Water Jet Cuting

8/10/2019 Water Jet Cuting

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How Waterjet Cutting Works

Introduction

In the battle to reduce costs, engineering and manufacturing departments are constantlyon the lookout for an edge. The waterjet process provides many unique capabilities andadvantages that can prove very effective in the cost battle. Learning more about thewaterjet technology will give you an opportunity to put these cost-cutting capabilities towork.

eyond cost cutting, the waterjet process is recogni!ed as the most versatile and fastestgrowing process in the world "per #rost $ %ullivan and the &arket Intelligence 'esearch(orporation). *aterjets are used in high production applications across the globe. Theycompliment other technologies such as milling, laser, +&, plasma and routers. onoious gases or liquids are used in waterjet cutting, and waterjets do not createha!ardous materials or vapors. o heat effected !ones or mechanical stresses are left on awaterjet cut surface. It is truly a versatile, productive, cold cutting process.

The waterjet has shown that it can do things that other technologies simply cannot. #romcutting whisper thin details in stone, glass and metals/ to rapid hole drilling of titanium/to cutting of food, to the killing of pathogens in beverages and dips, the waterjet has proven itself unique.

History of Waterjets

r. orman #ran! is regarded as the father of the waterjet. 0e was the first person whostudied the use of ultrahigh-pressure "102) water as a cutting tool. The term 102 isdefined as more than 34,444 pounds per square inch "psi). r. #ran!, a forestry engineer,wanted to find new ways to slice thick trees into lumber. In the 56748s, #ran! firstdropped heavy weights onto columns of water, forcing that water through a tiny orifice.0e obtained short bursts of very high pressures "often many times higher than arecurrently in use), and was able to cut wood and other materials. 0is later studies involvedmore continuous streams of water, but he found it difficult to obtain high pressurescontinually. 9lso, component life was measured in minutes, not weeks or months as it istoday.

r. #ran! never made a production lumber cutter. Ironically, today wood cutting is a very

minor application for 102 technology. ut #ran! proved that a focused beam of water atvery high velocity had enormous cutting power : a power that could be utili!ed inapplications beyond r. #ran!8s wildest dreams.

In 56;6, r. &ohamed 0ashish working at #low 'esearch, began researching methods toincrease the cutting power of the waterjet so it could cut metals, and other hardmaterials. r. 0ashish, regarded as the father of the abrasive-waterjet, invented the process of adding abrasives to the plain waterjet. 0e used garnet abrasives, a material

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commonly used on sandpaper. *ith this method, the waterjet "containing abrasives)could cut virtually any material. In 56<4, abrasive-waterjets were used for the first timeto cut steel, glass, and concrete. In 56<3, the world8s first commercial abrasive waterjetcutting system was sold for cutting automotive glass. The first adopters of the technologywere primarily in the aviation and space industries which found the waterjet a perfect tool

for cutting high strength materials such as Inconel, stainless steel, and titanium as well ashigh strength light-weight composites such as carbon fiber composites used on militaryaircraft and now used on commercial airplanes. %ince then, abrasive waterjets have beenintroduced into many other industries such as job-shop, stone, tile, glass, jet engine,construction, nuclear, and shipyard, to name a few.

How High Pressure Water is Created

The basic technology is both simple and etremely comple. 9t its most basic, waterflows from a pump, through plumbing and out a cutting head. It is simple to eplain,operate and maintain. The process, however, incorporates etremely comple materialstechnology and design. To generate and control water at pressures of =4,444 psi requiresscience and technology not taught in universities. 9t these pressures a slight leak cancause permanent erosion damage to components if not properly designed. Thankfully, thewaterjet manufacturers take care of the comple materials technology and cutting-edgeengineering. The user need only be knowledgeable in the basic waterjet operation.

+ssentially, there are two types of waterjets/ "5) pure waterjet and ">) abrasive waterjet.

&achines are designed to employ only waterjet, only abrasive waterjet, or both. *ith anytype, the water must first be pressuri!ed.

