Articles

Waterjet cutting prices are based upon the following:

• Setup time
• Material
• Machining time

Setup time includes the time to load the stock material on the waterjet machine, process and transfer the cutting program to the computer controlling the machine, and so on.

Material is the type of stock from which your part is made. Some materials simply cost more to obtain.

 

Generally more expensive per square inch <			Generally less expensive per square inch >
stainless steel	aluminum	steel	acrylic

 

Thicker material generally costs more than thinner material per square inch. The harder and thicker the material is, the take longer it takes to cut.

Machining time is just the amount of time that the machine has to run in order to make your part. The longer the cutting time, the more wear and tear on the machine, and the more electricity and other consumables that must be used.

 

Factors affecting machining time

Thickness

Keep in mind that it takes a lot longer to cut through 1/2 inch steel than it does 1/4 inch steel. 2.3 times as long, in fact; not twice as long as you might expect. Thus, you should possibly consider the stacking technique for thicker material. This means cuting two pieces separately, then fastening them together into one thick piece. In the case of a ½ inch thick part, you would stack two ¼ inch pieces to get the correct thickness.

Material type

Softer materials cut more quickly than harder ones.

< Generally hardest to cut			Generally easiest to cut >
alloy steel	steel	aluminum	polycarbonate

Convex and Concave Corner Design

Sharp inside (concave) corners and tight arcs are take longer to cut and will drive up the cost of your part. The sharper the corner and tighter the arc, the harder it is to make. Note that sharp outside (convex) corners are generally faster to cut than outside curves or arcs, due to a waterjet cutting technique called corner passing. However, rounded inside corners are often less expensive to cut than sharp inside corners, as the waterjet does not have to slow down as much.

 

 

Illustration : More expensive: rounded outside corners and sharp inside corners

 

Illustration : Less expensive 1: sharp inside corners

 

Illustration : Less expensive 2: round inside corners

Internal Holes

Every internal hole you add to your part means that the machine has to do extra work to cut each one out. The waterjet cutting head must traverse to each hole location and pierce the material before cutting around the cutting line. With fewer traverses and pierces, machining time is sped up.

 

 

Illustration : More expensive: more internal holes

 

Illustration : Less expensive: fewer internal holes

Returning to the clamp example from this previous article, let's take a closer look at how the bolt holes were made through the side of the clamp.

In the photo below of the two clamp halves, you will notice that the clamp has indendations along the cut line where the bolt holes go. This makes it easier for a drill press operator to precisely locate the position of the holes.

 

 

Illustration : The two halves of the split clamp before the bolt hole
is drilled in the cut edge. Note the indentation which indicates
where the bolt holes will be located.

 

Illustration : Drilling a bolt hole in the cut edge of the clamp with a drill press.
The indentation in the cut line helps locate the correct spot for the hole.

 

A similar indentation in the cut line can be used to mark the location of blind holes or other features created by secondary operations.

 

One common application of waterjet cutting is to make custom clamps to hold pipe or tubing in place. You can see an example of such a part in the photo below.

Note that the bolts holding the two halves of the clamp together pass through a hole in the cut edge of the part.

 

 

Illustration : A waterjet cut custom clamp holding two poles together.

 

 

For parts where t-nut construction won't work, you may consider bending flat parts in a sheet metal brake.

Be aware that getting precise results from bending can be difficult. There are many factors to consider, including the bend radius of the material, the amount the material will stretch, and so on. Wikipedia has a brief overview of the subject.

Typically, an experienced brake operator will perform several practice bends on test parts in order to get the setup and bending procedure just right for accurate production parts. Thus, you should have several extra parts made and plan on your final accuracy being no better than +/- 0.01 inches in a small production run. You can compensate for this inaccuracy by, for example, elongating mounting holes in the part.

If you plan to use aluminum in a part which needs to be bent, you should use 5052 alloy instead of 6061, as 6061 is prone to cracking when bent.

 

 

 

Illustration : Waterjet cut aluminum 5052 parts before bending on a sheet metal brake

 

Illustration : The same parts as above after bending on a brake.

The position of the ends of the bend line can be indicated by indentations along the cutting line. In addition, the material can be made easier to bend by cutting narrow reliefs along the bend line. A drawing of such a part is shown below.

 

 

Illustration : Drawing of flat piece to be bent.

The photo below shows the part waterjet cut from 0.08 inch thick aluminum. The reliefs on this piece are large enough and the material thin enough that this was able to be bent accurately by hand. For added strength, the length of the reliefs should be reduced.

 

 

Illustration : 0.08 inch thick aluminum 5052 piece bent (left) and original flat piece (right)

In this extension of the stacking technique, pieces are oriented face-to-face, but separated. Each piece is held in place on a longer bolt or threaded rod with a nut (and optionally, a washer) on each side.

You can also disguise the threaded rod and add a little more compressive strength by using a pre-formed standoff.

 

 

 

Illustration : Waterjet cut parts held apart by nuts and long bolts

 

Illustration : Another view of the assembly from the previous photo
showing how the parts are kept in place with nuts

Quite often designers need the equivalent of milled parts' boss: slot or pocket features. This can be accomplished through stacking. Parts are stacked with faces touching, then secured together with a nut and a bolt.

