Making things in the shop has changed significantly in the past decade. We are using all sorts of new tools and methods that didn’t exist or were unobtainable for reasonable cost during most of my career in metal work.
I’m using two of those great new methods in the motorcycle tube miter/weld fixture that I show below:
Fixture table systems and PLA extruders are an amazing force multiplier in a small shop. One of the more powerful methods we can deploy in fabrication is to mix these together to make part holding in very complex ways possible. The question then arises, what about precision and rigidity/strength? Let’s look into this.
This is important as it makes machining and fixturing complex weldments both trivial and very precise. Extremely complex alignments can just be printed and lock into a raster board. That’s wild!
In the world of cycle frame fabrication, we use tube blocks all the time. There is a reason, thin wall tubing needs to be well supported during machining so that it doesn’t get destroyed in the process. Thus, we understand that thin wall tubes have special needs that come about by their fragility and distortion under clamping load.
I’ve used additive plastic tooling in the past to hold tubes in odd ways. Most notably, to cut handlebar struts in a wild way. This was three years ago and I’ve developed significantly since.
While the blocks used for that handlebar functioned well and I was successful in my project, there was an issue that experienced engineers would recognize, the tool was incorrectly constrained. The term of art is Kinematic Constraints. It is simply not correct to use a cylinder to align a cylinder for precise repeatability. Other problems in the design cause ambiguous mates that are either under-constrained, over-constrained, or redundancies.
I’m going to use extrude printed blocks in conjunction with the fixture table components in my current project and I need to improve a part of that from what I’ve done in the past.
Extrusion prints are far from precise and are much less capable of the bearing the loads they will confront in the shop without deformation. Addressing these limitations is important if I plan on using them in my tooling.
The first step in this process must be calibrating the filament profiles and CAM settings of our slicer. We need to have repeatable and know results coming out of our machine. The part produced should represent the part modeled. Thus, filament extrusion properties need to be set and the size of the part must be calibrated.
In my shop, we are using an AnyCubic Kobra S1 extruder with OrcaSlicer 2.4.2. Most of the general printing we do is with AnyCubic PLA 3D Printer Filament 1.75mm, Black (#212721) and that is what I’m going to use in this example. There are obviously other materials that would be superior in this application but I am looking to design something that can work in the most basic of environments.
Certainly, a full filament calibration should be performed in the machine that is being used. Attention should be paid to whether material shrinkage is a factor. Then, X-Y hole and X-Y contour compensation needs to be examined. With proper measurement tools, we should be able to get ‘very close’ to the target size.
I modeled a calibration block that, since I’m working in a proper shop, I have measuring tools sufficient for understanding the form that it takes. I can measure the outside dimensions, the bore diameter, but also the pin to pin distance as that’s crucial for interfacing with the BuildPro 5/8″, 2.000″ x 0.625″ table raster.
Once the extruder process is calibrated I can move on to the shape that the block will take. Special care was taken in the geometry in the shape of the printed block. It was intended to be printed with the pins facing up to ensure the maximum quality of the surfaces that matter. Notice that only certain parts of the print need to be of high quality and other parts are free to have a lousy support remnant. That’s key to the design. I also provided flexibility in my CAD design for changing the vee angle with a single value change to update everything needed. Not used now but maybe later as I debate exactly what angle is best.
To optimize the part, and what makes this really special to this post, is the flat steel plate interface. This is crucial. By integrating a steel plate, many of the problems with PLA prints are mitigated. The pressure of the tube is redirected from a tiny focused edge to a broad surface. Pressure on the plastic at any specific point is minimized. The tiny deviations from the theoretical surface are averaged to help the reference be accurate. The surface is as hard as the tube is for increased repeatability. The amount of pressure that this system can handle and the rigidity of the hold, given that it’s really just plastic, is amazing. Best of all, adding this plate to the printed block is as easy as cutting a length from the strip and deburring it.
My friend, Ronen, suggested that I add a pocket for small magnet to the printed block. That’s a great idea but not really needed in the context that I’m using right now. Maybe in another case. He also agreed with me that stainless steel would be nicer as it wouldn’t rust but the additional cost of that material makes no sense right now.
I can check the height over the surface using various gauging components. The block shown has a 1/2″ gap over the table. Showing that with a 1/4″ and two 1/8″ block confirms that on either side.
Now that this detail is sorted and part of my methods, I can move forward with the rest of the fabrication of the chassis. I feel that this sidebar was well worth slowing me down as so much more is understood for making nice tools.
