…code, cg, electronics, 3d printing…

I built my X-Carve back in December:  It’s been a great new tool to learn.  I’m still very new to the world of CNC, and like to visually grasp the concepts.  So I decided to do a series of tests to understand how ‘stepover’ values effect the finish-pass quality of the surface both on X, and on the XY axes.

The MeshCAM blog does a great job of describing the fundamentals of stepover here.

Here are the stats for the cuts:

• Hardware:  Inventables 1000mm X-Carve.
• 1/4″ ballnose bit, 2-flute upcut.
• Feedrate 60ipm, DeWalt set 1 to 2.
• Wood type:  Unknown (came from an old bookshelf bottom), but if I had to take a guess, I’d say pine.
• 3d Design Software: Autodesk Maya
• CAM: MeshCAM
• Sender: Chilipeppr

The specifics from MeshCAM below. All values for all cuts were the same except of the stepover, and either “Cut along X”, or “Cut X then Y”.

I wanted really extreme examples, so I set the following stepover percentages for my test: 100% (1/4″), 75%, 50%, 25%, 10%, 5% (only done on X, not XY).

I started by designing a model in Maya that incorporates a variety of surface angles.  The inside volume is just over 2×2″, by about 1/4″ deep.

 (that’s a flattened sphere in the middle)

I then made multiple different gcode (nc) via MeshCAM, and started cutting them.

---

The whole piece for the X-cut:

And the whole piece for the XY cut:

 (note, no 5% test here)

---

Individual close-ups below.  X pass on the left, XY on the right.

Note the rough-cut for all pieces took just about exactly 2 minutes.  All the times listed below are for the X & XY-Axis Finish pass in min:sec.  So to get the total cut time, just add two minutes to the below values.

---

 

• 100% stepover, .25″ : This is obviously super rough.  I honestly expected the segment to be closer together.
• X Finish Pass Time:  0:47
• XY Finish Pass Time : 1:34

---

 

• 75% stepover, .1875″ : Not too much different than 100 really.
• X Finish Pass time : 1:03
• XY Finish Pass time : 2:03

---

 

• 50% stepover, .125″ : Still really rough, but arguably could do something artistic with the ridges at this point.
• X Finish Pass time: 1:30
• XY Finish Pass time : 3:00

---

 

• 25% stepover, .0625″ : Carry on, nothing to see here.  Even with the XY pass, it’s still pretty rough.
• X Finish Pass time: 2:50
• XY Finish Pass time : 6:40

---

 

• 10 % stepover, .025″ : Now we’re getting somewhere: Ridges are still visible, but small.  Pretty smooth to the touch, but you can still make them out.  Sanding could take care of this.
• X Finish Pass time: 7:10
• XY Finish Pass time : 14:00

---

• 5% stepover, .0125″ : Done.  Finished.  Can’t make out the ridges with the naked eye.  Very smooth to the touch.  No sanding needed really.
• X Finish Pass time: 14:20
• No XY pass done.  Not much point considering the quality already achieved.

---

Final thoughts:

• Notice on all X-cuts that the lower-left section of the hemisphere is rough.  Must have to do with the direction of the toolhead (left<>right on X) and the spinning of the bit (clockwise).  The XY cuts removed these issues.
• If you are ok with sanding, 10%/.025 stepover is ok.  If you want to avoid sanding entirely, go with the 5%/.0125″ stepover.
• Even though the 5% X-only stepover and  10% XY stepover took the same amount of time, the X-only has a far better surface quality.  You’d still need to sand the 10% XY one.
• What do I take away the XY Finish pass?  The XY Finish Pass times are generally 2x the X-only times, but don’t really increase the quality.  Not much point unless you’re looking for ‘that look’ in the cuts.
• I feel like the speeds could be greatly increased on the finish pass:  I was only running the router on speed 1 to 2.  The smaller the stepover, the smaller the amount of material you’re removing, so arguably the faster the toolhead could move to compensate for this under load:  There’s a lot of speed left in the router…. sounds like another good test to try.