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Parametric Brass Heat-Set Insert Library for Shapr3D

Technical Documentation & Variable Reference

A fully parametric CAD library for generating ISO metric brass heat-set inserts in Shapr3D, covering all 20 sizes from M2×2×3.2 through M6×10×8. Change three input values: thread size, length, and outer diameter, and every dimension of the insert updates automatically, including the helical knurls, internal threads, chamfers, pilot geometry, and necked gap.

Rendered brass heat-set insert from the Shapr3D parametric library

This document describes every variable in the file, the formulas that drive them, the design decisions behind each choice, and the practical limits of the current model. It is intended as a technical reference for using and adapting the released design files.

Downloads: Printables · GrabCAD

Table of Contents

  1. Overview
  2. Supported Size Range
  3. Quick Start
  4. Complete Variable Reference
  5. The Pitch Formula
  6. File Structure and Scope
  7. Known Limitations
  8. Customization Guide
  9. Files Included
  10. Credits and License

Overview

Heat-set brass inserts are small knurled cylinders that get melted into 3D-printed plastic parts to provide durable metal threads for screws. They're a staple of the 3D printing ecosystem because printed plastic threads are weak and wear out quickly, while brass threads are strong, reusable, and self-lubricating.

The "double-knurled" style this library models is the most common variant sold in assortment kits. It has:

  • A pilot end with a stepped shape that self-aligns during installation
  • A lower knurl band with diagonal grooves
  • A necked gap between the two knurl bands
  • An upper knurl band with diagonal grooves running the opposite direction
  • Internal threads matching standard ISO metric coarse (ISO 261)

The library generates dimensionally accurate models of this style across the entire M2–M6 range, with every feature parameterized so a single file can be reused for all 20 common sizes.

Supported Size Range

The library is calibrated for these nominal sizes:

| Thread | Available Lengths × Outer Diameters | |---|---| | M2 | 2×3.2, 4×3.2, 6×3.2, 8×3.2 | | M2.5 | 2.5×3.5, 4×3.5, 6×3.5 | | M3 | 3×4.2, 4×4.2, 6×4.2, 8×4.2, 10×4.2 | | M4 | 6×6, 8×6, 10×6 | | M5 | 6×7, 8×7 | | M6 | 8×8, 10×8 |

Notation is M{thread} × {length} × {outer diameter} in millimeters.

The ratios were derived from measurements of a 440-piece INCLY assortment kit. Other suppliers may use slightly different proportions. See Customization Guide for how to adapt.

Quick Start

  1. Open the Shapr3D file.
  2. Open the Variables panel from the History sidebar.
  3. Change exactly three variables:
    • threadSize — the M number (2, 2.5, 3, 4, 5, or 6)
    • length — total insert length in mm
    • outerDiameter — outer diameter of the brass body in mm
  4. All other dimensions recompute automatically. The model updates in real time.
  5. Export as STEP, STL, or your preferred format.

That's it. You don't need to touch any of the derived variables unless you want to adapt the library for a different supplier's inserts.

Complete Variable Reference

Dimensioned Shapr3D model showing the main derived zones and dimensions

Input Variables

These are the three, well, four: pitch is technically computed but you'll never touch it manually, variables you change when generating a new size.

| Variable | Type | Description | |---|---|---| | threadSize | Number | The M-number for the thread (2, 2.5, 3, 4, 5, or 6). Unitless. | | length | Length | Total axial length of the insert in mm. | | outerDiameter | Length | Outer diameter of the brass body (the widest point of the knurl bands) in mm. | | pitch | Length | Thread pitch. Auto-computed from threadSize via the pitch lookup formula. |

Body Length Proportions

The insert length is divided into five axial zones, each scaled as a fraction of length. These ratios were derived by measuring a physical M6×10×8 insert and verified across the other sizes in the assortment kit.

| Variable | Formula | Purpose | |---|---|---| | upperKnurlLength | length * 0.3 | Length of the upper knurl band | | gapLength | length * 0.166 | Length of the necked gap between knurls | | lowerKnurlLength | length * 0.33 | Length of the lower knurl band | | pilotUpperLength | length * 0.125 | Length of the necked-in upper pilot section | | pilotLowerLength | length * 0.095 | Length of the locating lip at the tip |

These five zones add up to length × 1.016, which accounts for the typical 1.6% manufacturing over-run observed in the assortment kit inserts: nominal 10 mm inserts physically measured closer to 10.16 mm. If this doesn't match your supplier, adjust the ratios. See Customization Guide.

