Thermal Spray Manipulators Articles: Part 1, Taking Notice of the Manipulator

Taking Notice of the Manipulator
The first manipulator article, titled “Taking Notice of the Manipulator”, considers the question “why automate the thermal spray application process?” The first objective is usually cost reduction, but there are other concerns to take into account.
Taking Notice of the Manipulator
by Plasma Powder and Systems, Inc.Take a look at a typical coating specification. You’ll find extensive details about the powder chemistry, morphology, porosity, thickness, gun configuration, gas flows, currents, etc. However, you will usually find little, if anything, regarding the manipulation of the part and gun during the coating process. Where should the coating start? What path should be followed? Should the coating be sprayed as a single layer or multiple layers? On cylindrical objects, should the coating be achieved in axial or radial paths?Until recently, the manipulator has not been given the notice it deserves. This is probably because coatings were initially hand applied and path decisions were left to the discretion of the thermal spray technician. However, with the advent of automated manipulation of the gun and workpiece, it is possible for the engineer to take charge of the manipulator and to consider its potential role in coating processes.Plasma Powders and Systems, Inc. has written a series of articles on manipulators: turntables, gantries, robots, etc. These articles are primarily oriented towards management to highlight issues that need to be considered. The information should also be of value to the thermal spray technician and the manipulator programmer when considering options for setting up and operating a manipulator.In this first article, we consider the question “why to automate the application process.” The first objective is usually cost reduction but there are other concerns to take into account.

COST. A large production run, where the same coating is applied to parts in a production line, is certainly a candidate for automatic manipulation. For example, the spinel coating on hot-gas oxygen sensors is an ideal candidate for automation. In a production-line process, only simple gun motions are needed and one operator can monitor multiple guns. The control program can be basic and can use hard-wired logic (limit switches, cam drives, etc.) since program changes are infrequent.

HUMAN LIMITS. Some coating processes, such as HVOF, require high-speed motion between the gun and the workpiece. In the case of cylindrical objects, this is often achieved by high-speed rotation of the part, allowing the gun to be manually controlled. However, when the part cannot be rotated or moved rapidly, man’s limits dictate the use of a high-speed manipulator. Such was the case when a tungsten-carbide coating had to be applied to large hydro-turbines destined for use with river water with high silt content. The coating specifications required a gun speed that could not be sustained by a man. Therefore, a robot was selected for gun manipulation.

ENVIRONMENT. Sometimes it is necessary to apply coatings in areas where a man cannot work, such as coatings on repair weld areas in the hot section of a nuclear reactor. In this case, a tracked robot provides remote positioning and control of the thermal spray gun.

QUALITY CONTROL. Even though a man may be able to apply the coating manually, it may be preferable to have the coating applied under the automatic control of a manipulator with full documentation of the process. Metallic implants are often coated with hydroxyapatite. Quality control of each part is significantly improved by using an automated manipulator with automatic verification of the motions and paths along with other spray parameters. Many factors need to be examined when evaluating manipulation systems for maintaining a competitive edge in thermal spray operations. These are addressed in the second and next article in the series “Which Manipulator Is Best For Your Shop Or Application.”