How does one go about categorizing a universal shot-peening machine? Should this machine be able to process multiple part geometries without changes to basic design elements and with common or reduced set-ups? Multiple media capabilities? Process ID and OD of components? The list could be endless and so could the machine features. Though theoretically, everything is possible (for the right price), there are practical limitations.
This article will explore whether it is practical to design a versatile machine. Would there be unavoidable limitations to the design? Langtry Blast Technologies (LBTI) in Burlington, Ontario, Canada had viable answers to the above questions. LBTI has, in the past, catered to their customer requirements and supplied this machine. What follows is an applications-based response to some of the questions raised above. Due to the confidential nature of the project, customer names and part pictures are not included in this discussion. Included are some models to explain the machine concept.
The criteria for designing this machine are listed as follows:
1. Batch style (operator loads and walks away)
2. Large production runs (OD and ID) of cylindrical parts
3. Parts with blind holes
4. Small production runs (OD and ID) of longer cylindrical parts than above
5. Wide variety of part styles
6. ID and OD capabilities
7. Off-center holes and slots
8. Irregular part geometries
9. Metallic and non-metallic media
Given that the Aerospace market is going through several challenges due to drop in passenger volume, it has become critical for users of shot-peening equipment to pack all possible features into a single machine. This is also dictated by the lack of floor space at most aerospace companies, requiring engineers to be innovative in their approach to equipment design.
Included with the above-listed requirements were the more obvious ones, such as closed loop monitoring of air pressure, media flow, classification of media size, shape and finally, part and nozzle movement. As with most shot-peening projects, this machine was also designed to conform to commonly used specifications and audit requirements.
Large production runs
Addressing questions related to production rate is usually not straightforward in shot-peening applications. Thankfully, planes are not manufactured and serviced at the pace of automobiles, sparing most engineers the need to respond to this challenging question! The general solution to increasing productivity is to bombard a great quantity of media at the component. Unfortunately, this is not always possible, especially when faced with tight geometries and blind bores without the risk of ‘shot on shot’ possibilities instead of the desired ‘shot on part’ requirement. High production rate in Aerospace is generally seen when treating engine blades, but since the part geometry is fairly open, this does not usually pose a major issue.
The common solution is to process these parts on an indexing turntable with multiple satellite stations. Each satellite station is provided with independent rotation, ensuring complete exposure to all OD areas. LBTI was faced with cylindrical parts that required peening on the ID and OD. Some of these parts had through bores while others had blind bores. Their engineers solved the productivity and blind hole geometry problems with a conventional indexing multi-table solution, but instead of the nozzle lance traveling from the top into the bore, the lance was located below the satellite station and traveled inside the bore from bottom to top, oscillating between pre-set travel limits. Rotation of the satellite table allowed all ID surfaces to receive proper coverage. Large table spindles with liberal bores allowed a 1” (25mm) lance to freely travel through the hub into the part bore. Most importantly, gravity assisted the design in allowing free drainage of peening media into the recovery system, without filling up the blind bore as would have been the case with a vertical lance from the top. This concept proved successful for both blind and through bores.
As for productivity, the main table was fitted with eight satellite stations, permitting the operator to load all eight stations, close the work door, start the cycle, and walk away to perform other tasks outside the machine area. The table automatically indexed an individual satellite to the blast station for processing. The indexing multi-table design is a proven "workhorse" in the Automotive world, and is now successfully employed in Aerospace applications as well.
Small production runs
LBTI engineers approached this project by categorizing small and large parts by their length. Small parts, with greater productivity requirements, were typically within 30” (762 mm) and large parts upto 60” (1524 mm) in length. Long parts (between 30” to 60”) were loaded horizontally between rollers mounted on a horizontal frame, with the frame located on the tabletop. The rollers were powered to allow exposure to the entire part OD. Important to mention here is the fact that this arrangement was designed to peen both OD and ID parts simultaneously. ID areas were peened by an individual nozzle lance with two axes of travel. The vertical axis allowed the lance to align itself for different part diameters, and the horizontal axis transported the lance along the part bore. The vertical axis also allowed compensation for any potential nozzle droop, especially when peening small and long bores.
