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WaterJet How To

WaterJet carved Bumper Bracket on '33 Plymouth Coupe.

Water Jet Cutting has become affordable and widely available. I recently had some bumper brackets for my street rod cut on the Waterjet cutter at the University of Utah’s Department of Engineering. What follows is how that process worked.
Waterjet is a tool capable of slicing into metal or other materials using a stream of water at high velocity and pressure, or a mixture of water and an abrasive substance. A benefit of the water jet cutter is the ability to cut material without heating it.. Minimizing the effects of heat allows metals to be cut without harming its properties. The kerf (width of the cut) are in the range of 0.040" to 0.050" but can be as narrow as 0.020". Waterjet cutters are capable of attaining accuracy of 0.005".

The particular machine used to cut these brackets had a maximum cutting area of 24” x 24” and a maximum metal thickness of 6” thick mild steel. Another vendor in my area offers a very large cutting envelope of 6ft x 12ft. . Another thing that impressed me was a display of waterjet cut aluminum Butterflies and X-mas ornaments from .020” thick sheet aluminum. Not only will this process cut large items and heavy steel, it will cut intricate filigree patterns for decorative work. Imagine the possibilities

The actual cutting of two brackets from a piece of surface rusted ½” mild steel plate (provided by myself) took approximately 20 minutes total. The fee charged is based upon the actual cutting time. There was no charge for setup. My total charge for cutting the two brackets was $28.00.
The waterjet works from a computer file. To generate the necessary .DXF (Drawing Exchange Format) file, I drew the part in AutoCAD. The .DXF file format is as close as the CAD industry has to a standard file. Almost any 2-D cad drafting software package offers the option to save file as a .DXF format and most programs will open a DXF file created in some other software package.

Here you can see a screen shot of the AutoCAD DXF drawing.

Click DXF image for larger view

I know that CAD isn’t necessarily a common tool in the hardcore fabricators tool box, but basic CAD software is no longer expensive, and a little CAD capability can be a real improvement in skills. To learn a bit about the subject ask around, it isn’t that uncommon or visit your local community college for a class.

Now while Waterjet is modern technology, I don’t want this project to sound like some sort of computer exercise. Lets get back to the old school basics of how I operate. In this photo you see a 1/8” thick plywood template I made to develop my design and verify fit.

Checking fit with Plywood Template

As a matter of fact, you can see that I needed to increase the depth of the cutout for the rear fender/gas tank cover. It’s easy to change details at this point, modifications are much more difficult when the steel part is finished.

Fully dimensioned CAD Model

This rather oddly shaped bracket required quite a few dimensions to fully define it. The process was pretty simple-- I made the plywood bracket fit well, then I measured and translated it into the CAD drawing. I had drawn two mounting holes to attach the brackets to existing holes in my chassis. I drew the holes at .530” diameter so as to have a clearance hole for a ½” bolt. I had expected to have to drill them after the parts were cut, but after some discussion with the machine operator about location accuracy and hole tolerance, I had him cut those holes on the waterjet. As you can see it worked great.

A .530" diameter mounting hole. Notice how the waterjet pierced in the center then cut out the slug.

The cutting process was quick and simple. They opened my DXF file, checked to be sure that I had created it at 1:1 and directed their software to create two instances of the part to cut (I needed brackets for both sides. They took my big heavy rusty plate of steel and put it into the cutting bath. The actual cutting is done with the part and cutting head immersed in water to control splatter. Then they loaded about 40 pounds of 80 grit Garnet Abrasive into the hopper and pushed “Go”. The water bubbled and the machine pulsed and hummed. I was able to watch the cutting proceed on an adjacent computer monitor. In about 20 minutes the job was done and I was inspecting my new bumper brackets.

Note the smooth finish, no cleanup required.

There is very likely someone offering a waterjet cutting service near you. Check your local yellow pages. This is a modest priced process offering amazing versatility and accuracy. Keep it in mind when you're considering how to whittle your next widget.

The end product is a "Gasser Style" rear bumper.