The Pump

The pump is the heart of the waterjet system. The pump pressuri!es the water anddelivers it continuously so that a cutting head can then turn that pressuri!ed water into asupersonic waterjet stream. Two types of pump can be used for waterjet applications :an intensifier based pump and a direct drive based pump.

Direct Drive Pump

The direct drive pump operates in the same manner as a low-pressure ?pressure washer@

that you may have used to pressure wash a house or deck prior to repainting. It is a triple pump that gets the movement of the three plungers directly from the electric motor. These pumps are gaining acceptance in the waterjet industry due to their simplicity. 9t the timeof this writing, direct drive pumps can deliver a maimum continuous operating pressure54 to >7A lower than intensifier pumps units ">4k to 74k for direct drive, B4k to =4k forintensifiers).


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The irect rive pump is a relatively new type of high-pressure pump"commercially available since late 56<48s).

Though direct drive pumps are used in some industrial applications, the vast majority ofall ultra-high pressure pumps in the waterjet world today are intensifier based.

Intensifier Based Pumps

Two fluid circuits eist in a typical intensifier pump, the water circuit and the hydrauliccircuit.

The water circuit consists of the inlet water filters, booster pump, intensifier, and shockattenuator. Crdinary tap water is filtered by the inlet water filtration system D usuallycomprising of a 5 and a 4.B7 micron cartridge filter. The filtered water then travels to the

booster pump, where the inlet water pressure is maintained at approimately 64 psi Densuring the intensifier is never ?starved for water.@ The filtered water is then sent to theintensifier pump and pressuri!ed to up to =4,444 psi. efore the water leaves the pumpunit to travel through the plumbing to the cutting head, it first passes through the shockattenuator. This large vessel dampens the pressure fluctuations to ensure the water eitingthe cutting head is steady and consistent. *ithout the attenuator, the water stream wouldvisibly and audibly pulse, leaving marks on the material being cut.


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The hydraulic circuit consists of an electric motor ">7 to >44 02), hydraulic pump, oilreservoir, manifold, and piston biscuitEplunger. The electric motor powers the hydraulic pump. The hydraulic pump pulls oil from the reservoir and pressuri!es it to 3,444 psi.This pressuri!ed oil is sent to the manifold where manifoldFs valves create the strokingaction of the intensifier by sending hydraulic oil to one side of the biscuitEplunger

assembly, or the other. The intensifier is a reciprocating pump, in that the biscuitEplungerassembly reciprocates back and forth, delivering high-pressure water out one side of theintensifier while low-pressure water fills the other side. The hydraulic oil is then cooledduring the return back to the reservoir.

Typical intensifier pumping unit. This unit is designed to stand alone, rather than integrated into motionequipment.

The advanced technology in the pump is found in the intensifier. 9s mentioned briefly inthe description of the water circuit, the intensifier pressuri!es the filtered tap water to upto =4,444 psi. Intensifier pumps utili!e the ?intensification principle.@

0ydraulic oil is pressuri!ed to a pressure of, say, 3,444 psi. The oil pushes against a piston biscuit. 9 plunger with a face area of >4 times less than the biscuit pushes againstthe water. Therefore, the 3,444-psi oil pressure is ?intensified@ twenty times, yielding=4,444-psi water pressure. The ?intensification principle@ varies the area component ofthe pressure equation to intensify, or increase, the pressure.

2ressure G #orce E9rea

If #orce G >4, 9rea G >4, then 2ressure G 5. If we hold the #orce constant and greatlyreduce the 9rea, the 2ressure will go 12. #or eample, reduce the 9rea from >4 down to5, the 2ressure now goes up from 5 to >4. In the sketch below, the small arrows denotethe 3,444 psi of oil pressure pushing against a biscuit face that has >4 times more areathan the face of the plunger. The intensification ratio, therefore, is >4H5.



In the illustration below, the biscuit and plungers are circled. The biscuit contains thesmall arrow suggesting movement to the left. The two water plungers etend from eitherside of the biscuit. 0igh-pressure water is delivered out the left side while low-pressurewater refills the right. 9t the end of travel, the biscuitEplunger assembly sequence isreversed.

