Joinery is a centuries old woodworking technique designed to remedy the problem of joint separation. Many joint styles rely on the addition of fasteners, bindings, or adhesives for structure, but fitted joints can also be designed in such a way that they connect exclusively by the force of friction. We can learn a great deal by studying the features of this historic craft, and — if we apply this thinking to digital design — can drastically improve the results of assembling multi-part models.

The great advantage of digitally designed and fabricated joints is that special layouts, tooling, and woodworking fixtures aren’t required to create successful mating features. If configured properly, a 3D printer, laser cutter, water jet, or CNC will produce repeatable features that press together with remarkable consistency.

EXTRUDED PARTING LINES

Creating joint details

In its simplest form, the process of creating joint details can be broken down into three simple steps:
1. Project a parting line / joint detail on an object

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2. Break the object into segments by extruding a cut through the object

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3. Save the individual segments as separate parts

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BENEFITS OF EXTRUDED PARTING LINES

Simplicity

Adjustments, if required, are performed on a single parting line feature — thereby simplifying the revision process.

Alignment

Component alignment can often be a challenge with 3D printed parts. Butt joints are difficult to align, and thin-walled lap joints can be fussy and fragile. This challenge may be compounded when bonding components together with instant adhesives that set quickly. Mating features takes the guesswork out of alignment and insures repeatable location of components time after time.

Unambiguous Orientation

Proper orientation of components isn’t always obvious with symmetrical designs. The inclusion of asymmetrical alignment features eliminates the ambiguity of orientation and can make it impossible to assemble segments improperly.

Mechanical fastening

Dovetails (and other parting line details with undercuts) provide the benefit of holding components together without the use of adhesives or external fasteners. This is particularly useful when components need to be assembled and disassembled.

Adhesion

Good adhesion of components is a byproduct of good part preparation and the selection of the proper adhesive, but also correlates directly with the amount of surface area being bonded. A contoured parting line increases contact area — which results in a stronger bond.

TIPS FOR SUCCESS

Joinery Design Guidelines

There can be a temptation to overthink design in pursuit of the ultimate solution, but perfection can be the enemy of progress. Keep it simple and remember these fundamentals of joinery:

Butt Joints: are easy to design, but can be difficult to align.

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Lap Joints: align nicely, but can be fussy and fragile when 3D printed.

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Dovetail joints: lock features together and can be assembled and disassembled easily.
Square keys: align features precisely and are well suited for gluing.
Asymmetrical location of keys: results in unambiguous orientation when assembling components.

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3D Printing Guidelines

Tolerances should always be considered when designing components intended to fit together. Dimensions of 3D printed parts will vary from the digital source files, and this discrepancy must be accounted for. Tight tolerances work with professional 3D printers, but hobbyist systems require a looser fit. We experimented both with a Stratasys uPrint and a Makerbot Mini, and found that the accuracy of the uPrint required minimal clearance (.005 in / .127 mm) to create a sliding fit between components, while the Makerbot required additional clearance (.015 in / .381 mm) to achieve the same kind of fit. There are many factors to consider, and in order to proceed with confidence it is essential to test.

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Factors That Influence 3D Printing Accuracy

1. Machine Type & Class
2. Material Selection
3. Component Orientation
4. Layer thickness

Procedural Best Practices

Testing will satisfy any uncertainty about achieving a desired fit. We always observe the following protocol:

1. Isolate mating features and print small sections to test the tolerances of your machine.
2. Fits that are too tight or too loose can be worked out with a minimal investment of time and material when isolated.
3. Print test parts with the same printer settings as final parts to get repeatable results.
4. Print test parts in the same orientation as final parts to get repeatable results.