Bend it, bend it ..... !!

Includes workshop practice, painting and weathering, model photography etc.
hughesp87
Posts: 115
Joined: Tue Aug 05, 2008 8:25 pm

Bend it, bend it ..... !!

Postby hughesp87 » Mon Jun 07, 2021 9:06 pm

I'm currently battling with a conundrum caused by the need to form a 3-arc roof profile for a diesel loco. The prototype is Danish and the model is in p87, but maybe this is a problem that someone else on this forum has resolved before.

The arcs at the edges of the roof are around 2.5mm radius, whereas the central arc is much shallower, as the following drawing and photograph show.

End profile.pdf
(61.56 KiB) Downloaded 38 times


DSCN1287.JPG


The material is an etch in 0.3mm nickel silver, which for much of the roof area is half-etched to reveal lines of rivets across the roof and a full thickness bead around all four edges.

I have a GW Models rolling mill, which will easily achieve the required radius for the centre arc, but the edges are beyond the scope of this tool. The only solution I could come up with was to solder the edges along a length of 3.2mm or 4mm diameter brass rod, clamp it lengthways in a vice, and use a piece of wooden dowel to carefully shape the roof around the rod.

The final profile can be achieved by soldering the roof to the existing ends and internal formers, or alternatively by using separate formers which might allow the roof to be attached to the body by magnets, as David Brandreth has successfully postulated.

Is this the best way to do it, or can anyone come up with a better idea?

Thanks in advance for any help offered.

Geraint
Geraint Hughes
Cromford & High Peak in P4
Danish Railways in P87

davebradwell
Posts: 499
Joined: Fri Jan 18, 2019 3:48 pm

Re: Bend it, bend it ..... !!

Postby davebradwell » Tue Jun 08, 2021 8:02 am

Most bending in our applications is done by clamping the piece to be bent in a vice with an appropriate bit of rod and pushing the sheet round the rod. It's unlikely your vice will be long enough for your roof and could well have serrated jaws so you will probably use a piece of flat steel or even hardwood and a bit of rod, say 4mm or slightly more in diameter held by an assortment of clamps.

Use narrow strips for trials - bits of scrap etch and cut them to exactly the same length as the width of the roof supplied. Curve in your rolls then assemble your bits of scrap for the bend. Yes, this is a fiddle requiring 4 hands. Check strip being bent is square to the rod and note measurement of long end which you will pull round the rod. Compare with roof formers, adjust dimension and repeat. You should be able to get an exact fit and some dimensions that mean you can do it repeatedly. Note also if overall length of strip is correct in case you need to adjust roof blank.

Finally bend roof - it will take more clamps and more effort, possibly using a bar to get the pressure close to the bending rod which may need clamping down (in vice, perhaps). The sheet will probably need annealing first if it's the usual half-hard supplied for etching. This will affect how much the metal springs away from the rod and should be considered first as the test strips will need to be similarly treated.

DaveB

User avatar
Guy Rixon
Posts: 833
Joined: Sun Mar 27, 2011 6:40 pm

Re: Bend it, bend it ..... !!

Postby Guy Rixon » Tue Jun 08, 2021 8:24 am

hughesp87 wrote:The final profile can be achieved by soldering the roof to the existing ends and internal formers, or alternatively by using separate formers which might allow the roof to be attached to the body by magnets, as David Brandreth has successfully postulated.


When I've tried to maintain a profile by soldering to ribs, against the spring of thin metal, it tends to pleat around the ribs. I fear for the flatness of your half-etched roof unless the profile is formed exactly before soldering.

What's worked for me is soldering to longitudinal stringers formed of angle-section brass, a couple of mm wide in the plane of the panel, and faired in to the profile by filing. This eliminates all the edges over which the panel can pleat. Your loco body already has slots in the middle of the roof to receive stringers, and the roof members could be notched lightly to accept L-sections. You'll need stringers at the cantrails, too.

User avatar
Will L
Posts: 2073
Joined: Sun Jul 20, 2008 3:54 pm

Re: Bend it, bend it ..... !!

