Springs Fabrication ATG Work Featured in Enclosure Magazine
Originally printed in the Summer 2020 edition of the American Glovebox Society Enclosure magazine.
Radiation was the theme for the 2019 AGS Conference in Boston and presentations covered topics ranging from design for internal equipment, processes and radiation shielding, to ergonomics for glovebox operators.
Many factors such as these are taken into consideration when designing a glovebox shell. The resulting glovebox designs directly impact fabricators’ abilities to manufacture them, conform to the requirements, and meet cost and schedule requirements.
As a result of the multiple attributes that need to be considered, glovebox shells may have unique challenges that fabricators strive to overcome and this paper will dive into several of those difficulties and some possible solutions.
One way to combat this challenge is to add relief cuts when making the mechanical layout for the shell. As there is less material, there is less force required to form plate. Although helpful, there are other drawbacks to this. When removing large pieces of the shell in order to form the parts, they have to be replaced and welded back in at a later time.
Corners in a 5/8-inch thick shell is the most complex part to form into shape. Not only is it difficult to properly form the pieces, but it is difficult to make the sides of them come together and line up for welders to have an even fit up. One way to prevent issues in the corners is to machine them out of raw billet and then weld those into the glovebox shell.
A major benefit to this alternative fabrication technique is that there is less welding required in the corners and fabricators do not have to manipulate the shell to properly fit up.
Assembling and sealing one glovebox shell is an accomplishment, but to assemble multiple boxes together and seal as a lineup is an entire challenge of its own. In order to achieve this, it is important to ensure the mating flanges are fl at and perpendicular to the shells they are welded to. Meeting tight tolerances on flatness and perpendicularity with conventional welding techniques can be done, however, it is difficult.
One way to ensure flatness and perpendicularity is the use of post-weld machining. Fabricators can weld thicker flanges onto the end of glovebox shells and then machine flat, parallel to the other side, and perpendicular to the bottom or top.
This technique does not negate the fact that fl anges have to be welded on to shells reasonably flat and perpendicular. Only so much material can be removed to meet the requirements and maintain minimum thickness requirements.
Another suitable way to maintain fl atness on end fl anges for enclosures is stiffening structures. This can be achieved with a bolt on stiffener design or one that is incorporated into design and welded to the shell. These structures are most helpful when it comes to shells with large panels.
There is a tendency for large wall panels to bubble. Long seam weld lengths and the weight of the material causes it to bubble. Stiffeners provide a structure suitable to maintain shape while maintaining containment; thus, making large lineup sealing achievable.
A persistent challenge in glovebox fabrication is excessive distortion as a result of containment boundary welds. In order to get a containment quality weld, there must be complete joint penetration, which means the weld melts through from one side of the base material to the other side. If not well-planned, excessive distortion can ruin an enclosure and make it irreparable for sealing.
One way to prevent excessive distortion during welding is to design the mechanical layout so there are no long shell seams. Sometimes it is unavoidable due to size and material availability, but it is important to lay out a glovebox shell as symmetrically as possible and with as short of seams as possible.
In order to evenly distribute shrinking during welding, it is important to make the shell layout such that the sides and ends have as much symmetry as practical and as few seams as allowable. If there is a concentration of heat in one section of the shell, the material will pull that direction and create a parallelogram rather than a square box.
The example provided is an enclosure shell with unique flat patterns and minimal weld seams. As shown, the corners have 45-degree seams. This slight change can take a total weld length from 12 inches down to 8.5 inches. For complex weldments, small adjustments to flat patterns can make a significant difference.
Fabricators typically use aluminum barring during welding to prevent shrinkage. There have been recent developments in welding equipment that make a signifi cant difference in heat applied to a full penetration weld. Heat Input is the energy put into the part during welding; it is calculated using travel speed, voltage, and amperage. New equipment being developed such as TIP TIG has provided a way to achieve full penetration welds in a fraction of the time.
TIP TIG is different from typical manual TIG welding because it is a hot wire process whereas manual TIG welding is not. In manual TIG welding, filler metal is maintained at room temperature and energy from the arc is used to melt both the base metal and weld wire. This slows down the process and creates a greater probability of defects in the weld. TIP TIG equipment sends a hotwire current to the filler metal, preventing the weld bead from cooling upon deposit and putting up to 40% of the energy in through the filler. This provides a quality weld and minimizes chance of lack of fusion.
On certain weld seams, full penetration can be achieved in a single pass.
For other enclosure applications that do not require the full penetration welding for containment purposes, other weld fit up designs are effective in preventing excessive distortion. Slot and tab weld design has become increasingly common. With precision cutting equipment available, fabricators are able to take advantage of tight-fitting parts to weld.
The example provided is a pharmaceutical enclosure that is not required to maintain containment, but to hold pressure. The slot and tab design on this shell provides tight fi t up for fabricators. This is beneficial for a couple reasons, one, it provides a guide for fabricators during fit up. Two, it maintains a tight fi t so when welding proceeds, the parts hold each other in place. This is not commonly used on gloveboxes, but has been effective for pharmaceutical applications that do not require radiation shielding.
The final challenge this article addresses is fabricating large, complex weldments. These are weldments that cannot be welded as a single shell; but more, an assembly of shell weldments to produce one containment boundary. Walk in enclosures are a good example of these. These can be assembled with bolted and gasketed surfaces or field-welded. The greatest challenge with these assemblies is welding and machining flanges in a way that they will come together with the other shell weldments and line up properly to assemble and seal.
To prevent mate up issues, fabricators can do post weld matchmarking and machining of holes. This is a time-consuming, challenging process, but it is the best way to ensure proper assembly and provides the best chance for leak prevention.
Enclosure designs evolve as applications and processes change and along with that, fabrication techniques also change. These are some of the most current, consistent fabrication challenges in enclosure building, along with various solutions.
Although the list can be continuous, the most critical aspects of an enclosure remain to be containment and sealing. There are various ways to achieve these, but it can require a combination of fabrication techniques in order to be successful.