistent low hum/buzz as it engages material, which will be less pronounced the smaller your tools are. With tiny tools, as the precise bits article says, “silence is golden”.

Therefore, if you were a production shop, you’d run test cuts at increasing loads until you broke a tool, and then you’d back off a step from those, and run at that load :wink:

Since we’re hobbyists for the most part though, I’d suggest finding suggested chip-load ranges for the material you wish to cut, and the cutter you wish to use, and then shoot the middle of the range to start out. Of course, you could also invest in a copy of [G-wizard calculator][2], which would help answer a lot of those questions for you as well!

Best,

Jonathan
[1]: http://precisebits.com/tutorials/calibrating_feeds_n_speeds.htm
[2]: http://www.cnccookbook.com/CCGWizard.html

To follow up on that thought, I have bought a bunch of inexpensive Chinese 1mm and 0.5mm carbide end mills on eBay. I’m too ignorant to assess the quality by looking at them, although based on the price point they can’t be outstanding.

I used Precise Bits’ equation that said for soft woods like pine or fir, feed rate should be 0.02 x mill diameter x no. flutes x RPM (2% chipload per tooth). In practice, this yields a result that’s too slow. I ended up gradually ramping up to values three times higher and have been mostly successful, which leads me to believe I misunderstood something somewhere.

I’m slowly building up an understanding of the materials I’ve been using - but my wife just gave a me a big box of hardwoods for my birthday, including pau ferro and other very hard woods. Others have suggested posting results in the Wiki somewhere. Although I may end up investing in G-Wizard too.

I think this forum is a good place to post details when we show off our stuff as gallery posts :slight_smile:

Don’t just post in the wiki, post here too! We’re working on documentation, and will draw heavily from what’s posted here from the excellent work by the community :relaxed: