Avian’s Blog

Two weeks ago I was writing about 3D-printable replacements for 3.5" drive rails used in an old Chieftec PC enclosure I have. The original plastic rails became brittle with time. They often broke when I was replacing hard drives and I've eventually ran out of spares. I've drawn up a copy in FreeCAD and modified the design so that it was easily printable on a FDM printer. At the time of my last post I was waiting to get a sample pair printed. This is a quick update on that. I've tried the new rails and found that they fit well, but need some minor improvements.

This is how the new rails look when mounted on a 3.5" hard drive. Unfortunately I've specified a wrong diameter for the holes in the STL file, so the imperial-sized screws that go into these drives don't fit. I've manually drilled the holes with a larger diameter on the photo above, but obviously it's better if the rails come correct from the printer in the first place.

These pieces were kindly printed for me by Matjaž on a Prusa i3 MK3S in PETG filament. They feel sturdy and more than strong enough for their purpose. The only thing I was slightly worried about is them getting softer when mounted on warm disk drives. According to the Prusa website the material should be good enough up to 68°C. My monitoring says that no drive got hotter than 50°C in the last 12 months so even in summer they should be fine.

I was happy to see that the drive with the new rails fits into the case perfectly. Much better in fact than with the original rails. Original rails were very difficult to handle, even when new. These slide in and out with just enough force. The plastic arcs on the top and bottom of the rails engage nicely with the guides and provide enough friction so that the drive doesn't rattle in the case. The strengthened tabs on the side also fit nicely. I saw no need to fix any of the basic dimensions.

Apart from the holes the only other part that needed fixing is the flexible latch. This latch has to be printed in a separate piece. My original idea was that I will glue it to the base part of the rail. However the latches all broke off when I was testing them. Part of the problem was that the cyanoacrylate (superglue) I was using doesn't seem to have good adhesion to PETG plastic. I'll probably find a glue that works better, but I still wanted to change the design so that it depends less on the strength of the bond between the two plastic pieces.

The picture below shows how I've slightly modified the latch. The red part is the latch and purple is the base part of the rail. This picture shows a cross-section in FreeCAD. See my last post for the renders of the complete rail.

The new latch design (top) has a tab that inserts into a slot in the rail. When the latch bends, the slot itself should take most of the torque and hold it in place. The glue should only hold it so that the latch doesn't fall out of the slot when not under tension. In the old latch design (bottom) the glue itself was taking all of the torque when the latch was bending.

I would be even happier with a design that wouldn't require glue at all and where the latch would click into place somehow. However I felt that coming up with the right tolerances for that would require several more round trips between FreeCAD and the printer and, since I'm already at version 4 of the design, I didn't want to waste any more time on this.

Anyway, I've put the updated STL files at the same place as last time. Again I'm waiting for the printouts. Hopefully when I get the chance to test them no further changes will be necessary and I can finally install a new stack of hard drives into my case.