Mitering Gap Tolerance - Lugs

[Marek - www.arkobici.sk]

Regards All.

From the little I know, I've always kept the tubes tight and work with the lug angles. Even if I hand miter no matter what. 

I've never had opportunity to learn from any framebuilder so I work with other frames and make frames ever since. Restoring frames gave me a lot, looking at the techniques and features. I've had few frame repairs also ST replacing of this Colnago as recently I had on my bench one of those newly-cheaply made so called steel is real lugged frame with cracked HT lug right on the junction of HT-DT
I knew it was caused by poor contact DT-HT - simply too much force on the lug instead of DT wall and lug wall. Am I right on this?

Now I've got to repair this Scapin with head on crash, TT DT totally snapped. Then I've realized sT lug was snapped on its long point and when was removing completely this lug spotted this gap all around TT not touching ST at all. All was held only by lug wall. 

And it's not the first time seeing this as I've cut two other frames in the past to see what are they like. 

I'd never doubt about it but is there a tolerance for mitering in terms of junction gaps? just wonder.

Many thanks

Marek. 

[Marek - www.arkobici.sk]

pictures

  

 

[Steve B]

I haven't got frame 1 out of the way yet , still learning to be a lot less heavy handed with the torch which was ok on mild steel hydraulic tubing, so take this as you will.

 

 I don't recall the exact number anymore, but I believe silver isn't good for joints with gaps bigger than about .005 in. What I'm seeing there looks like way way too much for silver, but maybe ok for brass if the braze gets complete penetration?

So probably too much gap, too little braze material to fill the voids. Pretty sad for a manufacturer. The test joints I did a while ago had better penetration, and I felt they were poor joints. ( Incomplete in the binder area with a small torch, and overheated with the real torch. gotta get back to practicing soon !)

Steve Birmingham

Lowell, Mass

USA 

 

[Johan Larsson]

I'm also curious about this, and would like to hear what someone with experience has to say. But of course a fully penetrated joint between the tubes is always a good thing. Good lugs are plenty strong though, and I've dissected a lot of old bikes and have seen really bad brazing and non-existent miters hidden inside lugs, and those bikes worked fine for many decades. It has been cheaper bikes though, margins are smaller on high end stuff when dimensions go down. I've never seen a cracked lug myself, only cracked dropouts and main tubes.

Johan, Sweden

[Mark Bulgier]

Steve B wrote:
> I don't recall the exact number anymore, but I believe silver isn't good for joints with gaps

> bigger than about .005 in. What I'm seeing there looks like way way too much for silver, [snip]

Steve, what you're saying is correct but not what Marek was asking about. The gap you're talking about is between the lug and the tube, where Marek is asking about the miter accuracy.

If the tube-to-lug fit is good for the filler you're using, then we know the miter doesn't need to be perfect or rammed all the way home, because the lug itself is often strong enough. We've seen plenty of examples of horrible mitering (much worse than the Somec pictures Marek shared) that held together for many years of pounding by a strong rider.

Conversely we have seen frames where a tube popped out of the lug, despite being properly mitered and inserted all the way so that the tubes were in contact all the way around. That can happen if the lug was too loose a fit for the silver being used – or inadequate pre-braze prep or any of a number of other causes. So we really do have two completely separate issues here.

I have seen frames where the lug cracked or ripped because the miter was bad, and the lug turned out to be *not* strong enough on its own. So don't get me wrong, I'm not saying good miters and full insertion into the lug aren't important. I'm just saying that *sometimes* you can get away with less than full contact between the miter and the tube it should be up against.

Since no one here is a mass-producer (that I know of), I hope no one here ever cuts corners to that degree. Miters in full contact, and fully brazed so there's even a little inside fillet, is not that tough a goal to achieve.

No one here should be satisfied to be only as good as a Colnago – with a little effort you can be so much better than that!

Mark Bulgier
Seattle

[Mark Bulgier]

Johan Larsson wrote:
> I've never seen a cracked lug myself [snip]

Here's an example, a nice classic Mondia cyclocross from about 1980.
https://www.flickr.com/photos/61312162@N02/27145813223/

If I think back on all the cracked lugs I've seen that were either proven or suspected to be from bad mitering, it would be a dozen for sure, probably much more.

Mark Bulgier
Seattle

[Steve B]

Cool, Learned a bit more of the important stuff today :-)  It's a bit too big, but I found a pic of that second try. Nailed the miter and got really great penetration, but totally cooked everything.

Ah, the bad habits from working with really forgiving materials for a few years.

I wonder if the lug being IC or stamped makes a difference? Not that I'd want to cut that corner. A good miter is pretty easy even with hand tools.

