What is Die Cutting?

Die cutting is an important part of the industrial manufacturing process, below is everything you would need to know about the process and then some covering the spectrum of die cutting techniques.

diesdie cutting may also be referred to as  dinking and sometimes blanking. this can loosely be described as a technique where patterns and shapes are cut from sheets of different materials. similar to everyday cookie cutters.  this specially shaped blade or cookie cutter presses the edge into one or more layers of sheeting.  these may be referred to as steel rule dies.  After the cutting, pressure by way of a hydraulic or mechanical press is applied.

This Piece first focuses on one type of die cutting, rotary die cutting. It outlines the function and ability of rotary die cutting, including types of applications, as well as some disadvantages and things to keep in mind. The second piece examines the different varieties of die cutting equipment, as well as the processes they use to fabricate materials. The machines discussed include, rotary, press, flatbed, laser, and water-jet cutters.

Rotary die cutting machines feature a cylindrical anvil and die fabricated from a single piece of tool steel. As material is fed through the machine, a series of quick and accurate cuts modify the metal. Rotary die cutting functions well on high volume projects, producing little waste and featuring quick turnover times. The machinery is well-suited for precision cutting at low tolerances and can also be used in conjunction with other processes, such as laminating and coating.

Rotary die cutting can be used for

* Multiple process projects, because rotary die cutting can be combined with coating, embossing, lathing and other processes.* Producing less waste than other, comparable methods;* Fast turnaround times;
* High volume projects of consistent cut;
* with CNC (Computer Numerical Control Machines)

diecuttingThe industrial process of die cutting has many applications in manufacturing such as, cutting, forming and shaping there are multiple varieties of die cutters, including rotary, press, and flat bed die cutting. All the methods of die cutting listed below may be used in tandem with a CNC machine, but it is worth noting that die cutting often does not use CAD software to the extent that it is used with milling machines. Die cutting is only 2D as opposed to the many different 3D milling applications. It is possible to use a CNC machine for die cutting applications, without any CAD interface at all and instead use the parameters in G code to program directly into the machine. G code is the language the CNC mill interprets directly. and it is what the CAD instructions are converted to through the stepper interface This is often simple instructions, xyz coordinates, and simple programming constructs such as loops and basic methods. There is less of a need for the complex 3D wire mesh patterns generated through CAD with die cutting as there is with 3D milling. Because of this when we refer to CNC die cutting, keep this in mind that it is a different process then what is most often mentioned and may not be using a CAD interface.

Different Methods of Die cutting

Press Die Cutting

Press die cutting machines are different sides from small desktop models to large machine shop types. They feature a cutting die that is raised and lowered. the die cutting can be thought of similar to a large paper cutter, found at Kinkos.

Flatbed Die Cutting

Flatbed die cutting machines are used for smaller volumes projects, and are good for many kinds of shapes; it is used when no material curvature is required. Flatbed die cutting is versatile and may be used to cut precise sheets, laminate, kiss-cut, butt-cut, and die-cut a wide array of materials. shapes are stamped out and are adjusted using different degrees of   material. Steel rule dies that are adjusted accordingly to varying degrees of hydraulic presses that power the die cutting process.

Laser Die Cutting

Laser die cutting is usually computer-controlled and follows instructions from the CAD program connected through a stepper into a CNC,  the CNC automation enables the production of a large volume of uniform parts. Laser die cutting applies a non-thermal, fully focused beam to force material into custom shapes and sizes. Different types of lasers used include C02 Lasers, which uses C02 gas in a airtight glass tube, and solid-state Nd:YAG lasers. See the post about the DIY C02 laser, to see a really cool home made hack project. Laser cutting often leaves very smooth edges, but has its limits on thickness. Safety precautions must be taken as to not get burned.

Water Jet Die Cutting

Water jet die cutters fire highly pressurized streams of water, this water moves at two and a half times the speed of sound. The stream is pressurized through a tiny opening (usually about 0.003 inches in diameter), and is able to cut through a wide variety of material. Water jet cutting is often very messy as water and debris can splash all over the place, and an extreme amount of pressure is needed to cut with water.  Laser cutting is often preffered for this reason.

