Avian’s Blog

I use an old Chieftec tower case for my home desktop computer. Instead of the usual screws the case uses purple plastic retainers for mounting 3.5" disk drives. These parts were quite fragile and even though the case came with plenty of spares I broke all of them over the years. I failed to find an off-the-shelf replacement so I went and designed a 3D printable part to replace them. I'm currently waiting to get the prototype printed.

I don't know the model name of the case or whether Chieftec had a specific name for this method of mounting drives. The only identifiable mark on the case is the name "Chieftec" embossed in the front panel. I wrote about it in this blog post so it must be at least 16 years old at this point. After quite a lot of web searching, this 2004 review of the BX-03BL-BL-BL model was the only other reference to this rail design I could find. It has a photo of the rails that look identical to the ones I have although the design of the rest of the case looks different.

I think these plastic rails were badly designed from the start. I remember even when the case was new it was very hard to insert or remove a drive. The rails are formed in the shape of a slight arc. When inserting a drive into the slot they straighten out which grips the drive in place. However the rails are fixed to the drive at two points so they can't flex freely. The other problem is that the triangular catch that latches into the metal case has no spring. You need to bend a relatively short and thick length of plastic to unlatch.

The plastic didn't age well. It got quite brittle and also changed color from a bright pastel purple to this purplish-gray. Very soon changing a hard drive in the case meant breaking one or both of the rails that held it in place. These days it begins to crack if I put even a slight pressure on it. An unfortunate flaw since otherwise I like this case and it has served me well for all those years.

The rails have a complex shape that isn't printable on a common FDM printer in one piece. To get a single, flat face that starts on the printer's build plate I removed the outward arc in the base shape. It was problematic anyway. Since holes for the sunken screw heads require an overhang in this orientation, I left a thin sacrificial layer that separates the two hole diameters. This needs to be drilled through before the screws can be inserted. I plan to use normal screws instead of the knurled pins the original rails used.

I moved the catch to a separate part that must be glued to the base part. It's mounted on a thin leaf spring that should provide the flexibility I was missing in the original. This part can be printed with layers following the spring shape which should make it more resistant to bending. I also extended the tabs on the sides to make them stronger. After the handle these were the second most common thing to break when unlatching the rail.

This was the first more complicated mechanical part I designed using the FreeCAD's Sketcher Workbench. I like the visual way of defining lengths and constraint on shapes mapped to object's faces. It's much clearer than the approach I took with my previous 3D printing projects - using cubes and other basic primitives from the Part Workbench, placing them using Python expressions and using unions and subtractions to get the final shape.

The problem I encountered however was that Sketcher isn't very convenient for experimenting. Because of the topological naming problem you can't easily go back and change things on a model. So far I've already ended up redrawing this shape 3 times from bottom up. Changing something in the sketch for the base shape tends to invalidate all other sketches for pockets and pads that are built on top of it. I don't know how to fix the missing edge and face references without starting all over again.

I'm now waiting to get these new rails 3D printed and I'm yet to see how they turn out in practice. This is the third iteration and I think I got all the measurements right. I'm mostly concerned if the new rails will fit snugly enough into the case to prevent the drive rattling and at the same time not be too hard to insert and remove. If they are too loose I may just add some rubber padding like the 2004 review I linked above suggests.

I've made the current version of my 3D printable design available for download here. I'll write another post when I have the chance to test these out and if I make any more iterations to the design.