Postby Will L » Tue Jun 08, 2021 3:09 pm

While its nice if you have the expertise to exactly match the curve on something to the place it is going to fit, it isn't actually necessary much of the time where there is a profile piece for the metal sheet to follow. Profile pieces you have lots of. Roughly (but gently) pre curving the metal particularly for the shape curves may be worth doing. But do it so it is still not curved enough. Its easier to bend it a bit further to fit than take out any over bending. You can consider annealing the metal so that it is significantly softer and easier to bend.

Thinking particularly of your roof, and given that its got etched detail on it which away makes bending a bit of a gamble, and that it would be well supported by the plentiful roof stringers and profile pieces, I would suggest the following
  1. Check carefully it is exactly the right width (that's what Dave's strips were for),
  2. Do anneal it (heat up to red heat and allow it to cool). (this should prevent Guys pealing problem)
  3. Do fit all the roof stringers, its now soft and it needs support. (but I doubt the need for L shaped ones)
  4. Solder the first edge in place along one side, but not at this stage to any of the roof profile pieces. (you don't want any strary solder between roof and profile piece
  5. Then just bend it over the profile of the roof using something flat to push it down onto the profile pieces. Tack solder it down as you go.
  6. Run all he joins when your happy

With the roof annealed it should do this no problem and is less likely to deform because of the etch detail. You might want/need to press a bit of a bend into the edges to so they roughly fit but don't over do it. The centre section can be left flat as it will bend to follow the required profile no trouble. You may want to be careful to avoid dropping things on the finished roof as it will be less resistant to denting than nu-annealed hard brass sheet.

You probably want to practice a bit before trying to make boilers this way.

Yes I think that nicely illustrates the difference between us kitchen table modellers and the proper engineers like Dave.

davebradwell
Posts: 499
Joined: Fri Jan 18, 2019 3:48 pm

Re: Bend it, bend it ..... !!

Postby davebradwell » Tue Jun 08, 2021 4:17 pm

It just seemed that using improvised bending bars is so quick and easy.......once you've found something the right diameter and if you can find someone to help you hold the bits while you do the clamp up. Unfortunately a pencil is too fat but something similar and an off cut of mdf and you're there. I did a Southern CCT roof in nickel silver like this once.

To bend it in situ you'd have to put stringers on edge halfway round the tight corner rads, as Guy points out, otherwise there's a good chance of it looking unhappy. You solder roof to side and bend - the first side is dead easy as there's plenty of roof material but the second gives you little to get hold of. Then what happens if end isn't at same height as start?

There seems to be some strange idea that the right way is somehow going to take an age and the amateur method much quicker. Somebody being paid is under considerable pressure to get the job done correctly at the first attempt with no reworking and will choose an appropriate method. It's just a pity you don't have my stash of silver steel offcuts.

When you anneal the roof avoid using too much heat or you can melt the edge - don't go past dull red if it's brass.

DaveB

Stephan.wintner
Posts: 33
Joined: Sun Mar 15, 2020 11:04 pm

Re: Bend it, bend it ..... !!

Postby Stephan.wintner » Wed Jun 09, 2021 4:55 am

I'm outside my experience. But, some speculation.

Take an overly wide strip, mark it in some durable fashion with five parallel lines on both sides. Not deep scratches though, that'd affect the stretch. Center, roof width, and a few mm beyond the roof width each side.

Now bend it, perhaps starting at one small radius and working across. Regardless, note where the marks are relative to the bending bar and resulting bend before and after bending.

Because it's too long, it's easier to work than the real roof. As Dave suggested, this gives you dimensions for the bending bars, etc. On the real roof, you'll need to locate the bends correctly relative to the edge - the marks allow you to determine what that relation should be, without needing to get it right the very first try. (It's not an issue if it ends up a bit off centered on the trial.) And, being too long, it's easy to see if it's curled too far, or not far enough (e.g. 50 or 90 degrees, when the finished part should be nearer 70 degrees, or whatever the case may be).

Once bent, a piece of paper could be used to compare those marks from earlier, both sides, and estimate the stretch.

Just a thought, perhaps it helps. I'm curious to see how you get on.