%ophisticated check valves ensure the low pressure and high-pressure water is onlyallowed to travel one direction. The high-pressure cylinders and end caps that encase the plunger and biscuit assembly are specially designed to withstand the enormous force andthe constant fatigue.

With a 1!1 intensification ratio and "#$psi

oi% pressure# what is the resu%tant water

pressure&

The resultant water pressure would be 54 times that of the

hydraulic oil pressure. The quick answer is 34,444 psi.

The entire unit is designed for long life, while also designed to fail in a safe way. *aterjetsystems fail in a gradual, rather than instantaneous way. The seals and connections beginto leak slowly through specially designed weep holes. The operator or maintenance person can see a drip escaping from a weep hole. The location of the drip and the amountof water indicate when maintenance should be performed. 1sually, the maintenance person can schedule the periodic maintenance of seals and check valves out 5 to > weeks



into the future by simply monitoring the gradual weeping. *arning and shutdown sensorsalso cover the pumping unit to further safeguard against pump damage.

Troubleshooting of an intensifier is quite simple. ripping of hot water from a weep hole indicates a high- pressure leak, cold water indicates low-pressure. In the actual image, those drops labeled hot or warm are inred, cold is in blue.

Two Types of Waterjets

The two types of waterjets are the pure waterjet and the abrasive waterjet. oth haveunique capabilities proven a benefit to industry.

Pure Waterjet

2ure waterjet is the original water cutting method. The first commercial applications werein the early to mid 56;4s, and involved the cutting of corrugated cardboard. The largestuses for pure waterjet cutting are disposable diapers, tissue paper, and automotiveinteriors. In the cases of tissue paper and disposable diapers the waterjet process createsless moisture on the material than touching or breathing on it. 1nplanned down time,common to other cutting processes, cost over >4,444 per hour in some diaper or tissue

plants. The waterjet provides the >B hour per day, ; day per week, 3=4 day per yearoperation required by such applications D maintenance can be scheduled into production.

Pure Waterjet 'ttri(utes

• Jery thin stream "4.44B to 4.454 inch in diameter is the common range)

• +tremely detailed geometry

• Jery little material loss due to cutting

• on-heat cutting


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• (ut very thick

• (ut very thin

• 1sually cuts very quickly

• 9ble to cut soft, light materials "e.g., fiberglass insulation up to >B@ thick)

• +tremely low cutting forces

•

%imple fituring• >B hour per day operation

How hot is the water in a )ach " waterjet stream&

The water is warmed as it is accelerated to high speed. #rictional forces andother factors warm the stream as it eits the orifice. Inlet water temperature provides the starting point. *ater temperature is then raised > to 3 degrees foreach 5,444 psi. The quick answer is the &ach 3 jet is approimately 5;4 to 5<4degrees #.

Pure Waterjet Cutting Heads

9s you may recall from an earlier section of this document, the basic waterjet processinvolves water flowing from a pump, through

plumbing, and out a cutting head.

In waterjet cutting, the material removal process can be described as a supersonicerosion process. It is not pressure, but streamvelocity that tears away microscopic pieces or grains of material. 2ressure and velocity aretwo distinct forms of energy. ut how is the pumpFs water pressure converted to this other form of energy, water velocityK The answer lies in a tiny jewel. 9 jewel is affied to theend of the plumbing tubing. The jewel has a tiny hole in it. The pressuri!ed water passes

through this tiny opening changing the pressure to velocity. 9t approimately B4,444 psithe resulting stream that passes out of the orifice is traveling at &ach >. 9nd at =4,444 psithe speed is over &ach 3.

2ure waterjet orifice diameter ranges from 4.44B to 4.454 inch for typical cutting. *henwaterblasting concrete with a no!!le traversing back and forth on a tractor, a single largeorifice of up to 5E54 th of an inch is often used.


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The three common types of orifice materials "sapphire, ruby, diamond) each have theirown unique attributes. %apphire is the most common orifice material used today. It is aman-made, single crystal jewel. It has a fairly good quality stream, and has a life, withgood water quality, of approimately 74 to 544 cutting hours. In abrasive waterjetapplications the %apphireFs life is that of pure waterjet applications. %apphires typically

cost between 57 and 34 each.