What is Water Jet Cutting?

Waterjet cutters are a type of device that rely on pressurized water mixed with abrasives to cut through metal material. Because abrasives, such as garnets, are added to the water, a waterjet uses both the force of the water and the abrasive to erode the surface of a designated sheet of material. Typically, waterjets are used to perform processes such as ablation, cutting, and structuring across a range of industries, including aerospace, industrial machining and manufacturing and construction. Depending on the parameters of various waterjet cutting components, a waterjet can be used to cut through extremely thick metal as well as to handle softer, more pliable material. Generally, material thickness should fall between 0.4 and 2.0 inches to benefit from waterjet cutting.

The Waterjet Cutting Process

Typically, waterjet cutting occurs in four steps. First, the water is pressurized using a hydraulic pump, until the water reaches between 36,000 and 80,000 psi, depending on the specific application. Next, the water passes through a nozzle, which helps concentrate the stream so it can be effectively controlled during cutting. However, before the stream is ready to cut, it passes through the abrasive hose directly below the nozzle, where abrasive material merges with the stream. Carbide or sintered boride is often used within the nozzle, to aid in focusing the stream of water during cutting. Finally, the stream exits the nozzle and cutting begins.

In addition to achieving these basic process steps, waterjet cutting machines can include up to five different axis. The greater the number of axis, the greater the range of cutting motion will be for a given waterjet cutter, which can be advantageous for specialty parts or components with difficult geometries.

Materials and Applications

When considering materials used in waterjet cutting, there are two kinds of materials to keep in mind: the abrasive material used in the cutting process, and the material being cut. Typical abrasive materials include garnet and aluminum oxide. Material waterjets frequently cut marble, granite, glass, and stone, in addition to a range of metal materials. However, waterjets cannot be used to cut diamonds or tempered glass. Diamonds are too hard, and tempered glass will shatter. Several kinds of ceramics are so hard that cutting them with a waterjet is not the most efficient process. Additionally, waterjets can erode lamination on composite materials, and typically are not used in such applications. When a waterjet is used without an abrasive, it can be used to cut softer materials such as wood and plastic. Often the actual cutting stream and workpiece are submerged in water, to minimize noise and splash.

Waterjet machines are not specialty machines, but general devices for a typical machine shop. Therefore, their applications are quite varied. Because waterjets can work with an array of materials from start to finish, they are frequently used in rapid prototyping applications where fast turn-around time is essential. In EDM shops, smaller waterjets with high precision can be used in conjunction with EDM machines. They can also be used alongside laser cutters.

Additional Considerations

Although waterjut cutters are precise, they depend on such a high level of force that using them to machine small, delicate components should be closely monitored. Burring and thermal stress are not concerns with waterjet cutting, but surface erosion can result in back-splash, and goggles should be worn. Additionally, the process can be messy and can require significant clean-up.

Abrasive jet and Water Jet technologies have been around for years.  Waterjet cutting has been a specialty technology used in a wide variety of industries since about 1970.  Around 1993, big advances in the technology were introduced that have caused this technology to become very popular for machine shops.  There are now a lot of companies making a lot of money by replacing and complementing conventional machining with water jet cutting methods.

Over the last 10 years, abrasivejet machining has taken off like wildfire.  Thousands of job-shops have sprung up around the world.

Why are so many people suddenly buying abrasive waterjet machine tools?  Because:

  • They are quick to program (make money on short runs.)
  • They are quick to set up, and offer quick turn-around on the machine.
  • They complement existing tools, used for either primary or secondary operations.
  • They make parts quickly out of virtually any material.
  • They do not heat your material.
  • All sorts of intricate shapes are easy to make.
  • They are money making machines.
  • We hope you find this information useful.

    Most of the information contained here specifically applies to 2D machining, but is general enough to be useful for those researching abrasive and water jets in general.


    Background:

    Waterjets (or abrasivejets) are fast, flexible, reasonably precise, and in the last few years have become friendly and easy to use.  They use the technology of high pressure water being squirted through a small hole to concentrate an extreme amount of energy in a small area to cut stuff.

