The Transmitter Rack: A Lesson in Distortion Control
My journey into metal fabrication began when I bought my first MIG welder to repair automotive sheet metal on a classic vehicle restoration. For years, I picked up the torch on and off between other projects, gradually improving with each build. I knew how to make a solid joint, but I hadn't yet been truly tested on precision tolerances.
That changed when I tackled my first major structural build: a custom 19" equipment rack for a local high school radio station. Constructed from 1" x 1" x 1/8" ASTM A500 square tubing with 1/8" thick steel rails, this project put my understanding of weld distortion control to the test on a rigid frame.
Over a decade ago, I was tasked with fabricating this custom rack for the station. Today, it’s still housing the core equipment without a hitch. Here is a look at the design decisions and fabrication process that ensured everything stayed square, solid, and reliable over years of continuous service.
That changed when I tackled my first major structural build: a custom 19" equipment rack for a local high school radio station. Constructed from 1" x 1" x 1/8" ASTM A500 square tubing with 1/8" thick steel rails, this project put my understanding of weld distortion control to the test on a rigid frame.
Over a decade ago, I was tasked with fabricating this custom rack for the station. Today, it’s still housing the core equipment without a hitch. Here is a look at the design decisions and fabrication process that ensured everything stayed square, solid, and reliable over years of continuous service.
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Having little experience or knowledge about rack mounted equipment, the first step was figuring out the exact requirements. The station needed to house the main LPFM transmitter, the EAS system, a multi-channel relay, digital HD exciters for main and sub-channels, and leave room for future expansion. It also had to be mobile, requiring heavy-duty wheels to roll easily out of a corner location.
After browsing eBay, I sourced pairs of 36-inch long, 1/8-inch thick, 20 RU low-carbon steel rack angles pre-drilled and tapped for #10-32 screws. To ensure the rails would stay perfectly parallel during welding, I ordered two 3 RU blanking plates to screw to the top and bottom of the front and rear rail pairs as temporary alignment jigs. With the hardware secured, I drew up a CAD model in SolidWorks. |
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With the design set, I sourced the steel through my cousin's metal fabrication shop using a precise cut list for 1” x 1” x 1/8” A500 square tubing. To achieve a cleaner aesthetic than a standard butt joint, the upper and lower frame members were cut with 45 degree bevels.
While waiting for the rack rails to arrive, I started fabricating the top frame. Although complete joint penetration (CJP) wasn't required for this application, I beveled all edges to 45 degrees so I could grind them completely flush for a seamless finish without sacrificing strength. I also hit the inner corners to create a flatter fillet profile. My workflow was simple: fit each joint tightly, tack all four corners, check for square, and lay down the final welds. |
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When welding square frames, alignment is critical and distortion is the main enemy. Lacking a guaranteed flat worksurface, I used pieces of 1” x 1.5” steel bar as alignment bars, fastening them to the top and bottom of the corners with C-clamps prior to tacking to keep everything flat. I started with two members, aligned them at 90 degrees using a framing square, secured them with C-clamps, and tacked all four corners of the joint before repeating the process for the opposing side.
Once the tacks cooled, I re-checked squareness. Because cooling weld puddles shrink and pull members out of alignment, minor adjustments were necessary. Working in my now late grandfather's shop, I used his large bench vise and plenty of leverage to bend any slightly skewed members back into true square. Finally, I aligned the two opposing "L"-shaped frame halves, cross-measured the outer dimensions in multiple places, clamped them to the flat steel bars, and locked them in with final tacks. |
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While completely eliminating distortion is nearly impossible, managing your weld sequence—both order and direction—makes a massive difference. Keeping the pieces clamped while cooling also helps lock in alignment.
I started by clamping a corner and welding the outside corner joint downhill. Downhill MIG puts less heat into the metal than uphill, which means less distortion, and outside corner joints typically pull the least as they cool. Because I wasn't rushing against a production clock, I let every single weld cool completely to the touch before moving on to the next. From there, I welded the top and bottom from the inside out. The end of the weld puddle is always the hottest zone; by terminating on the outside corner, the most distortion-prone point of the weld is already locked in place by the preceding passes. I saved the inner fillet welds for last, as they would be welded unclamped, and so would have to fight against the rest of the completed joint structure to distort. |
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Once the tops, bottoms, and outside corners were complete, the frame was largely locked into position. I removed the clamps, bumped up the voltage and wire-feed slightly, and ran the inside corner fillet joints unclamped. Because of the inner bevels, the deep penetration actually pulled a bit less than a standard fillet, due to the shortened lever arm available to the shrinking weld metal.
