A marking tool can look finished long before it works as intended. With a steel rule positioning block, the question is not only whether the body presents 45° and 90° edges. It is also whether the rule sits where you expect when you bring the tool to a piece of wood.
That small question shaped Version 4 of my 3D-printed positioning block.
Combining two useful functions
I began with a compact block that offered two common reference angles. The 90° face helps when a line must cross a board squarely; the 45° face offers another familiar layout direction. This is an example of the Combine step in SCAMPER: putting related functions into one tool, then checking whether they still work well together. It is the same question I explored when combining functions in another printed design.
The steel rule is the working edge, so the printed body needs to position it reliably. A printed thumb screw pushes from one side. The exterior reference faces then meet the edge of the workpiece while the ruler remains available for marking.
Physical photograph: the body and matching printed screw.
A small dimension changes the contact
In Version 3, the rule rested on a seat against a straight shoulder, with the screw contacting its opposite edge. The CAD section below makes that arrangement visible. It established the basic relationship, but it also drew attention to the rule's upper edge: a flat shoulder does not place a lip above it.
Version 4 adds a 2 mm-high, 2 mm-deep undercut with a 45° internal face along that shoulder. The rule tucks beneath the lip; the existing screw has enough extra travel to press it toward the sloping face. The screw trough was extended by 2 mm to make room for this movement. This is a digital design result, not a claim of measured holding force.
The number that matters first is the thickness of the actual steel rule. The CAD checks considered rules from 0.3 to 1.0 mm thick. Within that range, the geometry gives 1.7 to 1.0 mm of insertion beneath the lip. The 2 mm dimension describes the retaining recess, not a typical rule thickness. This follows the same principle as modelling the accessory before sizing the product around it.
Function and finish belong together
The body remains about 80 × 55 × 18 mm, with a 45° diagonal reference face, a 90° face and a round finger opening. The softened outside corners and edges carried forward from Version 3 make the form look more resolved and feel less abrupt. I kept the functional junction at the rule seat crisp, because rounding that point would change the contact with a thin rule. In a design drawing, form, function and critical dimensions should support one another rather than be treated as separate concerns.
The V4 download includes the body, matched thumb screw and a thread-fit coupon. The printed thread is a custom pair, so I would check the coupon first, inspect the retaining lip in the slicer, then test the assembled tool with the particular rule and workpiece before making a project mark. The supplied photographs show a printed example; they do not establish marking accuracy or long-term retention.
Download the Steel Rule Positioning Block V4 files on Gumroad.
Small details such as the rule's thickness, a two-millimetre recess and a carefully kept sharp contact edge decide whether combining useful features becomes a useful tool.
#SteelRule #MarkingGauge #SCAMPER #CriticalDimensions #3DPrinting #Woodworking #DesignAndTechnology #DesignJournalSOS
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