    “A machine shop without a waterjet, is like a carpenter without a hammer – Sure the carpenter can use the back of his crow bar to hammer in nails, but there is a better way…”You have already heard the terms “Waterjet” and “Abrasive jet”.  It is important to understand that Abrasive jets are not the same thing as water jets, although they are nearly the same. Water Jet technology has been around since the early 1970s or so, and abrasive jets extended the concept about 10 years later by adding abrasive to the mix.

    Both technologies use the principle of pressurizing water to extremely high pressures, and allowing the water to escape through a very small opening (typically called the “orifice” or “jewel”).  The restriction of the tiny orifice creates high pressure and a high velocity  beam, much like putting your finger over the end of a garden hose.

    Water jets use the beam of water exiting the orifice (or jewel) to cut soft stuff like diapers, candy bars, and thin soft wood, but are not effective for cutting harder materials.

    The inlet water is typically pressurized between 20,000 and 60,000 Pounds Per Square Inch (PSI).  (Or 1300 – 6200 “bar” if you prefer metric). This is forced through a tiny hole in the jewel, which is typically 0.007″ to 0.020″ in diameter (0.18 – 0.4mm)  This creates a very high velocity beam of water!

    Abrasive jets use that same beam of water to accelerate abrasive particles to speeds fast enough to cut through much harder materials:

    _

    People often incorrectly use the word “waterjet” when they really mean “abrasivejet”.  Also, people sometimes say “abrasivejet”, “abrasive waterjet”, or “AWJ”, which mean the same thing.  Don’t worry.  If you accidentally call an “abrasivejet” a “waterjet”. Nobody will laugh at you, as it is fairly common to do so.  Likewise, their are multiple spellings for the terms “water-jet”, “waterjet”, “water jet”, etc.  Any of these variations is ok to use, though perhaps “waterjet” and “abrasivejet” are the most common.

    Which nozzle is best for my material?

    Water Jet Nozzle AbrasiveJet Nozzle
    Soft rubber Hardened tool steel Plastic
    Foam Titanium Nylon
    Extremely thin stuff like Foil Aluminum Graphite
    Carpet Hard Rubber Many ceramics
    Paper and cardboard Stone Carbon Fiber
    Soft Gasket material Inconel® Composites
    Candy bars Hastalloy mild steel
    Diapers Copper Stainless Steel
    Soft, or thin wood Exotic materials Kevlar
    …All sorts of other soft stuff Hard, or thick Wood Granite
    Glass (even bullet proof!) Mixed materials
    Marble Brass
    In Fact, there are very few materials that abrasivejets can’t cut!

    Note: Many machines let you swap nozzles in a matter of minutes. Alternately, you can simply turn off the abrasive, and get a somewhat inefficient water jet from your abrasive jet nozzle.

    Cost comparison:

    Complete water jet nozzle assemblies cost around $500.00 – $1000.00 (US), while abrasive jet nozzles cost from $800 – $2000. The abrasive nozzle also requires support hardware for abrasive feed which can cost anywhere from $500 to $2,000 (These numbers are not precise – for exact pricing, contact a waterjet supplier or waterjet equipment manufacturer.) Cost of operation is much higher for the Abrasive jet because of mixing tube wear, and abrasive consumption.

    Limitations to water only nozzles:

    Typically, the only problems that arise with a water only nozzle will be with the jewel (the orifice with the tiny hole that the water squirts through).

    Jewels can crack, plug, or form deposits on them. Cracking and plugging happens as a result of dirty inlet water, and is typically avoided with proper filtration. Deposits accumulate gradually as a result of minerals in the water. Depending on your water supply, slightly fancier filtering may be necessary. Jewels are easily replaced in about 2 – 10 min., and are typically cheap ($5-$50). There are also diamond orifices for sale for $200.00 and up, which can last longer in many applications.  Which is better, will depend on your exact needs.

