McMaster Carr is an amazing resource. It's well laid out, informative, comprehensive..but even better, they provide free 3D models for most of their parts. Not all, but many.
If you have access to a 3D printer, you can print these parts yourself. Pretty cool, eh?
Step 1: Find your parts
First, point your browser to...
See? Easy peasy. Just clickity click. Not all parts on McMaster have 3D models for download. For this example, I'm going to be looking for sprockets. I've navigated to the part I want, and clicked on the part number. A little box pops up with some information.
More importantly, this particular part has the little symbol next to it that denotes a 3D model is available. I've highlighted it with an arrow in the picture.
Step 2: Download the file
After you click the product detail icon, this is the page you come to. I want a STEP file, as I'll be going into Fusion360.
If you're a user of CAD software that takes any of the files shown, congrats! You can now take those files and use them for modeling or 3d printing. Me, I'm continuing on to Tinkercad, to edit the part a little and to 3D print.
To do that I need to convert STEP into STLs.
Step 3: Convert file to STL in fusion360
Once you've downloaded your file, open up Fusion360 and click 'upload'.
When the upload is completed, open it. You can only export single bodies (solid shapes) so you have to either delete the ones you don't want, or download them one by one. In the navigator menu click on 'bodies'. In the tree that shows up select the bodies you want, either to delete or download.
In this case the sprocket came with a set screw in the model, which I removed.
In the menu that pops up, click refinement as 'high'. Do not change any of the measurement settings.
Step 4: Upload to Tinkercad
Head over to Tinkercad.com and open up a new project. On the right hand side you'll see an option for 'upload file'. Navigate to your STL and upload it, again without changing any size parameters.
Here is the file for this sprocket, uploaded to TinkerCad:
Step 5: Tinker!
If you need to, edit the model. For example I might want a different shaft diameter; but since I uploaded this as a solid body I'll have to do a little workaround. First filled the shaft bore that was there, and grouped the two shapes. Then I made a cylinder of the shaft size I wanted, made it a hole, centered it on the model, and grouped them.
In this step you could add multiple STLs if you wanted a complex part, say, where gears were interacting. The beauty of 3d printing is that you can both make multiple iterations of custom parts, that would normally take a lot of time and money otherwise.
You could even do specialized shape shaft drive- like a hex!
Step 6: 3D print
I've printed the sprocket on an objet connex 500. The one on the left is the part as ordered from master, the one in the right is my printed part.
You can use this technique for all sorts of parts; and many on master will print just fine on a lower resolution/extrusion 3d printer like a makerbot. Kings like motor mounts, pillow blocks, chain tensioners, specialized brackets, pulleys- will all print well on an extrusion printer.
I encourage modification as well! Once you're 3d printing you can add decorative elements, or even combine parts!
Another thing you can do is use the product file as a subtraction/hole. For example, you can download the file for a press fit bearing, convert to 'hole' in TinkerCad, and subtract if from a base to make a perfect press fit. You can use that to create perfect recesses for bolts and fasteners, as well.
There are some more in depth techniques I'll post about in later instructables, but for now, go out and grab some parts!
Another website that offers CAD files is SDP (standard drive products):