PultMaster 3000
Tolerance-driven mechanism design
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Two-player catapult game assembly with the slotted crank mechanism visible
- Client
- Robust Design of Products and Mechanisms, DTU
- Year
- 2025
- Role
- CAD, prototyping and tolerancing
- Team size
- 6
- Tools
- CAD · GPS/GD&T · RD&T · Tolerance stack-up · FDM printing · Laser cutting
- Contribution
- Team of 6, joint CAD, 3D prints, tolerance stack-up, testing
A slotted crank mechanism with a 5/8 gear reduction converting rotation into controlled translation, designed to a tolerance scheme and then built and measured in order to check whether the scheme survived real manufacturing variation.
The brief
Design a working mechanism where the outcome rests on the tolerancing rather than on the concept. Ours drove a two-player catapult game, where a side scroll wheel controls the catapult's position across the field, with a second axis for tilt.
The mechanism
A rotating gear drives a crank arm pinned at one end to the gear and at the other to a slotted link, and the pin-in-slot joint lets the crank rotate while the slot guides the motion, which converts rotation into non-linear reciprocating translation. Here the 5/8 gear reduction buys precision before the conversion happens.
Tolerancing
GPS tolerances were specified on the part drawings, followed by a stack-up and a sensitivity analysis. It is important to note that a clearance fit between pin and slot is what makes the whole thing work, since too tight and it jams, while too loose and the slot stops controlling anything.
What the RD&T analysis found
The gear centre pin came out at RSS 21.45, and a high value normally means an underconstrained part, however this pin is small and heavily located, so the number was instead evidence of overconstraint with conflicting constraint directions. Point-to-point measure stability across the assembly, from the small gear centre to the cover edge, came out at 1.26, which is moderately sensitive to variation and the clearest indicator of where the robustness could be improved. Furthermore a mobility analysis confirmed that the mechanism as a whole was not overconstrained.
Built and measured
The locating panel, pins and gears were 3D printed and the housing laser cut, and the printed parts were then measured in order to compare the actual manufacturing variation against what had been assumed.
What I owned
I was jointly responsible for the CAD, the 3D prints and the tolerance stack-up in a team of 6.
What I took from it
A mechanism that works in CAD stops working once it gets the clearances it is actually given, and the ambiguities that mattered were exactly the ones dependent on print accuracy, being the pin in the slot and the pin trying to locate several components at once.
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CAD render of the assembled catapult mechanism on its mounting board
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Tolerance-chain diagram showing key distances from the gear centre to the arm hole and slot
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Exploded CAD view of the catapult mechanism, showing gears, crank arm and mounting boards separated
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RD&T software view of the crank arm and gear assembly with GPS datum markers
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Table of nominal versus measured dimensions for the 3D printed gear parts, with print settings