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Tube coping/fishmouthing

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Tube coping/fishmouthing

Old 08-09-17, 11:49 AM
  #1  
Chaspar
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Tube coping/fishmouthing

How precise does the tubes have to fit togheter for a frame without lug?
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Old 08-09-17, 04:38 PM
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unterhausen
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it's fairly important. It can lead to frame alignment problems and fatigue failures if done poorly
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Old 08-10-17, 01:54 AM
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Originally Posted by unterhausen View Post
it's fairly important.

I think Unterhausen is understating by using the word 'fairly'. I see your 'fairly' and raise it to 'very'.

An early frame (#4) for my 9yo son had a very small - barely visible - mitering gap in the crotch of the DT/ST where it met an already brazed ST/BB joint. When I finished the DT/BB joint and it cooled, the front triangle closed up enough that the top tube was too short, BB too low, ST too steep relative to original design. it's clear when he was riding it that it isn't right. He doesn't have a reference point to say 'Dad, this sucks!' but, I could see it in the way it handles - it sucked.

So, I would say the miter in a non-lugged frame can make or break the end product from an alignment/geometry point of view. Tight miters also help make brazing easier and more consistent and a stronger joint.

With lugs, we've seen over the years, that some production bikes aren't mitered at all, simply cut to length square and inserted into the lug socket for brazing.
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Old 10-04-17, 05:53 PM
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ksisler
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From my opinion base, I put both tubes back in the tacking jig, line them up and lightly snug the clamps. Then I turn off the shop lights and stick a small LED MagLite down each tube, one at a time. If I don't see any light sneaking out at the intersection, then its is spot on perfect. If there is much light is sneaking out, it is time to do a bit of filing to correct the fit or to cut a replacement tube after sorting out where and how the error occurred... and just relegate the miss-cut one into the barrel for when doing a slightly smaller frame.
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Old 10-04-17, 05:58 PM
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Originally Posted by duanedr View Post
I think Unterhausen is understating by using the word 'fairly'. I see your 'fairly' and raise it to 'very'.

An early frame (#4) for my 9yo son had a very small - barely visible - mitering gap in the crotch of the DT/ST where it met an already brazed ST/BB joint. When I finished the DT/BB joint and it cooled, the front triangle closed up enough that the top tube was too short, BB too low, ST too steep relative to original design. it's clear when he was riding it that it isn't right. He doesn't have a reference point to say 'Dad, this sucks!' but, I could see it in the way it handles - it sucked.

So, I would say the miter in a non-lugged frame can make or break the end product from an alignment/geometry point of view. Tight miters also help make brazing easier and more consistent and a stronger joint.

With lugs, we've seen over the years, that some production bikes aren't mitered at all, simply cut to length square and inserted into the lug socket for brazing.
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Ditto Duane on all points. I am thinking that Trek was the first of the full production brands that really paid close attention to mitering on lugged frames (I remember from 1974 or so). But they did later develop investment cast lugs and fittings that accepted straight cut pipes into their sockets.
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Old 10-06-17, 01:16 AM
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Originally Posted by ksisler View Post
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Ditto Duane on all points. I am thinking that Trek was the first of the full production brands that really paid close attention to mitering on lugged frames (I remember from 1974 or so). But they did later develop investment cast lugs and fittings that accepted straight cut pipes into their sockets.
Yes, there have been several that I can name that probably attempted to remove the variability of tight miters from the equation. My guess is that was what was behind the Raleigh 753 bikes that had internally sleeved lugs. I never liked those bikes, although I have a deep fondness for 753 tubed bikes. Trek did something similar. I cut apart a Bianchi I was given with a broken top tube and it was appalling how wonky the miters were. It's all about the flow of units!

As I read my earlier post, I wasn't disagreeing with Eric, just that as a newbie and someone who doesn't use a jig, I find tight miters to be critical to a successful outcome. I'm sure the experienced builders can pretty much do whatever with whatever they are given and it'll be fine so for them tight miters maybe aren't as critical.

