How To Mill A Bridge

A port bridge too big to fit a Bridgeport, that is.

Weymouth Town Bridge is a twin-bascule lifting structure, built in the 1930s I think, that in the past served mainly a commercial ship-repairer and paddle-steam operator, Cosesn & Co. That is long gone and the Inner Harbour - "the Backwater" to use its proper name - is now populated with fishing-boats, dive-charter boats and private yachts.

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Its deck asphalt originally covered cast-iron grids infilled with oak blocks, but this gives a rather fragile surface prone to breaking up earlier than on a solid hard-cored road. So quite some years ago now (1970s?) the iron and wood were replaced with steel plates stitch-welded to their under-frames. The welds are each about 150mm long.

The bridge is heavily-used, including by some buses,, and over the years the welds have deteriorated, started to crack and need replacing.

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This entails not grinding, but milling, the old welds out to the appropriate depth and profile.

The MIG welding is performed within a tent to protect everyone else including the public - one footway is still in use, alongside the fenced working area. I noticed the welder himself wears not only a full helmet but one with a filtered air-supply, that unit worn on a waist-belt.

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The bridge is still opened at at least one of the two or three daily scheduled times, which must be very inconvenient for the contractors. Probably quite annoying as it only really needs be opened for the tall-masted yachts - surely their owners could have had their pleasure-sailing curtailed slightly for a couple of weeks, to expedite the repairs?

With the agreement of the rather startled operators, I photographed their milling-machine, built, they told me, by the company itself. I'd disturbed their afternoon tea-break but without my even asking they obligingly moved out of their gazebo!

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As illustrated, Allen-screws secure the machine to the deck via faced steel blocks temporarily tack-welded to the deck itself. The long and cross-feeds have power was well as manual feeds, the cutter is held in an ER collet.

The photos tell the rest of the story, with some of the mounting-blocks and weld-grooves visible, surrounding objects giving scale, and a snap of the van sign-writing as credit.

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I should add I have no connection with the contractors Mactech (Europe) Limited, to whom I give thanks.

Nor to Weymouth or Dorset Councils or the Harbour authority, except as being a local Council Tax paying resident and services-user!

(Apologies for the black corners on 2 pics: sticky shutter-cover.)

Edited By Nigel Graham 2 on 28/10/2022 14:55:41

Fascinating! I see that a robot Henry was supervising the work.

Well done you, the only real hassle will be humping it about

Very impressive, and probably much more pleasant [for the workers and the public] than grinding would be

Thanks for for documenting it, Nigel

MichaelG.

An optimum proflie would be easier to achieve by milling than grinding. Notice the hydraulic motor for the mill spindle, and the powered leadscrews.

Edited By old mart on 28/10/2022 19:19:53

Edited By old mart on 28/10/2022 19:21:53

I once worked on a job where the equipment bedplates were cast into concrete structure, but had to be accurately aligned. The designers (Davy McKee of Sheffield, now gone I fear) just set them all high, and after all the concrete had been poured bought in a company who set up their equipment and machined the bed plates in situ. Most impressive. In their works Davy had a huge tee slotted table set in the floor. For machining big rolling mill side frames, they bolted the job to the table and then craned in various machines to bolt down and do the actual work locally.

What an interesting bit of kit!

How are they managing the bridge lifts? Do they have to pack up all their tools plus tent and vacate the bridge?

I was informed by a bridge operator that most lift bridges have to remain active, short of mechanical failure. Whether it be designated on demand or timed openings, maritime passage is protected by law.

Thanks for sharing

By Weymouth Harbour's own web-site, the The Town Bridge is opened at set times of the day from April to September; and by request with a minimum notice, October - March; so the work was timed to cater for this.

It spans the entrance of an enclosed harbour, not along a canal or strait.

I did wonder about the bridge lifts. I passed the harbour late yesterday afternoon, by bus, along a road some distance from the bridge. I could see it was open, just starting to close; but I do not know if for boat passage or in connection with the repairs. By then it was past 5:30 with the daylight fading, so the contractors had probably finished for the day anyway

The contractors seem to use part of the approach ramps to keep their vehicles, and as much plant such as the welding generator as possible, off the lifting parts, to minimise the equipment movement. The milling-machine and its fastenings might be robust enough to be safe to lift with the bridge. The gazebos would be a bit awkward but the rest is hand-tools or like the vacuum-cleaner, on wheels.

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I attach this photo with full acknowledgements to Weymouth Harbour, as it is from the official web-site so its copyright, but I don't think they'll mind me giving it free publicity! The view is up-stream, with The Backwater beyond. The actual town of Weymouth is on the left ( the estuary's true right bank); the much larger Melcombe Regis, forming the bulk of the commercial centre, to the right.

"Regis" after that renowned Ornamental Turner, King George III, visited the town for the sea-bathing his physicians thought might relieve his illness. (Items made on the lathe by Geo. Rex are in the Royal Collection.)

That's a neat bit of gear. Thanks for the pics.

Onsite machining is a big thing these days. And on big jobs with long straight welds, they have a similar looking machine that crawls along a pre-laid track and does the welding, no human hands involved! A perfect weld every time. It is how they weld submarine and ship hulls together these days.

Mining and marine industries use a lot of line-boring machines that are similarly portable and set up on the job. Quite intriguing to see a flimsy looking boring bar set up and running through a series of very large holes, but of course it is usually only taking out a few thou to correct ovality or to fit an oversized shaft or bush etc. And they even have portable lathes now that will machine a power station steam turbine rotor right there on the powerstation floor instead of trucking a 100-ton rotor across the country to a specialist machine shop. Pretty amazing stuff.

First time I have seen a mobile milling machine though. Yes, much tidier and quicker than angle grinding by hand.

If the welding process was MIG then the tent would be to protect the weld area from draft which would blow away the sheilding gas which would result in poor welds. HSE would need the area that arc welding is being done in screened to prevent rays from the process burning skin or eyes ! Know the bridge well. Noel.

Impressive machining there, Hopper!

Noel - the milling-machine is being used in a sort of heavy-duty gazebo, the welding in a tent; and a third reason for these is to keep the cut steel and the welds dry.

I did notice a large gas blowlamp among the tools, which I guessed is for drying the weld grooves if necessary.

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Was it on Tornado they realised they'd forgotten to bore out and tap the centre cylinder drain-cock mounting, after the cylinders had been mounted in the frames; so built a simple but effective horizontal-borer bolted to the frames, to complete the work?