Over the last few months, I've been working to rebuild a ShopBot Alpha CNC machine that Hammerspace Workshop received as a donation. The machine was only a frame when we received it, so its been a long road to collect all the parts needed to get it running again.
In rebuilding this machine I've had an eye towards experimenting large scale 3D milling operations. The ShopBot Phoenix is much lighter then our other CNC machine at Hammerspace, letting it run at feed rates 3 to 4 times faster, but can't take as deep of a cut per pass.
With everything finally assembled, and some small test jobs under my belt, It was time to try something more ambitious!
My father restores Little British Sports Cars as a high level hobbyist. He tracks down hard-luck cars in need of heavy mechanical work, gets them back into working order, then sells them on to other enthusiast looking for mechanically solid project cars. Its not really a business, more of a personal mission to save cars from being parted out and sold on Ebay.
A hobby that cool requires an awesome sign! So I decided to use my newly resurrected ShopBot 'Phenix' to 3D carve one for him as a Fathers Day Gift.
3D Modeling the Sign in Fusion 360
The sign was designed in Fusion 360, which has become my CAD platform of choice in recent months. Fusion 360 integrate CAD and CAM tools into a single piece of software, making projects like this one much more approachable.
The 3D model needed to respect a few basic principles.
- No Undercuts! The ShobBot Phoenix is a 3 axis machine so it can only carve straight down in the Z axis.
- No Small Details! Life is too short use bits smaller then 3mm
- Had to fit within a 24" x 24" x 4" volume. The size of the prepared MDF Blanks I had on hand from an earlier project
3D CNC Milling the Sign with Toolpaths from Fusion 360
The 3D milling Toolpaths in Fusion 360 where divided into 6 operations using 4 different tools. We'll look into each of them individualy here.
Clearing - Pass #1
- 1/4" 2 Flute Ball Endmill -
The goal of this pass was to cut away the majority of the the material to be removed, and provide space for the tool/collet/spindle body to access the lower levels of the part in subsequent operations. For this Operation, I used a Pocket milling path with the spindle running at 12k RPM, a cutting feedrate of 210 Inches Per Minute (IPM), and a stepdown of 1/2 inch. The 'Stock to Leave' is set to 0.1 of an inch, leaving enough material behind for subsequent passes. The bottom height of this path is set to 1 inch above the bottom of the model, so this path dosen't get bogged down in any of the more detailed areas that can be handled without pre-clearing.
The 1/4" ball endmill is almost 3" long, giving it the ability to access much deeper into the block then the 3mm endmill that will be use for the final detail paths. To provide access for these later tools, I set the machining boundary to 'Silhouette' with the tool on the outside, and added and additional offset of 1.25"
Cutting this path took about an hour and a half. I think the machine could have handled a higher feed rate. Next time I'll try something in the 250 ipm to 275 ipm range.
Rough Contour - Pass #2
- 1/4" 2 Flute Ball Endmill -
Using the same endmill as the first path, this second path refines the shape of the sign down to 0.05 inch from the final surface, clearing most of the remaining spoil material. For this path, I used a Spiral milling path, will machining boundary to 'Silhouette' with the tool on the outside, and and additional offset of .05 inch.
This path took about 30 minutes to carve at 210 ipm.
Flat Surfaces - Pass #3
- 3mm 2 Flute Flat Endmill -
To carve the final detail on the sign, I switched to using 3mm endmills. This first finishing pass is with a flat 3mm endmill, to smooth the tops of the leters and other areas.
An important note: I have found that most of the inexpensive end mills sold as being 1/8 inch are really 3mm. Its always good practice to verify specifications before using any tooling. 3mm is a far more common available size globally than 1/8 inch making it much less expensive. I speculate we'll soon see the same thing start happing with 1/4 inch and 6mm tooling.
The path was generated as a Horizontal milling path with the spindle set to 10k RPM and traveling at 120 ipm.
This path took less then 15 minutes to complete.
Final Contour - Pass #4
- 3mm 2 Flute Ball Endmill -
The Final 3D contour pass carved the majority of the sign's final surface. This path was a 3D Contour pass with only basic modifications. The speed was set to 120 ipm, the RPM to 10k, and the bottom of the pass was offset from the bottom of the model by 0.4 inch. This offset keeps the sign firmly attached to the rest of the mass of the blank, and is designed into the model as a straight 1/2" vertical depth all the way around the model.
This path took more than 5 hours to carve. Honestly, I should have run the machine much faster then 120 IPM, but those little end mills are so easy to break that i've become a little paranoid.
Projected Line - Pass #5
- 3mm 2 Flute Ball Endmill -
Rather then model the elements of the billowing UK flag into 3D, I used a really neet milling trick Fusion 360 can do called 'Project'. A project operation takes a flat line from a sketch, and mills it into a selected surface by a specified depth.
This path took less then a minute to cut, as was so much easier then trying to build all of that contour into the 3D model.
Outer Contour - Pass #6
- 1/4" 2 Flute Ball Endmill -
This final path was a 2D contour used to cut the sign free from the remains of the original block. I used tabs, but they were likely unnecessary.
This path only took a minute or so, and was followed by sanding... lots of sanding.
Painting
Painting the sign was a fun and fairly straightforward task.
After an initial sanding, I sprayed the sign black to seal the surface before sanding again.
A final coat of black formed the base of the color. The areas that are going to receive bright colors where brushed in with a base coat of white acrylic.
The remainder of the color was brushed in with thick body acrylics, then a sealing coat of clear was applied.
Finaly, high quality metal effect paint was used to paint the boarder and raised letters. This paint must go on after the clear to keep its shine and pearlescent effect.
Lessons Learned
I wouldn't use MDF again. I had been advised this before hand, but it was the material I had on hand, and this was a non-revenue project. The final surface of the MDF really left a lot to be desired. Even repeated cycles of sanding and sealing couldn't fully smooth the final surface.
In the future, I'm going to look at different types of wood, or possibly a machinable foam board products for doing this type of work. One idea i've had is to build up the blank from multiple layers of different woods, then carve down to each layer to create a multicolored 3D object.
I will also push the machine to run faster in the future. I honeslty think in soft material and a 1/4 inch bit, this machine will happily run at more then 250 IPM.
Thanks for Reading! I hope you are Building Something Awesome!
-Michael
