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Wednesday, 17 September 2014 13:53

Materials

If you're trying to find a material for your project, the options can be overwhelming. Waterjet cutting offers so many choices that it can be difficult to pick the material that fits both your design and budget.

The truth is most projects can be done with 6061 aluminum, 304 stainless, cold roll steel, or clear polycarbonate. Our customers have found that each of these four fills a unique niche.
Here's a chart that ranks the three materials against each other in terms of cost, appearance, and specific strength (also known as strength-to-weight ratio). 4 is best, 1 is worst.

  Cost Appearance Specific Strength
Aluminum 6061 2 2 4
Stainless Steel 304 1 4 2
Cold Roll Steel 3 1 3
Polycarbonate 2 3 1

 

It's worth noting that though we've ranked them against each other in terms of appearance, none of these materials look particularly bad. Stainless steel is more durable than aluminum, and thus holds its finish better. If you need a transparent material, then, of course, a metal won't work at all and you need a plastic like polycarbonate.
Cold roll steel doesn't come out on top of any of the above categories, but it's still useful. Why? It is harder (good for sliding or wearing parts), denser, and can be welded more easily than the other materials. It's also magnetic.

 

If you're considering But Need Try
Aluminum 6061 Better formability (ability to bend the material into shape) Aluminum 5052
Aluminum 6061 Better electrical conductivity Copper 110
Carbon Steel Higher strength Prehardened steel alloy 4130 or similar
Polycarbonate Lower cost PETG, Acetal, UHMW-PE or Laser cut acrylic
Stainless Steel 304 Maximum corrosion resistance (like in salt water environments) Stainless Steel 316
Stainless Steel 304 Lower cost Aluminum or one of the laser cut metallic appearance acrylics (Brushed Bright Nickel, for example)

 

Of course, there are quite a few materials which can be cut on a waterjet that don't appear on any of these charts: various kinds of wood, stone, metals, and plastics. If you're considering these, you probably already have a good idea of what you need.

One of the great advantages of waterjet cutting is that prehardened metals can be cut quite easily. Certain metals can be purchased in bulk as bar or sheet, and then cut on the waterjet. This saves the extra step of having the material heat treated for hardening after machining.

 

Cutting glass on the waterjet

Most ordinary glass will shatter when cut on the waterjet. Certain types of untempered glass can work, but will typically need to have a test cut performed on them first in order to make sure that they can be cut without breaking.

 

Medium Density Fiberboard (MDF) and plywood on the waterjet

Unfortunately, Medium Density Fiberboard (MDF) is quite sensitive to moisture. This means that when the waterjet stream starts to cut into MDF, it immediately swells up and makes a big mess.
Most quality plywood will cut on the waterjet and can be a good substitute for MDF in some projects.

 

 

Wednesday, 17 September 2014 11:50

Gears

Traditionally, spur gears are made using specialized tooling and setups. This means that small runs of custom gears can be expensive; much more expensive than most other types of machined parts.

However, the flexibility of a waterjet cutting machine means that gears are no more difficult to waterjet cut than any other part. At Big Blue Saw, we have made many gears for customers who needed unusual sizes of gears or gears with custom mounting holes. Other customers have wanted, for example, polycarbonate plastic or titanium alloy gears which were impossible to find off-the-shelf.

 

 

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Illustration : Steampunk costume by Chris Lee with waterjet cut aluminum gears which
actuate a set of wings http://apparitionabolishers.com/2012/09/01/steampunk-wings-mechanism-close-ups/

 

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Illustration : Steel (left) and aluminum (right) waterjet cut gears

Waterjet cutting works best with gears of 12 diametral pitch (DP) or larger (this is roughly equivalent to module 2.25 in metric gears). As you can see in the diagrams below, the waterjet cutting stream is small enough to easily machine the features of this size gear. Additionally, a geartrain of this size or larger can tolerate the roughness of the cut edge on a waterjet cut gear. With a waterjet stream diameter of 0.04 inches, however, you can make gears down to about 20 DP (module 1.25). I would only recommend this size for thinner gears in applications where there is some “slop” in the geartrain due to the surface roughness of the cut edge as well as other irregularities which can arise from waterjet cutting.

Waterjet stream of  0.04 diameter shown with a 12 DP gear

Waterjet stream of 0.04" diameter shown with a 12 DP gear

 

Waterjet stream of  0.04 diameter shown with a 16 DP gear

Waterjet stream of 0.04" diameter shown with a 16 DP gear

 

Waterjet stream of  0.04 diameter shown with a 20 DP gear

Waterjet stream of 0.04" diameter shown with a 20 DP gear

 

Waterjet stream of  0.04 diameter shown with a 24 DP gear

Waterjet stream of 0.04" diameter shown with a 24 DP gear

 

Waterjet cut gears are rough along the cut edge where the gears mesh together. This can cause a shorter gear lifetime due to wear than you might expect from a traditionally cut gear. So for applications where this might be a problem, you should consider making the gear larger, either by making it from thicker material, or decreasing the DP so that the teeth are larger.

When first using new waterjet cut gears, it is a good idea to let the gear train run continuously for a few hours without load and with a light lubricant. Then clean the gears and re-lubricate before placing the gear in service. This will even out the rough cut edges of the gears and allow the geartrain to run smoothly.

