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Adjusting Lug to Tube Fit

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Adjusting Lug to Tube Fit

Old 10-27-16, 04:48 PM
  #1  
WheelNut2
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Adjusting Lug to Tube Fit

Hey Everybody,

I'm having some trouble with the DT-HT lug fitting correctly around my DT on the frame I'm building (only my second build). The angles are all good and I'm not modifying the shorelines of the lug, but the inner diameter of the lug is not round at all. I've got big gaps on the top and bottom of the tube (probably 1mm). I've tried to use a hammer and punch to bang the lug into shape, but it is quite strong in the areas I need to move downward. I could probably fill these gaps with brass, but I'd prefer to have the lugs fit nicely around the tube and use silver.

I need some tips on shaping lugs, please.

Some photos of the lug in question are attached (sorry the lighting is so bad I need more lights in the garage).
Attached Images
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IMG_4491.JPG (92.4 KB, 118 views)
File Type: jpg
IMG_4492.jpg (87.6 KB, 116 views)

Last edited by WheelNut2; 10-27-16 at 05:12 PM.
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Old 10-27-16, 06:55 PM
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Andrew R Stewart 
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Did the lug socket fit the tube properly prior to the bending? If not then the lug is wrong, but I doubt that IME. If the distortion is from your angle shifting this is to be expected. As the lug is very strong/stiff at it's root the hinge point will be further out the socket then the tube to tube contact point. This is why pre filling with brass, grinding the other side of the root to allow the second tube to shift within the socket or just brazing the joint with brass is often done. (When I built the handful of Terry type frames with 66ish* lower head angles I had to move the lug 4*-6* and used brass). But another option is produce a hinge point by cutting a dart out of the lug then either expanding of contracting the angle, then brassing it back and file/grinding the lug back to good prep state.This cut or wedge is inbetween the two tubes and goes much of the way through the lug but not all the way.


This is one more reason why being willing to practice with scrapable stock is a good thing. these methods and the skill to do them well (and look nice after) is not a turn key process. This is another reason that fillet brazing or making one's own lugs from 0.058 walled tubing that's 0.125 larger in diameter is done. This is also one reason why some builders still like pressed formed lugs and not cast ones. Andy.
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Old 10-29-16, 02:56 PM
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Harder to do with Investment cast Lugs , than stamped formed sheet Lugs..

Brass filler will be drawn in and fill gaps better (way) than silver (near fits needed)

Last edited by fietsbob; 10-29-16 at 02:59 PM.
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Old 10-29-16, 08:24 PM
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So I revisit this thread and add comment. In looking at the first photo the lug tip bending looks typical of initial lug angle steps. The next step i'd consider is to bend the lug's socket/shoreline on either side of the tip. This produces a cupped lug socket interior. The shoreline gets proper contact with the tube. The angle the tubes rest at is correct (if done well). but the interior of the socket has gaps like shown in the second shot.


Some will say that even with silver the lugged junction is so much overbuild that as long as a good fill with silver is attained the joint will be good. I certainly have done this and been OK with this before. But at some point one's questions will nag at one and a more full method will be sought. I suspect this is where you are.


Then more sophisticated methods as I mentioned are considered. The first is just to use brass. Of course this requires some practice if all you've flowed is silver. If you have formed the lug shore well and can file well then all is good for many angle changes.


The slitting method I referred to would be, in this case, hack sawing a slit of maybe a wedge out of the lug from between the top of the top tube and the top of the head tube. This cut would travel to at least half way across the tubes intersection and staying roughly between the shorelines of the TT and HT. Then the lug's sockets get bent to a tighter angle. Brazing this slot closed and filing/grinding to end up with both smooth exterior and tube fitting interior is needed before final tube fit up is done. Re contouring the shorelines might be needed too.


The last method is to build the bike twice, make your own lugs. Using brass one joins tubes of, usually, .058 wall and .125 larger diameter at the angles needed. Then file the fillets down to look as lug like as you wish then scallop the sockets too.


It will be interesting to see what you chose. Andy.
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Old 10-30-16, 06:50 AM
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is that a Pacenti lug? It looks really thick at the junction of the two tubes. I think you're going to have to thin it in order to beat on it successfully
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Old 10-30-16, 11:54 AM
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Do you have a "junk" tube you can insert? If so, stick it in the lug, heat the lug up so it's soft, and tap gently with a hammer to bring closer to the tube.
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Old 10-30-16, 08:58 PM
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Thanks for all the replies so far!

The lug is a Long Shen that I got from Nova as part of a classic road lug and tube set. I actually have not changed the angle at all, but I did have to do some bending to the get the head tube to clear. The part of the lug that sort of extends or projects from the DT was too tight to fit the head tube. The lower ring was okay though. Its possible that this manipulation deformed the top/bottom fit of the lug and ovalized it.

It seems to me that heating the lug and giving it a tap would be pretty effective. If that doesn't work thinning the top out to make it easier to manipulate might work. I suppose filling it with brass really makes the most sense. I'll goto the ol' metal shop this week and pick up some metal to mess with so I'll have a dummy tube to work with and I can setup some practice joints too.
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Old 11-02-16, 09:40 AM
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Modern lugs have a generous inside radius at the tube to tube junction- supposedly to facilitate filler flow to insure a good bond at the tube miters. The fit you see is common, especially on short point lugs.


It may be an illusion in the photo but it appears that the mitre angle is off a bit on the head tube, contacting the head tube at the bottom, but not at the top of the top tube. If the angle is such that there's full contact with the head tube, the additional inside radius will not matter. It should fill fine. Poor mitre angles will allow filler material to "leak" at the head/top tube joint and you'll never be able to fill it.


The mitres should be tight enough that when fixturing the tubes without the lugs one could fillet braze or tig the joint.


Head tube doesn't fit the lug well? I see some casting flash on the inside of the "ht ring". Proper lug prep may help.


To fit the points at the shoreline, the use of a lug vise, or a solid tight fitting mandrel can be used so you can massage the shorelines. A good solid backing in the socket will allow you to give it a good whack or two.


In Nova's own words for these lugs- "Because the sockets are very short, these lugs require precise mitering of the tubes.They will not hide poor quality miters. These are not recommended for beginning builders."


Tighten those mitres up and you'll be OK.
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