Views: 106 Author: Site Editor Publish Time: 2025-12-18 Origin: Site
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● Sources of Shrinkage in Parts That Reach the CNC Machine
● Measuring Shrinkage Accurately
● Practical Scaling Approaches
● Best Practices and Common Pitfalls
Machining parts that come from other processes can be tricky when dimensions don't line up exactly with the drawing. Parts from 3D printing, injection molding, or die casting often end up slightly smaller than intended because of how the material cools and contracts. When you take those parts to the CNC mill or lathe for finishing operations—drilling holes, facing surfaces, cutting threads—the features you machine can end up in the wrong place or the wrong size if nothing is done to account for that earlier contraction.
The fix that many shops rely on is scaling. You make the model or the program slightly larger (or adjust it directionally) so that after the initial process shrinks the part, the final machined dimensions match the print. This approach has been used for years in tool rooms and production floors dealing with hybrid workflows. The goal here is to walk through where shrinkage comes from in processes that feed into CNC work, how to measure it reliably, and the practical ways to apply scaling compensation, with examples pulled from real parts in aerospace, automotive, and medical components.
We'll cover uniform scaling for straightforward cases, non-uniform adjustments when shrinkage behaves differently along different axes, and iterative methods for more complex geometries. Along the way, we'll look at how material choice affects the amount of compensation needed and how modern software and machine controls can make these adjustments easier.
Pure subtractive machining from solid bar or plate rarely shows noticeable shrinkage because the starting material is already stable. The problems start when the blank or near-net-shape part arrives from a thermal process.
In fused deposition modeling (FDM) with thermoplastics, material is deposited hot and cools quickly on the bed. Contraction happens as each layer solidifies, and residual stresses can pull features inward. Typical overall shrinkage for common materials falls between 0.5% and 1.8%, depending on print settings and part geometry.
Injection-molded parts shrink as the molten plastic cools in the mold. Amorphous materials like ABS or polycarbonate usually shrink less (0.4–0.8%) than semi-crystalline ones like polypropylene or nylon (1.0–2.5%). Flow direction, packing pressure, and gate location all influence how much and in which direction the part contracts.
Die-cast aluminum and zinc alloys show lower shrinkage—often under 0.6%—because of alloying elements that improve flow and reduce contraction. Even so, thick sections cool more slowly and can form slight sinks that affect machined surfaces.
Metal additive processes like direct metal laser sintering (DMLS) involve significant shrinkage during the build and subsequent stress-relief heat treatment, sometimes requiring 1–3% compensation before any CNC finishing.
Example: A supplier producing aluminum die-cast transmission cases found that bearing bores machined after casting were consistently 0.15 mm undersize because the casting shrank slightly more than the datasheet predicted. The thick walls around the bores cooled last and pulled the diameter inward.
Another common case: 3D-printed polymer fixtures for assembly lines. Large flat surfaces warped inward after printing, moving mounting holes off location by up to 0.4 mm before any secondary machining.
Compensation only works when the numbers are reliable. Datasheets give starting points, but actual parts almost always deviate because of specific geometry, cooling conditions, and machine settings.
The standard method in most shops is to produce representative test coupons or full prototype parts under production conditions, let them stabilize for at least 24 hours (longer for hygroscopic plastics), and then measure critical dimensions with a CMM, optical comparator, or structured-light scanner.
Shrinkage percentage is calculated simply as (nominal – measured) / nominal × 100. The scaling factor applied to counteract it is then 1 + (shrinkage percentage / 100).
For directional differences, measure along flow or build axes separately.
Example: An injection molder running glass-filled nylon brackets measured 0.9% shrinkage in the flow direction and 1.4% across flow on a 150 mm long part. They applied separate scaling factors for length and width in the mold design.
In an FDM shop printing ABS enclosures, a benchmark block showed 0.7% in X-Y and 1.1% in Z due to weaker interlayer bonding. They adjusted scaling accordingly before printing parts that would later have threaded inserts machined.
For die-cast zinc housings, shrinkage was low overall (0.4%), but bosses around mounting holes sank an extra 0.2 mm. Measurements focused on those local features guided the compensation.
When shrinkage is reasonably isotropic, a single scale factor applied to the entire model is the fastest solution.
In CAD packages, most systems have a direct scale command that enlarges the solid body uniformly around a chosen origin.
Example: A medical device company printing PLA prototypes scaled the entire STL file by 1.006 before slicing. After printing and cooling, overall dimensions fell within ±0.05% of nominal, allowing CNC-drilled holes to align perfectly with mating components.
Many processes produce anisotropic shrinkage. Injection molding and FDM are classic examples.
CAD tools allow different scale factors along X, Y, and Z. Some advanced packages let you define a scaling tensor or apply different factors to selected features.
