V-bit cuts away more in the corners?

I noticed when running some text that my 60 degree v bit takes more material out of corners than it should be and doesn’t fully clear the bottom of the pocket.

It seems similar to dog bone-ing, but the shapes are definitely more angular. I tried making an Easel equivalent to the bit angle testing described in this video. I don’t think any of the lines from 62-58 degrees look particularly egregious though, and I can’t tell if the reason I’m thinking of them as curved is just the extra material taken off the corners.

I’m running it at 30in/m, 10in/m, 0.03" passes. Is there something I can do differently to fix or at least minimize this?

What is your Vbit step over in the machine settings?
V bits have to be set at a very small step over to produce a flat pocket floor. Kinda like asking a needle to do a chisels work.

The V-Bit Detail Step Over is at 1%, and normal Step Over seems to be 40%. The phrasing including “detail” makes me worry if it’s only using 1% step over if the v-bit is assigned as the detail bit, but even when I was using an 1/8" straight for the rough cut, the floor still looked like my first image.

I just went through a whole hassle with poor finish on pocket floors, turned out my machine was lifting itself & over the course of about 9 months actually losened the hold down screws.
Look at your 1/4" bit is it flat on the bottom, or is it angled like a drill bit (it will not be that sharp but if it is pointed it is going to leave grooves or ridges. All of my 1/4" upcut bits are a little pointed on the bottom and none of my down cut bits are. I suspect that is just coincidence because all my metal milling bits are flat on the bottom.
Edit to add
the only time I use 40% step over is if I am using a big flattening bit and entering it as a 1/4 inch bit. Normally I use between 10 and 20% on the 1/4" bits. Takes a little longer but gives generally better finish.

Which screws are the hold down screws? I figure I might as well check that while I’m on this.

The ends of drill bits are convex, but my 1/8" straight endmill (just the kind that Inventables sells) is honestly more concave on the end, with center being the highest point and the outermost edges of the flutes being the lowest points.

I’ll try running a 20% stepover. Do you think that will help with the corners as well, or just with the floor quality?

The hold down screws hold the machine to the table, caused by my dust boot design, which I did, but the movement of the machine relative to the spoilboard left me quite a bit of sanding till I discovered the problem.

So your 1/4 inch bit is a little bit pointy? just not so much as a drill bit? that means you will get small lines on the pocket floor. will see if I can find a picture or two of what I mean.
The 1/4 inch bit is not going to affect the corner, But the ramp angle setting on your software might, only for the Vbit. you will have to try it. (might is the operative word)

Google square end end mills to see what I mean.
pics will show you, a square end mill will give a smoother floor.

Is this on a Carvey?

The floor looks like different depths with the endmill and the v-bit.
The corner issue looks like some mechanical play…something is loose. What happens if you rerun the v-bit? Basically run the file twice.

Ah, I don’t think the hold down screws are relevant to me since I have an unmodified Carvey.

See how it’s a little concave? Is that flat enough count as a square endmill?

It looks like I was running my tests with the 20 degree ramp setting, so I don’t think that’ll help.

It is on a Carvey :upside_down_face:.

I was thinking that it might be something with the endmill and the v-bit, but the same effect happens when I run that text with just the v-bit.

Run the file twice in the same spot or next to each other to compare?

Yep, the hold down screws are only a flag for others to make sure your machine is staying relative to the spoilboard, mine was being lifted by the dust brush and it took me weeks to spot it.
Try that bit, but I would start out with 10% just to go to the extreme, unless it is taking too long to carve. You can tell with simulate, and it will tell you an aprox time to machine.
Does the software allow you to change the ramp angle if so try it at 5deg and see if it changes. Someone more knowledgeable may be able to tell you exactly what is happening, I am just a CNC beginner with 60 years woodworking experience.
You have started on a learning journey that will continue for as long as you keep CNC’ing.

Each of us here has made mistakes, wrecked pieces, busted Bits, you name it someone here has done it. Wait a year and see how much you have improved.

Same spot.
If you’re seeing the same issue with just the vbit, you may be seeing an issue with your Z axis. Can you move the spindle up and down at all by hand?

Alright so I changed the Step Over value to 20%, with the V-bit Detail Step Over still at 1%. The first triangle of this next set has the 20 degree ramp I was starting with. The next two triangles were only run one time each, with the variables being 5 degree ramp or no ramp.

The one I ran twice definitely has the best finish of any v-bit cuts I’ve made recently.

I don’t feel any play in the spindle when I try to move it. It feels very solid.

This is hard maple if it makes any difference.

Those corners are due, most likely, to deflection. The lateral forces on the endmill are different depending on the depth. There could also be mechanical slop. Have you done any maintenance on the Carvey?

Yes, I’ve replaced the spindle, replaced the smart clamp, adjusted the G28 location to be more centered over the smart clamp button, and more. This machine belongs to the library makerspace I work at, so I know someone completely disassembled it for some reason over 2 years ago before I started working here, but I don’t know the full maintenance history on the machine. I think it’s safe to assume that nothing is untouched lol

Just for fun try changing the V bit step over to 0.5% which will reduce the side loading on the bit.
PS, my favorite wife chairs the local library board here, you have my sympathy & support.

Looks like Easel won’t let me use a step over value that’s less than 1. I did change the normal step over value down to 10% and it increased the length of the cut slightly, but the corners are still the same as the triangles I’ve only run once.

Also thanks :slight_smile:

Did the smaller step over help with the smoothness of the sides that were Vcarved, just curious because if it did I will pay attention to that for my sloped sided cuts. Cheers.

When I run my nails up and down on the walls of the 10% step over, they’re much smoother and don’t catch like on the 20% step over walls. I don’t visually see a difference though, so whether it’s worth it or not may just depend on how much time it adds to your designs.

This time, when I ran it twice (using the hack from here), it went deeper the second time. The above picture is from when I paused it in the middle of the carve. Anyone have any more ideas for what could be causing this?