'uby can also be used in abrasive waterjet applications. The stream characteristics arewell suited for abrasivejets, but are not well suited for pure waterjet cutting. The cost isapproimately the same as the sapphire.

iamond has considerably longer run life "<44 to >,444 hours) but is 54 to >4 times morecostly. iamond is especially useful where >B hour per day operation is required.iamonds, unlike the other orifice types, can sometimes be ultrasonically cleaned andreused.

*ife +se Comments

,apphire74 to 544hours

2ure*aterjet

Meneral purpose, though life reduces by for abrasivewaterjet applications

-u(y74 to 544hours

9brasive*aterjet

%tream not suitable for pure waterjet applications

Diamond <44 to >,444

hours

2ure $

9brasive

54 to >4 times more epensive than 'uby or %apphire

'(rasive Waterjets

The abrasive waterjet differs from the pure waterjet in just a few ways. In pure waterjet,the supersonic stream erodes the material. In the abrasive waterjet, the waterjet stream

accelerates abrasive particles and those particles, not the water, erode the material. Theabrasive waterjet is hundreds, if not thousands of times more powerful than a purewaterjet. oth the waterjet and the abrasive waterjet have their place. *here the purewaterjet cuts soft materials, the abrasive waterjet cuts hard materials, such as metals,stone, composites and ceramics. 9brasive waterjets using standard parameters can cutmaterials with hardness up to and slightly beyond aluminum oide ceramic "often calledalumina, 9 66.6). In the net section we will eplore abrasive waterjet attributes andhow the abrasive waterjet cutting head works.


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B@ titanium shown, courtesy of T(I 9luminum

%tone, tile, and glasscutting

(omposites, graphite, epoy,and ballistic

#rom china bowls to aluminumoide "alumina)

'(rasive Waterjet 'ttri(utes • +tremely versatile process

• o 0eat 9ffected Nones

• o mechanical stresses

• +asy to program

•

Thin stream "4.4>4to 4.474 inch in diameter)• +tremely detailed geometry

• Thin material cutting

• 54 inch thick cutting

• %tack cutting

• Little material loss due to cutting

• %imple to fiture

• Low cutting forces "under 5 lb. while cutting)

• Cne jet setup for nearly all abrasive jet jobs

• +asily switched from single to multi-head use

• Ouickly switch from pure waterjet to abrasive waterjet

• 'educed secondary operations• Little or no burr

'(rasive Waterjet Cutting Heads

*ithin every abrasive waterjet is a purewaterjet. 9brasive is added after the pure



waterjet stream is created. Then the abrasive particles are accelerated, like a bullet in arifle, down the miing tube.

The abrasive used in abrasive waterjet cutting is hard sand that is specially screened andsi!ed. The most common abrasive is garnet. Marnet is hard, tough and inepensive. Like

the pink colored sandpaper found at the hardware store, different mesh si!es are used fordifferent jobsH

• 5>4 &esh D produces smooth surface

• <4 &esh D most common, general purpose

• 74 &esh D cuts a little faster than <4, with slightly rougher surface

The miing tube acts like a rifle barrel to accelerate the abrasive particles. They, like theorifice, come in many different si!es and replacement life. &iing tubes areapproimately 3 inches long, P inch in diameter, and have internal diameters rangingfrom 4.4>4 to 4.4=4 inch, with the most common being 4.4B4 inch.

9lthough the abrasive waterjet machine typically is considered simple to operate andQbullet proof,F the miing tube does require operator attention. 9 major technologicaladvancement in waterjet was the invention of truly long-life miing tubes. 1nfortunately,the longer life tubes are far more brittle than their predecessors, tungsten carbide tubes. If the cutting head comes in contact with clamps, weights, or the target material the tubemay be broken. roken tubes cannot be repaired. TodayFs most advanced systemsincorporate collision detection to spare the miing tube.

)ateria% *ife Comments

,tandard

Tungsten

Car(ide

B to =hours

These were the original miing tubes. They are no longerused, due to their poor performance and cost per hour ratio.They tend to wear out of round, require very frequentreplacement.

