Introduction
Prototyping is an essential part of the mechanical design process. It allows engineers to quickly create physical models of their designs to test form, fit, and function before committing to full production. With the rise of computer-aided design (CAD) software and 3D printing technology, engineers now have more tools than ever to prototype their ideas.
CAD software provides the digital design space for creating 3D models of mechanical parts and assemblies. Popular options like SolidWorks, AutoCAD, and Fusion 360 enable designers to sketch 2D geometry that can be extruded and manipulated into complex 3D shapes. CAD allows for rapid iteration by tweaking dimensions, geometries, and features in the digital model.
Once the CAD model is finalized, 3D printing can then be used to physically realize designs as tangible prototypes. 3D printers build up parts layer by layer using plastic, resin, metal and even advanced composites. The proliferation of desktop 3D printers has put this capability within reach of mechanical engineers. Different 3D printing technologies trade off build speed, part strength, feature resolution, and material capabilities.
Prototyping with CAD and 3D printing facilitates learning through hands-on testing. By printing testable prototypes early in the design process, engineers can identify improvements and optimize the design prior to large-scale production. This step-by-step guide will explore the key steps involved in leveraging CAD and 3D printing to prototype mechanical designs.
Choosing CAD Software
Computer Aided Design (CAD) software is essential for creating 3D models ready for 3D printing. There are many options to consider when selecting CAD software for mechanical design prototyping:
SolidWorks - A premium CAD package with powerful design, simulation and documentation features. Widely used in engineering firms and ideal for complex mechanical designs. The high cost may be prohibitive for hobbyists.
AutoCAD - A well-known CAD software with a full feature set. AutoCAD has a steep learning curve but is a robust all-rounder. The licensing model can get expensive for commercial use.
Fusion 360 - A newer cloud-based CAD package from Autodesk. Has integrated simulation and CAM features. Hobbyist licenses are free, attractive for makers. The cloud dependency may be an issue.
Onshape - A fully cloud-hosted CAD package with built-in version control and team collaboration features. The free plan has limited private storage and feature restrictions.
Free CAD - A free and open source CAD package. Not as polished as commercial options but has solid modeling capabilities and an active community.
Tinker cad - A free browser-based CAD tool from AutoDesk. Very easy to learn but limited to basic shapes. Best for beginners.
When selecting a CAD package, consider upfront cost, the learning curve, features, collaboration needs and your level of experience. Spending time gaining CAD skills will pay dividends when designing prototypes. Leverage free trials to test different options.
Selecting a 3D Printer
When it comes to 3D printing your mechanical prototype, you'll need to select the right 3D printer for the job. The main options are desktop 3D printers and industrial 3D printers.
Desktop 3D Printers
Desktop 3D printers like the MakerBot, Ultimaker and Prusa models are affordable, accessible options well-suited for prototyping mechanical designs. They have build volumes generally under 10 inches cubed, with resolutions around 100 microns. While not as precise as industrial machines, desktop printers can achieve sufficient detail for most prototypes. Most common materials are PLA and ABS plastics, but more exotic filaments are available too. The tradeoff is print speed, part strength and maximum size. But for early stage prototypes, a desktop 3D printer is easy to use and provides valuable design feedback quickly.
Industrial 3D Printers
For high resolution prototypes with industrial grade materials, an industrial 3D printer is required. Companies like Stratasys, 3D Systems and HP offer machines starting around $100,000 with much larger build volumes, multi-material capabilities, and resolutions down to 10 microns. Materials like production-grade thermoplastics, metals or photopolymers allow very accurate prototypes that mimic final performance much better. The extended time and cost is harder to justify for initial prototyping phases though. Industrial printers come into play once the design is more finalized and testing requirements are stringent. Their high throughput is also better suited for small batch production rather than one-off prototypes.
When selecting a 3D printer, analyze your specific prototype requirements - size, feature resolution, material choices, budget and timeline. Desktop printers are great for initial testing, while industrial machines are necessary if the prototype must mirror production exactly. Choosing the right printer between these options ensures you can iterate quickly without overspending.
Designing your Prototype in CAD
Once you have chosen your CAD software, it's time to start designing your prototype. The key steps are:
1. Create 2D sketches - The foundation of your 3D model will be 2D sketches. Use the sketching tools to layout profiles and cross-sections of your part. Pay close attention to dimensions and constraints to define the size and relationships between features.