You can also produce an undercut effect by stacking larger pieces on top of smaller pieces.

 

 

 

Illustration : Waterjet cut parts stacked to simulate boss features

One of the most popular ways to create a 3 dimensional assembly of waterjet cut parts is via the t-nut technique. In this technique, a bolt or screw is passed through a round hole on the faces of one waterjet cut part, and then through a special slot perpendicular to the cut edge of another piece. The bolt is threaded through a nut which is held in place in a perpendicular slot.

The mating edges of both parts are usually cut with tabs and slots to further secure them together. This is similar to the finger joint or comb joint sometimes found in woodworking.

 

 

 

Illustration : CAD drawing of parts to be joined at their edges with the t-nut technique

 

Illustration : Waterjet cut pieces joined at the edge with the t-nut technique

 

 

Illustration : Another view of the t-nut technique with the same part from the previous photo

The slots cut into the face can be placed along the edge for maximum space savings. For more stability, the slots should be surrounded by solid material. This is analogous to the mortise and tenon joint of woodworking.

 

 

Illustration : Parts to be assembled using the t-nut technique;
the slots are placed in a location which provides more stability.

 

Illustration : The t-slot technique in a more
stable configuration: the slots in the center of the part

 

Illustration : Another view of the more stable variation of the t-slot technique

 

Note that the slot which holds the bolt extends slightly beyond the nut. This is to make sure that the bolt fully engages all the threads on the nut. It also gives some flexibility in the length of the bolt used.

You might need to consider making the tabs slightly smaller than the slots or overcutting the corners in order to compensate for rounded inside corners due to the waterjet's kerf, as shown below. (More information on kerf can be found in our article on the limitations of waterjet cutting).

 

 

 

Illustration : Inside corner reliefs on tab section to compensate for kerf

 

Illustration : Inside corner reliefs on slot section to compensate for kerf

 

Since the cut edge of the part is used in contact with a flat face, you should consider the use of low-taper waterjet cutting for this type of construction.

You should decide whether it's more important to have a tight fit or easy assembly.

If you go with "tight fit", you should design the slots slightly undersized (0.005 or so), and fix it by filing away any excess. Heating the hole section with a torch or heat gun so that it expands will make putting the pieces together easier. You can force the pieces together with a press or even a hammer.

If you want "easy assembly", you can get the parts to hold together more tightly using retaining compound (Loctite makes a good one) or epoxy.

Here is a complete box made using this technique. The box sides have been laser cut from clear acrylic, which lets you see how the whole thing is assembled.

 

 

 

Illustration : Box assembled with the t-nut technique

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.

SDP/SI

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

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

When using SolidWorks to design a part to be waterjet cut, you will need to create a drawing file from the part.
Let's take a look at a part designed in SolidWorks which we want to cut. The part is open in SolidWorks in the picture below.

 

 

Illustration : A part in SolidWorks

To turn this into a drawing, choose File | Make Drawing from Part from the menu.

 

 

Illustration : Creating a drawing

A dialog box will appear. Make sure you have un-checked the “Display sheet format” box, then click the OK button.

 

 

Illustration : Removing the sheet format

The drawing sheet will appear. Drag the drawing view (1) onto the drawing (2). You will typicaly want the Top or Bottom view. You should then press the escape key or click the green checkmark to indicate that you are done adding drawing views.

 

Illustration : Adding the part view (1) to the drawing (2)

You then need to make sure that the drawing scale is set correctly. Click on the drawing view within the drawing. Then choose the “Use custom scale” radio button and pick “1:1” scale from the drop down list on the panel.

 

 

Illustration : Setting the part scale in the drawing

Finally, you should save the file in a format that's compatible with the waterjet cutting system. Most waterjet cutting systems accept the DXF file format. Choose File | Save As... from the menu. Pick DXF from the “Save as type” drop down list. Enter the file name and click the Save button.

 

 

Illustration : Saving the drawing

At this point, you now have a file which can be used to turn your SolidWorks design into a real part.

Waterjet quoting and final cutting is driven from 2D vector format files. However, many people use 3D modeling tools for their designs. Popular software packages in this category include Pro/Engineer, Sketchup, SolidWorks, Inventor, and Geomagic (Alibre). Generating an appropriate 2D file generally means going though an export process to get the correct projected version of the parts to be made.

The biggest stumbling block here is that many 3D design tools will add perspective information, such as hidden lines, to the final output. Lines in the drawing to indicate perspective simply confuse the waterjet software. Below is an example of this problem. The original design was exported as a 2D drawing from ViaCAD (though this problem is by no means unique to that package). When zoomed out, the part looks like a simple outline.

 

 

 

Illustration : ViaCAD part export, zoomed out

However, when you zoom in on the upper right hand corner of the image, you can see that the software has drawn the front and back of the part in perspective, plus a middle line. The extra lines will simply confuse the waterjet software, as there is no real indication which line represents the outline to cut.

 

 

Illustration : ViaCAD part export, zoomed in on the problem area