Broken Brompton hinge

[lkingscott]

As this frame's been modified a bit, I wanted to do something with the logo, but couldn't find anything off the shelf that I liked.

So I came up with a design I like and then had to think how I'd get it onto the frame. Initially I thought I could print on paper, then cover with self adhesive polyimide film (a high temperature resistant plastic film) with a release film and use a sharp knife to cut around the lettering. This would have worked, but to get a good result would have been rather tedious.

I have a cheapo Chinese laser CNC with 10W diode laser, so browsed YouTube videos, initially with the idea of using it to selectively melt fresh powder coat, but on seeing the mixed results, discounted that and next thought about cutting the stencil from the polyimide film with the laser.

This could work well, I found initially that the laser can easily vaporise the film (see initial test photo). I forgot to photo subsequent tests with lower power settings, which cut very neatly.

However I then wondered about using the laser to just blacken the powder coat. I sprinkled some powder on a piece of aluminium and baked it to use as a quick and dirty test plate, so obviously the coating was a bit bumpy, as I discovered in my initial powder coating tests. The initial test at 10% laser power hardly marked the coating, so I dialled it up to 30%, which probably vaporised a bit too much, discolouring some of the surrounding coating in places and cut quite deeply into the coating where it was thicker. I'll run some more tests between 15 and 25% power and also multiple passes at lower powers and different speeds.

I don't know what effect laser marking on the round tube will have. I think I should run a test on a round tube before ruining the real thing!!!

The blackened coating seems as robust as the underlying coating, so a coat of clear powder lacquer over the top should be all that is needed. I'm tempted to laser mark the serial number with barcode similarly (the old label disintegrated on removal). The front tube had a seller label, so a laser marked logo could go nicely in its place.

[rogerzilla]

Bilt-Hamber Dynax S50 is what I use.  Runny, and comes in a big aerosol with a fabulous injection lance included.

[lkingscott]

Bilt-Hamber Dynax S50 is what I use.  Runny, and comes in a big aerosol with a fabulous injection lance included.

 

[lkingscott]

The test lasing the name onto the tube worked fine. I dialled the power down to 16%.

The pics are of a smaller diameter tube and zoomed right in the individual scans are very visible, but hardly noticeable on the real thing.

With some clear powder lacquer they should not be at all visible.

I think I might try the power at 10% again. When I do the real thing, so long as I do not move it, I can do multiple passes.

[lkingscott]

No I haven't stopped work in this, but I had to do some work on my every day trike.

I've mostly been cleaning everything else up, but I have refined the logo lasering. I increased the resolution and I worked out that the logo g-code I was producing was for some reason varying the laser power, although it should be solid black. I manually edited the file, making it full power whenever it is on and I can now vary the power of the whole logo with the laser cnc software. 

I also bought the seat post liner. After buying it, I realised it will need reaming, so once fitted, I'll take it to a local Brompton shop to be reamed. I thought about doing it myself, but if they do it, it's their responsibility if they mess it up.

Anyway, hopefully I'll have some pics with the logo burnt on shortly.

[rogerzilla]

I have the Brompton reaming kit.  It appears to be trivially easy.

[lkingscott]

I have the Brompton reaming kit.  It appears to be trivially easy.

Yes, dead easy IF you have a suitable reamer!!!

I looked at sandpaper on a tub or making a simple reamer from 32mm tube, but as the Brompton shop isn't far away, they should be able to do it.

[lkingscott]

Waaahhhh.... I found another crack!!!

Luckily it's not on the main frame, which is now powder coated. See the pics below. I'll braze it up. Shouldn't take long, just yet another issue...!!!

It actually looks as though the crack either runs ender the brazed joint, or right along the edge, as there's some rust there. However extends beyond the end of the brazing, heading off across the tube.

I also made a mistake on one side with the laser marking, so had to recoat part of it. It's created s some orange peel around the edge of the recoating, but I'll flatten that slightly then clearcoat the whole frame.

[lkingscott]

While I was discovering all that, the laser was remarking the other side of the frame.

I think it came out pretty well.

Now it just needs a clear powder coat. I'll do the rear frame, then clear coat everything.

[lkingscott]

Crack brazed, corrosion ground away and the various scratches flattened out.

Just a degrease and I can powder coat.

[lkingscott]

Here's the coated frame while baking and when finished. The coating is darker than expected, it's metalic gunmetal colour.

[rogerzilla]

Those rear triangles rust from the inside out; despite claims by Brompton, the open tubes are not well protected, or haven't been over the years.

[lkingscott]

Those rear triangles rust from the inside out; despite claims by Brompton, the open tubes are not well protected, or haven't been over the years.

I've got some rust proofing wax to spray in all the tubes. It's made for enclosed car panels, but should do the job.

[lkingscott]

I've just completed the clear powder coat layer on the rear triangle frame. I'm REALLY pleased with the clear powder coat. I hope it performs as well as it looks.

It gives the metallic colour coat an appearance of depth and a nice gloss. I'm not sure whether it comes over in the photo.

I've not coated the luggage rack and supports, but sanded parts of it back to aluminium and I think I'll just clear coat it.