2. Extrude sketches into 3D - Once you have detailed 2D sketches, you can use commands like Extrude to turn them into 3D geometry. Think about the overall thickness and depth you need for your part.
3. Add additional features - Use CAD tools like fillets, chamfers, holes, shells, etc. to add details to your 3D geometry. This will make your prototype look and function closer to the final product.
4. Refine the geometry - Continue to refine your 3D model by tweaking dimensions, adding or removing features. Strive to only include geometry critical to the function of your prototype. Simple is better for initial prototyping.
5. Focus on critical dimensions - As you design, focus on getting the most important dimensions and tolerances correct according to your specifications. The key functional features must be precise, even if the overall shape is approximate.
6. Simulate motion and assembly - Use CAD tools to simulate motion of components and to test fitment with other parts. This will help identify areas for improvement before 3D printing.
Following these steps will result in a clean CAD model ready for 3D printing and functional testing. Don't try to make your prototype model too perfect. The goal is to quickly get a design printed that validates your concept. Refinement comes later in the design process.
Preparing Files for 3D Printing
Before sending your 3D model to the printer, it's important to prepare the CAD files to ensure a successful print. Here are the key steps:
Check for Errors and Create a Watertight Model
Any gaps or intersecting geometry in your CAD model can cause errors when slicing or during printing. Perform a thorough check for errors to confirm your model is "watertight" with no holes or issues. Most CAD programs and 3D printing software can run automated checks to detect problems. Fix any errors by repairing geometry and ensuring a fully enclosed solid.
Add Supports as Needed
Depending on the geometry of your part, you may need to add support structures to prevent overhangs from drooping or sagging during printing. Enable automatically generated supports in your slicer software and adjust as needed. Supports can be tricky to remove post-print, so only use them where absolutely required. For mechanical parts, well-designed assemblies often need minimal supports.
Choose Layer Height and Infill Settings
When slicing your model into printing layers, you'll need to choose a layer height and infill percentage. Lower layer heights around 0.1mm produce a smoother surface finish. Layer heights of 0.2mm are more common for faster prints. Infill refers to the interior density of the part, with 100% being completely solid. For functional prototypes, 40-60% infill is recommended for good strength vs print time. Denser infill increases strength but also uses more material and takes longer to print.
3D Printing Your Prototype
With your CAD model prepped and sliced, it's time to start 3D printing your mechanical prototype! Here are the key steps in the 3D printing process:
Level The Print Bed
Start by ensuring your print bed is clean and free of any residue from previous prints. Wipe it down with isopropyl alcohol.
Level the print bed following the instructions for your specific 3D printer. This is a crucial first step for getting good adhesion.
Many printers have an auto-leveling probe that can measure variances and compensate. Still do a manual check periodically.
Use sheets of paper, feeler gauges or calibration objects to help get even spacing between the nozzle and print bed.
Start The 3D Print
Load your chosen filament or resin and get it feeding into the extruder/vat.
On the printer interface, select your sliced model file to start heating up the nozzle or resin vat.
Once at temperature, the print will begin with starting gcode to prime the nozzle and start extruding onto the print bed.
Monitor The First Layers
Stay close and watch as the first several layers print. The most common print failures happen early.
Check that filament is sticking to the bed and layers are bonding correctly without gaps.
If adhesion is poor or layers seem spaced, stop the print and re-level the bed.
If all looks good, let the print continue on its own with occasional check-ins.
Resist making adjustments or touching the model for the first few layers to prevent shifts. Allow it to stabilized and bond well to the bed first.
With good first layer adhesion achieved, you're on your way to a successful 3D print! Now just wait for all the layers to complete.
Post-Processing
Once your part is 3D printed, there are often additional steps required before it is ready for testing and evaluation. Here are some of the typical post-processing techniques for 3D printed mechanical prototypes:
Removing Supports
If your part needed support material during printing, this will need to be removed after the print is finished. Support removal is usually done by hand, twisting, cutting or dissolving away any excess material. Go slowly and be careful not to damage the part.
For soluble supports, soak the print in the appropriate solvent to dissolve away supports. Waterworks well for PVA while limonene or other solvents can be used for dissolvable filaments like HIPS.
Sanding and Smoothing
3D printed parts will show layer lines based on your print resolution. For prototypes not requiring ultra-high accuracy, sanding can help smooth the layer steps. Start with a coarse grit sandpaper like 120 and work up to finer 400-600 grit for a smooth finish.