Last edited by duanedr; 10-06-17 at 01:21 AM.
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Old 10-08-17, 10:44 AM
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I must have given the wrong impression somehow -- I'm pretty obsessive about mitering. And don't forget that two perfect miters can be clocked, I've seen that in a cutaway frame. It's also easy enough to get them so they don't actually touch inside a lug. Actually, my theory is that the old timers made sure their horrible miters didn't touch the mating tube inside lugs so that there was nothing pushing the frame out of line. I don't recommend it, but it does seem to work. Plus, back then, they could just bend a frame back into plane. Nowadays you'll buckle a tube trying that.

I tried to miter on my lathe, but that was before I fixed the cross slide and it wasn't stiff enough. Kept breaking teeth on the saws. Maybe I'll try again now that I replaced the lead screw.
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Old 10-09-17, 01:57 AM
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Originally Posted by unterhausen View Post
I must have given the wrong impression somehow -- I'm pretty obsessive about mitering. And don't forget that two perfect miters can be clocked, I've seen that in a cutaway frame. It's also easy enough to get them so they don't actually touch inside a lug. Actually, my theory is that the old timers made sure their horrible miters didn't touch the mating tube inside lugs so that there was nothing pushing the frame out of line. I don't recommend it, but it does seem to work. Plus, back then, they could just bend a frame back into plane. Nowadays you'll buckle a tube trying that.

I tried to miter on my lathe, but that was before I fixed the cross slide and it wasn't stiff enough. Kept breaking teeth on the saws. Maybe I'll try again now that I replaced the lead screw.
-------------------------------------------
Unterhausen;

About 5 years ago I was contemplating getting back to building frames so I decided to replace my frame making tools which had all been lost due to a shop break-in some time before. After sourcing a new Thermadyne 0384-2045 Victor 510/540 torch set, a new TIG unit, a 14" chop saw, and a generous sets of good files, my budget didn't have much room left. I did a ton of reading and researching and searching as I had not bought tools for a couple of decades and didn't have a good sense on what was available. In the end I decide to move up to a tube notching system as all my prior years had been hand filed, but the hands aren't what they once were.

Here is my basic initial order list related to the notcher. I don't know if the prices are still good or not, but all the items have worked out well and are a lot faster and more accurate than than I could have achieved manually. At the time of the order, I did not jump into a larger quantity of the consumables as I was not sure I was going to really really start making frames again and as said, I needed to keep to a hobby level of budget.

<Source: Medford Tools and Supply <orders@medfordtools.com

Item Ref. Price ea. Qty. Description
====================================
101200 $279.00 1 Notch Master Tube Notcher
BIF-16 $9.95 1 5/8 fine tooth hole saw
BIF-19 $9.95 1 3/4 fine tooth hole saw
BIF-25 $10.95 1 1 inch fine tooth hole saw
BIF-32 $10.95 3 1-1/4 fine tooth hole saw
BIF-35 $11.95 2 1-3/8 fine tooth hole saw
BIF-38 $11.95 2 1-1/2 fine tooth hole saw
BIF-44 $12.95 1 1-3/4 fine tooth hole saw
BIF-51 $13.95 1 2 inch fine tooth hole saw
LEN-30857 $3.95 3 Hole Saw Adapber 1/2x20 to 5/8x18
SYNC-SCL114T $9.95 2 Snap Collar 1-1/4 Tube
SYNC-SCL14K $89.95 1 Snap Collar 14pc Tube Kit
T-WITS-19 $19.95 1 3/4 TWiTS
T-WITS-25 $19.95 1 1 TWiTS
T-WITS-29 $19.95 1 1-1/8 TWiTS
T-WITS-32 $19.95 2 1-1/4 TWiTS
--------------------------------------------------------------
Item Subtotal: $638.85
Shipping Cost: $34.01
GRAND TOTAL: $672.86>

Hope that helps a bit. If you hit the Medford site and give their tools a look if might aid your effort. There was a good bit of helpful information (in PDF's) also.

/K
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