    Limitations of Abrasive Jet nozzles:

    Despite their simple design, abrasive jet nozzles can be troublesome at times.  There are many designs, but they share the same problems:

    • Short life of an expensive wear part: The mixing tube. Like I said, the abrasive jet can cut through just about anything – including itself. This will be a large part of your operating cost. (more on operating cost later)
    • Occasional plugging of mixing tube: Usually caused by dirt or large particles in abrasive.  (This used to be a big problem with abrasivejet nozzles, but not so much any more.)
    • Wear, misalignment, and damage to the jewel.

    What other components make up a typical abrasivejet / waterjet machine?

    A typical abrasivejet machining center is made up of the following components:High pressure water starts at the pump, and is delivered through special high pressure plumbing to the nozzle.  At the nozzle, abrasive is (typically) introduced, and as the abrasive/water mixture exits, cutting is performed. Once the jet has exited the nozzle, the energy is dissipated into the catch tank, which is usually full of water and debris from previous cuts.  The motion of the cutting head is typically handled by an X / Y axis structure.  Control of the motion is typically done via a computer following the lines and arcs from a CAD drawing.As with automobiles, there are loads of other accessories and options, such as automatic tank clean-out systems, water recyclers, special tilting heads, fixturing, or motorized Z axis, etc., but the above compromise the basic system, and pretty much everything you need for making most 2 dimensional parts.

    Advantages to abrasive jet machining:

    Extremely fast setup and programming

    No tool changes required, so there is no need to program tool changes or physically qualify multiple tools. For some systems, programming simply involves drawing the part. If you customer gives you that drawing on disk, half the battle is won.This means that (for some machines) you can make good money off single part and low volume production!

    Very little fixturing for most parts

    Flat material can be positioned by laying it on the table and putting a couple of 10 lb weights on it. Tiny parts might require tabs, or other fixturing.  At any rate, fixturing is typically not any big deal – though it is important to secure the material in the X, Y, and Z directions.

    Machine virtually any 2D shape (and some 3D stuff)

    Including tight inside radii, Make a carburetor flange with holes drilled and everything. Some exotic machines are capable of 3d machining, (robot arms, (x,y) machines with lathe axis, and (x,y)-(u,v) axis machines).  (3D machining is especially tricky, however, due to issues regarding control of a “floppy tool”.  For this reason, 3D machining is reserved strictly for specialty applications.).  In other words, abrasivejets are exceptional at 2D machining, but limited in 3D capability.

    Pictured here is a dragon machined from 1″ thick bullet proof glass, and inlay of marble and granite. Notice the fine detail possible.

    Very low side forces during the machining

    This means you can machine a part with walls as thin as .01″ (0.25 mm) without them blowing out. This is one of the factors that make fixturing is so easy.  Also, low side forces allow for close nesting of parts, and maximum material usage.

    Almost No heat generated on your part

    Machine without hardening the material, generating poisonous fumes, recasting, or warping.  You can machine parts that have already been heat treated with only a tiny, tiny decrease in speed. On piercing 2″ (50mm) thick steel, temperatures may get as high as 120 degrees F (50 C), but otherwise machining is done at room temperature.

    Aerospace companies use abrasivejets a lot because of this.

    No start hole required

    Wire EDM, eat your heart out. Start holes are only required for impossible to pierce materials. (Some poorly bonded laminates are sometimes the exception.  In which case pre-drilling or other special methods may be employed)

    Machine thick stuff

    This is one huge advantage Abrasive jets have over lasers.While most money will probably be made in thickness’ under 1″ (25mm) for steel, It is common to also machine up to 4″ (100mm). How thick it is possible to cut is dictated by the time it takes.  Cutting speed is a function of thickness, and a part twice as thick will take more than twice as long. People make low tolerance parts and roughing out metal up to 5-10″ thick (125mm-250mm), but those people are very patient, and probably have no other way to do it.  Typically, most money is made on parts 2″ (50mm) thick or thinner.

    Pictured here is a 2″ (50mm) thick piece of 304 stainless steel. In 1993 when this part was first cut, It took just under 3 hours with a very small 10 horsepower pump and old control software to machine this to a tolerance of +/-.005″ (0.125mm).   Today, using a 40 HP direct drive pump, and modern control software, this could be machined to the same tolerance in under an hour (including programming, setup, etc.) .

