Rotoforge 1/23/2023

Today I spent 3 hours 3D modeling some modular insert tooling for the rotoforge printer in fusion360. Previously I spent a good deal of wasted money ~1500 USD having failed tool designs manufactured by contract manufacturers. boy that was dumb. 

All of the resultant designs clogged or failed and were essentially 1 time use. How disappointing! 

After seeing a video by clickspring, and learning about spade bits and early history of drill bits I realized that going back to the past of drilling might lend us some benefits in the present in terms of the total open area in our rotoforge tools and the total chip load, IE the thickness of chips that our tools can generate per rotation. This helps us build thicker layers of material and hold more practical layer height tolerances while printing.

Figure 1. image from the bottom of the insert tool showing the slot all the way through the tool perpendicular to the threads. The slot is ground to prevent the tool from rolling. 

 

The concept is very simple, find a way to incorporate a spade bit tip, like the ones seen here on clickspring's video,into a rapidly spinning tube. 

Why would one want to do this? 

For a few reasons of lesser known importance...

1.) Spade bits, though incapable of clearing their own chips without peck drilling or other assistance are extremely resistant to clogging, welding and bit breakage. This is due to their simple round chisel cross section, and relatively reduced surface area at the cutting edge. This cross section and reduced contact stress gives them excellent minimum runout capability for precision drilling, but also yields a larger open area when inverted in the way of a rotoforge tool. 

2.) Spade bits act effectively as negative rake angle tools, which facilitates large plastic deformation, IE material extrusion at the surface of the work piece in conventional machining and thus increases the total rate of material removal, and reduces heat loss to the substrate and tool. That is, more energy goes into plastic dissipation in the chip as it flows away from the cutting surface. For rotoforge, this is what we want, to facilitate thick layers, and chips at a high enough temperature to be soft when they contact the underlying build surface, thus facilitating easy and through stir welding to the build plate and subsequent layers. 

3.) Spade bits, as with many ancient technologies, are completely open source, extremely replicatable, and easy to make from basic materials and tools. This makes them much more attractive than contract manufactured monolithic tools, as they essentially make it possible to rapidly test geometries at lower cost and in away that others could potentially follow.

I accomplish the task of effectively inverting the spade bit, by drilling out a piece of A2 tool steel rod, 7 mm OD, blind threading it to M5, and placing a 2.2mm OD hole at the end opposite the threading. as shown in figures 1 and 3.

Figure 2. Side view of the tool holder, 7mm OD, with the ground slot for holding the tool, made from ground HSS (high speed steel) round stock.

The tool holder consists of 7mm OD A2 tool steel round rod. This is for heat and abrasion resistance and the ability to resist welding to hot plasticized aluminum.

The tool itself is to be made of M2 HSS round bar stock 0.032" OD. It will be ground to the shape of a spade bit with a 118 degree angle point, and 45 degree rake and relief angles.  this is to prevent welding and abrasion damage and to facilitate edge retention on the tool, as well as increasing material throughput by increasing the maximum tolerable chip load. M2 high speed steel has higher hot hardness than A2 tool. This means that even under the highly undesirable (from a machinist point of view) conditions in the Rotoforge tool, the material should retain its edge and ability to cut and deform material.

Figure 3. showing the M5 thread, blind cut to ~2 mm from the end of the 7 mm long stock. In the center is the 2.2mm OD hole drilled in the end that will serve as our "nozzle" or die orifice. Where the plasticized chips of material that are cut from the feedstock (typically a metal wire) will be ejected and stirred to 3D print a layer.

The quest continues as I await materials to actually fabricate this part myself and then test it on my rotoforge prototype to try and 3D print metal, plastic and ceramic on my home desktop for cheap. All Files will be available on the Github, and a video will follow in a few days/ weeks showing assembly steps for how you too can build a rotoforge if you so desire.