Your printed part does not fit because 3D printing has real dimensional error — typically in the range of a few tenths of a millimeter for FDM and around a tenth or better for resin, and worse on large or tall parts — and because holes and internal features shrink more than external ones. The model is exact; the process is not. Once you know where the error comes from, you can design a part that fits on the first print instead of the third.

Why the part comes out different from the model

Every additive process converts a mathematical surface into physical material, and each conversion step introduces deviation:

  • Thermal shrinkage. Molten plastic contracts as it cools. Slicers compensate with an average factor, but shrinkage varies with geometry, so thick sections and thin sections do not shrink equally.
  • Layer discretization. Height is quantized to whole layers. A 10.05 mm feature printed at 0.2 mm layers becomes 10.0 mm or 10.2 mm, never 10.05.
  • Extrusion width. An FDM nozzle deposits a bead with finite width and rounded edges, so corners are never truly sharp and small features grow or shrink depending on how the toolpath resolves.
  • Light bleed and over-curing. In resin printing, light scatters slightly beyond the intended pixel, which enlarges external features and shrinks internal ones.
  • Post-processing. Resin parts change dimension during washing and post-curing. SLS parts shrink as the powder cake cools. Sanding, bead blasting, and painting all remove or add material.
  • Warp and internal stress. Uneven cooling curves large flat parts off the plate. A warped part measures correctly in one place and wrongly in another.

What published accuracy numbers actually mean

Printer specifications usually quote something like plus or minus a fixed value, or plus or minus a percentage of dimension, whichever is greater. Two things get missed.

First, that number is a capability under good conditions, not a guarantee for your geometry. It was measured on a calibration artifact designed to print well — compact, symmetric, thermally benign. Your bracket with a thin flange and a thick hub is a harder case.

Second, percentage-of-dimension terms dominate on big parts. The same printer that holds a small part comfortably can be off by most of a millimeter across a long enclosure, and stacked errors across a multi-part assembly are what usually break a fit.

The factors that matter in practice

  • Process. Resin and material-jetting are the most accurate, SLS sits in the middle with excellent repeatability, and FDM is the loosest — but also the cheapest and fastest.
  • Part size. Error grows with dimension. Split long parts or design in adjustment.
  • Material. High-shrink materials such as nylon and ABS move more than PLA or PETG. Filled filaments are more dimensionally stable, which is a real reason to choose them.
  • Geometry. Tall thin features lean, wide flat areas warp, and unsupported spans droop. Uniform, compact geometry prints truest.
  • Orientation. The Z axis behaves differently from X and Y. A hole printed vertically is round and undersized; the same hole printed horizontally is slightly oval.
  • Machine condition. Belt tension, nozzle wear, bed level, and resin age all shift results. Two prints of the same file on two machines will not match exactly.

How to design a part that comes out right

  1. Decide which dimensions are critical. Most are not. Identify the few that control fit — a bearing bore, a screw spacing, a mating face — and give the rest generous latitude. This is the same discipline as GD&T on engineering drawings.
  2. Machine the features that must be precise. Print undersized and drill, ream, or tap to final size. A drilled hole in a printed part is dramatically more accurate than a printed one.
  3. Design clearance into every fit. Do not rely on nominal dimensions matching. Use the gaps in our design for 3D printing guidelines.
  4. Print a test coupon first. A small block with a few reference holes, slots, and pins on your actual machine and material tells you the real offsets in under an hour.
  5. Prefer adjustable interfaces. Slots instead of round holes, shims instead of exact stack-ups, and set screws instead of press fits absorb process variation for free.
  6. Avoid stacking tolerances. Locate features from one datum rather than chaining them part to part.
  7. Match material to stability. If dimension is the priority, choose a low-shrink material from our 3D printing materials guide.

When printing is not accurate enough

Some features simply need a subtractive process. If you need bores held to a few hundredths of a millimeter, precise flatness, real threads, or a smooth sealing surface, machine those features — either from a printed blank or from solid stock. CNC machining for prototypes covers the tolerances and costs, and the FDM vs SLA comparison shows how far resin printing closes the gap.

For production quantities, the calculation changes again: injection molded parts hold tighter and more repeatable dimensions than printed ones once the tool is dialed in, which is one of the arguments in 3D printing vs injection molding.

Measure, verify, and move on

Measure with calipers at multiple points, on multiple parts, and record what you find. One part tells you an offset; five parts tell you whether the process is repeatable, and repeatability is what lets you compensate confidently. Surface treatment comes last, since sanding and painting change dimensions — see painting and finishing prototypes. More on process selection in the 3D printing pillar.

Struggling with parts that will not fit together? Send us the model and what is going wrong through our contact form and we will tell you whether it is a design fix, a process change, or a machining step you need.