Originally I didn't think a tape measure would be that interesting... I mean, it doesn't even have gears. Once I started piecing it together in my mind and determining the acceptable "cool factor", I realized that the parts count alone was skyrocketing. My calipers had 9 pieces, this tape measure would have well over 100... Now things were getting interesting.
I decided to attempt this based on the parts count and the fact, that if successful, I would be able pull out over 4ft of tape from something about 3" sq. Also, I had no better ideas at the time.
I designed the tricky parts first and printed little test pieces here and there to validate the design. Right around the time I starting adding all the cutouts in the main body (for cleaning purposes), it started to look pretty cool.
The entire tape measure would be printed already assembled, wound up (pictured above), with support material. The support material is necessary to support all the complex geometry and separate pieces. Support material will be in every void not already occupied with the printing plastic (UV hardened resin, actually). This support material needs to be cleaned out after printing, which is what all the cutouts are for. For me, not being artistic in any way, adding lots and lots of cutouts in a way that didn't look stupid was the hardest part of the design.
Now for some tech specs! There are 114 parts in this tape measure, 52" of tape links labeled every 1" with graduations down to 1/8", a lock for the tape, a belt clip, and a fold-out crank with free-spinning handle to reel the tape back in. Measurement accuracy over the 52" is about 1/16". The belt clip, lock, and crank all have flexible features on which stress analysis was preformed to be sure they wouldn't break. The printed material is fairly brittle which means if it breaks it does so like glass. Unless the stresses in the material are low and are distributed properly it will break.
The crank has an over center cam-like feature and hard snaps in both the open and closed positions to make it bias towards those. The tape lock knob has a detent (visible on the backside) to make it lock in the unlocked position so it doesn't rattle around and lock up the tape when not in use.
I would really have liked to print a one-piece flexible tape or even a spring retractable tape but due to limitations in printing technology and material I couldn't do that. I was also resistance to turning this into a giant research project. Springs are one of the things that are difficult to print, especially in a brittle material. Springs with a preload are, as far as I know, impossible to print.
In this case, if I were to print a retraction spring, it would have to be a very large spring that would have to work over 10+ rotations and not be susceptible to material creep. Not to mention it would have to provide enough force over those 10+ rotations to reel the tape back in. That is not an easy problem to solve.
The tape itself can also be considered a spring; assuming its the same curved cross-section profile as a normal steel tape. If the tape was printed outside of the tape measure body, out of a flexible material, and that material wasn't very creep sensitive it would be possible to print a one piece tape that was rigid when extended outside the tape measure body. The printer required to do this would have to be very large though since the tape would have to be fully extended during printing.
The other, simpler, option is to just print a simple ribbon of tape out of a flexible material. This tape would not be rigid but it would be one piece and could be printed coiled up inside the tape measure body. The down side to this method is that it requires the use of 3 different materials during printing: rigid modeling material, flexible modeling material, and support material. This is currently possible but all but a few printers in the world cannot do this. Moreover, my preference is to keep my designs uni-material (not including support material) so they can be printed on a wider range of printers.
All that being said, I still think it would be cool to further investigate some kind of spring retraction option.
For the curious, this tape measure was printed on an Objet Eden 3D printer.
And lastly: Yes. This is in Imperial units. Why? Because this is America. We are simultaneously innovative and stubborn. Mostly though, its what I am used to. Although on the small scales I greatly prefer SI units. ... Ok, I just looked up Imperial units and came across some of the lesser known ones... All I can say is wow...