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3D printing replacement feet for test equipment

Using DesignSpark Mechanical to model parts for a Rohde & Schwarz power meter

Losing or breaking a small part of an expensive or cherished piece of equipment can be frustrating, particularly when the part in question is no longer made or hard to find. In our workshop is a Rohde & Schwarz power meter that was once rack mounted, and therefore lacks it’s original feet.

Now used on a workbench, replacement feet are required to allow for the vents on the bottom of the enclosure to function correctly. Fortunately we have another piece of Rohde & Schwarz equipment from the same range, so we can measure and copy the feet from there.

In this post we will do just that, using DesignSpark Mechanical for 3D modelling, and the Ultimaker 2 desktop 3D printer for fabrication.

Evaluating the original

Closer inspection revealed that the front feet are slightly different to those at the rear of the unit, so two different parts will have to be made. To save time I intend to model one, then make adjustments to create the second, rather than designing two separate units from scratch.

DesignSpark Mechanical, (DSM), is a powerful CAD package from RS Components that is free to use, with plenty of tutorials and resources to get you started. This post is not intended as an introduction, rather as inspiration of some modelling techniques and methods that can be employed when using DSM.

First draft

After looking at an original foot, I decided to sketch a rough profile of the side, then use the Pull tool to extrude it to the width it needed to be. After sketching the outline and pulling it into a 3D object, I wanted to change the angle of the sides, but keep the 2mm lip around the base vertical. To do so, I needed to sketch additional lines on each side of the part, since the 2mm lip was present on the front and back, but not the sides, as shown below.

Using the Sketch mode and Line tool, and choosing first one side, then the other as sketch planes, I was able to quickly and easily add the lip as I wanted, especially as the Line tool snapped to where the front and back lips were.

I could then use the Pull tool, in Pivot Edge mode to select and move vertices either side of the part, resulting in angled sides down to the newly-sketched lip, as shown below, in a top view screen shot.

I then saved the model using File > Save As > Save As New Version. Doing so after completing parts of your design is a good habit to get into, since the Undo function may not go back as many steps as you wish, and some processes and tools in DSM can be hard to work back from.

Adding features

With the main body shape now drafted, I began to sketch shapes on the base and again use the Pull tool to extend them out. There are three pegs on the original foot that extend into the metal case body, two with flexible parts that clip in, and one plain peg simply for location.

Sketching circles for these pegs in a rough place to start with, I used the Move tool and it’s Ruler option to position them accurately. More information on using the Move tool in this way can be found here, part of the DSM reference library.

It may not always be possible to use the Move tool as above with features that you have modelled. In these situations, you can draw extra features, or use construction lines to help. I knew that the smallest peg needed to be 24mm apart from the two larger pegs, along one axis. To make this work, I drew a construction line between the centres of the two larger pegs, that I could use a reference from which to Move the smaller peg, shown in the screen shot above.

By copying the larger pegs and using the Pull tool to carve out holes in the foot, then adding caps and chamfers, I soon had a fairly close on-screen representation of those on the original foot, except for the cut-outs that gave the pegs their ‘springiness’.

When working with several parts, it makes sense to create new components, within the Structure panel, in the top left of the main interface. When you have multiple components, remember that you must Activate the one you want to work on, either by right-clicking the component in the Structure window, or the part in the main drawing, and selecting Activate Component.

The component that is currently active will be displayed in bold in the Structure panel.

To make the cut-outs, I first sketched two rectangles on the top surface of one of the pegs, checking the Define rectangle from centre option to make it easier to get them in the right place. With these drawn, I used the Move tool, holding the Ctrl key down whilst dragging, to copy these rectangles to the other peg.

I could then use the Pull tool to drag the rectangles downwards, creating the cut-outs required.

The original pegs have threaded centres with M5 grub screws, to keep the feet attached to the case. Since I don’t have much M5 hardware, but plenty of M4 hardware, I added circles of the correct size for tapping for M4 hardware.

With the pegs completed, I saved a new version of the file. Next, turning the foot over, I created a recess for an adhesive rubber bumper. This is different to the original design, but suited the stock of bumpers I had to hand.

Once happy with the design, I saved it before using the Export options button to export the model as an *.stl file. This could then be converted into instructions for our Ultimaker 2 3D printer, using their Cura software package. This process has been covered in a previous post and so will not be discussed in further detail here.

Prints and revisions

The first print suffered from some printing problems due to a loose nozzle, leaving erroneous blobs of plastic all over the place, which called for cancellation of the job before the end, tightening of the nozzle and starting again. The second time I got a successful print, and the part was almost right, except for the retianing pegs not being quite long enough to click into place correctly.

With some adjustments made, the updated model was saved, exported and again processed in Cura, before being printed out. This time, the part fit and looked to be suitable, and so I started to print a second identical part.

Whilst this was printing, I made a copy of the file and modelled the front feet. These include a hinged 'leg' that can fold out, raising the front of the enclosure for an improved operating position. For this, I designed the leg separately, and used it to 'carve out' a section of the foot.

Since the foot and leg would be printed separetly, then pushed together, I added a sprung 'clip' part to the design, shown above. When printed, this turned out to be a lot stronger than I had first expected, and will be something that I employ in future 3D modelling.

Again, I was able to assess the printed part, make revisions and re-print within a very short timeframe. I was impressed with the strength of the parts too – I am using PLA filament – and was very happy with the results.

Summary

With four new feet designed and printed, I could add them to the base of the power meter. Whilst I could have used some basic thick pieces of wood or plastic, the ability to design and print new parts is much more satisfying.

With our newly-refurbished power meter ready to use, I am on the look-out for more equipment in the workshop that requires new parts. With DesignSpark Mechanical and the Ultimaker 2, there is plenty of scope for in-house repair and improvement!

maker, hacker, doer

7 Oct 2016, 8:45

Comments

April 28, 2018 15:25

This is one of my favorite uses for 3D printing - making parts to fix, enhance, or adapt existing products. It is so nice to be able to repair a product, keeping it out of the waste/recycle stream.

Sometimes the "repair" part can integrate hindsight about failure modes that the original designer lacked, making the repair better than the original.

While some parts may be difficult or impossible to create 3D-printed replacements for, others can be quite trivial.
One example is caps to protect the terminals of batteries when not in use, especially for batteries that have all their terminals on one end, like 9V consumer batteries, or many lithium batteries for phones or cameras.
Another example is caps to protect the terminals of connectors when not in use.
In both of these cases, the original product might not have even had a cap, but an appropriate cap can be very simple to model - pretty much just a shell very slightly larger than the cross-section of the battery or connector, closed at one end. A few extra ribs protruding from the outside of the cap can make it easier to get a hold of for pulling it off of the battery or connector. (These ribs are seldom present on the original, because it would significantly complicate the mold or molding process to incorporate them.)

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