Bar Clamps Build Process

3/5/2021

Having decided that everything is going to be TIG welded, the next job was to clean all the zinc off the threaded rod and nuts and to clean the mill scale off the box section and flat bar. Getting rid of the zinc plating is really easy as it's a thin layer that can easily be removed with citric acid, which is cheap and easy to buy in powder form (and I bought a big tub of it a few years ago). For the mill-scale, I could remove it with an angle grinder, but that's a really unpleasant process and the citric acid is a reasonably effective alternative.

As most of the parts I'm working with are long and relatively narrow, I used a couple of lengths of "square line" gutter as a citric acid bath. There's an end cap on each end of the guttering; I put some grease on the seals to help ensure the acid mixture stayed in the gutter, but I don't know whether that was necessary. I'd worried that these end caps are designed to stop small amounts of water (which is typically flowing away from the end cap) leaking through and I wasn't sure how it would cope with a filled-up gutter.

These photos show the box section and threaded rod before and after being in the citric acid "tank". I used about a mug full of citric acid powder in each bit of gutter and filled it up with cold water. It's much quicker with hot water, but I had a lot of pieces to go through and figured it would be cold for most of the time so it wasn't worth the effort of heating the water. The zinc comes off in a matter of minutes, but I tend to leave each set of pieces in the gutter overnight to be safe.

When the parts come out, they need a bit of immediate attention. The threaded rod just needs to be thoroughly dried (I use my air compressor to blow-dry them) as they will rust if left dripping. The box section needs to be wiped clean of any remaining oil and then needs a quick rub with a Scotchbrite pad. I used a maroon (360 grit) pad (and actually a "Hermes Webrax Hand Pad" rather than genuine Scotchbrite). I also use the air compressor to blow out excessive water from the inside of the box section.

The mill-scale on the flat bar (I'm using 3 mm × 30 mm and 3 mm × 50 mm bar) is a lot thicker than that on the box section. The citric acid doesn't completely shift it (at least in the time I've been allowing it), so I use a "non-woven preparation wheel" on the angle grinder to clean the last bits of scale off. I've tried using this wheel on a bit of the flat bar that hadn't been in the citric bath and it took much, much longer to get the steel clean, so it's definitely worthwhile putting it in the citric acid before trying to clean it. With hindsight it probably would have been easier to buy some 3 mm cold rolled sheet, but this would have been a lot more expensive.

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