The FLIR photo shows the temperature as it's cooling down, but I used the camera to make sure it got up to temperature and not too hot, for the duration of the cooking. The coating has to melt, but then stay hot for 10 minutes or more to allow the coating to cure. If this dwell time isn't applied, then the coating will not be as resilient. Acetone can be used to test whether it's cured.

[lkingscott]

The main frame is now lasered and clear coated. I'm really pleased with the result, although I think I might have overheated part of it and the clear coat has bloomed a bit which you can see close to the main hinge as it looks a little lighter. If I have time I'll try polishing it out.

The lasered name has come out nicely. I could have used the standard Brompton, but I thought this would differentiate it.

I found a laser setting which just took the blue colour out of the coating, leaving it more golden. However the distance from the laser affected the colour significantly, whereas only 1% more laser power would produce a more consistent black, which I used in the end.

[lkingscott]

I'm finally putting everything back together. I've started with the rear frame, including upgrading the mini wheels to inline skate wheels. The ones I got may be a bit big, but we'll see. The ones at the back fitted fine, but then the real light fouled them.

The seat tube braze is hardly visible now. I left it proud for additional strength.



The rear frame braze is only visible if you know what to look for. The pic below doesn't really do it justice, as it was a bit dusty.



The luggage rack came out well too. I just rubbed it down and lacquered it. The top I rubbed down to the aluminium to leave a brushed aluminium finish. I left a couple of deep scratches, otherwise I'd have needed to rub too much away.



And here's the rear frame assembled.

[Goldcrank]

Looks great!

[lkingscott]

Looks great!

 

[lkingscott]

I got a local Brompton dealer to ream the seat tube insert. I think they reamed it too much. The post is really loose unless the clamp is tightened quite a bit.

Ironically the shop said they couldn't do it for weeks, but leave it with them and they'd do it when they could. I therefore ordered an adjustable reamer, cost under €30 inc delivery but the shop rang today said they'd done it already and I only dropped it there yesterday, charge €20.

The sleeves come in pairs, so if this really is too loose, now I have the reamer, I can replace it and ream the other one.

I got a can of automotive enclosed panel interior wax, which included a flex tube with side spray nozzle. It's been brilliant to coat the interior of all the tubes.

The repaired hinge works really well and the rear hinge is amazing. I have no idea why Brompton continue doing it the way they do. The Brompton rear hinge bushes, although brass are actually plastic lined and the plastic is the bearing surface. The nylon side washers are supposed to minimise dirt and moisture ingress as well as taking up end float. Nylon top hat bushes do both jobs and the metric 12mm OD, 8mm ID fit perfectly, the compression holds them in nicely and after reaming provide a perfect fit. Countersunk stainless cylinder nuts with 8mm OD and M6 stud between will mean there is never again any difficulty to remove.

[lkingscott]

Almost finished!

I got the bike stand out to make it easier for assembly. As you can see I got the handlebars on back to front initially. I didn't photo or take notes when I disassembled, so I've been discovering where everything fits.

The rear frame has no clip on the suspension bush, so I'm using a big cable tie to hold it in place.

The main hinge alignment is as perfect as it could possibly be. I used the inline skate cylinder nuts in the end instead of the tube I'd made. They are a bit undersize, but when the bushes are compressed into the tube, they have no play.

After all the work on the powder coat, it's a shame I overheated it on the main frame. It looks fantastic, but cracks easily. Recoating would be a huge PIA.

[Paul]

Thank you so much for this thread and all the work you have put in to it, on top of all the actual work you put in to your Brompton.
It’s been a real joy to follow your - frankly bonkers - story. Now and again I have wondered what this has cost in parts and (nominally) labour, but that clearly was never the point. It is irrelevant that you might now have the makings of a Brompton factory in your shed, I think?
It’s also delightful that your saga was almost exactly as long as the season: A Brompton Winter.
Nice work, lkingscott.
 

[Kim]

This! ^

[PaulF]

Amazing stuff. Where did you get the wheels for the rack from?

[lkingscott]

Thank you so much for this thread and all the work you have put in to it, on top of all the actual work you put in to your Brompton.
It’s been a real joy to follow your - frankly bonkers - story. Now and again I have wondered what this has cost in parts and (nominally) labour, but that clearly was never the point. It is irrelevant that you might now have the makings of a Brompton factory in your shed, I think?
It’s also delightful that your saga was almost exactly as long as the season: A Brompton Winter.
Nice work, lkingscott.
 

Many thanks for the compliments! Yes it has been a bit bonkers, but that wasn't the point. The cost was not huge, just time. The cost of making the powder coat oven and buying the gun were the biggest costs.

 

[lkingscott]

Amazing stuff. Where did you get the wheels for the rack from?

Many thanks

Amazon!!! They are a bit bigger than I'd planned, but they suit it well.

I got the idea from this video:

https://www.youtube.com/watch?v=K-AZOjqeADw&ab_channel=VietPhan

The ones I got had 8mm centre holes, but the sleeve nuts for inline skates are generally 6mm. I therefore had to make some sleeves from 8mm stainless tube. Also to fit on the rear triangle, the frame is threaded for M6. I used M6 stud with nylock nuts and also had to make some sleeves for these too, however the wheels are a bit big, so the nuts stick out a bit and I will buy some bolts with flatter head, which will look better too.