Body Diameter Proportions

The insert has three distinct diameter tiers, each expressed as a fraction of outerDiameter. The knurl bands themselves sit at the full outer diameter.

| Variable | Formula | Purpose | |---|---|---| | gapDiameter | outerDiameter * 0.86 | Necked gap between the two knurl bands | | pilotUpperDiameter | outerDiameter * 0.83 | Narrowest point of the pilot (the upper neck) | | pilotLowerDiameter | outerDiameter * 0.88 | The locating lip at the very tip |

The knurl bands themselves are drawn at the full outerDiameter. There's no variable for this because it's just the input value.

Thread Geometry

The internal threads are real ISO metric coarse helical threads, modeled by revolving a triangular profile up through the bore. The bore starts as a smooth cylinder at the tap drill diameter, and the triangle cuts helical valleys into it.

Top view of the modeled internal threads and upper knurl band

| Variable | Formula | Purpose | |---|---|---| | boreDiameter | threadSize - pitch | ISO tap drill diameter (standard rule). For M6 = 5.0 mm. | | threadLength | length * 0.75 | Portion of the insert that gets threads cut into it | | baseWidth | pitch * 0.75 | Axial length of the triangle base (on the bore wall) | | threadDepth | baseWidth * 0.866 | Radial depth of the triangle (keeps apex at exactly 60°) | | revolveAngle | threadLength / pitch * 360 deg | Total revolve rotation (one full turn per pitch period) |

Why baseWidth = pitch × 0.75 instead of pitch: Using the full pitch for the triangle base causes adjacent thread profiles to touch at their corners during the revolve, which Shapr3D rejects with an "invalid contact" error. Shrinking the base to 75% of pitch leaves small crest flats between adjacent threads, which is how real ISO threads are actually shaped anyway. The 25% reduction in base width gets redistributed as uncut bore wall, crest flats, at the top and bottom of each pitch period.

Why threadDepth = baseWidth × 0.866 instead of pitch × 0.866: The 0.866 multiplier, √3/2, is what makes a triangle's apex exactly 60°, but it has to be applied to whatever the base actually is. If base shrinks to pitch × 0.75, depth must also shrink to (pitch × 0.75) × 0.866 = pitch × 0.6495 to preserve the 60° angle. Driving depth from baseWidth instead of pitch keeps them synchronized automatically. You can change the 0.75 multiplier later and depth updates on its own.

Why revolveAngle is written as ... * 360 deg: Shapr3D is strict about unit types. threadLength / pitch is dimensionless, length divided by length, and you can't multiply a dimensionless scalar by a plain number to get an angle. The deg suffix forces 360 to be interpreted as an angle, and scalar × angle = angle, which is what the Revolve tool expects.

Chamfers

The top and bottom of the bore get 45° chamfers to create a lead-in for screws and to smoothly terminate the internal threads.

| Variable | Formula | Purpose | |---|---|---| | chamferSize | pitch * 0.5 | Equal-distance chamfer size for both top and bottom bore edges |

For M6, pitch = 1, this produces a 0.5 mm chamfer that opens the bore mouth from boreDiameter, 5 mm, to exactly threadSize, 6 mm. This is standard machining practice for tapped holes. The chamfer's wide end equals the nominal screw diameter, providing a clean lead-in without cutting into the threads themselves.

Bottom view of the insert showing the chamfered bore opening and internal threads

Knurl Grooves

The diagonal grooves on the two knurl bands are created by sketching a small circle in a square, revolving it 45° along a short helical path to form a single groove, then circular-patterning that groove around the insert.

| Variable | Formula | Purpose | |---|---|---| | grooveCount | floor(outerDiameter * 3.14 / 1 mm) | Number of grooves in the circular pattern | | grooveByOD | outerDiameter * 0.11 | Ideal groove diameter based on outer diameter | | grooveByGap | gapLength * 0.8 | Maximum groove diameter based on gap length | | grooveDiameter | (grooveByOD + grooveByGap - abs(grooveByOD - grooveByGap)) / 2 | Final groove diameter, the smaller of grooveByOD and grooveByGap |

Why grooveCount uses floor(OD × π / 1 mm): Groove density should be constant around the circumference, not a fixed count. Larger inserts have more circumference and need more grooves to look right, while smaller inserts need fewer. The formula gives roughly one groove per 1 mm of circumference, which visually matches the reference inserts. The / 1 mm strips the length unit so floor() operates on a plain number.