Steve Birmingham

Lowell, Mass

[Eric Keller]

there was a forum thread where someone cut apart a number of
production frames from bitd. In a number of cases, they didn't bother
to miter well at all. Sometimes it ended up with cracks, but in most
cases there were no problems at all. The example in the OP is much
better than many bikes. I think the idea of lugs was originally that
you didn't have to miter, but then the lugs started shrinking.

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[Jon Norstog]

I'll second that.  I've taken apart a few frames, including some expensive ones.  The level of workmanship is often a little, aaah, shoddy to my way of thinking.  And a lot of them are not all that well aligned.

That said, just do your best and you will be doing better than about 80% of famed clAssic framebuilders, no names mentined.

jn

"Thursday"

[www.arkobici.sk]

That's it Mark, I was asking about the tolerance for tube miter. I'd never thought frames were produced in such a manners. mass produced frames indeed.

That Mondia lug crack is massive, will you replace it? 

Thanks

[Mark Bulgier]

Arkobici wrote:
> That Mondia lug crack is massive, will you replace it? 

I’ve had that frame for years, can’t decide what to do. There are cracks in the head tube, down tube and lug, so all three need to be replaced. The only thing I really like about the frame is the paint job, and a repair would blow that away, so for now it just hangs around as a “cautionary tale”.

What would you do with it?

Mark Bulgier
Seattle

[www.arkobici.sk]

  The only thing I really like about the frame is the paint job, and a repair would blow that away, so for now it just hangs around as a “cautionary tale”.

What would you do with it?

One interesting fact came on my mind last night apart I've had hard sleep because of thinking about my work sort of: what if ... end so on (that's probably logic if one builds frames) 

I often meet people who would be into bikes and cycling for decades, that making frames is such an easy cake! Almost all I've spoken to, see lugged frames as simple as tubes sleeved into lugs and brazed. No angle mitering, no gap reduction, just like you do plumbing!

 Back in the days, steel frames were produced in thousands so I guess the number often resulted in same as the pictures above. All low quality as no high end frame were available and therefore no space to compare. In former Czechoslovakia, this part of Europe was completely sealed from outside world and therefore no chance to come across any good bike.

 

Absolutely Mark, I'd prefer to keep the frame because of the paint too.  I've had a gent asking for repainting his 1972 racing frame but the tubes were in such bad state - extremely ridden but very nice paint and water paper decals - I've only washed it, cleaned of the greasy parts and put clear varnish over. Lovely wall hanger at his house. 

thanks

Marek.

 

[Dave Yates]

Now then Gentlemen

Apologies to any who have received this before, Google was using an old e-mail address that I no longer use and hence bounced this back from the list.

I have "lurked" on this list for many years, had some laughs and learned a bit. As a bit of background I have been building frames since 1976, professionally since 1980 under my own name, my business partners name Joe Waugh and the name of our company M.Steel Cycles. We employed nine people in the workshop in the late 80's early 90's. I have repaired literally thousands of frames in that time and endorse all the comments by the previous posters. Certain names crop up with monotonous regularity, I have dismantled many of the so called finest frames by well known names and been appalled by what I found. Having said that 99% of the jobs were as a result of crash damage and the frames had been giving good service up to that point. The pic Mark showed is a classic example of crap building on a production frame. It is a Prugnat S4 lug the like of which we used to use by the thousand. Pressed mild steel, nice fit on the tubes but needed a good fit on the mitres and full penetration of the braze material as the lug itself was next to useless as a load bearing component. I have been running courses for the last 12 years (not to produce framebuilders but to allow people to build a frame under close supervision) and the point I always hammer home is that the two tubes forming the joint must be stuck together independant of the lug, the lug simply acts as a locator. To achieve this the mitre must be a good fit.
Define "good" !!
It depends on your filler material, I have always used brass which will fill gaps much better than the higher percentage silver fillers. If you can get the joint to within about 10 thou visually most of the way round and get full penetration then that joint will never break under normal usage. Low melting point silver needs a slightly better fit. The trick is to feed the brass or silver in at one side and pull it through the joint until it emerges at the other side. NEVER EVER feed it from both sides, use gravity to help you by manipulating the frame during brazing and remember the filler will always follow the heat.


Cheers

Dave Yates

www.daveyatescycles.co.uk

daveyat...@yahoo.co.uk

--------------------------------------------
On Sun, 19/6/16, www.arkobici.sk <marek....@gmail.com> wrote:

Subject: [Frame] Re: Mitering Gap Tolerance - Lugs
To: "Framebuilders" <frameb...@googlegroups.com>
Date: Sunday, 19 June, 2016, 20:28

That's it
Mark, I was asking about the tolerance for tube miter.
I'd never thought frames were produced in such a
manners. mass produced frames
indeed.

That Mondia lug crack is massive, will you

replace it? Thanks