A small, but fatal error in part positioning

Blognosticator head

In my last post I discussed making parts to hold a rack on the back of my Superstrata carbon-fiber bicycle. To read Part 3 of these posts, click here.

I have been machining those parts, and it has been a humbling experience!

On a CNC machine made primarily for cutting wood and wood composites (plywood, particle board, etc.), the cutting of aluminum requires that the machine be run more cautiously, more slowly, and with greater discipline.

Though I have been very cautious, I got a bit ahead of myself yesterday when I positioned the parts in a grouping that was very efficient for a scrap end of an aluminum plate I had on-hand. The machining went well for the first two parts, then I got into trouble. My error was in positioning the parts too close to one another, and in places where their support tabs would be cut off by subsequent cutting paths.

This is an example of tabs machined into a part (shown in red). These tabs keep the part attached to its parent, and they are cut/sanded off after the part is complete. This is one of my bike parts, cut after the others, to serve as a back-up part in case something went wrong in drilling and tapping.

This was a serious mistake on my part, and it caused me to remember an adage in machining circles: holding the material is more than half the work!

I make almost all of my aluminum parts with a .25 inch single-flute cutter from Vortex Tools. That means that there will always be a quarter-inch channel around the periphery of the pieces I cut.

I knew this, of course, and I thought that I had planned for it in the positioning of my parts in the job. But, as my cutting continued – more than an hour into the process – I realized that I had lost control over two of my parts, and I still needed to make more passes to finish those parts. I stopped the machine and redesigned the cutting paths for those two parts to try to salvage the operation. It worked, to a point.

This is the approximate position of my parts as I had put them for machining. The light brown areas are the 0.25-inch channels cut by the Vortex cutter. The darker brown areas are channels where the cutter overlaps the previously-cut channel (this is OK). The yellow rectangles are the tabs that were appropriately positioned. The green rectangles represent the tabs I placed that were later cut off or rendered useless because I had cut through their support to the parent metal (not good!).

When cutting parts on the CNC machine, I usually insert small tabs along the edges of those parts that are left behind in the cutting steps. These tabs keep the machined pieces attached to the source material. At the end of the process, I cut the parts free by breaking or sawing the tabs, then I sand the remnants of the tabs off to remove them.

Yesterday, while preparing my file for cutting the parts, I put two tabs into the mix that would be cut off in subsequent cutting steps, leaving the original part unsupported. This is not acceptable, because I am pushing a rotating cutter up against the edge of a plate of aluminum with close to six horsepower of machinery, and expecting that little chunk of aluminum to sustain its tentative hold on the parent plate with just one tab!

In other words – I really messed up.

I restarted the project today, and I was be able to salvage the two parts by super-gluing them to the spoil board, a trick I learned from a machinist friend. The super-glue held on one, but broke free on the other, causing an unsightly divot in the surface of that part (not structurally significant). My parts are now cut, but still need to be drilled and threaded.

These are the tabs, reconsidered. I should have put these in the areas marked in yellow. Those would have remained connected to the metal from which I was cutting. Note that I also flipped the part in the upper-left to put its tabs on the top. Hindsight is always 20-20! By drawing the 0.25-inch offset in Adobe Illustrator, shown in the two tints of brown, I would have foreseen the trouble I caused on the machine. I’ll do that next time.

Here is what I did wrong: I positioned the pieces, nested together, into a small grouping that would fit into the area on the metal plate. I failed to make note of the tabs and their positions in the path of subsequent cuts.

What I will do from now on: Whenever positioning parts for nesting, I will draw an offset of 0.25 inch around every part, then analyze where those paths overlap in Adobe Illustrator. If they overlap in places without tabs, that’s OK; if they overlap in areas where there are tabs, then I will have to move the parts away from each other or reposition the tabs in some fashion to prevent those fatal intersections.

My future projects will come out better for this.

This is Part 2 of a three-part post. In the first part, I discussed designing parts to hold a rack on the back of my Superstrata carbon-fiber bicycle. To read Part 3 of these posts, click here.

About Brian Lawler

Brian Lawler is an Emeritus Professor of Graphic Communication at California Polytechnic State University, San Luis Obispo and was a Guest Professor at Hochschule München from September, 2021 to September, 2022. He writes about graphic arts processes and technologies for various industry publications, and on his blog, The Blognosticator.
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