3D offset Finishing cut reverses direction halfway !

This section is for useful tips and tricks for Aspire
Post Reply
Darryl Kerney
Posts: 4
Joined: Sun Oct 18, 2020 10:24 pm
Model of CNC Machine: Ultimatik Mini

3D offset Finishing cut reverses direction halfway !

Post by Darryl Kerney »

I am trying to make a simulated speaker, so basically a straight angled bowl with a dome in the center for the voice coil dust cap, and it all goes well until running the simulation for the finishing pass of the bowl part, it starts at the edge of the inner circle that is the dome boundary and works it's way round and round outward and upward (after roughing pass of course) BUT then about half way it reverses direction and leaves a slightly wider offset ! I am intentionally using a fairly large stepover with a ballnose bit so it will leave concentric rings like some speaker cones have, but it leaves a very visible ring where it switches direction !
I cannot find anything that would even allow me to do that if actually wanted.
(I will finish the dome in the center with a small stepover so it ends up nice and smooth.)
WHY the heck is it doing that ?
Attachments
speaker 3d finish issue.png

Darryl Kerney
Posts: 4
Joined: Sun Oct 18, 2020 10:24 pm
Model of CNC Machine: Ultimatik Mini

Re: 3D offset Finishing cut reverses direction halfway !

Post by Darryl Kerney »

ALSO, i am using a 1mm stepover retract hoping it will avoid the line you get if you don't....

Darryl Kerney
Posts: 4
Joined: Sun Oct 18, 2020 10:24 pm
Model of CNC Machine: Ultimatik Mini

Re: 3D offset Finishing cut reverses direction halfway !

Post by Darryl Kerney »

SOLVED, i think,
changing from 25% stepover (using 1/4" ballnose) to 23%, it still appears to reverse direction in the preview simulation but the noticeable one wider ring seems to be gone.
Hopefully it actually carves that way.

User avatar
adze_cnc
Vectric Apprentice
Posts: 1052
Joined: Sat Jul 27, 2013 10:08 pm
Model of CNC Machine: AXYZ 4008
Location: Vancouver, BC, Canada

Re: 3D offset Finishing cut reverses direction halfway !

Post by adze_cnc »

That "reversal" happens when you are doing a 3D finishing path between two (or more) vectors. The program is cutting from one vector halfway to the other and vice-versa

One way to get around it is to model a fake dome just higher than your really central area. That way you only need the outside border. You can then turn the fake dome off and do whatever cuts you're doing for the inside part.

The downside is the extra time to "air cut" the fake dome.

Darryl Kerney
Posts: 4
Joined: Sun Oct 18, 2020 10:24 pm
Model of CNC Machine: Ultimatik Mini

Re: 3D offset Finishing cut reverses direction halfway !

Post by Darryl Kerney »

Thanks !
i might have to also do something like that if i size it down to use as a smaller 3d clipart that i saved it to, i found that going too small ruins it, the 1/4" ballnose is too big at some point of course...

User avatar
adze_cnc
Vectric Apprentice
Posts: 1052
Joined: Sat Jul 27, 2013 10:08 pm
Model of CNC Machine: AXYZ 4008
Location: Vancouver, BC, Canada

Re: 3D offset Finishing cut reverses direction halfway !

Post by adze_cnc »

I was thinking. You could use "cuts halfway" method. You'd need to know the distance between the two vectors on the surface of your object. Add any boundary offset you might be using. Divide that by your stepover. Round that result to the nearest whole number. Divide the distance by that number and use that result as your stepover. This should eliminate the leftover line.

As for your bit. I'd say your 1/4" bit is too small. Attached is a project I helped a student out with. It's a 8.25 diameter wooden lid that is similar to a speaker cone.

The finishing bit was a 3/8" ball end bit using a 0.025" step-over. The object required no sanding (as it's wall thickness is 1/8" that's probably a good thing).

I was going to use a 1/2" ball end with the same stepover distance but the tool changer was already loaded with the 3/8" one.

Steven
yy-lid.jpg
lid-concave.jpg
lid-convex.jpg

Post Reply