49759178 366098043968191 6440236965668323328 oCustom waterjet cut 3 inch span wrench for BattleBots team Hypothermia


When I’m cutting a set of parts for one of my projects, I often look around the shop for tools or tooling that I can add to the order that would upgrade the shop just a tiny bit better to make it easier to work in. One of my favorite adds to the low taper cut list is custom wrenches. The reason being is that one more part usually doesn’t increase the cost of cutting or shipping too much, and having a designated wrench for changing the lathe tooling that lives with the rest of the lathe tools keeps my wrench set from roaming around the shop in a unorganized jumble.

So let's design a wrench! First we need to identify where the working surfaces are on the recipient of our wrench are. For a hex headed bolt, the working surfaces are the faces that I’ve highlighted in pink below:

Notice that the force lines don’t extend to the convex corners. The reason for that is that there is less material on the convex corners than there is on the centers of the flat faces. If your wrench grabs the corners during use and applies the force there, they won’t hold up to much force before they strip out.

So let’s mock up a crescent wrench to match the hex bolt.

As you can see, we are only applying force to 4 of the 6 faces and if we were to cut the green wrench as shown, it would have to cut perfectly and the hex head would have to be forged perfectly to fit together. Since we are all working within a tolerance range lets add some ease into the places where the faces are interacting to make these work in real life. For starters, we are going to scale our example hex head up by 0.005”-0.01” larger than the actual bolt head. That will allow the crescent to slip onto the bolt without aligning it at the perfect angle.

In the image above, the dotted orange hexagon is the 1” height of my bolt. The black hexagon is 0.01” larger than the bolt head size. That extra room will make it so that my wrench will slip on the hex bolt easier and allow for that bolt to have some variation in size from the perfect CAD model. This is particularly important because the available hardware out there sometimes shifts during stamping or is heat hardened, and all of those factors can affect the finished size of the bolts.

Next we need to address where the wrench is going to touch the corners of the bolt head. As mentioned before, putting pressure on those corners is a fast way to strip a bolt. I hate pulling stripped hardware, so I’m going to add radii to the cutout that will keep the wrench from touching my bolt head.

Those 1/16” circles should keep the corners from getting dinged up nicely. Now to merge them with the larger hexagon. It’s starting to look like a wrench, but I need to adjust the depth that the bolt head slides into the spanner. The corners of the wrench I’ve highlighted below could slip during use and cause stripped corners.

So lets move our bolt head and hole a bit to the right.

Now let's extend the top and bottom parallel faces outward using the line tool to create where the mouth of the wrench will be.

And merge.

Now I just need to cut the green working face shape out of the blue outer wrench shape.

And now it is ready for cutting on the low taper waterjet. Why the low taper waterjet and not the regular one? Because taper in the cut will affect the performance of the wrench and possibly lead to having to dress the working faces. Which would be really difficult for the two faces that are closest to the handle. The low taper waterjet will get you a cut that is as close to 90 degrees through the thickness of the material as possible, leaving much less cleanup. Learn more about taper and low taper waterjet cutting in our FAQ article.

Having a few extra tools and parts in your file library to add to an order to spread out the costs of setup and cutting makes per part prices lower and helps you expand shop tools without a huge investment. There are also a few tools in my shop where replacement parts are no longer available or hard to find, so having a wrench made out of brass or HDPE that will fail before I strip out a specialty setting screw is also handy.

49285372 366101180634544 2033712232071692288 oAnother view of the Hypothermia wrench


Happy cutting!