More on tabbed parts

In an earlier post, I described how tabbing was used to keep small parts from dropping into the waterjet tank when cutting. This can mean that the must be broken off by working the part back and forth until it snaps off from the tab and the part is left with a vestige of the original tab.

Here is a set of waterjet cut stainless steel parts which demonstrate this.

 

Tabbed parts with US Quarter (24.26 mm/0.955 inches diameter)

Tabbed parts with US Quarter (24.26 mm/0.955 inches diameter)

 

A closeup of a part removed from the tabbed group. Note the vestigal tab at the bottom of the part.

A closeup of a part removed from the tabbed group. Note the vestigial tab at the bottom of the part.

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Limitations of waterjet cutting, Part 4

Hole size

With a stream width of 0.04 inches, you might think that this would also be the smallest accurate hole that can be made with the waterjet. But this is not so. The waterjet stream must break through the material before cutting along the cut line. In this process, called piercing, the machine sends a powerful blast of water and abrasive at the material surface in order to create the initial hole. This initial hole is fairly irregular. Once the material is pierced, however, the cutting is much smoother. If you want a hole to be reasonably round, I recommend a diameter of no smaller than 0.1 inches. Of course, a waterjet can make hole smaller than that, but be aware that its diameter and shape will be proportionally less accurate.

In the photos below, you can see holes of various sizes cut by a waterjet in ¼ inch thick aluminum. The part is designed so that equal sized round holes are arranged in columns. You can see the irregularities on the closeups of the holes. These irregularities make up a higher fraction of the diameter as the hole size gets smaller. On the right is a pierce only hole, the smallest hole that the waterjet can make.

Various hole sizes, arranged in columns. From left: 0.25", 0.1875", 0.125", 0.1", 0.05" and a pierce only hole approximately 0.04" diameter.

Various hole sizes, arranged in columns. From left: 0.25″, 0.1875″, 0.125″, 0.1″, 0.05″ and a pierce only hole approximately 0.04″ diameter.

Closeup of the 3 smallest hole sizes from above.

Closeup of the 3 smallest hole sizes from above.

Accuracy

Positioning of the waterjet stream is generally very accurate; close to 0.001 inches. However, several factors can affect the accuracy of the part, including wear on the mixing tube, vibration within the part, taper, and marks created by lead-in and lead-out, as shown below. Thus, we typically quote +/- 0.005 inches (5/1000 of an inch) as our accuracy. This means that you should not depend upon edges of features to be more accurately placed than 0.005 inches. Holes may be up to 0.01 inches smaller or larger in diameter than your specification. (I’ll be writing about creating accurate holes in a later blog post.)

Lead in / lead out

You will occasionally see small indentations where the waterjet stream begins and ends cutting a piece.

Indentation at lead-in/lead-out point

Indentation at lead-in/lead-out point

Indentation at lead-in/lead-out point

Indentation at lead-in/lead-out point

Limitations of waterjet cutting, Part 3 (Taper)

Taper

Parts cut using ordinary waterjet cutting will have a slight taper along the cut edge of the part, typically between 0.0005 inches and 0.01 inches. This is due to the waterjet cutting stream spreading out as it gets farther from the nozzle. The top face is cut accurately, and the accuracy gets worse as you get toward the bottom face.

Metals will taper outward from the top face when cut on the waterjet. This means that the overall size of the part will be larger on the bottom face than on the top face. Holes will be smaller on the bottom face than on the top face.

With plastics, it is the reverse. Plastics will taper inward from the top face when cut on the waterjet. This means that the overall size of the part will be smaller on the bottom face than on the top face. Holes will be larger on the bottom face than on the top face.

Big Blue Saw’s low-taper waterjet cutting service eliminates this taper for an extra charge.

You can see the effect in the photo below. Both parts are made from aluminum 6061, but the one on the right is made using a low-taper waterjet cutting process.

Low-taper waterjet cutting is almost never worth the extra expense for parts 1/8 inch (3 mm) thickness or below. The taper on parts this thin is much less pronounced than on thicker parts and is typically less important to the design anyway.