*ow Cost

Composite

Car(ide

37 to =4hours

Mood for rough cutting or when training a new operator.-Qbic 10-

)id$*ife

Composite

Car(ide

<4 to 64hours

9 good all around tube. -Qbic 20-



Premium

Composite

Car(ide

544 to574hours

The top-of-the-line. This popular tube ehibits the mostconcentric and predictable wear. 1sed for both precisionwork and everyday work.-Qbic 30-

The standoff distance between the miing tube and the target material is typically 4.454to 4.>44 inch. Larger standoff "greater than 4.4<4 inch) can cause a frosting to appearatop the cut edge of the part. &any waterjet systems reduce or eliminate this frosting bycutting under water or using other techniques.

The consumable items in an abrasive waterjet are the water, abrasive, orifice "usually'uby) and miing tube. The abrasive and miing tube are eclusive to the abrasivewaterjet. The other consumables are also found in the pure waterjet.

)otion ./uipment

&any different styles and configurations of waterjet motion equipment, or machine tool,eist. esides just providing motion, the machine tool must also include some means ofholding the material catching the jet and collecting the water and debris.

,tationary and 1$Dimension )achines

The simplest of machines is the stationary waterjet. Looking much like a band saw, it is

usually used in the 9erospace industry to trim composites. The operator feeds thematerial through the stream much like a band saw. 9fter the material has been cut, thecatcher collects the stream and debris. 1sually outfitted with a pure waterjet, somestationary waterjet machines are equipped with abrasive waterjets.

9nother version of a stationary machine is a slitter. 0ere a product such as paper is fedthrough the machine, and the waterjet slits the product into specific widths. 9 cross cutter is another eample of a machine that moves in one ais. 9lthough it is not trulystationary, this simple machine often works in conjunction with a slitter. *here the slittercuts product to specific widths, the cross cutter cuts across a product that is fed beneath it.Cften the slitter and cross cutter work together to create a grid pattern in materials such as

vending machine brownie cakes.

It is generally not recommended to use an abrasive waterjet manually "either moving thematerial by hand or the cutting head by hand) : it is very difficult to manually move at aspecific speed. &ost manufacturers will not recommend or quote a manually operatedabrasive waterjet. Cnly in special cases were operator safety is not in question should anabrasive waterjet be used in a manual mode.


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%litterH ; stations, with > heads at each station.2rovides >B-hourEday operation.

(ross (utterH 1sually angled across and timed withthe travel path of the product. 2recise speed controlcan then produce a perpendicular cut.

0 Ta(%es for 2$Dimension Cutting

RS tables, sometimes called ?flat stock machines,@ are the most common forms ofwaterjet motion equipment. These machines are used with pure water jets to cut gasketsand plastics, rubber and foam. 9brasive water jets utili!e these tables to cut metals,composites, glass stone and ceramics. #lat patterns are cut, in every imaginable design.9brasive waterjet and pure waterjet tables may be as small as > B feet or as large as a34 544 ft. The basic components of an RS areH

• (ontrolled by either (( or 2(

• %ervo motors, usually with closed-loop feedback to ensure position and velocity

integrity• ase unit with linear ways, bearing blocks and ball screw drive

• ridge unit also with ways, blocks and ball screw

• (atcher tank with material support

&any different machine styles are available, however two distinct styles dominate theindustry/ mid-rail gantry and the cantilever.


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#latstock machines "often called RSs) come in a variety of configurationsto match specific applications. %ome are small precision machines, whileothers are large production machines.

The mid-rail gantry machines have two base rails and a bridge. The cantilever system hasone base and a rigid bridge. In each of the sketches below the green bridge moves in onedirection while the red arrow "signifying the cutting head) moves in the other. 9llmachine types will have some form of adjustability for the head height "the head height is

controlled by the N-ais). The N ais adjustability can be in the form of a manual crank,motori!ed screw, or a fully programmable servo screw.

The catcher tanks on flat stock machines are usually water filled tanks that incorporate

grating or slats to support the work piece. These supports are slowly consumed during thecutting process. (atcher tanks can either be self cleaning, where the waste is depositedinto a container, or manual, where the tank is periodically emptied by hand.