Why grooveDiameter uses a min() workaround: Shapr3D has no min() function, so the algebraic identity min(a, b) = (a + b - |a - b|) / 2 is used instead. This caps the groove diameter at whichever is smaller: the OD-based value, good for most sizes, or 80% of the gap length, which prevents grooves from spilling into adjacent knurl bands on small, squat inserts like M2×2×3.2. Without this cap, the M2×2×3.2 groove circle would be slightly wider than the gap between its knurl bands, and the revolve would chew into the wrong zone.

The Pitch Formula

ISO 261 metric coarse thread pitches don't follow a clean mathematical relationship to thread size:

| Thread Size | Pitch | |---|---| | M2 | 0.40 mm | | M2.5 | 0.45 mm | | M3 | 0.50 mm | | M4 | 0.70 mm | | M5 | 0.80 mm | | M6 | 1.00 mm |

There's no simple formula that maps M2 → 0.40 and M6 → 1.00 and all the values between them. Shapr3D has no if-statements or lookup tables either. The workaround is to build a lookup using the sign() and abs() functions:

pitch = (0.4  * (1 - sign(abs(threadSize - 2   mm)))
       + 0.45 * (1 - sign(abs(threadSize - 2.5 mm)))
       + 0.5  * (1 - sign(abs(threadSize - 3   mm)))
       + 0.7  * (1 - sign(abs(threadSize - 4   mm)))
       + 0.8  * (1 - sign(abs(threadSize - 5   mm)))
       + 1    * (1 - sign(abs(threadSize - 6   mm)))) mm

How it works: Each term evaluates to 1 when threadSize equals the target value, because 1 - sign(abs(0)) = 1 - 0 = 1, and 0 otherwise, because 1 - sign(abs(nonzero)) = 1 - 1 = 0. Multiplying by the desired pitch value and summing all six terms gives a selector. Exactly one term is nonzero at any time, and its coefficient becomes the output. The final mm suffix converts the scalar result back into a length.

Known limitations of this approach:

  1. Only works for exact matches. Setting threadSize = 3.5, which isn't a real ISO size but might be typed by mistake, returns pitch = 0, which will break downstream calculations. The formula only recognizes the six values baked into it.
  2. Adding more sizes requires editing the formula. Want to add M1.6 or M8 to the library? You have to expand the expression with additional sign(abs(...)) terms.
  3. Floating-point precision. Shapr3D evaluates sign() and abs() with Parasolid's 1e-8 tolerance, which is more than enough headroom for the integer and half-integer M-sizes this formula uses. No precision issues observed in testing.
  4. Unreadable at a glance. Six months from now when you open the file, this expression will look like line noise. Paste the pitch lookup table as a comment or sticky note in the Shapr3D workspace so future-you remembers what it does.

If Shapr3D adds conditional logic or lookup tables in a future update, this workaround can be replaced with a cleaner expression.

File Structure and Scope

This library is meant to be a reusable reference file. The .shapr file already includes the editable history, so this document focuses on what each variable controls, why certain formulas exist, and where the simplifications are.

At a high level:

  • You only need to change threadSize, length, and outerDiameter to generate a supported insert size.
  • The external body is driven by five axial zones and three derived diameter tiers.
  • The necked gap, pilot geometry, chamfers, knurls, and internal threads are all derived from formulas in the Variables panel and update automatically with the three user inputs.
  • Internal threads are included for visual completeness and learning value, but suppressing them still leaves a perfectly usable reference model for enclosure design and fit checks.
  • The file is intended for CAD reuse, fit checking, and visualization of common M2–M6 brass inserts. It is not intended to serve as manufacturing documentation for producing brass inserts from scratch.

Known Limitations

Thread geometry is a simplified sharp triangle, not a true ISO trapezoid. Real ISO metric threads have small flats at both the crest and root of the thread profile. This library uses a sharp triangle, apex = 60°, with a deliberate crest flat created by baseWidth < pitch, but no flat at the apex. The visual difference is imperceptible at M2–M6 scale, but technically the modeled thread has a slightly sharper root than a spec-compliant thread. For CAD visualization purposes this is fine. The library shouldn't be used to CNC-machine functional threads that need to pass inspection.