Be careful not to sand away any critical dimensions or features. Sanding can also help reduce blemishes or blobs on the print surface.
Drilling and Machining
Often you'll need to add tapped holes, press-fit inserts or other precise features to your 3D printed prototype. Using a drill press, tap set or basic machine tools, you can modify your print as needed.
Reference your original CAD design and mark the locations for any holes, cuts or mounts. Go slowly and carefully to achieve the right fit. 3D printed plastics machine similarly to traditional thermoplastics.
Testing and Evaluating Your Prototype
Once your prototype is fully post-processed, it's time to test it out. This is one of the most important steps to refine your design.
Carefully check that all the features and parts fit together and function as intended. Try assembling it and note any areas that are too loose or too tight.
Test the moving parts and mechanics. Do hinges, gears and other components operate smoothly? Identify any binding, friction or interference.
Evaluate the overall strength and durability. Will it withstand expected loads and usage? Consider testing to failure to understand the weak points.
Aesthetically, does it achieve the look and finish you want? Pay attention to smoothness of surfaces, visible print layers, etc.
Get feedback from peers, colleagues or users. Have someone else try out your prototype and get their impressions. Different perspectives can reveal issues you may have overlooked.
Make notes on all the areas that need improvement or refinement. Don't rely on memory alone. Documenting this will help with redesign.
If possible, capture insights quantitatively through measurements. But qualitative feedback is also extremely valuable at this stage.
Thoroughly testing your prototype and gathering feedback is crucial to refining the design. Focus on learning as much as possible from this hardware incarnation, before making tweaks in CAD and printing the next iteration.
Redesign and Re-print
Once you've tested your initial 3D printed prototype and evaluated its performance, it's time to make design tweaks and print an improved version. Don't expect your first prototype to be perfect - redesigning and reprinting is part of the iterative process of 3D printing prototypes.
Go back to your original CAD model and make changes based on your learnings from testing and evaluation. Focus on addressing any flaws or issues that came up with the first prototype. For example, you may need to:
Adjust dimensions that didn't fit quite right
Modify features that didn't perform as intended
Redesign areas that had print errors, poor strength or tolerances
Improve the overall form and ergonomics based on evaluation
Don't go overboard with changes - keep tweaks focused on your key learnings. Significant redesigns can come later once the concept is proven.
Once you've updated your CAD design, go through the full process again:
Prepare and slice the revised model for 3D printing
3D print the new prototype
Post-process as needed
Test and evaluate the new version
Be methodical with your redesign process. Keep notes comparing versions and document what changes you made. Take photos of prototypes side-by-side to analyze iterations. Getting into this habit will streamline future redesigns.
Expect to go through several rounds of reprinting and refinement before achieving your optimal prototype. With each version, you'll bring your design closer to its full potential. 3D printing enables fast redesign iteration so take advantage of it.
Considering Production
Once you have iterated on your prototype and are satisfied with the design, the next step is considering production methods if you want to manufacture multiple units.
While 3D printing is great for prototyping and low volume production, for higher production volumes you will likely want to look at other manufacturing methods like injection molding, CNC machining, die casting, etc. Each method has its own pros and cons in terms of startup costs, per unit costs, production time, part properties, and more.
Some key factors in choosing a production method:
Volume - How many units do you need to produce? 3D printing tends to make the most sense for shorter run production of up to 1,000 units or so. For higher volumes, other methods become more efficient.
Part geometry - Does the part have complex organic shapes and details best suited for 3D printing? Or precise, geometric features that would be easier to injection mold?
Materials - What material properties do you need for the application? 3D printing gives a wide range of plastics, resins, metals and more. Other methods may be better for specific material needs.
Cost - What is the overall budget for startup and production? 3D printing has low startup costs but higher per unit costs. Injection molding has high startup costs but lower per unit costs at high volumes.
Timeline - How quickly do you need to produce the units? 3D printing gives fast turnaround for the first unit but becomes slower for mass production.
Once you've selected a production method, you'll want to partner with a reputable manufacturing company specializing in that process to turn your prototype into a finished product. They can help refine the design for manufacturability and provide quotes for different volume scenarios. Be sure to get quotes from multiple vendors.
Having open communication with your manufacturing partner throughout the development process is key. Provide them with detailed 3D models, ideally your original CAD files if using a process like CNC machining. Be clear about target volumes, timelines, and budget. This will ensure the best results as you scale up from rapid prototyping to full production.
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