Hole Depth Gauge

TLDR: Simple depth gauge for holes etc with a minimum diameter of 5mm

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This is my take on a design I first saw in 1976 during my very first term of high school metalwork. It was drawn on the 'shop blackboard by our teacher, “Digger”. His real name (and the name you used when in his presence) was “Mr Gardiner”. But to every pupil throughout the school - and quite possibly to some of his colleagues in the staff room - he was “Digger”.

Anyway Digger's design consisted of a small chunk of metal (to be cut and filed into a coathanger-shape by each student), a short length of metal rod (to be cut, cross-drilled, and a thread added by the student), a slightly larger diameter length of dowel (to be drilled, tapped, and then knurled by the student), and the final part was a plain 6" (150mm) length of 1/8th" (approx 4 mm) thin steel rod to act as the depth probe.

Our next three months of one hour classes were completely given over to each student fabricating the design into real metal, whilst Digger sat and did teacher things at his desk. By pure luck nobody was seriously injured and, at the end of the three months, I was especially proud to earn an ‘A’ from Digger, even though I was "ham-fisted, clumsy, and would never amount to anything". But, to put it in Clarkson's favourite exclamation: I did a thing!! So I took it home, showed it to my parents, and then kept it in my bedroom right up till I left home. It also turns out that my older brother still has the one he made in the same class the year before. He sent a great picture of it for me to show here - thanks K!

Roll forward 48 years to today, and I finally realised just how useful it would be if I still had it. Well, I have FreeCAD and a 3D printer, so let's recreate it in plastic:

I made minor changes to the original design: (1) I made the depth probe a square section and added a millimetre scale, and (2) I didn't make the knob knurled because I still haven't got the hang of arrays in FreeCAD yet!

I printed the main block with the large face on the bed. I used PETG at 0.16 layer height, and a 90% infill. The depth rod was printed flat, in two colours of PETG at 0.12 layer height. The colour change is only there to make the scale easier to read. If you don't have a multi-colour printer, you can just use one colour. Or you can pause just before the last couple of layers and change the filament reel. Finally, the clamp and knob parts were both printed at 0.08 layer height, purely to ensure good quality thead reproduction.

I've had a supertack cool plate kicking around for about a year, and finally got around to trying it. After some issues with the printer not reading the plate's ID code, I found it great for printing those little items that are vulnerable to falling over during printing. Wish I hadn't waited a year to try it out!

I didn't need support for anything, but I did have to run a square needle-file through the holes in the block and clamp to remove some rough edges preventing the rod from being pushed through. Nothing major, though, and the whole process took 30 seconds tops.

Don't be put off by the amount of files. There's actually only four printed parts. I've simply included the FreeCAD file, the .step files, the .3mf files and the .stl files so that you can select whichever file type you prefer to use.

So that's my school homework project done, only 48 years too late. But at least I now have the ability to probe all sorts of orifices to find out if they're deep enough to accomodate my tool!

Happy printing!