Knurl grooves are approximated as circular cuts, not true diamond knurling. Real knurling is formed by rolling hardened tooling into the brass, which creates pyramidal diamond shapes rather than clean round grooves. This library approximates the visual effect with circular profiles revolved 45°, which reads as "knurled" from a normal viewing distance but wouldn't pass close inspection as anatomically correct. Again, this is fine for visualization but not for representing the exact surface for friction analysis.

Ratios are calibrated to INCLY-brand inserts. The length and diameter proportions come from measuring a specific assortment kit. Other suppliers, ruthex, CNC Kitchen, McMaster, may use different proportions, particularly for the pilot geometry. See Customization Guide for adapting.

Inserts are all double-knurled style. The library doesn't cover tapered inserts, McMaster style, single-knurl inserts, RS Pro style, or press-fit inserts. A similar parametric approach would work for any of these, but the underlying body geometry would need to change.

Pitch formula only covers six thread sizes. M1.6, M8, M10, and imperial sizes aren't included. See The Pitch Formula for how to extend.

Groove count can't exceed what fits around the circumference. On very tiny inserts, M2×2×3.2 and smaller, the default floor(OD × π) formula may produce fewer grooves than the reference real-world inserts have. The visual difference is subtle but noticeable on close inspection.

Customization Guide

Adapting to a different supplier's inserts

  1. Get a few physical inserts from your supplier, in different sizes if possible.
  2. Measure each zone with calipers and record the dimensions.
  3. For a single size, divide each length measurement by the total length to get ratios. Divide each diameter measurement by the outer diameter to get diameter ratios.
  4. Update the five length ratios and three diameter ratios in the Variables panel.
  5. Check that the ratios sum to approximately 1.0 for lengths. If they don't, the nominal length on your supplier's label differs from the actual measured length by that factor. You can either accept the discrepancy or apply a fudge factor like length_real = length_nominal * 1.016.

Adding new thread sizes to the pitch formula

To add M1.6, M8, or another size, append a new term to the pitch formula:

+ 0.2 * (1 - sign(abs(threadSize - 1.6 mm)))    // for M1.6
+ 1.25 * (1 - sign(abs(threadSize - 8 mm)))     // for M8

Look up the correct ISO pitch value for the size you're adding, ISO 261 tables are freely available online, and plug it in as the multiplier.

Disabling internal threads for faster performance

If the file is slow on very small inserts or you do not need internal threads for your use case, suppress the thread-cut history step. The bore remains as a smooth cylinder at boreDiameter, which is still perfectly usable for enclosure design, spacing checks, and most reference workflows.

Files Included

Published packages (see Printables or GrabCAD) typically ship:

  • brass-insert-master.shapr — the master Shapr3D file with all variables and history
  • brass-insert-[size].step — individual STEP exports for each of the 20 common sizes
  • brass-insert-[size].stl — individual STL exports for each of the 20 common sizes
  • README.md — a condensed version of this documentation for quick reference
  • pitch-lookup-table.png — optional visual reference for the six ISO pitches (when included in a given release archive)

Credits and License

This library was designed, measured, and parameterized by Shreyash Gupta as part of the OffGrid Devices project, a small hardware business making MagSafe-compatible enclosures for mesh networking devices. The work was done in Shapr3D 5.8 or later. Variables and expressions were introduced in 5.800, February 2025.

Reference materials consulted during development include CNC Kitchen's heat-set insert design guide, the ISO 261 metric coarse thread standard, and the Shapr3D community forums. Dimensions are calibrated to a 440-piece INCLY brand assortment kit.

The file is released for free use in any project: personal, educational, or commercial. Attribution is appreciated but not required. If you extend it, different suppliers, more thread sizes, different insert styles, I'd love to see what you build.

Reporting issues: If a specific size generates incorrectly or you hit a Shapr3D error on a size I haven't tested, let me know and I can verify the formulas or adjust the file.

For the story behind this project and the lessons I learned building it, see the companion blog post: What a Bag of Brass Inserts Taught Me About Parametric CAD.