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Monday | 13 April, 2020 | 11:52 am

Meeting standards

Written by By Joshua Swainston, OMAX Corp.

Manufacturers are adapting machine operations to manage quality assurance tasks simply and quickly

April 2020 - Choosing the correct tools can be the defining factor in a true representation for testing. In an ideal scenario, testing preparation shouldn’t impart any additional changes to the original material but many traditional cutting machines leave residue or distort the material.

As stated in ASTM E8/E8M–13A under Subsection 6.1.3, “Improperly prepared test specimens often are the reason for unsatisfactory and incorrect test results. It is important, therefore, that care be exercised in the preparation of specimens, particularly in the machining, to maximize precision and minimize bias in test results.”

However, there is an option for test prep that is fast, clean and versatile yet might be overlooked. Abrasive waterjets can work as a valued solution for all types of metallurgy testing. By cutting testing coupons on a waterjet, some manufactures are seeing up to nine times faster preparation speed.

When cutting any type of metal coupon, traditional CNC machines impart heat, add oil or coolant, and leave burrs. Cutting with laser, plasma, EDM or traditional saws each transfer some level of heat that will need to be dealt with in a secondary process, in addition to leaving slag or chatter marks.

ASTM standards require specimens for testing should be “free of cold work, notches, chatter marks, grooves, gouges, burrs, rough surfaces or edges, overheating or any other condition which can deleteriously affect the properties to be measured.”

Waterjets impart no heat, meaning additional processes can be eliminated. Cutting material on a waterjet with a slow speed can ensure a smooth finish for testing specimens. Ultimately, this means that the test sample will be truthful to the original material, leaving no doubt about the results.

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With taper compensation, the software controlling the jet understands the way the waterjet behaves and places the taper on the remaining stock rather than the cut product.

Time savings

One manufacturer used four CNC machines to produce dog bone coupons for ASTM E8 tensile testing. The company required seven dog-bone coupons for every heat-treated lot, and the preparation process took more than 45 minutes per coupon. Delays in testing time impacted part shipments to customers or rework based on nonconforming product lots. When the manufacturer adopted its waterjet to prepare test samples, it was able to cut parts ready for testing in about five minutes. Similar applications for cutting coupons on a waterjet are ASTM E9-19 compression testing, destructive testing and cast porosity verification.

Another use of the waterjet for cutting coupons is Charpy testing. Depending on the material being tested, milling for Charpy testing can use up expensive bits; other methods impart heat, which changes the metal’s properties. Charpy testing is also very specific on depth and width of the Charpy’s coupon’s notch. ASTM E23, “Notched Bar Impact Testing of Metallic Materials,” under Section 7, gives exact specifications on variations of the notch and coupon required. With a waterjet, the notch, V and straight indentation can be made perfectly.

Destructive testing

In cases where destructive testing is required, cutting castings with CNCs or lasers is not ideal, particularly if the finished product comprises multiple materials, and because of the pockets and variation in density. Because there are no bits to change and the abrasive cutting medium is always being refreshed, waterjet cuts straight through nearly any material. Again, because waterjet doesn’t impart heat, the finished sample won’t be distorted like it would be with laser cutting.

With finished product destructive testing, there may be several different materials in a dense part. This can be an issue for CNC machines where different materials are cut with different bits at different speeds. Waterjets will cut cleanly through several layers of different materials without changes to machine setup.

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Waterjet software typically comes with standard ASTM shapes for quick test prep.

Process of elimination

More advanced waterjets perform taper compensation and etching, which allow it to be a complete machine tool for applications testing. What could take two or three separate processes to cut a dog bone coupon is now a single-step process.

Because waterjet is not a hard tool but rather a flexible cutting medium, there is a tendency for the jet to leave taper. However, with taper compensation, the software controlling the jet understands the way the head behaves and places the taper on the remaining stock rather than on the cut sample.

OMAX Corp.’s taper compensation intelligently adjusts the cutting angle at every position along the tool path, which increases the precision of the cutting edge, especially in arcs and during acceleration/deceleration.

Etching lot numbers on a sample is typically done in a costly laser etching machine, creating yet another step in the testing process. A few waterjet brands already have the capability to etch built into their software package. OMAX’s IntelliETCH can engrave patterns, logos or lot numbers on surfaces. The OMAX Controller varies speeds to etch the desired pattern. This is in addition to other standard etching and scribing tools. The ability to cut and mark your test samples with a single machine saves time and money.

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One customer switched preparation methods to abrasive waterjet, so they were able to cut parts ready for testing in about five minutes.

Clean and quiet

Abrasive waterjets are safe and comfortable to be around in a testing environment. When cutting under water, machining is both cleaner and quieter. There is significantly less material dust than with a saw. OMAX brand machines run at 76 decibels, making it about as loud as a household garbage disposal. In addition, since the cutting medium is water and garnet, it’s often safe (depending on local regulations and what material you’re cutting) to dispose of the water through a city water system, and the garnet can be recycled.

Versatility

One product that may fit well into the testing applications space is the ProtoMAX. With the development of the ProtoMAX, OMAX incorporated all the versatility benefits of large abrasive waterjet cutters into a small, sleek and economical abrasive waterjet that’s suited for quality assurance laboratories, prototyping and engineering shops.

With a cutting envelope of 12 inches by 12 inches, the ProtoMAX is powerful but compact. The pump and cutting table are on casters for easy relocation. Work material is submerged under water for quiet cutting that won’t disrupt work spaces. With a clamshell cover, the ProtoMAX also provides a safer environment. MM

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