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CAD the Easy Way

Getting started is often the hardest part of creating a design for a custom part. I often have customers come to me looking for a particular mechanical part that they just want tweaked a little bit. I often tell them that their best bet is to find a CAD file that's similar to what they need, and make the changes that they require. My two favorite resources for CAD files for mechanical parts are McMaster-Carr and SDP/SI. In this article, I will show you how to use these incredible free resources to jump start your project.


SDI/SI is one of my favorite sources for parts for my robots. They sell a variety of mechanical drive components including bearings, gears, pulleys, and the like. Their online catalog also lets you download CAD files in a variety of formats for almost everything that they sell. From their home page, you can click the "BUY ONLINE" tab to get to their online store where the CAD drawings are located.



Illustration : SDP/SI Website

For example, let's suppose we want to find the CAD drawing of a gear. Their online shop has an expandable list of items that they have for sale. First we can click on "Gears" to see the different types of gear.



Illustration : Choosing a category from SDP/SI

Then under "Spur Gears", we can select "Metal".



Illustration : Choosing a subcategory from SDP/SI

On the right hand pane, a list of all the metal spur gears will appear. We can click on the part number to show the details of that item.



Illustration : Choosing a gear

The product details will appear in a new window. There's a link that says "AutoCAD Drawing", but this link didn't work for me. However, I was able to get the drawing via the "3D CAD Models" link.


Illustration : SDP/SI product details page

SDP/SI wants you to register before they will let you download any CAD files. You can use your existing SDP/SI login or click the "Click here" link to create a new account. The new user registration page (not shown) is easy to fill out and doesn't require you to jump through any hoops like a confirmation e-mail before allowing you access to the CAD models.



Illustration : Creating an SDP/SI account

After you register, clicking the "3D CAD Models" link will now show you a download page like the one shown below. Before you download, you need to set the file format in the drop down list, as well as any dynamic attributes for the part.



Illustration : The SDP/SI download page

For most of my CAD work involving waterjet or laser cutting, a simple 2D DXF file is quite sufficient to model my final parts. This is one of the formats that SDP/SI can provide.



Illustration : Getting an SDP/SI file in DXF format

This may help with rendering performance in your CAD tool. In this case, I want to get a model of the whole gear, so I enter 8 teeth for the "# of Teeth to Display on CAD Model".



Illustration : SDP/SI dynamic attributes

After clicking the "Download 3D Model" button, the web page will indicate that the CAD model is being generated. When it is done, you will see a link that lets you download the CAD drawing. When you click the link, your web browser will begin downloading a ZIP archive file containing the CAD drawing.



Illustration : Downloading an file from SDP/SI

Here's what the downloaded file looks like in QCad.



Illustration : A downloaded file from SDP/SI

Note that this design isn't quite ready for waterjet cutting yet. If you wanted to waterjet cut this gear, you would have to delete the extra views, as well as any lines other than just the outline of the part.


McMaster-Carr is legendary among makers. They carry nearly half a million products of every sort, including nuts and bolts, raw materials, hydraulic components, cleaning supplies, and tools, just to name a few things. If you haven't seen their website yet, take a look.



Illustration : The McMaster-Carr home page

Their website does a good job helping you sort through the giant variety of stuff that they sell, but getting to a particular part might mean selecting 5 or 6 different parameters. For instance, for machine screws, you have to choose the material, head type, thread size, length, and so on. I'll spare you all of that and just choose a particular screw with McMaster-Carr part number 91241A083. To get to this part on the McMaster-Carr website, just enter the part number in the search box on the home page and click "Find".



Illustration : Getting a part by part number

This will bring up the product detail page for the part, a type of socket head cap screw. You will notice that this page has a link on the left-hand side for a "Technical Drawing". (Note: this link will not be present if McMaster-Carr doesn't have a CAD drawing for the item).



Illustration : The link for the CAD file

Clicking on the "Technical Drawing" link brings up a measured drawing in your browser. This is nice, but not quite a useful as a CAD file. You can click the "DOWNLOAD" link at the top of the page to select a CAD format file to download.



Illustration : Download a CAD file from McMaster-Carr

In this case, I want a 2D DXF file, so I will select that and click the "SAVE" button. This starts the download process.


Illustration : Choosing a file format on the McMaster-Carr website

Here is the drawing from McMaster-Carr as shown in QCad.



Illustration : A file downloaded from McMaster-Carr

Again, files you get from other people will