    Environmentally friendly

    As long as you are not machining a material that is hazardous, the spent abrasive and waste material become suitable for land fill. The red color of garnet abrasive also looks nice in your garden. If you are machining lots of lead or other hazardous materials, you will still need to dispose of your waste appropriately, and recycle your water.  Keep in mind, however, that very little metal is actually removed in the cutting process.  This keeps the environmental impact relatively low, even if you do machine the occasional hazardous material.

    In most areas, excess water is simply drained to the sewer.  In some areas, some water treatment may be necessary prior to draining to sewere.  In a few areas, a “closed loop” system that recycles the water may be required.

    The pumps do use a considerable amount of electricity, though, so there is some additional environmental (and cost) impact due to this.

    Your clippings are valuable

    When machining or roughing out expensive materials such as titanium, your scrap still has value. This is because you get chunks, not chips. You can also get more parts from the same material because of the abrasive jets low kerf width.

    There is only 1 tool

    There is no need to qualify multiple tools, or deal with programming tool changes. Programming, Setup and Clean up time is reduced significantly, meaning you make more money because you can turn more parts faster.


    Myth Buster: Wow! you can cut 6″ (150mm) thick Tool Steel with Water!?

    Nope! If you are seeing 6″ (150mm) thick steel being cut on a “waterjet”, what you are really looking at is an “abrasivejet”.  The water is accelerating the abrasive.  The steel is being cut by the abrasive, not the water!

    Life of Cutting Nozzles:

    How long will a mixing tube last?

    A “worn” mixing tube is like a worn tool bit: It is difficult to say when a mixing tube is fully worn, but as it wears, it becomes a less effective cutting tool. (although once it starts to go bad, the wear rate accelerates). For precision work, a new mixing tube performs better than a used one.  How long a mixing tube will last depends on a number of factors, including the sales person that you talk to.  Numbers from 20 to 80 hours are fairly typical, although it is possible that they may wear faster, or last longer, depending on circumstances.

    So what’s the real cost?

    When looking at costs such as mixing tubes and jewels that are expensive wear parts, consider the “total cost of operation”, and compare it with the productivity of the machine.  When you make such a comparison you will quickly see that an abrasive jet will probably be the most profitable machine tool in your shop – by far.  Consider that your operating cost of the machine will vary between $20 and $35 per hour, but for “typical” jobs you will earn between $60 and $150 per hour, with $120/hour being quite typical*.

    * I have also seen shops do special work at prices between $500 and $2000 per hour.  Although not the norm, I occasionally see shops that find some niche market that cannot be done any other way, or where the alternate methods are very expensive.  These guys make loads of money, and often are quite secretive about how they do it.

    Price varies considerably depending on regional factors such as competitors and local markets.  Research this carefully when looking to purchase a machine.

    When pricing the work, it is often more sensible to price based on a “per part” price, instead of “per hour”.  Often profits can be maximized this way, and it is possible to then realize the benifits of faster cutting machines, and/or machines with multiple nozzles.

    Obtainable tolerances:

    It is important to have a machine with good precision to get precision parts, but there are many other factors that are just as important. A precise machine starts with a precise table, but it is the control of the jet that brings the precision to the part.  A key factor in precision is software – not hardware.  This is also true for cutting speed.  Good software can increase cutting speeds dramatically.  This is because it is only through sophisticated software that the machine can compensate for a “floppy tool” made from a stream of water, air, and abrasive.Obtainable tolerances vary greatly from manufacturer to manufacturer.  Most of this variation comes from differences in controller technology, and some of the variation comes from machine construction.  Recently, there have been significant advances in the control of the process allowing for higher tolerances.  A machine from 1990  may be capable of tolerances of 0.060″-.010″ (1.5mm-0.25mm) Today, some machines are capable of making some parts +/- 0.001″ (0.025mm), or even better in special circumstances (though +/-0.002″ is perhaps more realistic).