On the other hand, if you have thicker parts that make contact with other pieces in an assembly along their cut edge (rather than just on the face), you should strongly consider low-taper waterjet cutting.

Two parts cut from 3/4 inch thick 6061 alloy aluminum and measured against a machinist's square. Note the gap at the bottom between the part on the left and the square.

Two parts cut from 3/4 inch thick 6061 alloy aluminum and measured against a machinist’s square. Note the gap at the bottom between the part on the left and the square.

Below are more photos of the same two parts. This time, each is placed on its un-machined face and the side is measured against the square. The cut edge on the piece cut with low-taper waterjet cutting measures perpendicular to the reference surface.

0.5 inch (12 mm) thick low-taper waterjet part with reference machinists square

0.5 inch (12 mm) thick low-taper waterjet part with reference machinists square

0.5 inch (12 mm) thick regular taper waterjet part with reference machinists square

0.5 inch (12 mm) thick regular taper waterjet part with reference machinists square

Low taper and width

In some areas on low taper cut parts, it’s not always possible to completely remove the taper. This can happen if there is an especially small hole or slot in thick material. In these sections, the waterjet head will tilt to compensate for the waterjet stream spreading out, but then the side of the stream opposite the cutting line tends to cut into the material on the far side of the slot.

Limitations of waterjet cutting, Part 2

Tabbing

Small parts must be connected either to each other or to the sheet of material to prevent them from falling into the waterjet tank when they are cut. This can mean that the part is left with residual tabs or must be broken off by working the part back and forth until it snaps off from the tab. At Big Blue Saw, we offer various finishing options, like Basic Finish, to remove the tabs for you when necessary.

Loosely connected segments and vibration

This limitation does not come up too often, but it is worth keeping in mind, particularly if you are making signs with lettering or logos. If your part has an “island” connected to the main part by a single very thin segment (also called a bridge), the island area will often vibrate as it is being cut, ruining the smooth cut line. For this reason, we suggest making connectors at least 1/8 inch (3 mm) thick, and placing at least 3 on each island area.

Island connected with thin segment.

Island connected with thin segment.

Thin segments/features

Similarly, connecting segments themselves can vibrate, causing the segment to be destroyed during cutting. Again, make sure that the bridge is at least 1/8 inch (3 mm) wide.

Limitations of waterjet cutting, Part 1

Like any tool, waterjet cutting has limitations. The good news is that the limitations of waterjet cutting are easy to understand. This makes it possible to develop your design around those limitations (which is what I’ll be covering in many of these blog posts).

Absence of depth control

This is a limitation for many people, but there are design techniques that can help you compensate, as you will see.

In my experience, it is almost never practical to use a waterjet to engrave or cut only part way through a material. For one thing, as it is difficult to get accurate depth control. Another problem is that the area which is removed is limited to the thickness of the waterjet cutting stream, typically 0.04 inches or 1 mm.

Thus, for a part which requires marking or etching, we would typically use laser engraving or printing to apply the design.

A design which would require a slot or pocket feature would be better done though modifying the design (see the stacking technique which I’ll cover in a later blog post) or, as a last resort, via a secondary operation such as milling.

Kerf width

Kerf refers to the material removed by a cutting implement. For example, imagine that you are cutting a 2 foot board in half with a hand saw. If you were to join the finished halves back together, you will find that the total length is less than 2 feet due to the material removed by the saw. The amount of material removed and turned to sawdust would be roughly the width of the saw.

The width of the waterjet cutting stream is typically 0.04 inches or 1 mm. The less common microjet waterjet cutting heads have a smaller nozzle and stream width of 0.02 inches (0.5 mm).

The shape of the waterjet stream means that inside corners cut on the waterjet cannot be perfectly sharp. For some applications, this is not significant, but if you want to fit a part with a sharp outside corner into a waterjet cut sharp inside corner perfectly (for example, when using the tab and slot technique in Section XXX), it is best to make the design with a rounded corner that “overcuts” the corner by a little bit. For an 0.04 inch wide waterjet stream, I would recommend designing for an inside corner radius of slightly larger than the stream radius: 0.025 or even 0.03 inches (0.05 or 0.06 inch diameter).