Wide variety of part styles
This is likely one of the most sought-after features in a "universal" shot-peening machine. Wide variety, in addition to the two-part styles described above, included irregular-shaped parts that often make one wonder about their location in an aircraft! But to add to an engineer’s excitement, complex parts do exist and also need to be peened. Going back to over 25 years ago, when nozzles were manipulated by carriages alone, such parts likely did not get peened, or were processed manually. In re-defined processes, these parts are also required to be peened as part of an automated operation. Parts with complex geometry require multiple axes of manipulation to access all areas at the required 45-plus degree target angle. However, we are here in 2020 when robots are widely employed in shot-peening applications. LBTI design incorporated a standard, pedestal style, six axis robot carrying two venturi-style nozzles for all OD peening activities. The robot was located outside the cabinet, behind the back wall and accessed the interior through a flexible seal. Though, with creative programming, the robot could be utilized for carrying a lance and peening ID areas as well, but this would have required increased floor space and possibly a larger model of robot, so the robot was mainly used for processing part ODs.
Parts with irregular geometry (picture trunnion braces in a landing gear, links, and other structural members) are placed horizontally on the tabletop with the robot processing all OD areas at the required target angle. Some components incorporate off-center holes that also require peening. LBTI utilized its proven rotary nozzle lance arrangement to attach to the end of the robotic arm in order to process off-centre holes. The robot was sized to handle the load introduced by this arrangement and allow for manual tool change when this feature had to be incorporated.
Fixturing plays a critical part towards the successful execution of such projects. Given the variety of parts involved, sizes and exposure requirements, LBTI decided to employ the capabilities of a 3-D printer to print easily adaptable fixtures for parts loaded on the satellite stations. Part fixtures allowed the majority of the OD and all ID areas to be exposed in a single set-up. Parts that required 100% of the OD to be covered had to be processed for a second cycle to allow for the fixture-masked areas to be shot peened.
Metallic and non-metallic media use
The design required two options for media in the peening process: (a) two sizes of cast steel shot, and (b) ceramic. The means to handle two shot sizes was fairly structured. The sizes were S70 and S230. Since there is a significant difference between the two sizes, the machine was able to avoid cross-contamination by employing a multi-deck vibratory classifier. Further, the peening system also incorporated a spiral separator to remove non-rounds from the useable peening media.
The obvious answer to handling metallic and non-metallic media in the same machine is to use a magnetic separator. However, cross-contamination from either side is not permitted by specifications. This is also the reason some large Primes do not subscribe to the technique of using magnetic separators in the machine and opt instead for two separate machines – one for metallic and the other for non-metallic media peening.
LBTI’s customer informed them about the low volume of parts that had to be peened with ceramic, leading to the choice of a separate cabinet for processing parts with ceramic.
Total solution in one – limitations?
The LBTI system design utilized all design standards used in multiple past projects into this universal solution for their Aerospace customer. Machines of this type are best suited when the end-user is faced with a ‘job-shop’ situation and volumes of individual parts are not high enough to justify individual machines. In a metal laundry (cleaning), it is common to see a table or a spinner hanger processing a variety of parts, demonstrating the versatility in the type of parts they are capable of handling. However, this is not as easy in shot-peening applications. In a shot-peening machine, especially one that is computer-controlled and built to conform to AMS 2432 or similar requirement, the cost of the controls system, as a percentage of the overall cost, is rather significant, so the temptation to capitalize on the controls investment and club multiple operations is understandable - in other words, a machine that does it all for the price of a single control system. It is the equipment engineer’s responsibility to lay out the limitations of this machine type and manage unreasonable expectations.
Limitations of this machine include:
This is a mainly air blasting concept, as this is the only way two different media sizes can be used in the same machine. A single blast wheel will flow at least ten times the media flow of a single nozzle and commercial classifiers are not designed to handle resulting volumes.
Fixturing could get onerous when working with a wide variety of parts. It is important that the end user takes ownership of the process and starts creating ‘families’ of parts that could benefit from common fixturing.
One size could never fit all, especially in sophisticated peening systems. That said, the above is an example of one size fitting most. Aerospace is destined to go through radical transformation post-COVID. Will this create pockets of specialization or require MROs to adapt by processing a variety of parts? – likely the latter rather than the former. The machine discussed here will certainly address this trend.
Langtry Blast Technologies Inc
5390 Munro Court, Burlington
Ontario, L7L 5N8, Canada
Tel. +1.905.681 2030, Fax +1.905.681 2814