9ll RS tables have critical specifications in the following areas that suggest, but do notguarantee, the performance of the machine on your shop floor.

elow are brief descriptions of waterjet machine tool motion specifications commonlyfound in quotations and literatureH

.nve%ope ,i3e The length of travel found in each ais of movement. The mostcommon si!es for flat stock cutting on a waterjet or abrasivewaterjet machine are >m 3m 4.3m, or approimately =545 ft.The catcher tank is usually at least = inches larger than the travellength and width, aiding in heavy plate loading, allowing forclamping, and allowing for variability in raw sheet si!e.



*inear Positiona%

'ccuracy

&easures how accurately the machine can move. Cne ais ismeasured at a time from one point to another. %peed is not aconsideration here.

)achine

-epeata(i%ity

9bility of the machine to return to a point.

-apid Traverse

,peed

'apid traverse is the top speed a machine can move withoutcutting. The control system simply sends a signal to the drivemotors saying, ?go as fast as you can in that direction.@ Theaccuracy of machine motion is usually compromised during rapidtraverse. 'apid traverse is used to move from one cut path "e.g., ahole cutout), to another cut path "e.g., another hole cutout).

Contour ,peed The top speed that the machine can move while maintaining all theaccuracy specifications "i.e., accuracy, repeatability, velocity). Thisis a critical specification as it relates to part production cycle timesand part accuracy

45 '6is )achines for "$Dimension

Cutting

&any man-made items, like airplanes, have fewflat surfaces on them. 9lso, advances in comple3 composites and metal forming technologiessuggest fewer flat parts are in our future. Thus, theneed for 3-dimension cutting increases each year.*aterjets are easily adapted to 3 cutting. The

lightweight heads and low kickback forces duringcutting give machine design engineers freedomnot found when designing for the high loadsfound in milling and routing. 9nd the thin, high- pressure plumbing : through use of swivels : provides freedom of movement.

The simplest of the 3-dimensional cutting systems is the 1niversal *ater'outer. Thisdevice is moved by hand, and is only intended for pure waterjet cutting of thin materials

0andheld ?1niversal *ater'outer.@ 9 pointcatcher safely stops the jet immediately aftercutting the thin material.


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"e.g., aircraft interiors and other thin composites). The handheld gun is counterweighted,and provides all degrees of freedom. This device was popular in the 56<4Fs as a superiormethod of trimming composites. 9s a replacement for the router, the operator would press a special no!!le against the template, turn on the jet with dual thumb triggers, andtrace the template with the no!!le as heEshe walked around the part. The jet would shoot

into a point catcher as after cutting the material. &any of these safe and effective toolsare used today in thin aerospace composite cutting and other applications.

The need for enhanced production and the desireto eliminate the epensive templates required forrouters and 1niversal *ater'outers lead to theuse of fully programmable 7-ais machines. *iththese machines a programmer creates the tool path in an office and downloads the program tothe operatorFs machine control system that thencuts the material.

+ven with the improvement of advanced 3-offline programming software, 3 cutting provesmore comple than >-, regardless of whether thecutting process is waterjet, router, or another process. #or eample, in the composite tail

illustration at left, many steps are taken to trim the part. #irst, the programs for the cut path and the fleible ?2ogostick@ tooling are downloaded. Then the material is flown invia overhead crane while the pogosticks unscrew to their pre-programmed height. 9fterthe part has been roughly located and the pogostick vacuum tops have secured the part, aspecial N ais "not used for cutting) brings a touch probe into location to precisely locate

the part in space. The touch probe samples a number of points to ensure the part elevationand orientation is known. Then the program part transformation takes place. 0ere the program is re-oriented to match the actuallocation of the part. #inally the touch probe N isretracted and the cutting head N swings intoaction.

The cutting of relatively thick composites "4.47inch) or any metal requires the use of abrasive. %ohow do you stop a 74 horsepower jet from cuttingup the pogosticks and tooling bed after cuttingthrough the materialK The only way known to dateis to catch the jet in a very special point catcher. 9steel ball point catcher can stop the full 74 02 inunder = inches, then the slurry is vacuumed awayto a waste handling tank. 9 ( shaped frameconnects the catcher to the N-ais wrist. This (-frame "shown in bright orange) can rotateto allow the head to trim the entire circumference of the wing part.