Stephan

davebradwell
Posts: 499
Joined: Fri Jan 18, 2019 3:48 pm

Re: Bend it, bend it ..... !!

Postby davebradwell » Wed Jun 09, 2021 7:55 am

There's also the 'rolling it on the carpet' technique as taught by, I think, Malcolm Mitchel for boilers. The circa 3/16" rod you'd be using might be a bit bendy, though. This mimics a technique for forming sheet involving pressing a former into rubber.

DaveB

User avatar
Will L
Posts: 2073
Joined: Sun Jul 20, 2008 3:54 pm

Re: Bend it, bend it ..... !!

Postby Will L » Fri Jun 11, 2021 9:07 am

davebradwell wrote:It just seemed that using improvised bending bars is so quick and easy......

My point was not that you shouldn't using something to apply an approximate bend, just that, as you have an available profile it didn't need to be anything like exact.
To bend it in situ you'd have to put stringers on edge halfway round the tight corner rads, as Guy points out, otherwise there's a good chance of it looking unhappy.

Don't agree, sheet materials bend easily in one dimension, this significantly reinforce them so they resist manfully any attempt to bend them in another dimension. Which is what would need to happen for the profile pieces to show through.
You solder roof to side and bend - the first side is dead easy as there's plenty of roof material but the second gives you little to get hold of. ]Then what happens if end isn't at same height as start?

I did say
Will L wrote:Check carefully it is exactly the right width (that's what Dave's strips were for)...
and...
You might want/need to press a bit of a bend into the edges to so they roughly fit but don't over do it.
To address these issues.
There seems to be some strange idea that the right way is somehow going to take an age and the amateur method much quicker.
My purpose, Dave, is not to po po doing it the proper engineers way, but to convince those who don't have /are yet to develop these skills nor posses the approved equipment, that this prevents them the from having a go or getting good results,
When you anneal the roof avoid using too much heat or you can melt the edge - don't go past dull red if it's brass.

Good advice. For those that don't possess a gas torch, but do have a gas hob, the danger of getting the metal too hot is much less likely

User avatar
Will L
Posts: 2073
Joined: Sun Jul 20, 2008 3:54 pm

Re: Bend it, bend it ..... !!

Postby Will L » Fri Jun 11, 2021 9:10 am

davebradwell wrote:There's also the 'rolling it on the carpet' technique as taught by, I think, Malcolm Mitchel for boilers. The circa 3/16" rod you'd be using might be a bit bendy, though. This mimics a technique for forming sheet involving pressing a former into rubber.

Essential the same as my, on your thigh like the Cuban ladies making cigars, method. The reason why the fact that my thigh isn't flat is not a problem is as, mentioned above ,that the metal only wants to bend in one dimension

davebradwell
Posts: 499
Joined: Fri Jan 18, 2019 3:48 pm

Re: Bend it, bend it ..... !!

Postby davebradwell » Fri Jun 11, 2021 10:28 am

There's no right and wrong way to do this sort of job and none of us are qualified to judge anyway. You just have to weigh up the potential disasters using past experience and choose the method that you think gives the best chance of success. Available tools will be part of the choice or you might just fancy a change from last time. On the day, of course, it's something unseen that trips you up but that's all part of learning - we're in a game where we learn by failure.

DaveB

PS surely the job's been done by now?

User avatar
Will L
Posts: 2073
Joined: Sun Jul 20, 2008 3:54 pm

Re: Bend it, bend it ..... !!

Postby Will L » Fri Jun 11, 2021 10:40 pm

davebradwell wrote:There's no right and wrong way to do this sort of job and none of us are qualified to judge anyway. You just have to weigh up the potential disasters using past experience and choose the method that you think gives the best chance of success. Available tools will be part of the choice or you might just fancy a change from last time. On the day, of course, it's something unseen that trips you up but that's all part of learning - we're in a game where we learn by failure.
.
Now that I can definitely agree with
PS surely the job's been done by now?

Not at the rate I'm working at at he moment I'm afraid


Return to “Tools and Techniques”

Who is online

Users browsing this forum: No registered users and 4 guests