    When purchasing a machine, be sure measure parts that come off the machine you are going to buy.  Some manufactures stretch the truth a bit when quoting tolerances, or they quote the positioning accuracy of the mechanics of the machine, which does not necessarily translate into the cutting accuracy in the final parts.  The reality of it is that Manufactures of abrasive jet equipment are in a tough spot when trying to advertise obtainable tolerances because of these and other factors:

    Material to machine

    Harder materials typically exhibit less taper, and taper is a big factor in determining what kind of tolerances you can hold.  It is possible to compensate for taper by adjusting the cutting speed, and/or tilting the cutting head opposite of the taper direction.

    Material thickness

    As the material gets thicker, it becomes more difficult to control the behavior of the jet as it exits out the bottom. This will cause blow-out in the corners, and taper around curves.  Materials thinner than 1/8″ (3mm) tend to exhibit the most taper (which is perhaps the opposite of what you might expect.), and with thicker materials, the controller must be quite sophisticated in order to get decent cuts around complex geometry.

    Accuracy of table

    Obviously, the more precise you can position the jet, the more precise you can machine the part.  Generally speaking, though, it is much easier to find precise tables, than it is to find machines that can make precise parts.  (More on why this is in “control of the abrasivejet” below.)

    Stability of table

    Vibrations between the motion system and the material, poor velocity control, and other sudden variances in conditions can cause blemishes in the part (often called “witness marks”), as shown in a severe case below:

    The hardware that is out there varies greatly in stability and susceptibility to vibrations.  If the cutting head vibrates relative to the part you are cutting, then your part can be ugly.

    Control of the abrasive jet

    Because your cutting tool is basically a beam of water, it acts like a “floppy tool”. The jet lags between where it first enters your material and where it exits.

    Above: Bottom of jet lags behind cutting head.  The controller needs to be aware of this behavior, and compensate for it, in order to get high tolerances.

    This can be a source of error in the following places:

    Around curves

    As the jet makes its way around a radius, the jet lag causes a tapering effect. Therefore it is necessary to slow the jet down, and let the tail catch up with the head. (And / or tilt the cutting head to compensate)

    Inside corners

    As the jet enters the corner, the traverse speed must slow down to allow the jets tail to catch up. Otherwise the tail lag will cause the corner to “blow out” a little.

    As the jet exits the corner, the feed rate must not be increased too quickly, otherwise the jet will kick back and damage the part.

    Feed rate:

    When the jet slows down, its kerf width grows slightly.

    Acceleration / Jerk:

    Any sudden movement (like a change in feed rate) will cause a slight blemish as well.  Thus for highest precision it is necessary to control the acceleration as well as feed rate, and even Jerk (“Jerk” is a change in acceleration.).

    Nozzle Focus

    Some nozzles produce more taper than others. Longer nozzles usually produce less taper. Smaller diameter nozzles also produce less taper.  Holding the nozzle close to the work piece produces less taper as well.  (And, of course, it is possible to tilt the cutting head to elliminate the taper in most cases.)

    Speed of cutting:

    Typically, the slower the cutting, the higher the tolerance.  This is because as the cutting is slowed down, the surface finish improves, and the taper begins to disappear, and the jet exhibits less lag.  However in some cases it is possible to slow the cutting down so much that tolerances begin to get worse due to reverse taper, unless the head is tilted.

    Active taper compensation:

    Some newer machines now have the option of tilting the cutting head against the taper.  This can be used to virtually eliminate the taper, or to purposely add taper into a part.  The big advantage to active taper compensation is that taper can be reduced without having to slow the cutting down. (“Taper” is when the edge of the part is not 100% perpendicular.)  I have an entire page dedicated to this topic elsewhere in this web site.  If you want to go there now, click here.

    Kerf width

    Kerf width, which is the width of the cutting beam, determines how sharp of an inside corner you can make. About the smallest practical abrasivejet nozzle will give you a kerf width of .015″ (0.38mm) in diameter. Higher horsepower machines require larger nozzles, due to the amount of water and abrasive that they flow through.

    Some waterjet (water only) nozzles have very fine kerf widths (like .003″ / 0.076mm).  Likewise, it is possible to make ultra-small abrasivejet nozzles, but they can be problematic.