9 large 3457B ft. machine cuts tailcomponents for the oeing ;;;. ?2ogostick@tooling unscrews to specified height, thenvacuum cups hold the part in place.

(-frame holds the unique point catcher. Thiscatcher can stop a 74 02 abrasive waterjet in just = inches. 9 vacuum constantly removesthe slurry.



The point catcher consumes steel balls at a rate of approimately to 5 pound per hour.The jet is actually stopped by the dispersion of kinetic energy. 9s the jet enters the smallcontainer of steel balls, the balls spin. The spinning balls then rub their neighbors andthey spin. The spinning balls in the point catcher will consume the energy of the jet,leaving the cutting slurry to harmlessly leak out the screened catcher bottom. These point

catchers have proven so effective that they can run hori!ontally, and even sometimescompletely upside-down.

The compleities of locating the part in space, adjusting the part program, and accuratelycutting a part are magnified as the si!e of the part increases. &any shops effectively use3 machines for simple > cutting and comple 3 cutting every day. 9lthough softwarecontinues to get easier and machines continue to get more advanced, parts continue to getmore comple. Ueep in mind that the compleities associated with 3 cutting are presentregardless of the cutting process.

Characteristics of Part 'ccuracy

9 distinct difference eists between part accuracy and the accuracy in which a machinecan move. %imply buying a 4.44444444444445@ accurate machine, with perfect dynamicmotion, perfect velocity control, and dead-on repeatability will not mean you will cut perfect parts. It will, however, mean you will spend a lot of money on the super-accuratemachine. #inished part accuracy is a combination of process error "the waterjet) Vmachine error "the RS performance) V workpiece stability "fituring, flatness, stable withtemperature).

The table below describes part errors which would occur even if the waterjet machinewas perfect. The waterjet beam has characteristics that greatly effect part accuracy.(ontrolling these characteristics has been the focus of waterjet suppliers for many years.%imply put, a highly accurate and repeatable machine may eliminate machine motionfrom your part accuracy equation, but it does not eliminate other part errors "such asfituring errors and inherent waterjet stream errors).

*hen cutting materials under 5 inch thick, a conventional waterjet machine typically cuts parts from VE-4.443 to VE-4.457 inch ".4; to .B mm) in accuracy. 9 machine equipped withynamic *aterjet can cut parts as accuracy as VE- 4.445 inch. #or materials over 5 inch

thick the machines will produce parts from VE- 4.447 to 4.544 inch ".5> to >.7 mm). 9high performance RS table is designed to have an accuracy of about 4.447-inch linear positional accuracy or better. %o where do the part inaccuracies come fromK

Part .rrors Description


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Beam Def%ection

or 7,tream *ag8

*hen the waterjet, or other beam type cutters like laser or plasma,are cutting through the material, the stream will deflect backwards"opposite direction of travel) when cutting power begins to drop. This problem causesH increased taper, inside corner problems, andsweeping out of arcs.

'educe this lag error by increasing cutting power or slowing downthe cut speed.

Increased Taper

9 ?J@ shaped taper is created when cutting at high speeds. Taper can be minimi!ed or eliminated by slowing down the cut path orincreasing cutting power. Image is eaggerated for descriptive purposes.

Inside CornerPro(%ems

*hen cutting an inside corner at high speed, the streamcan dig into the part as it comes out of the corner. Thisimage is of the hole that is left when cutting a square cutout, viewedfrom the eit "or bottom) side. The image is eaggerated fordescriptive purposes.

,weeping 9ut of

'rcs

*hen cutting at high speed around an arc or circle the stream lagsweeps out a cone. Image is eaggerated to illustrate the error.

:i6turing

+ven though the waterjet delivers under pound of vertical forcewhen cutting a high quality part and under 7 pounds when roughcutting, proper fituring is required to produce accurate parts.

