Tumble deburring is a process designed to remove burrs from manufactured parts. Burrs can often hurt brazing as well as people. However, the tumble-deburring medium used ends up on the parts being brazed even though you may not be able to see it or feel it. Some common tumble-deburring media used today are aluminum oxide in pressed cone/pyramid form (Fig. 1), marble chips, stone, walnut shells and corn cobs. These all have a negative impact on the brazeability of the surfaces tumbled in them. All of these media are nonmetallic, and leave a nonmetallic residue on the surface, which will prevent brazing, or cause voids in the brazed joints. To eliminate this situation, only metallic media, such as small stainless steel balls, screws and nuts, should be used, or parts can be gently tumbled on themselves. The liquid in the tank should be some type of alkaline cleaner, which is effective for both water-based oils and mineral-based lubricants.
The same principles and considerations for tumble deburring should be used for grit blasting. Suppliers should not use aluminum-oxide grit to blast parts that subsequently will be brazed. Aluminum oxide is the basis of many common stop-off compounds, and effectively prevents brazing. Thus, braze shops should always approach such grit, tumbling media, grinding wheels, emery-type paper, etc., as functioning as a stop-off.
Some customers may inadvertently hurt a brazing operation because they do not understand what makes a good or bad brazement, such as fillet size, braze joint void content and joint design/application requirements.
Consider customers specifying large fillets. Some customers believe from past practices that large fillets are a primary visual criterion for a good braze, contrary to the fact that large fillets are unimportant for a good braze, but that attempts to create large fillets can be a waste of time and effort, leading to a lot of unnecessary rework, and part rejects.
Voids in brazed joints is another area of misconception. Some customers specify the amount of voids that will be acceptable in a braze joint or in a fillet. Such attempts to establish void-content criterion can create problems for both the brazing house and the customer. The reason: What is specified must be measured.
Every brazed joint will contain some voids due to wide variety of sources. Voids by themselves will not usually cause problems in service; some brazed components having up to 50% void content in the joint (fairly evenly distributed) have had no problems in service.
Inappropriate joint design for service conditions also presents a problem. Some customer designs for brazed components are not designed with brazing in mind, but instead, are based on welding-design criteria. A common example of this is fabricating a large diameter tube containing punched holes into which smaller diameter tubes are to be brazed. If the brazed joint will see further bending in subsequent manufacturing processes, it is not uncommon for the customer to demand large fillets to handle the stresses from bending, etc. The problems this creates for the brazing shop are trying to effectively and reliably braze tubing into punched holes in the walls of larger tubes, and then handle all the rejects and rework that may result due to the fractures occurring in those improperly designed brazed joints.
Brazing personnel need to work closely with both their customers and suppliers to ensure that:
- Parts to be brazed are properly designed for brazing at the start by the customer
- Customers understand brazing well enough so inappropriate demands are not made on the brazing process with respect to fillets, void-content, inspection techniques, brazing filler metal choices, etc.
- Suppliers are sending parts that are ready for good brazing (i.e., all lubricants, blasting and tumbling media, grinding processes, sand paper, etc., are acceptable for the brazing process
Controlling customers and suppliers makes your own in-house brazing operations much more effective. Which of the following two statements (identical in meaning) often mentioned in brazing shops describes your operation? “This process is not yet fully in control, but we’re working on it,” or “This process is out of control, but we’re working on it.” Few shops choose the second statement although it draws attention to the problem more quickly. The first statement sounds less panicked, but if a process is not in control, it is out of control. It is important to work closely with suppliers and customers to make effective brazing a shared goal, and add to everyone’s bottom line.