Simulating a Bevel Effect Using Stacking

At Big Blue Saw, we occasionally get requests from customers to put a bevel or chamfer on the edge of a waterjet cut sign or control panel.This is usually done for aesthetic purposes, to allow the piece more to show more highlights and shadows.

While we can’t put a bevel on the edges of parts using the waterjet, we can create a similar effect using the stacking technique.

Let’s take a look at how this works with a simple example created in Inkscape. The techniques shown here can be replicated using almost any vector drawing or CAD program.

Using the text tool, we will add a number “8” to the drawing.

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Next we will turn the character “8” into a set of curves by selecting it and choosing Path > Object to Path from the menu. We want to see the curves, so we choose View > Display Mode > Outline from the menu.

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We then make a copy of the figure using the Copy and Paste commands.

To make one of the figures into the skinnier upper layer, we use Path > Inset on the menu or Ctrl+( on the keyboard a few times. If you’re doing this, be careful that you don’t make the thin areas too thin to be cut by the waterjet.You can also try making the lower layer larger using Path > Outset or Ctrl+).

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If we were going to mount this number using adhesive, we would be done at this point. We could just save the file as a DXF and upload it to Big Blue Saw to be cut.

However, for this example, we will make mounting holes so that the numbers can be mounted with screws or nails.

We need to make sure that the mounting holes align on both parts. We select both figures and align them using Object > Align and Distribute on the menu. The shapes need to be aligned both horizontally and vertically using the buttons in the Align panel shown below.

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Now the two outlines will look like the figure below. This gives us an idea of how the final parts will appear.

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Let’s add a couple circles for mounting holes.

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We duplicate the entire drawing using Copy and Paste.

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And finally delete the outer outline from one of the copies, and the inner outline from the other copy. Now the drawing has both parts with the mounting holes correctly aligned.

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Finally, here are the assembled parts with the simulated bevel waterjet cut from 0.135 inch (3.4 mm) thick stainless steel, ready to be mounted.

 

Numeral 8 With Simulated Bevel

 

Locating Marks for Secondary Operations

Returning to the clamp example from the previous post, 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.

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.

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.

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.

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.

Custom Clamps

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 to halves of the clamp together pass through a hole in the cut edge of the part.

 waterjet cut custom clamp holding two poles together.

waterjet cut custom clamp holding two poles together.

More on T-nut Construction

In an earlier post, I mentioned overcutting the inside corners of the parts so that they fit together better. Below are diagrams that show what this looks like.

Inside corner reliefs on tab section to compensate for kerf

Inside corner reliefs on tab section to compensate for kerf

Inside corner reliefs on slot section to compensate for kerf

Inside corner reliefs on slot section to compensate for kerf

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.

Construction Technique: Extended stacking assembly using long bolts or threaded rod

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.

 

Waterjet cut parts held apart by nuts and long bolts

Waterjet cut parts held apart by nuts and long bolts

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

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

Construction Technique: Stacking

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.

 

Waterjet cut parts stacked to simulate boss features.

Waterjet cut parts stacked to simulate boss features.

Construction technique: Tab and Slot with T-nut

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.

 

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

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

 

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

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

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

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 mortice and tenon joint of woodworking.

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

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

Another view of the more stable variation of the t-slot technique.

Another view of the more stable variation of the t-slot technique.

 

 

Note that the slot which holds the bolt is 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. (See my earlier post on this subject).

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

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.

Box assembled with the t-nut technique.

Box assembled with the t-nut technique.