As a rule of thumb, steel butt welds require roughly 1A per 0.001 inches of thickness, translating to about 125A for this project. I used a Millermatic 135 transformer welder, which maxes out at 135A. Since these older units lack digital displays, I relied on the chart inside the cabinet door to set the dials. Because the joints were beveled, I backed off the settings slightly for the tops, bottoms, and outside corners while still achieving excellent penetration. For the final fillets, I pushed the machine right to the top of its chart. To ensure flawless, sound welds, every bit of mill scale was stripped from the joint areas, and everything was wiped down with acetone right before striking an arc. |
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Once all the welds were finished, I ground them flush using a 4-1/2” angle grinder paired with an 80-grit flap disc. To refine the corners and give them a professional look, I hand-filed the radii to match the factory-formed corners of the square tubing. With that cleanup complete, the top and bottom frames were officially finished.
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With the rack rails finally delivered, I prepped the top section to mount the transmitter, using a drill press to machine 82 degree countersunk holes for its flat-head screws. To give the final assembly a clean, seamless look—making the rails and uprights appear as one solid piece—I also drilled out the existing inner holes to 3/8” roughly every six inches to allow for plug welding the rails to the uprights.
Once plug welded and ground flush, I turned my attention to the uprights. Since the fronts and rears of the rails needed to be in the same plane, I temporarily screwed the eBay purchased blanking plates to the top and bottom holes, taking frequent measurements to keep everything square. Knowing the shop floor wasn't perfectly flat, I supported the frame using pieces of 1” x 1-1/2” bar stock so the thin blanking plates wouldn't flex. Finally, I tack-welded a piece of ¼” flat bar across the front to lock the alignment in place. I then removed the temporary blanking plates and repeated the process for the rear rails. |
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Next, I stood the uprights vertically and balanced the top frame on top, using shims to compensate for the uneven floor while constantly checking alignment with a framing square. Once everything was sitting true, I carefully held the assembly in place and tack-welded the front and sides of the top frame to the uprights to lock in the geometry.
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With the top frame securely tacked, I flipped the entire assembly over and repeated the process for the bottom frame. Once everything was tacked, I removed the front and rear braces and smoothed down the tacks with an 80-grit sanding disc before laying down the permanent welds. To minimize distortion and heat input, I started with the fronts and sides, jumping around the frame rather than welding continuously, and finished up with the internal fillets. Just like the upper and lower frames, the ends of the uprights had been pre-beveled for a tight fit, allowing me to grind the final welds completely flush for a professional, seamless appearance.
After finishing all the welds, I ground them flush using the sanding disc. While flap discs remove metal quickly, I prefer a sanding disc for finishing due to its superior control, smoother finish, and easier blending. Finally, I hand-filed all the outer corners to create clean, uniform radii. |
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The next step was mounting the casters: two locking swivel casters in the front and two fixed casters in the rear (though on later projects, I've shifted to using four swivels for much better maneuverability). The caster frames were zinc-plated, which is a major hazard for welding. Breathing in zinc fumes causes metal fume fever, leading to severe, multi-day flu-like symptoms.
To mitigate this, I ground away the zinc plating in the immediate weld zones using a pneumatic angle die grinder and an 80-grit roll-lock disc. With the casters positioned, I plug-welded three holes on each caster directly to the frame. As extra safety precautions, I wore my standard P100 respirator, worked right next to the open shop door, and held my breath whenever my head was near the puddle. The faint white powdery residue left on each plug weld is the telltale sign of the remaining zinc. |
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With all the welding complete, I stripped away the remaining residue and oxides using a drill-mounted 3” steel wire wheel. Then came the moment of truth: I flipped the entire assembly over and tested it out. Success! The frame rolled smoothly and sat completely stable on all four casters.
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The next step was prepping the frame for paint. I started by carefully masking off the casters using Ziploc bags sealed with duct tape—a bit makeshift, but it got the job done with what was on hand. Next, I used acetone and paper towels to scrub away all oils, grime, and surface rust to ensure a clean bonding surface for the primer.
While heavy mill scale is notoriously bad for paint adhesion and should normally be stripped, the layer remaining on the square tube was thin and smooth. Since the rack was destined for indoor use only, skipping the extra grinding saved a lot of time. With prep complete, I moved the rack onto a pallet outside and got ready to prime. |
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For the primer, I applied 3 coats of Rust-Oleum Automotive Primer-a modified alkyd spray-can primer with good metal adhesion and solid fill-waiting a few minutes between coats for proper flash time. After letting the primer cure for 4-5 hours, I lightly sanded it smooth with 600 grit sandpaper to prep the surface for top-coating.
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For the topcoat, I initially tried using Rust-Oleum gloss-black enamel, but the finish turned out blotchy and uneven. Getting a consistent, smooth gloss with a standard spray can without extra post-finishing is notoriously difficult. I quickly pivoted to Rust-Oleum flat black enamel—an oil-modified alkyd resin spray paint that lays down a remarkably smooth and attractive finish. By applying multiple light coats, I achieved a gorgeous, even look without needing any additional buffing or finishing.
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Once transported to the radio station, it was time for the ultimate test. With the transmitter mounted on its sliding rails and secured in the frame, everything operated smoothly without a single hitch, and all the auxiliary components fit into place precisely as planned. The entire project was a complete success.
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