After I built my MPCNC and installed the recommended Dewault router I started going through 1/8 bits like crazy (bending and breaking) so I switched to 1/4 bits. Bought the Freud and Whiteside CNC bits and they’ve worked great, but the cost is pretty high.
So my question is this: What are the advantages to the 1/8 inch bits over 1/4 bits?
Tighter corners (inside corners) is one of the big advantages. I mostly use 1/4” bits as well. With the 1/8”, what settings are you using for DOC and travel speed? Sounds like one (or both) of them might be too high for the narrow shank of the 1/8” bit.
It was both. I was pushing them to the limit, mainly as I wanted to understand the limits of the size. My major problem came when I wanted to do a 3D carving and I didn’t have 36 hours for it to run, 
I agree something is off can you show us your tool settings? Most everything I do is with 1/32"-1/8" bits. 1/8" bits leave a cleaner bottom in pockets, they are cheaper and have a tighter radius.
The long run times on some of the things I’ve been messing with are killing me as well. I was using Fusion and for one of the things I was looking at 4’x8’ carving for a sign in foam board was showing over 166 hours!!! Lol. Needless to say, that’s not gonna happen!!
Holy smokes! I think my longest cut has been 6 hours. If it’s a sign I’m assuming you were cutting everything down so the letters stood proud? What about cutting letters out then doing a shallow pocket in the backer for letter placement and glueing them In place?
That’s exactly the process I am working with now. When I saw that 166 number I thought it was in minutes at first. lol. Cutting the letters out and then gluing them is the way to go.
If you’re cutting out parts, then you really only care about the linear time, and you can cut that faster with a 1/8" because you’re removing less material. Pockets could probably done a little faster with a 1/4", but not by much, because you’ll be going slower to have the same force. 1/4" bits can have the same absolute stepover, lower relative, which will speed things up.
IMO, the 1/8" is a great balance. I think there’s a reason why miter saw and table saws have a standard 1/8" kerf.
I don’t see breaking bits very often though (except in aluminum). They should get dull before breaking. The trick is deeper passes, slower, which will make the force of the work distributed across more of the bit, and make for longer lasting bits. The limit to that is if you are cutting a deep slot, you need to get the chips out quick.
Pockets are the worst.
If you have bent a bit you are not buying the right thing.
So what I think you’re saying is that if I use a 1/8 endmill I should make my Z step 4mm @ 10ms as opposed to 1mm @ 30ms. With the 1/8 and smaller bits I’ve been very conservative with both linear speed and and z step. There doesn’t appear to be a clear consensus on a rule of thumb (i.e linear speed x bit diameter / 2 is max Z). If you have any advice I’d sure appreciate it.
Ryan the one time I bent the bit was when I was moving too fast.
I think my overall take away here is that 1/8 and smaller bits are useful, but maybe less forgiving until you understand the best movement parameters to use.
I want to do more detailed 3D carvings like animals to adorn box tops and such and probably need to learn to use 1/8 and smaller bits… but I don’t want the 5+ hour times. So any advice would be greatly appreciated.
Experience really helps, each cut try something a little bit different. There is a calculator on the basics page if you really want numbers but for no machine ever are they perfect. Test cuts are your friend.
I have never seen carbide or even HSS bend, normally they shatter. Not sure if they can or not but bending sounds like a quality issue or just the wrong bit overall.
Test cuts. Instead of a 5 hour cut why not try a small section a few times. Sure at first it takes longer but then you will understand from there on out how to get in the ballpark of max capacity. The difference between 1% and 5% step over on a 3D carve, to the eye maybe nothing, to the machining time 5X less. You have a ton of variables to adjust, feedrate has very little effect as compared to DOC on times. Even depth passes, bit sharpness, chip evacuation, quality and quantity of knots in the wood. Experience is the best way.
It is much easier for you to make a few cuts and then ask for changes that can be made, providing all the specs, gcode, and some pictures, but just asking for the best numbers will not work.
I’m assuming you meant mm/s?
But yes, that’s right. using just 1mm will mean all the force is at the very end of the bit, that part of the cutter will get hotter, and it’s doing all the work. Meanwhile, you’ve got perfectly good cutting surface right above it that’s not doing anything. Plus, if you’re going at 30mm/s and you hit something that makes you bite 4mm instead of 1mm, you’ll break the bit or skip steps.
The limits to that logic is, you need to get the chips out, so 16mm depth of cut wont work on a slot (usually).
Every machine is different, but 30mm/s is probably too fast, and deeper up until the point where you’re recutting too many chips is better.
I’ve done some dumb things with my machine, but I skip steps before breaking 1/8" bits.
Thanks Jeff, Ryan and others. Time to rethink my work flow and approach.