The part must not move during cutting or piercing, and it must notvibrate. To minimi!e these errors try to butt the workpiece up againstthe edge of the catcher or a solid bar stop secured to the table slats.Look for material vibration or movement during cutting the firstarticle.

)ateria%

Insta(i%ity

%ome materials, like plastics, can be very sensitive to temperaturechanges. (alled thermal epansion, these materials may epand whenslightly heated or shrink when cooled. uring waterjet cutting thematerial does not get hot, but it can get warm.

9lso, be especially careful of air gaps in cast material, as the streamtends to open up in air gaps.

The 9*W will not induce warpage in sheet material. It will, however,



relieve stresses. If you are working with a sheared material X4.5>7inch thick and you start your cut path off the part, enter into the part,and then cut the part, you may see the material twist and warp. 9voidthis warpage whenever possible by beginning cut paths from within

the material "pierce a hole and begin cutting) as opposed to beginningfrom outside of the material.

Pump Issues

eyond the obvious pump issues such as ensuring that the pump isdelivering water at the set pressure, other issues can also impact partaccuracy. If the pump has > or more intensifiers, do the intensifiersalways stroke at the same timeK If so, then look on the part forvertical marks on the cut edge that match in frequency with thestroking.

(heck valves should be in good working order.

Water Pressure

at the ;o33%e

(ut speed can be lost if ecessive pressure drops "greater than >,744 psi) eist in the high pressure plumbing run from pump to head.

+nsure the in-line filter, usually located near the cutting head, is freeof ecessive buildup.

If you have made any changes to the plumbing run "changed theroute, replaced a large line with a smaller replacement line, etc.) thenensure that you have not created larger pressure drops. 9ny loss of pressure between the pump is to be minimi!ed. 2ressure is cut speed,

cut speed is money.

Cutter Comp

.rror

(utter compensation is the value entered into the control system thattakes into account the width of cut of the jet/ in effect, you are settingthe amount by which you are enlarging the cut path so that the final part comes out to proper si!e.

efore you perform any high precision work where finished parttolerances are better than VE- 4.447 inch, cut a test coupon and ensureyou have properly set the cutter compensation. &any a good drawinghas been cut wrong because the operator did not take the time to

establish the best cutter compensation value.

Programming

.rror

Cften the most difficult of all part accuracy errors to find is a programming error where the dimension of the part program does not perfectly match the dimension of the original (9 or hand drawndrawing. 2art programs that appear graphically on the screen of anRS control typically do not display dimensions. Therefore, this error



can go undetected. *hen all else fails, double check that thedimensions on the part program match eactly those of the originaldrawing.

'(rasive )esh

,i3e

Typical abrasive mesh si!es are 5>4, <4, and 74 "similar to sand paper you might use for woodworking). The different mesh si!es do nothave a significant impact on part accuracy. They have a greaterimpact on surface finish and overall cut speed. #iner abrasives "larger mesh number) produce slower cuts and smoother surfaces.

)achine )otion

The positional accuracy and dynamic motion characteristics of amachine have an impact on the part accuracy. There are many aspectsto machine motion performance. 9 few areH

acklash in the mechanical unit "changes in direction, is there slop inthe gears or screw when the motor changes from clockwise tocounterclockwiseK), repeatability, will the machine come back closeto the same point over and overK %ervo tuning is important. Impropertuning will cause backlash, squareness, repeatability errors, and cancause the machine to chatter "wiggle at high frequency) whenmoving.

2osition accuracy is important, as well as straightness, flatness, and parallelism of the linear rails. %mall part, under 5> inches in lengthand width, do not demand as much from the RS table as larger parts.

9 large part measuring, for eample, BB ft, will be greatly impacted by machine performance. 9 small part will not see position accuracy,or rail straightness as a major impact on finished part tolerancesimply because the small part masks machine errors. Large partsepose such errors more evidently.

'emember that a machine motion characteristic does not directlycorrespond to finished part tolerance. 9n epensive super-precisionmachine "linear position accuracy of, for eample, VE- 4.445Y overfull travel, will not automatically generate a finished part of VE-4.445Y other part accuracy factors are still there "see above).

Documents

Water Jet Cuting