Print-in-Place and Tolerance Design#

Overview#

A print-in-place design is an assembly that comes off the printer already assembled and working: hinges, chains, gears, ball joints, captive bearings, and flexible mechanisms. It is possible because layer-by-layer processes can build separate bodies in the same job, as long as there is a gap between them that is both large enough not to fuse and small enough to function. The same discipline of "design for the clearance" applies to any printed fit: sliding lids, press-fit pins, snap-fits and threaded joints.

This page covers how to choose clearances, how to model assemblies in Fusion so that clearance is parametric, how to handle supports and orientation for moving parts, how to design common mechanisms, and how to calibrate with test coupons. The numeric values are typical for FDM at 0.4 mm nozzle and 0.2 mm layers; resin, SLS and metal differ (see their pages). Always calibrate on your printer.

Where to find it#

Step by step#

1. Choose clearance by process and fit#

Define named parameters for each fit class. Starting values for FDM at 0.4 mm nozzle:

Create parameters like clr_slide, clr_pip, clr_press and use them in all relevant dimensions.

2. Model a print-in-place hinge#

1. Sketch the hinge knuckle profile and extrude two interleaved knuckles as separate bodies, A and B.
2. Create a pin as part of body A that extends into the knuckle of body B. Make the pin diameter pin_d.
3. Make the hole in B diameter pin_d + 2 * clr_pip.
4. Offset the end faces of the knuckles so there is clr_pip of gap between A and B axially (use Offset Face on one body or cut a slot).
5. Chamfer the pin ends and hole entries at 45 degrees so the surfaces are self-supporting.
6. Make the pin and hole axis vertical if practical. Horizontal pins print with flat-bottomed (oval) shapes and need supports in the gap, which are hard to remove. If the axis must be horizontal, use a diamond or teardrop section for the pin and hole so no gap needs support.
7. Check with Inspect > Interference that the bodies do not overlap, and Measure minimum distance to confirm the gap.
8. Export both bodies together as a single 3MF.

3. Other print-in-place mechanisms#

• Chain links. Each link is a body with a loop; the clearance between loop and neighboring link is clr_pip. Print flat; links lie in the XY plane and the gap is horizontal, which is more reliable than vertical gaps at small values.
• Gears and gearboxes. Use a gap between tooth flanks of 0.2-0.4 mm in FDM and axle clearances 0.3-0.5 mm. Use fillets at tooth roots.
• Ball joints. Sphere in a socket, separated by clr_pip; the socket needs an opening smaller than the ball diameter to retain it, and the lower hemisphere of the ball is a steep overhang; orient so the ball's support is minimal, or use a diamond-faceted ball.
• Captive balls or cages. A ball inside a larger cage prints loose with clr_pip all around; the cage openings must be smaller than the ball.
• Flexures. Thin cantilever strips (0.6-1.2 mm) that flex; orient so layers run along the strip's length.
• Bearings. Print-in-place bearings often have a ring of balls or rollers, gaps of 0.3-0.5 mm; best in stiff materials and printed at low layer heights.

4. Orientation and supports for moving parts#

1. Orient so the clearance gaps are vertical (perpendicular to the plate) where possible: vertical gaps are made of stacked layers and keep their dimension well. Horizontal gaps depend on the Z resolution and the first bridging layer above.
2. Avoid supports inside gaps. Design chamfers so each surface on the roof side of a gap is 45 degrees or less from vertical.
3. Increase the bottom layer separation: the first layer above a gap of 0.3 mm may sag; use a Z gap at least equal to two layers.
4. Disable support generation in the slicer with blockers around joints; allow supports elsewhere.
5. After printing, break the join: moving parts may be fused by "elephant foot" at the first layer; twist or flex to free them.

5. Calibrate with test coupons#

1. Create a small test part in Fusion: a plate with five pegs of the same diameter and five holes with clearances 0.1, 0.2, 0.3, 0.4, 0.5 mm per side. Add text labels.
2. Alternatively, create a mini hinge test with gaps of 0.2, 0.3, 0.4 mm.
3. Print on the same material and settings as the final part.
4. Record which clearance gives the fit you want.
5. Update your clearance parameters. All dependent features update automatically.
6. Store the coupon in your Fusion project so that it can be re-run when you change filament or printer.

Options reference#

Tips and pitfalls#

Tip: Keep a single clr_pip parameter and tie all print-in-place gaps to it. After one test, you can retune the entire assembly.

Warning: Do not assume the CAD gap equals the printed gap. Extrusion width variation, flow rate, and heat spread eat into gaps. A 0.2 mm CAD gap may be a fused solid in print.

Tip: Make print-in-place parts with a break-free feature: a tiny chamfer or clearance at the first layer. Without it, a thin layer of squashed plastic at the bed can weld the pieces.

Warning: Large temperature changes (a heated chamber, outdoors) alter fits for plastics with high thermal expansion. If your parts must work across temperature ranges, widen the clearances.

Tip: Use the Fusion joints and motion study to check range of motion before printing. Add joint limits with Joint > Motion tab. See the joints page in the components section.

Warning: Do not model a fit using a negative clearance and rely on the slicer to "fix" the overlap. The slicer sees overlapping solids as a union, not as a press fit.

Tip: Press fits are best done on features that tolerate small adjustments, like a pin with a lead-in chamfer, or an oversized pin that is sanded or reamed.

Warning: Threads printed in FDM need generous clearances and coarse pitch. An M8 bolt in a printed nut typically needs 0.3-0.5 mm radial clearance; a fine thread simply fuses. Consider a printed thread design with a trapezoidal profile designed for printing, or use inserts.

Practice exercise#

Build a tolerance coupon in Fusion with parameter clr and a rectangular pattern that increments clr by 0.1 from 0.1 to 0.5 mm (you can create five separate holes with different parameter expressions, such as d_hole_1 = pin_d + 2 * 0.1 mm). Print it. Pick the clearance you consider "slide fit" and the one you consider "loose". Update clr_slide and clr_pip accordingly. Then design a simple print-in-place hinge using those parameters and print it. Expected result: the hinge moves freely after a gentle twist. If it is fused, increase clr_pip by 0.1 mm and reprint only the hinge; do not edit individual sketches.

Related#

• Preparing Models for Printing
• FDM Design Guidelines
• Joints (05-components-assemblies)
• Lip, groove and snap-fit (08-sheet-metal-plastic)