When I was recently visiting my parents, I noticed that the mounting system for their Torqeedo electric outboard motor had seized up due to corrosion. The easy fix would have been to go to the hardware store and buy a wing nut. However, I decided instead to fix it the hard (and more fun) way. This spring, during the first COVID-19 wave while we were all sequestered at home, I was incredibly fortunate to get approval for a Tormach PCNC 440 Mill.
This part seemed like a perfect project, and would also give me some experience CNC’ing stainless steel (I had previously been focusing on Aluminum, which is considerably easier to cut). It was easy to design the replacement part in Onshape and then bring it into Fusion 360 CAM to set up the toolpaths, which would in-turn instruct the mill how to cut the metal.
I decided to use the 303 Alloy for stainless steel, as it is easier to machine for a first project, but at the cost of some corrosion resistance. McMaster Carr had the bar stock to me in a day. One of the first steps in machining is to set up a common reference system (aka Work Coordinate System - WCS) which the machine uses as a basis for all of the tooling paths. Because my design required flipping over the part to machine the bottom side as a separate operation, it was important to create common references that would work in both orientations. I accomplished this by facing one side of the stock as an initial step.
Note to self - End Mills are sharp - wear gloves next time.
The riskiest machining operation, and also the coolest, was thread milling the M6x1 internal threads. If you haven’t checked out thread milling before, it’s worth watching some YouTube videos - super cool! I prioritized this operation first so that if it failed, I wouldn’t have wasted time on the other operations.
Once the thread milling was complete, it was straight forward to contour out the top profile.
Using the edge I had faced in the first step, I flipped the part over, re-referenced the machine, and was able to cut away the rectangular section of stock that I had used to clamp the part in the vice to hold it while machining the top.
Once the machining was complete, I sent the part through a vibratory tumbler to deburr the rough edges, and installed it on the clamp bracket. Success!
This project required simple CNC machining skills but was also a good reminder that if I wanted to build this product in high volume, we’d have to consider other fabrication methods to get the cost down. The original part consisted of a stainless steel nut captured by an injection-molded plastic wingnut, which would have been considerably cheaper to build from both a material and a processing standpoint. The greatest expense would have been the injection molding tooling to support high volume production. For my application, because I only needed one unit, CNC was the best approach because it is tool-less.
At Dragon Innovation, we have the honor to work with amazing factories whose collective expertise spans the full spectrum of fabrication techniques. I have a lot of fun working on personal CNC projects and tend towards the conservative approach - more gentle cuts, less aggressive feeds, adding an extra pass above the material so I can do a dry run before making the first chip, and manual tool changes so I have an opportunity to double-check that everything looks good before committing.
For a commercial shop, all of these safeguards would erode their already thin margins. The more I machine as a hobby, the more I am in awe of the constraints, competitive pressures and resulting skills the commercial shops must contend with daily to stay alive. It is a pleasure to work with these factories and appreciate all of the skills and experience that they bring to our customers to help them succeed.
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