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.

Finishing waterjet cut parts

For many kinds of parts, the surface appearance of the part is of no great importance. For example, parts used deep inside a machine can be scratched or scuffed, and still work perfectly well. However, for many applications, having a regular, smooth appearance is important. In this category are things like car dashboard panels, musical instruments, and signs.
Several things can contribute to a blemished or uneven appearance in a waterjet cut part. To begin with, the stock material from which the part is cut may have surface scratches or marks. This is particularly true of aluminum and carbon steel plate, as these are often not considered as being for decorative use by their manufacturers. The waterjet cutting process itself can also cause irregularities. As mentioned above, parts cut on the waterjet will usually have frosting on them from stray particles from the cutting stream hitting the part.

Additionally, most parts waterjet cut from metal will have a small burr around the cut line on the bottom face. This is very undesirable in parts that must be touched or held, such as tools or handles.

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 Parts after waterjet cutting: Stainless steel which began with a 2B finish on the left, aluminum on the right

Parts after waterjet cutting: Stainless steel which began with a 2B finish on the left, aluminum on the right

In the photo above, you can see the “frosted” areas around the part’s cut line, and the typical, slightly rough surface on the cut edge of a waterjet cut part. Parts with no finish may also have scuff marks due to handling and easily removed printed lettering from the mill that produced the raw material.
In the unfinished state, parts will typically have a small burr where the waterjet exits the part. Softer metals, such as aluminum, are more likely to have this burr. The burr can be easily removed with a sharp implement like a knife, or sandpaper. You can see an example of a burr in the photo below if you look very closely at the edge.

Closeup of the burr on 6061 aluminum

Closeup of the burr on 6061 aluminum

A waterjet cut part made from  soft aluminum showing burrs and frosting on the bottom face.

A waterjet cut part made from soft aluminum showing burrs and frosting on the bottom face.

There are several techniques for cleaning up a waterjet cut part. The simplest is sanding with medium to fine grit sandpaper. This works well on most metals, including steel and aluminum. To get the most even surface, sand in only one direction, following the grain of the metal.

A waterjet cut aluminum part hand finished with sandpaper

A waterjet cut aluminum part hand finished with sandpaper

You can remove burrs by hand using a sharp knife or deburring tool. The photos below show the process of removing the burrs from around the large circular hole in an aluminum part.

The bottom face of a part waterjet cut from soft aluminum. Note the burrs around the cut lines.

The bottom face of a part waterjet cut from soft aluminum. Note the burrs around the cut lines.

Removing burrs from the part with a utility knife.

Removing burrs from the part with a utility knife.

The large hole on the right is now free of burrs.

The large hole on the right is now free of burrs.

A faster way clean up the face of the parts and to deburr at the same time is to use a power rotary buffing or sanding tool loaded with a ScotchBrite or similar wheel. At Big Blue Saw, we call this “Basic Finish”.

This produces an even, fairly shiny finish as shown below on stainless steel (left) and aluminum (right). Like sandpaper, this process can remove all mill identification writing. It also removes any machining marks from the face of the part, including the waterjet “frosting”. Some of the deeper marks which were present in the original raw material may be deeper than  can be removed with this process.
With this treatment, burrs on the outside convex corners of your part are removed, and most other burrs are reduced somewhat. Burrs can still be left in small holes and in deep inside (convex) corners.

Parts cleaned up with a buffing wheel (Basic Finish). A stainless steel part is on the left, an aluminum part on the right.

Parts cleaned up with a buffing wheel (Basic Finish). A stainless steel part is on the left, an aluminum part on the right.

Aluminum finished with Basic Finish

Aluminum finished with Basic Finish

Stainless steel finished with Basic Finish

Stainless steel finished with Basic Finish

One of the best looking ways to clean up small to medium quantities of parts is through the use of sandblasting or bead blasting. In this process, the parts are placed in a sealed cabinet and sprayed with a high pressure stream of sand or tiny glass beads. Finishing through bead blasting produces a more consistent surface finish the the Basic Finish, at the expense of some shininess. All machining and handling marks will either be eliminated, or made very hard to see. It is produced by spraying the parts with a high pressure, dry stream of tiny beads. The photo below compares the Bead Blast Finish on stainless steel (left) and aluminum (right).

With this process, burrs are reduced across the entire part, but may remain if there are large burrs in hard-to-reach places.
The Bead Blast finish also gives the face of the part an appearance consistent with the edges which were produced by waterjet cutting.