    Kerf width is typically compensated for by the controller by specifying a “tool offset”, where the jet is moved 1/2 of its diameter away from the edge of the part when it cuts.

    Consistency of Pump Pressure

    Variations in waterjet pump pressure can cause marks on the final part. It is important that the pump pressure vary as little as possible while machining is in progress to prevent these. (This becomes an issue only when looking for better than +-.005″ (0.125mm) tolerances, however). Typically it is older Intensifier type pumps that exhibit this problem. Some newer intensifiers, and as far as I know all crankshaft driven pumps have smoother pressure delivery, and this is usually not an issue.

    Operator experience

    Abrasive jets are capable of anywhere from +-0.02″ to +-0.001″ (0.5mm – 0.025mm) depending on the above factors. What distinguishes one machine from another is how easy those tolerances are obtained. If you had a nozzle attached to any X, Y table capable of positioning to +-.001″ (0.025mm), then, in theory, in 0.5″ (13mm) thick steel, you could perhaps machine +-0.002″ (0.05mm) or so. This is given either software to compensate for jet behavior, and/or an experienced operator tweaking the machine through trial and error. I have personally been able to produce parts in the slightly better than +-0.001″ (0.025mm) range on an OMAX 2652, which as far as I know is the most precise machine on the market (other than an OMAX 2626xp), but that usually requires cutting the part once, measuring the error, then cutting it again, and is only possible on certain materials and geometries.

    Buying a machine? Look at, and measure parts that come off the machine. Measure the first part, then cut the same part at different locations on the table to get an idea of repeatability.  Ideally, have the seller do so while you watch, to prevent cheating.  (One way to cheat is to slow the cutting way down, and another is to simply use a different machine – It happens.)

    Cutting speeds:

    Ideally, you want to make the most precise part possible in the least amount of time, and for the least amount of money.  Cutting speeds are a function of the the material to cut, the geometry of the part, the software and controller doing the motion, the power and efficiency of the pump making the pressure, and a few other factors such as the abrasive used:

    The chart below should give you some idea of what a few typical waterjet parts might take to machine:

    Picture Description Approximate Cutting time
    2.5″ x 2.5″ Box cut from 0.5″ thick mild steel
    (63 x 63mm from 12mm steel)
    5 minutes
    the same part as above, only in 3″ (76 mm) mild steel 2.25 hours
    8″ wide Electrical Panel cut from 0.06″ mild steel
    (200 mm from 1.5mm steel)
    1-3 minutes
    3″ wide gear cut from 0.25″ thick nylon
    (75mm from 6mm nylon)
    1.25 minutes
    10″ wide thingy cut from 1″ thick titanium
    (254 mm wide from 25 mm thick titanium)
    22 minutes
    7″ tall horse cut from 0.25″ thick aluminum
    (178 mm cut from 6 mm thick aluminum)
    4.8 minutes

    Of course, the above times and such are totally ball-park. How long the above parts actually take to make depend on a lot of different factors…

    Here are the primary factors that determine cutting speed:

    Material being cut (And how thick it is)

    Hardness: Generally speaking, harder materials cut slower than soft materials.  However, there are a lot of exceptions to this.  For example, granite, which is quite hard, cuts significantly faster than Copper, which is quite soft.  This is because the granite easily breaks up because it is brittle.  It is also interesting to note that hardened tool steel cuts almost as quickly as mild steel. (Though “absolute black” granite, which is tough as nails, actually cuts a bit slower than copper.)Thickness: The thicker the material, the slower the cut.  For example, a part that might take 1 minute in 1/8″ (3mm) steel, might take a half hour in 2″ (50mm) thick steel, and maybe 20 hours in 10 inch (250mm) thick steel.

    Geometry of the part

    It is necessary to slow the cutting down in order to navigate sharp corners and curves.  It also takes additional time to pierce the material.  Therefore, parts with lots of holes and sharp corners will cut much slower than simpler shapes.