This is the caption

Diagram showing a closeup of an inside corner which is unreachable due to the width of the waterjet cutting stream. This corner will be slightly rounded in the final part.

If must remove the material from an inside corner,  you should create a path which cuts beyond the corner.

If must remove the material from an inside corner, you should create a cut line which cuts beyond the corner.

If must remove the material from an inside corner,  you should create a path which cuts beyond the corner.

Similarly, slots narrower than the kerf width of the waterjet stream cannot be cut. If you have a narrow slot which leads to a larger open area, this means that the larger area cannot be reached by the waterjet cutting stream either, as shown in the diagram below.

When you have an area too narrow for the cutting stream, there is no way to reach larger connected areas.

When you have an area too narrow for the cutting stream, there is no way to reach larger connected areas.

Compensating for kerf

Most waterjet cutters, including Big Blue Saw, compensate for the kerf as best as we can in order to produce a part that’s as close to your CAD design as possible. In other words, you don’t have to worry about making your drawing larger in order for the part size to come out correct. So, for instance, if you send us a CAD drawing with a 3 inch diameter design, your final part will come out to be 3 inches in diameter, not 2.96 inches. (This is a change for many people who are used to working with most laser cutter shops, where they typically do not compensate for kerf. When doing laser cutting at Big Blue Saw, we also compensate for kerf in order to keep it consistent with the waterjet cutting process.)

Diagram showing a closeup of an inside corner which is unreachable due to the width of the waterjet cutting stream. This corner will be slightly rounded.

Why choose waterjet cutting and what it can do

Waterjet cutting offers several advantages over other machining technologies:

1. Lower cost. Waterjet parts are typically cut from sheet or plate material and require no special clamps, fixtures or tool changes. This translates to quicker production and lower cost.

2. Wide variety of materials supported. Waterjet cutting be used on most metals, many plastics, wood and stone. It can be used to cut many materials that would be difficult to machine using other methods, such as heat treated alloy steel.

3. No heat-affected zone. Machining methods like laser cutting and EDM heat the material and the parts will need to be heat treated again after machining. Waterjet cut parts retain their heat treatment when cut. This means that you can start with pre-heat treated material and keep your costs down.

Applications

I’ve been running Big Blue Saw for a few years now. When people ask me what I do, I explain that I help people make custom parts out of metal, plastic, or wood. Their next question is usually “Who would need that?” or “What kinds of things do people want?” At Big Blue Saw, we have dealt with thousands of applications for waterjet cut parts. Here is a just a partial list of the types of applications we have seen:

3D printer frames

Animal cages

Arcade game joystick mounts

Bicycle dropouts

Business cards

Computer casemods

Control panels

Costumes

Dashboard panels for airplanes and airplane simulators

Dashboard panels for cars

Desk nameplates

Drain covers

Electronic bus bars

Electronics enclosures

Fighting robot frames

Fine art

Fireworks control panels

Fixtures for science experiments

“Free energy” machines

Gears

Go kart frames

Ground straps

Guitar capos

Guitar frames

Guitar pick guards

Handles

Head gaskets

Heat sinks

Industrial controls

Industrial HVAC equipment

Kite fabric templates

Knife blanks

Lighting fixtures

Machining fixtures

Motor mounting brackets

Motorcycle dogbones

Movie props

Nixie clocks

Oil pipelines

Painting jigs

Prop replicas

Rope belaying devices

Signs

Sporting goods

Stencils

Test tube racks

Trophies

Ventilation registers

Video monitor brackets

Wedding cake toppers

Wheels

Windmill contactors

Exporting from SolidWorks to DXF format for waterjet or laser cutting

When using SolidWorks to design a part to be waterjet cut, you will need to create a drawing file from the part.

Here is a part file open in SolidWorks which we want to cut.

10000000000003440000027EAF50A187
A part in SolidWorks

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

10000000000001AD00000123D20BDA81
Creating a drawing

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

100000000000020D0000014512FAF387

 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.

1000000000000478000002FB51273F9F
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 on the list.

100000000000023F000001F6C9B404FF

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.

10000000000001540000016DAC9C3587
Saving the drawing

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