Sausage Factory Finale – Milling a Hammond Stomp Box Enclosure

Hi everyone!

With the holiday season behind us it’s time I give you the last installment regarding my Sausage Factory overdrive/distortion project.  I’m from Australia originally, so I grew up with Christmas and New Year’s Day holidays in the sweltering heat of summertime. For most of the readers of this blog, you’re in the Northern Hemisphere so just imagine having Christmas or Hanukkah in July, trying to stay cool in 35 to 40°C (95 – 104°F) in high humidity. Oddly enough we had Christmas trees and watched American Christmas movies wondering what it must be like to have all that soft powdery snow!

These days I live in America, and I’ve come to love the holidays when it’s cold outside. Why? More time for staying indoors doing electronics projects of course!

So these few days out of the office have given me a chance to get some stuff finished with the Sausage Factory project. And thanks to our friends at Bantam Tools, I’ve been able to do some neat things with the Bantam Tools Desktop PCB Mill. In this final project blog for the Sausage Factory I want to show you how to use the PCB mill for something every project needs – a good quality enclosure to mount it in.

A variation of this blog with more machine setup details will be shared from the Bantam Tools site soon – so be sure to subscribe to their blog as well. I’ll come back here and provide the link when it’s ready.

I can’t thank Bantam Tools’ pro’s Zach and Kim enough for their help and guidance – I found little to no information on the web about how to mill a metal or plastic project box with this device, and I know very little about CAM, so this was a first for me and probably will be for many electronics engineers and hobbyists out there.

I broke a couple of bits while getting this right, so read this so you won’t have to!

Here goes…

Designing the PCB for a Hammond 1590BB metal box enclosure

From the start I planned to use a Hammond 1590BB aluminium (that’s Aussie for “aluminum”) project enclosure. These boxes are probably the most popular on the planet when it comes to guitar effects building – even many commercial pedal manufacturers use these because they’re very strong, have a great finish, are available pre-painted, and can easily withstand being stepped on by brutal guitar players.

I chose the 1590BB which is about a double-width of the more frequently used 1590B because I’m essentially putting two stomp-boxes (an overdrive and a graphic EQ) into a single unit:

20181023_145551

Here’s the datasheet: https://www.hammfg.com/files/parts/pdf/1590BB.pdf

Hammond documentation is really good – their data sheets even have a spinning 3D model of the enclosure and lid alongside the 2D dimension drawings. I used those drawing dimensions to set the size of the PCB outline in the Upverter “Mechanical Details” layer, using the dimension tool to make sure I got it right. Most users of course would normally have a DXF/DWG file from 2D CAD as a PCB outline drawing, but I am starting in the PCB and deriving all my data for construction from that.

Since this project was done in Upverter it was quick and easy enough just to draw it based on the datasheet dimensions, but next time I’ll download the DXF from Hammond (they make those available too!)

2019-01-14 14_08_45-SausageFactory (public design) - Opera.png

What you see here is a screenshot from Upverter with just the Mechanical Details, Top Package Outline, and Holes layers enabled. All the slider pots when I created the footprints in Upverter I used the Top Package Outline to accurately place the necessary rectangular slots and 2mm holes for the development of just such a drawing, and slot pattern for CAM in any panels they may be mounted to.

Just a not about the jack portion and room for the footswitches: you can see in the image above, that I placed slots in the PCB between the main circuitry (to the left) and the audio and power jacks (to the right). The design intent was to be able to test the board after initial assembly (which I did in my sound test video in the last blog) and then separate the jack board with a Dremel and connect it with wires for final mounting in the box – there are photos of this below, so read on.

I used the 3D model with the component detail layers exported from Upverter, and those in turn allowed me to make a 2D dimensioning drawing in MCAD:

2019-01-14 14_25_52-Sausage Factory 3D Model from Upverter - Alibre Design Expert.png

I use a few different MCAD tools at different times, but since all the Bantam Tools help blogs and documentation centers around Fusion 360, and since Fusion 360 also includes CAM tools for generating the gcode for CNC milling, I caved in a decided to use it for this project. The tool is nice with a modern UI but doesn’t follow paradigms I’m more familiar with from my experience with Solidworks or Alibre, so it was a bit of a learning curve, but before too long I had imported the STEP model of the 1590BB enclosure, and generated an extrusion for milling the top of the box. Later on I added the larger holes as well for the footswitches. Here’s the screenshot of the enclosure model ready for generating CAM toolpaths for the Bantam Tools Desktop CNC Mill:

2019-01-14 15_24_27-Autodesk Fusion 360 (Startup License).png

Not be glib – I’ll elaborate in a future post as a guest on the Bantam Tools site as to the process, but to summarize, here’s how I made this model:

  • Imported the 1590BB STEP model, downloaded from Hammond Manufacturing’s website.
  • Created three sketches, aligned with the plane of the box surface:
    • One for the slots and potentiometer holes, drawn from my 2D dimensions diagram (above) – this method is very precise because the positions come directly from the PCb components.
    • Another for the footswitch mounting holes (they are not PCB mounted, so these are positioned aligned with box centerline).
    • And the last for the text.
  • Extruded all the holes and slots through the box surface (the top of the actual stompbox pedal is actually considered the “bottom” of the mechanical model for some reason, so I had to re-orient it by flipping the assembly in Fusion 360).
  • Extruded all the text to a shallow depth (1mm or so) from the surface – this is for using the CNC mill to engrave the text into the box surface.

After creating the model, I switched Fusion 360 into CAM mode, and created three separate tool paths for milling.

  • The larger holes are all milled with a 1/8” flat end-mill, to save time. This is setup as a 2D Adaptive cut with the spindle at 16,500 RPM and 150mm/min plunge rate – because we’re using a larger bit we can be a little more aggressive to speed things up.
  • The slots are very fine, as are the 2mm holes for the sliders, so these needed to be done with a 1/16” flat end-mill, using a 2D Contour cut. At first pass I used the 1/32” end mill which was working great and making a very precise rectangle – until it got to the 2mm hole breakthrough and the excess material left in the center of the hole broke the bit! Moving to the 1/16” tool saved that problem because the tool path overlapped enough to not leave any loose materials in the holes or slots. Similar feeds and speeds were used.
  • The text was hard to figure out in Fusion 360. Somehow I figured a 1/32” flat end mill would be able to make nice 3D engraved markings on the box, but no matter what I did it refused to generate a tool path. Then after many attempts and trying different cut types, I used a 2D Contour cut with the engraving bit, and it successfully generated a tool path for the text. This still needs some tweaking – on the first non-painted 150BB box, the tool path was too deep. The text is legible, but looks a bit rough. But by setting the depth of cut shallower by about 0.5mm and milling the pre-painted enclosure, the engraving bit follows the letter outlines and it looks very nice.

Here’s a screenshot of the text engraving toolpath in Fusion 360:

2019-01-14 15_44_11-Autodesk Fusion 360 (Startup License).png

Then, it’s just a matter of post-processing the tool paths into GCODE files for the mill. I mounted the box including it’s base attached with the four screws, using the double stick tape that came with the Bantam Tools setup, to the front-left (lower left) corner of the spoilboard. Aligning this was easy, because the machine is highly accurate and zeros itself whenever you power up.

I created a new mill plan in the Bantam Tools software (formerly known as “OtherPlan”), and set the material to “generic” with the material dimensions just big enough to exactly fit the space of the box plus the mounting tape. Each GCODE pass was loaded into the plan and the tools specified to run them one at a time. Here’s a video I put together to show the process once I had the GCODE ready to go:

The end result looks like a professional, boutique guitar pedal which, well, hey that’s precisely what it is!

 

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