By Andrew Edman, industry manager product design, engineering and manufacturing
Formlabs
Jigs and fixtures are used to make manufacturing and assembly processes simpler and more reliable, reducing cycle times and improving worker safety. Today, there are a number of functional resins well suited to 3D printing jigs and fixtures, which can reduce costs, shorten development time and create more effi cient production workfl ows at the mold building facility.
Complexity is (almost) free
Since 3D printing allows for “free” complexity (increased complexity doesn’t increase part cost), take some time to consider what additional functionality can be built into the jig or fixture at the design stage to take advantage of this principle. Small features that would be difficult to machine, as well as geometries considered impossible due to tool clearance in milling or turning, are all within the scope of additive processes. Serial numbers, fabrication dates and other relevant data can be built into the part for digital inventory management and easy tracking without requiring secondary engraving steps.
What would typically be two components in a machined fixture can be built into a single part, which helps prevent buildup of dust or chips by eliminating gap space. For instance, instead of using inserted straight dowels or cylinders for part location, spherical or diamond-shaped structures are built into a single, gapless part. Using diamond- or sphere-shaped locators reduces or eliminates binding of parts during loading on and off by minimizing the contact area.
Build datum features into fixtures and jigs
Part of the process of implementing jigs and fixtures in an assembly or manufacturing context is verifying the dimensional accuracy of the fixture. The amorphous part structures that 3D printed fixtures are often designed to address can mean that the fixtures themselves tend toward more esoteric forms. These designs can be difficult to easily inspect with standard metrology tools such as calipers and micrometers. Building datum features into printed jigs and fixtures makes inspection easier and more accurate.
A datum is a theoretically geometrically perfect reference – a totally flat plane, the axis of a cylindrical hole, etc. A datum feature is the reality of that concept in the context of the part, which is used as a principle reference point for other measurements. Datum features should be relevant to the requirements of secondary operations and to the functional requirements of the part in end use.
Whenever possible, include fl at faces or right-angle geometries within the fixture to aid inspection and determine overall accuracy. With any jig or fixture, accuracy is proven when inspecting parts after processing, as operating conditions like deflection in the part or tool can create errors requiring alterations to the fixture design. In applications where precision is of utmost importance, use digital metrology tools such as 3D scanners or touch probes to inspect more organic geometries.
Increase rigidity
The typical way to increase stiff ness of a machined fixture is to leave extra material in locations prone to bending under loading. In additive processes, minimizing material consumption keeps part costs low. Using reinforcing ribs and fillets provides additional structure without dramatically increasing the cost or build time of the part.
Increase durability of mechanical connections
Using tapped holes in 3D printed plastic parts is an ineffective method for joining parts for fixturing; these parts are more prone to breakage or wear with repeated use than metals. Instead, use more resilient assembly methods, such as threaded inserts or a pocket to restrain a nut while a bolt is tightened. Alternatively, a 3D printed fixture may have clearance holes to run bolts through to T-nuts or a fixture plate below. To prevent elastic deformation of the part when bolted down to the work surface, through holes should use clearance-fit tolerancing.
Make printed parts go farther
In many cases, 3D printed parts for jigs and fixtures are augmented using stock parts from industrial supply companies. This approach works well when some components need the specificity and design flexibility of 3D printing, but the overall working envelope or other requirements, such as stiff ness or conductivity, cannot be met through an additive process.
Common stock parts to add extra functionality to printed jigs and fixtures include metal shaftings for spanning greater distances while maintaining rigidity, or washers for distributing screw clamping loads over a larger footprint. Stock parts in combination with additive processes quickly add mechanical functionality –linear or rotary indexing, for example – at a lower cost than machining.
Consider creep
Some SLA resins experience creep (permanent elastic deformation) if continually loaded, as in the case of a printed fixture clamped to a work table for long periods of time. To avoid warping of parts due to continued loading, it’s best to loosen any bolts and relieve clamping forces after completing secondary operations.
Replace worn components on demand
Even under normal usage conditions, fixtures, assembly tools and jigs commonly become broken or worn to the point where they are no longer effective. By creating jigs and fixtures with additive fabrication, a facility takes control of its own production and gains the ability to replace tools on-site on an as-needed basis, rather than counting on external vendors with minimum order quantities. Replacing worn fixtures with in-house equipment shortens the supply chain and reduces downtime risk.
Validate printed fixtures
Once the fixture has finished printing, clean and cure it according to the material specifications. If support structures have been generated on the model, remove the supports and carefully file or sand away any remaining bumps, maintaining a fl at, even base.
At this point, inspect the printed part against the original CAD model. Use a caliper or micrometer to check dimensions of the print against a dimensioned drawing or annotated CAD model, and take note of any discrepancy that could negatively affect the performance of the jig or fixture.
If everything is dimensionally sound, the next step is to test the functional performance of the fixture. When the part is loaded onto the fixture, pay close attention to how well it is seated against locating surfaces and supports. A properly designed and built fixture will support the part, eliminating any movement once clamping force is applied. Clamping forces should push the part into the fixture evenly without any tilting, shifting, or bending of the part.
For processes with higher operating forces, such as milling or drilling, calculate clamping requirements based on feeds and speeds, the power of the machine, and the selected material, as well as safety. During initial use, take light cuts to ensure everything functions as planned.
After performing any secondary operations on the part, another inspection will verify tolerances held, along with fitting an acceptable cycle time. When first deploying a new fixture or jig, more frequent quality checks will reveal any unpredicted operator errors or wear that might result in quality failure. Those errors can be caught early and corrected either through training or modifications to the fixture design.
Of course, not all fixtures or jigs can or should be 3D printed. Always select materials on the basis of functional requirements of the task to be performed. In cases where 3D printed material is not suitable for end use, SLA-printed parts can still be used for validating fit and function instead, saving time and money compared to milling solid blocks of aluminum.
Engineers familiar with the exact context of jigs and fixtures in production do a much better job of building the right tools, the right way. Low cost, high-precision 3D printers make it possible for even small organizations to close the gap between concept and reality to improve manufacturing performance and remain competitive.
Formlabs designs and manufactures powerful and accessible 3D printing systems. Headquartered in Boston with offices in Germany, Japan, China, Singapore, Hungary and North Carolina, the company was founded in 2011 by a team of engineers and designers from the MIT Media Lab and Center for Bits and Atoms. Formlabs also develops its own suite of high-performance materials for 3D printing, as well as best-in-class 3D printing software. For more information, visit www.formlabs.com.