Example: Automotive interior trim panels molded in ABS-PC blend showed 0.5% along the main flow path and 1.0% perpendicular. The mold was built with non-uniform scaling, resulting in machined clip towers landing exactly on center after post-mold trimming operations.
In metal additive for Inconel combustor liners, build direction shrinkage was higher. The STL was scaled 1.018 in Z and 1.012 in X-Y before printing, followed by CNC finishing of flange faces and holes.
Complex geometries with varying wall thickness or large flat areas often need more than one scaling pass.
The common workflow: produce a first article, scan it, overlay the scan on the nominal CAD, calculate local deviation fields, then warp or selectively scale the model to counteract measured errors.
Example: Wind-tunnel models printed in high-temperature resin required multiple iterations. First uniform scaling reduced average error from 0.8% to 0.3%. A second pass using scan-derived local adjustments brought critical airfoil contours within 0.05 mm over 600 mm span.
Not every adjustment needs to happen in CAD. Many CAM systems allow toolpath scaling, and several machine controls support programmable scaling codes.
On Fanuc and similar controls, G51 with a P factor scales the program around a set point. This is handy when running families of castings from slightly different lots.
Example: A shop machining multiple batches of cast iron valve bodies tested one part per heat lot, calculated the needed factor (typically 1.003–1.005), then applied G51 scaling to the proven program for the remaining parts in that batch without regenerating toolpaths.
Scaling works best alongside good process discipline—consistent mold temperatures, controlled cooling rates, proper packing in injection, enclosed chambers for FDM, and annealing cycles for additive parts.
Example: A composite tooling shop printing carbon-fiber nylon layup tools found that adding a 200 °C annealing step after printing reduced residual shrinkage from 1.2% to 0.6%, allowing a smaller and more predictable scaling factor.
ABS remains popular for appearance parts. Expect 0.4–0.7% shrinkage. Glass fill reduces it but increases anisotropy.
Example: Instrument panel clusters molded in ABS, then CNC routed for display openings. Uniform 1.0055 scaling in the mold cavity kept machined edges flush after shrinkage.
Polycarbonate lenses for lighting needed crystal-clear edges machined post-molding. Careful drying and 0.6% uniform compensation prevented stress cracking during machining.
PLA is forgiving with low shrinkage (0.3–0.6%) and minimal warpage, making it common for fit-check prototypes.
Example: Functional end-use drone arms printed in ASA required 0.9% XY and 1.3% Z scaling plus annealing to hold machined motor mount holes within 0.05 mm.
ADC12 aluminum offers good dimensional stability. Overall shrinkage around 0.4–0.5%.
Example: Engine mounting brackets cast in ADC12, then CNC bored for precision dowel holes. Minor uniform scaling plus extra stock on thick sections ensured bore location after local sink.
Zamak zinc alloys for small housings show similar behavior but slightly higher shrinkage (0.6–0.8%).
Titanium aerospace brackets built via electron beam melting needed 1.5–2.0% compensation plus heat treatment before CNC machining of critical interfaces.
Example: Hip implant prototypes in Ti64 scaled aggressively before printing, stress-relieved, then finish-machined to mirror polish on articular surfaces.
Always validate compensation on production-intent material and equipment. Measure parts after full stabilization—plastics can continue absorbing moisture and changing size for days.
Design fixtures with the expected shrinkage in mind. Use expandable locators or oversize pins that register on as-processed surfaces.
Watch for over-compensation. Too much scaling can push features outside tolerance in the opposite direction.
Control machining heat. Flood coolant and appropriate feeds prevent thermal expansion that masks or adds to shrinkage effects.
Example: A shop initially over-scaled molded nylon gears by 1.8% based on early prototypes. Machined tooth profiles ended up oversized, causing backlash issues. Dialing back to measured 1.4% plus tighter mold temperature control solved it.
Getting parts to measure exactly to the print when they pass through thermal processes before CNC finishing comes down to understanding and counteracting shrinkage through thoughtful scaling. Uniform scaling handles many everyday cases, while directional and iterative methods tackle the tougher anisotropic or complex parts. Material behavior, accurate measurement, and process consistency form the foundation of reliable compensation.
The examples we've gone through—whether die-cast brackets holding bore locations, 3D-printed fixtures keeping holes aligned, or injection-molded enclosures fitting perfectly after trimming—show that these techniques deliver real results on the shop floor. Shops that build empirical data for their specific materials and machines, then apply scaling systematically, cut scrap rates dramatically and hit tighter tolerances without heroic rework.
As hybrid manufacturing keeps growing, with more parts moving from additive or casting straight to CNC for final features, mastering these compensation strategies becomes a core skill. Start collecting shrinkage data on your current jobs, test scaling adjustments on prototypes, and incorporate the successful factors into your standard workflows. The payoff is consistent, on-print parts delivered on time.