    Desired Result

    If you want a high tolerance part and / or a smooth surface finish, then the part will take longer to make.   Note that you can make some areas of a part high tolerance and other areas fast, so you can mix and match to get the optimal balance between cutting speed and final part quality.

    Software controlling the motion

    This is probably one of the most overlooked aspects of abrasivejet machining by novice users.  You would not think that software would have much to do with the speed of cutting.  In fact, this is (mostly) true if all you are doing is cutting in a straight line.  However, as soon as you introduce any complexity to the part, such as a corner, there is great opportunity for software to optimize the cutting speed.Below, is a part to be machined from 1/2″ (12mm) mild steel:Notice the subtle difference between the two pictures.  (The colors represent cutting speeds, with yellow being the fastest areas, and blue being the slowest.)  The part on the left took 3.3 minutes to machine, while the part on the right took 4.4 minutes to machine.  That’s a 1 minute difference, or about 25%.  The difference, as it turns out, is all through software that automatically optimizes the tool path to provide the desired precision in the least amount of time.  Basically, what the software does, is looks at the geometry of the part, and then modify the feed rates and add “tweaks” to the cutting in order to squeeze the maximum amount of speed.  It does this by finding the optimal speeds and accelerations for all curves and corners, setting the optimal length and feed rate for all pierce points, adding special “corner pass” elements at corners to allow the cutting to go right past the corners where it can, etc.

    When I first started working with these kinds of methods back around 1993 or so, we found that by simply optimizing the corners that we could get about a factor of two in cutting speed over a hand-optimized part.  Then, over the next 10 years or so, we added a lot more optimizations in terms of faster piercing, corner passing, improved cutting models and such, and were able to get another factor of 2 in cutting speed for some parts, while at the same time increasing the precision.  There is still a lot of room for software to continue in this trend, though eventually it will level out.

    So, software matters!  And one of the most beautiful things about optimizing in software, is that it does not cost any money.  In fact, it saves money, because if you cut faster through software, you use less abrasive and put less wear and tear on all the high pressure components!  A 20% improvement in cutting speed through software is 20% pure profit.  A 20% improvement in cutting speed by increasing pump power, is not nearly as attractive because it cost more electricity, water, sewer, abrasive, and maintenance.

    Note: It’s even possible to speed up straight line cutting through software, by controlling how the jet “tilts forward” into the cut.  This allows for better surface finish at the same speed, or the same surface finish at higher speeds.  The results are not as dramatic as software improvements in other areas, but still significant.  This option, of course, requires a tilting cutting head, so it is technically a combination of software and hardware.

    Power at the nozzle (pressure and water flow rate)

    The more horsepower at the nozzle, the faster it can cut.  How much horsepower makes it to the nozzle is a function of the pressure and the orifice that it is being squeezed through.   (Note: do not confuse “horsepower of the motor” with “horsepower at the nozzle”.  It is the power that actually makes it to the nozzle that is most important.  Having a big motor makes no difference, if the power all goes into wasted heat!)Simply put,  the higher the pressure, the faster the cut.  The more water you flow, the faster the cut.   Unfortunately, as the pressure increases, so does the cost and maintenance, so this is not as simple as it seems.A good way to learn more about how pressure and jewel size effect cutting rates, and to calculate “nozzle horsepower” is to run the Abrasivejet Feed Rate Calculator,

    Quantity and Quality of abrasive used

    Type of abrasive: In the industry, most machines run 80 mesh garnet for abrasive.  However, it is possible to cut slightly faster with harder abrasives, but the harder abrasives also cause the mixing tube on the nozzle to wear rapidly.  So, pretty much everyone uses garnet.  It is worth mentioning that not all garnet is the same.  There are big variations between purity, hardness, sharpness, etc, that can also effect the cutting speed, accuracy, reliability, and operating cost.

    Quality of abrasive: Typically, abrasivejets consume between 0.5 and 1 Lb (0.25 and 0.5Kg) of abrasive per minute.  There is a sweet spot for every nozzle size and pressure as to what amount of abrasive flow rate will cut the fastest, and what amount will cut the cheapest.

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    3 thoughts on “What is Die Cutting?

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