 

bead blast finish

Closeup of Bead Blast on an aluminum part

Closeup of Bead Blast on an aluminum part

: Closeup of Bead Blast on a stainless steel part

: Closeup of Bead Blast on a stainless steel part

Waterjet cutting for 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.

 Steampunk costume by Chris Lee with waterjet cut aluminum gears which actuate a set of wings

Steampunk costume by Chris Lee with waterjet cut aluminum gears which actuate a set of wings

 Steel (left) and aluminum (right) gears

Steel (left) and aluminum (right) gears

Waterjet cutting works best with gears of 12 diametral pitch (DP) or larger. 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. 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 16 DP gear

Waterjet stream of 0.04″ diameter shown with 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.

Simplify lettering and signs by using the right font

In the last post on making signs, we saw that it was possible to make a sign with lettering in four different ways.

When you want the sign to be one piece, it can be tedious work to generate all the necessary bridges. This is true both when the letters are positive space (solid material) or negative space (holes). Fortunately, by using the correct font, you can save time and get a result that looks good.
When the letters form positive space, one good choice is to use a script font.
In the example below from David Kaufman, the Santa Fe script font was used to design two nameplates which were then waterjet cut from 1/4 inch thick aluminum. The right hand side of the “f” had to be modified to connect with the “m”, but the rest of the letters naturally run together with this font.

Nameplate signs from David Kaufman

Nameplate signs from David Kaufman

The font used in the examples above: Santa Fe LET http://www.fonts101.com/fonts/view/Script/28879/Santa_Fe_LET

The font used in the examples above: Santa Fe LET

 

When the letters are negative space, you can use a stencil font. Below are a few examples of the varieties of stencil fonts which might be useful for your project.

 Bodoni Becker Stencil Bold http://www.fonts101.com/fonts/view/Uncategorized/43971/Bodoni_Becker_Stencil_Bold
Bodoni Becker Stencil Bold
Tomorrow People. Note for this font: some numbers and symbols may not have appropriate bridges in this font. http://www.dafont.com/tomorrow-people.font

Tomorrow People. Note for this font: some numbers and symbols may not have appropriate bridges in this font.

Accurate holes with waterjet cutting

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.

Many waterjet cut parts are designed to be fastened together using bolts or pins. This comes up when using the using the stacking or t-nut construction techniques, as well as when using a waterjet cut part as a custom flange, bracket, bearing block, or control panel. When making a part which must accept a bolt or pin, you should modify your design to deal with the accuracy limitations of the waterjet process.

In short, you should plan to enlarge the hole with the right size twist drill or reamer. Since most of the material has already been removed from the hole, this is easy to do. The existing waterjet cut hole will guide the drill or reamer down through the material. It is easiest to do on a drill press, but it can even be done with a handheld power drill.

For instance, you might need a hole for a bolt with a 0.25″ (1/4″) diameter. Because the width of the waterjet stream can vary, you might end up with a hole 0.26″ or 0.24″ in diameter. If you have a 0.25″ post or bolt, the best thing to do is to specify a size of 0.24″ in diameter, then finish the hole with the appropriate drill or reamer. The exact size drill or reamer will depend upon how close of a fit you need: whether it’s an interference, close running, or free running fit. For most ordinary fastening applications, an ordinary twist drill will work.

If you don’t want to enlarge the hole with a drill, you’re going to have to specify a hole 0.262″ in diameter, and be prepared to accept that the bolt may wiggle around a bit in the hole. This is OK for many applications.

If you are unsure of the size of the bolt, pin, or post, you should double-check the diameter of the posts with a precision caliper or a micrometer.

Also, you should consider the effect of taper on your holes. For metal material that’s 3/16” (5 mm) or thicker in diameter, taper will be significant enough to interfere with most through bolts, as hole diameter on the bottom face will be smaller than the hole diameter on the top face. Again, it is probably best to undersize the original hole and finish it with a drill. If you are using low-taper waterjet cutting, this is less of a problem.

Tapped/threaded holes

For tapped holes where the amount of thread engagement is not critical, I similarly recommend undersizing the hole to just below the minor diamter of the thread, then enlarging the hole with a handheld power drill before tapping. Where you’re not concerned about the exact amount of thread engagement, you can usually get away with making the hole in your drawing exactly minor diameter of the thread, then tapping directly into the waterjet cut hole.

Choosing a waterjet cutting material for beginners

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 wtaerjet. 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.

Moving it again: a combination motor mount/bearing block for the LOLrioKart

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 (described in the post on stacking) 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.

 

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

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

 

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

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