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Tuesday, 23 September 2014 21:21

Reducing Costs

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.

 

 100002010000046E000003DF95845C71

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

 100002010000043E000003DF4E00C2C0

Illustration : Less expensive 1: sharp inside corners

 1000020100000464000003DFC35B5CFA

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.

 

 10000000000000B4000000B4CF4B32E0

Illustration : More expensive: more internal holes

 10000000000000B4000000B4FACAD6F8

Illustration : Less expensive: fewer internal holes

Tuesday, 23 September 2014 17:31

Bending/Braking

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.

 

 

 1000000000000B0000000840436513F4

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

 1000000000000B00000008409728658B

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.

 

 100000000000037B0000035D927D2A8D

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.

 

 1000000000000FA000000BB8C9643B34

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

Monday, 22 September 2014 21:35

CAD the Easy Way

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.

 

 1000000000000280000001D37ADE3BCC

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.

 

 100000000000028000000201706156D3

Illustration : Choosing a category from SDP/SI


Then under "Spur Gears", we can select "Metal".

 

 10000000000000CD0000018DA964E6F7

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.

 

 10000000000002800000013CDD9280A0

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.

 10000201000001930000028022D73528

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.

 

 10000000000001DB000001D9EB3C1B2A

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.

 

 10000000000002800000023BCBCB9072

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.

 

 10000000000001540000016DAC9C3587

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".

 

 10000000000002600000004CFDF72E88

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.

 

 10000000000001EB000000B4EBEED785

Illustration : Downloading an file from SDP/SI


Here's what the downloaded file looks like in QCad.

 

 1000020100000280000001C8E8A430BA

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.

 

 10000000000002800000019EEE9AB1C3

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".

 

 10000000000001AF0000004E15501690

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).

 

 1000000000000280000001658E33F62C

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.

 

 1000000000000280000001EAE9B24C47

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.

  1000000000000280000000C6799A4503

Illustration : Choosing a file format on the McMaster-Carr website


Here is the drawing from McMaster-Carr as shown in QCad.

 

 1000020100000280000001D7A5529424

Illustration : A file downloaded from McMaster-Carr


Again, files you get from other people will

Monday, 22 September 2014 21:08

SolidWorks

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.

 

 10000000000003440000027EAF50A187

Illustration : A part in SolidWorks


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

 

 10000000000001AD00000123D20BDA81

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.

 

 100000000000020D0000014512FAF387

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.

 1000000000000478000002FB51273F9F

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.

 

 100000000000023F000001F6C9B404FF

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.

 

 1000000000000377000002139481EABE

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.

Sunday, 21 September 2014 19:19

Exporting from 3D modeling tools

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.

 

 

 10000000000003B9000002DA9C91F919

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.

 

 100000000000029C000002341A799DBD

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

Using your favorite design software

Most graphic design and CAD software allows you to export your design into a DXF format compatible with most waterjet cutting services.
Note that DXF format is a vector file format, as opposed to a raster or pixel based file format. Vector formats allow precise creation of curves, lines and other shapes. Examples of vector based desgin software include Inkscape, Adobe Illustrator and Corel Draw.

Exporting from design software that supports vector drawing to DXF is typically a matter of choosing File | Export from the menu or File | Save As..., and then picking the DXF format.

 

 1000020100000190000001265D11EE43

Illustration : Exporting in DXF format

If you don't yet have design software

If you are just getting started and don't yet have CAD or graphics design software, don't worry. There are many great software packages out there, all of which are free to use and are of professional quality.

1. Free vector based drawing software for Windows, Macintosh, and Linux:

Inkscape 

2. Free Computer Aided Design (CAD) software for Windows:

Solid Edge 2D

3. Open source Computer Aided Design (CAD) software for Windows, Linux, and Macintosh:

QCad or Librecad

4. Freec Computer Aided Design (CAD) software for Windows, Linux, and Macintosh:

DraftSight

5. The Big Blue Saw Designer, an online tool for Java-enabled web browsers.

 

 

Sunday, 21 September 2014 19:08

Holes

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. When 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.

 

 

Sunday, 21 September 2014 12:29

Finish

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.

  

 1000000000000B0000000840B842D1EE

Illustration : 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.

  

 1000000000000400000003012A6E75EB

Illustration : Closeup of the burr on 6061 aluminum

 

 1000000000000FA000000BB88594507A

Illustration : 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.

 

 1000000000000FA000000BB8400517FB

Illustration : 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.

 

 100000000000095C00000704AF3CDB96

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

 1000000000000FA000000BB84D550E48

Illustration : Removing burrs from the part with a utility knife.

 1000000000000A1D0000079587EB9BD2

Illustration : 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.

 

 

 1000000000000B00000008404016E97A

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

 100000000000020000000180D03BA6FB

Illustration : Aluminum finished with Basic Finish

 10000000000001D40000015FF5FA55E3

Illustration : 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.

 

 1000000000000B0000000840254A2973

Illustration : Bead blast finished parts with stainles steel on the left and aluminum on the right.

 100000000000018400000123BD64EA90

Illustration : Closeup of Bead Blast on an aluminum part

 10000000000001CC00000159428611F6

Illustration : Closeup of Bead Blast on a stainless steel part

 

Rotary tumblers or vibrators loaded with polishing media are a good hands-off way to clean up parts. Some thinner parts may be too delicate for this process, however. Larger parts will require an especially large tumbler.

Wednesday, 17 September 2014 14:29

Deciding on a material thickness

If you don't have a set of calipers or a micrometer with which to gauge the thickness of the material you would like to use, you can use common household objects instead. Use chart below to get a feel for various thicknesses. You could also use any of these objects to measure existing parts.

For thicker measurements, stack several parts together. For example, 3 CDs are 3 X 1.2 mm = 3.6 mm or 0.142 inches thick.

 

 

  Inches Millimeters

Dollar bill

0.0043

0.1092

#9 Standard Razor Blade

0.009

0.2286

#12 Heavy Duty Razor Blade

0.012

0.3048

Credit Card

0.030

0.76

CD, DVD, or Blu-Ray Disc

0.047

1.2

US Dime

0.053

1.35

US Penny (Cent)

0.060

1.52

US Quarter

0.069

1.75

US Nickel

0.077

1.95

Canadian Polymer Banknote

0.004

0.091

Canadian 10 Cents

0.048

1.22

Canadian Cent

0.057

1.45

Canadian 25 Cents

0.062

1.58

Canadian Dollar

0.069

1.75

Canadian 5 Cents

0.069

1.76

 

Table : Thicknesses of common objects

 

Wednesday, 03 September 2014 07:46

Limitations of waterjet cutting, Part 4

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 they must be broken off by working the part back and forth until it snaps off from the tab; the part is left with a vestige of the original tab. At Big Blue Saw, we offer various finishing options, like Basic Finish, to remove the tabs for you when necessary.

Below is a set of waterjet cut stainless steel parts which shows what this looks like.

 

 100000000000040000000300BD90627D

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

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