The final steps of building my custom 3D Printing work cabinet were to design and install the drawer pulls (handles) and knobs. Sure, I can go to the local hardware store and buy low-cost hardware that will be very adequate for this purpose, but I thought it was appropriate to design and print these pieces myself. Besides, I have some future needs for devices with captive nuts for attaching to machine screws, so this gave me an opportunity to try out a few techniques and designs.
I decided to consider two methods of attachment: using wood screws to self-thread into the PLA plastic parts, and also using captive nuts for attachment to standard machine screws. The latter method allows me to keep the cabinet compatible with standard hardware available here in the USA: 4-inch (101.6 mm) spacing between the handle mounting hole centers, and 8-32 machine screws.
I used wood screws to self-thread into the simple cylindrical knobs. These knobs are 25mm diameter and 30mm long, with a hole depth of 25mm. For #8 wood screws, the hole diameter setting of 4.3mm was ideal for my slicing profile and printer, but various slicing profiles will result in variations in hole dimensions, so it’s important to do some test prints to be sure of proper sizing, and perhaps to tune to the ideal dimensions.
The knobs are simple designs in OpenSCAD. One cylinder defines the body, and a second smaller cylinder is subtracted from it to define the hole. This resulted in a design that gave a snug but usable fit for the wood screw, and once installed is more than strong enough for the application.
The drawer pull handles were a bit more work. I also designed these in OpenSCAD, using the Minkowski transformation to apply rounded corners to rectangular cubes.
I wrote a separate module to define the internal cutout for the screw, nut pocket, and passageway for insertion of the nut. This portion is subtracted from the inside of each side of the handle, and the rectangular piece on the side forms the passageway that allows insertion of the nut into the solid handle.
The above nut pocket subtractor assembly is used if the drawer pull handle is printed horizontally; that is, the same orientation as when it is installed on the drawer front. The nut subtractor module also has an option to allow the handle to be printed with the mounting surface down against the print bed (vertical.) In this case a dome is added over the nut chamber to make it easier for the printer to span this distance.
This method may require some trial and tuning of the nut chamber dimensions, so that the nut fits in reliably, but does not turn when threading the screw into it. My OpenSCAD model uses 9.4mm as the distance across opposing flats for a #8 nut. Again, this may vary across slicers and printers.
Once printed, we have handles with hex-shaped nut pockets to prevent the nuts from spinning while the screws are being threaded into them. We also have clearance for the screw to extend through much of the handle end if needed, and we have the passageway for insertion of the nut. The handles are mounted with the passageways down so they are not normally seen during use.
For the handles with nut pockets, installation requires a bit of preparation. The problem is that the nut does not stay in place when the handle is placed into position. I tried three installation methods, and all worked fine for me.
For the first method, I used the end of an old plastic cable tie, bent for springiness, to hold the nuts in place until the screws began threading into them. Once threading begins, the cable ties can be pulled out.
The next method is to use hot melt glue to hold the nut in place. For this method a screw must be inserted fully before the hot melt is applied, to reserve space for the actual mounting screw. I sprayed the screw with a bit of silicone first, as a release agent, to make sure the hot melt would not adhere to it. (Warning: The last thing you want on the print bed of your 3D printer is silicone — we’re always trying to get our prints to stick, and if you get silicone on the bed, nothing will stick. So if you use silicone, keep it far from your printer!)
A similar method that I have heard about but did not try, is to perform “plastic soldering.” For this method, a soldering iron is turned down to an extremely low temperature, and the printer filament is used as the “solder.” The advantage is that the color can match that of the part (although the texture surely won’t match.) Color was not important in my case because these passageways are not visible when installed.
An advantage of hot melting the screw into place is that the part becomes very much like a typical manufactured component. It can be given to anyone to install with no special considerations, because it installs just like a product purchased from a store.
A third method is to simply remove the drawers and position them face up during installation. The nut can be lying in the nut pocket and the screw can be carefully manipulated to get threading started without pushing the nut out of place. In my opinion, this was more trouble than the other methods, for my application.
That’s a summary of this week’s experience with fastening 3D printed parts with screws. I’d like to hear your thoughts, comments, and suggestions. Please leave a comment below.
Article text and photographs copyright 2014 by John Burt. All rights reserved.
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