CNC 2D pocket too narrow

[paulgregorytaylor]

Hi all, I did a test piece to dial in some CAM operations for Fusion 360 before I build a bookshelf for a client.  I'm trying to create a 2D pocket in plywood that will be wide enough to accept my shelves.  The material thickness is .73" and I created a pocket that was .745" wide, allowing .015" of slop for the dado joint.  I ran my operations and when I measure the pocket width it is .72", so .025" under what I was intending.  I don't think this is an issue with my machine, or the CNC bit (1/4" compression bit), because my overall profile pass created a part that was within a thousandth or 2 of the intended 12" x 8" piece.  I have checked all my settings in the 2D pocket menu and can't find anything that would account for the discrepancy in width--I unchecked "material to leave" for example.  I will attach my file.  If anyone has any insights they would be much appreciated!

[jhackney1972]

The model file you attached is empty in both the Design and Manufacturing Environment.  Please try again.

[paulgregorytaylor]

Sorry about that, not sure why that happened . . .  re-attaching, I checked this one and it should work.

[jhackney1972]

It is hard to check my idea because I do not have the ability to measure the component after I run a simulation and I do not have a CNC Router.  My only suggestion is to turn off Both Ways to keep the tool in Climb Milling mode.  This will make sure the Sideways Compensation, which is set to Left, is effective.

[paulgregorytaylor]

I can give that a try when I do some more tests tomorrow.  Ideally, I'd use 2-way when I scale this up as that would make the operation much faster.  I'll give it a try, though.

[seth.madore]

@paulgregorytaylor what post processor are you using?
Looking at it in CAM Simulation with "Comparison" for Colorization setting, I'm not seeing any additional stock (no more than .0002" or so)

How did you verify the correct size of the outside profile?

Posting out the code and looking at the backplotter, I can confirm that the width of the programmed slot (tool centerline) is .4950" wide. When we add in the diameter of the cutter (.250), that brings us to .745" wide.

 

[paulgregorytaylor]

thanks for doing that analysis.  It's helpful to see that, at least according to the gcode, the width should be .745".  I'm using the "Mach4Mill" post processor (vendor Artsoft).  

I checked the pocket width with calipers and tried to physically test by slotting in the .73" material, which didn't quite fit. . .   I was thinking maybe my bit was worn down slightly, given it a slightly reduced diameter.  However, the fact that the 2D contour pass produced a part that was basically the correct size gives me pause.

[ryan.bales · Accepted solution]

Just to tack onto what these guys have said. Anytime I cut plywood or solid wood the fibers seem to 'relax' and often this means the finished hole or pocket is smaller than designed. In this case we are talking .015" or 1/64" hardly imprecise for woodworking standards. 

One quick tip - you can export the stock as mesh and open it to measure: 

When i do this i get .745" 

@seth.madore already checked the posted code so I would wager that it's either something at the machine or the normal behavior of wood to expand, considering its stretching back into the pocket less that 1/64" per shrunk side.

So what i do is always leave stock to leave on for Pocket and Adaptive and clean it up with contour. My goto is .02 stock to leave (probably default setting) then come in with a slow and smooth 2D contour to clean it up.

So i'd try running a 2d contour spring pass along the pocket walls after it completes.

[paulgregorytaylor]

Thanks Ryan.  That makes sense.  I definitely know what you're saying about the "bounce back" that wood can sometimes have.  I think I'm tripping a little because I have done similar things in Vcarve, giving 15 thou wiggle room, without an issue in the past.  Thanks to everyone's help it seems like I just need to go with a bit wider of a pocket.  I also realize that I was doing a climb cut before, I will try conventional tomorrow, as well as making sure I go with a fresh, sharp bit.  Ryan, I like the idea of "stock to leave" with the pocket, and then a slow, smooth 2D contour to finish.  I'll see how it all works when I can get on my machine tomorrow and let you all know!

[paulgregorytaylor]

Thanks @seth.madore and @ryan.bales , followed your suggestions and they worked like a charm.  Thanks again for verifying that the gcode looked like it was producing the correct width.  I ended up switching to conventional cut, used a brand new bit, and did finishing pass and it is almost exactly .745".