Gears above 1/8 inch (3 mm) thick should usually be cut using low-taper cutting. This allows waterjet cut gear to mesh as accurately as possible.

Some people have reported success with regular (non-low-taper) waterjet cut gears by placing two waterjet cut gears so that their tapers face the opposite directions. In other words, the top face of one gear will be on the same side as the bottom face of its matching gear.

When building or customizing robots, motorcycles, go-karts, electric vehicles, conveyor belt systems, or just about anything that moves, you will eventually run into the problem of having to tie all of the motion components together into a single system. If off the shelf components don't meet your needs, you can waterjet cut custom frames, motor mounts, and bearing blocks.

Let me show you an example from my fighting robot “Jaws”. I needed a bearing block which would give maximum support to the wheels, but would occupy a minimum amount of space and weight. The solution I came up up with was a ¾ inch thick 6061 aluminum bearing block with a bronze bushing inserted into it. Using a bronze bushing kept the size down compared to using ball bearings.

After the bearing blocks were waterjet cut, I drilled holes in the edge of the bearing blocks in order to base mount to mount them. The bearing blocks rest on their cut edge and are held in place with bolts and nuts. Below you can see a rendering of the part, and a photo showing it installed inside the robot.

 

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Illustration : Rendering of the 3/4 inch thick aluminum bearing block from Jaws.

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Illustration : The bearing block installed inside Jaws.

 

Note that if you are base mounting this type of bearing block (mounting along the cut edge),  you will want to use low-taper waterjet cutting in order to keep your driveline straight. For face mounting, you might be able to get away with using regular cutting if you can drill or ream the hole where the shaft passes through in order to give it an even diameter all the way through the material.

Now let's take a look at a different bearing block system, this one from MattyCiii of the Endless Sphere Technology forums. He needed a special bearing block for a custom electric bicycle he was building for himself. He designed a bearing block to be made in two pieces, each 0.625 inches thick. When stacked together, they are exactly the width of the 1.25 inch thick tube that they mount to. An aluminum tube fits in the large hole in the blocks, and a ball bearing fits inside that, which supports the shaft. You may notice that there is a small gap at the bottom of the bearing blocks. According to MattyCiii, this allows the bearing block to grip the aluminum tube tightly when the bearing block is clamped in place. Here is how he describes how it works:

The gripping capability is actually a combination of two things: First the gap, and second, the flat plane you see on either side of the gap is not fully flat – it’s angled in slightly. So take for example that last picture: imagine only one of the hose clamps is clamped down tight. There would be about about a 1 degree angle between the other flat part of the mount and the square tube it’s mounted on. As you tighten the second hose clamp, it pulls the piece tight against the flat and slightly shrinks the diameter of the hole. I’m not an engineer by trade, I didn’t do any calculations to find a ‘best’ angle for this purpose – but it works as designed (miraculously!)

 

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Illustration : A stacked bearing block before mounting. Photo courtesy of
MattyCiii from http://endless-sphere.com/forums/memberlist.php?mode=viewprofile&u=567 .

 

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Illustration : The bearing block from the previous photo mounted in
place on the electric bike. Photo courtesy of MattyCiii from
http://endless-sphere.com/forums/memberlist.php?mode=viewprofile&u=567 .

 

 

When building things that move using a waterjet, you're not limited to just bearing blocks. One common problem faced when building an electric vehicle is mounting the motor to an existing frame. Waterjet cutting allows you to easily make custom motor mount plates that match the bolt pattern on the motor, as well as the mounting points on the frame.

Here is a great example from Roberto Melendez of MIT. In building the Cholocycle electric motorcycle, they needed a way to mount their 25 horsepower electric motor to a Kawasaki motorcycle frame. With waterjet cutting, they were able to construct mounting plates which fit both the motor and the frame perfectly.

 

 

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Illustration : The waterjet cut motor mount attached to the front of the electric motor
for the Cholocycle. Note the rear mount disassembled in the background.
Photo courtesy of Robert Melendez http://rjmelendez.blogspot.com/ .

 

 

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Illustration : The motor installed on the Cholocycle motorcycle frame.
Photo courtesy of Robert Melendez http://rjmelendez.blogspot.com/ .


Now let's take a look at an assembly of waterjet cut parts that combines a motor mount with bearing blocks. For the LOLrioKart project, Charles Guan wanted to add a powerful electric motor to a shopping cart (yes, really). In order to do this, he needed a way to attach the motor to the frame, as well as a way to hold it in line with the driven axle. His solution was to use the waterjet to make a combination motor mount and bearing block out of ½ inch thick aluminum 6061. He used the stacking technique to make the plates thicker and thus provide a more stable attachment where the assembly rests on the cross member tube. You can see the separate pieces as well as the assembled drivetrain below.

 

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Illustration : 1/2″ 6061 aluminum plates and motor for the LOLrioKart drive
train. Photo courtesy of Charles Guan http://www.etotheipiplusone.net/

 

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Illustration : The assembled LOLrioKart drive train.
Photo courtesy of Charles Guan http://www.etotheipiplusone.net/

 

 

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