[CR]Re: Lugs ?

(Example: Framebuilders:Rene Herse)

Date: Mon, 26 Aug 2002 11:12:19 -0700
From: "Brian Baylis" <rocklube@adnc.com>
To: Bob Reid <bob.reid1@virgin.net>
Cc: classicrendezvous@bikelist.org
References: <B9900106.F7BB%bob.reid1@virgin.net>
Subject: [CR]Re: Lugs ?

Listees,

Originally I wrote this responce to Bob Reid regarding a question he had about mitering a top tube. I spent some time on this and thought that a little peek into how handbuilding is done (from only one perspective, there are zillions of methods I'm sure) might be informative and interesting to some on the list. This method has been in use since the early 70's in my shop. It has never failed me.

Bob,

Well you're obviously making progress. Great question. I'm impressed. This is a simple way I use, done by hand and eye with materials on hand.

Miter the head angle first. I use a scrap piece of head tube about 6" plus long with a centerline scribed on it. Just draw a 6" smooth file over the top of the tube while holding the file parallel to the ground as the tube is lightly clamped in the vice jaws (equipt with softjaws of copper or aluminium) horizontal to the ground. Might as well make a tube with lines scribed opposite of one another for mitering use. I also use this head tube piece to check the fit of the miter and it's centeredness. The centerline aids in checking this stuff out by eye. You will be scribing the same centerline on the TT and DT also. I use a bevel protractor to check the angle of the miter. You'll see that everything lines up when it's all done.

Once you have a head tube miter that is centered, fits properly, and is at the correct angle, clamp the tube in your 1" wood block in your vice parallel to the ground and hold the head tube to the miter and adjust it until the head tube is parpendicular to the ground. I assume you have a "good eye" because it is required in custom framebuilding and is plenty accurate in the places it is used. The fixturing is responsible for the precision beyond that point. This is what makes framebuilding so much fun and so rewarding. Anyway, once you have the top tube clamped in the vice with the plane of its' centerline exactly 90 degrees to the ground, you can scribe a centerline at the head tube miter both top and bottom (making sure to always keep the file drawing parrallel to the ground) and do the same for the length of the unmitered end of the tube, making sure that the lines continue past the ultimate length of the tube a ways in both directions so you have some referance as you continue to work on the frame. These lines are also used to check the centeredness of the lug points, which becomes more difficult as the length increases. At this stage, the plane of the centerline of the top tube that you were seeking has been established. Now you must miter the seat tube end of the top tube which is actually the most difficult miter to do on a frame. There are several reasons for this. First, one more element is added to the qualifications for a perfect miter at this junction. Now it must be centered, fit properly, be at the correct angle, AND be the correct length and in plane once it's finished. This is one of the best examples of who useful it is to work from a full scale drawing of the frame. BTW, I use a fine ballpoint pen to make my drawings on butcher paper 36" wide that I cut off a roll as I need it. The lines are fine and hold up well to age. Since you now have top and botton centerlines on your top tube and one end has been eatablished as to where the actual end is, you can measure your drawing and the exact length of the top line on the drawing is the length to mark on your tube. Turn the tube upside down and mark the length of the bottom line on your drawing on the bottom of the tube. I always mark the head tube miter as to TOP and an arrow as to the forward direction of the tube because it is easy to loose track of which is which sometimes as you handle the tube and check everything.

Now take an investment cast lug and use it to scribe the miter on the seat tube end after cutting the majority of the extra length off. Leave a good 3/4" or more past your marks or you will be cutting off some of the tube that should be there when the miter is done! Put the top tube in the block, position exactly perpendicular in the vice, slip the lug on and align the lug as perpendicular as possible, parallel with the head tube. You now have a pretty much in plane miter plotted onto the end of the tube. You should be able to walk over to your drawing and place the unmitered tube exactly over the drawing and see the ends of the tube align with your drawing and your marks on both ends. If not, you just prevented yourself from cutting the tube before it was too late. Actually I check the marks before I scribe or cut anything off since that's when you need to know that you have everything in the right place.

The miter at the seat tube is complicated a little bit additionally because a 12" bastard file makes a pretty close radius for an 1 1/4" tube but is too big for the 1 1/8" seat tube miter. A 10" bastard makes a radius slightly under the proper radius. Therefore, it is neccessary to creep up on it and it's better to start undersized than oversized. Once the metal is gone you can't put it back. Take it slow and careful and check the fit and angle (get to fit first and check the angle second) often. You can now check the in plane of the head tube and seat tube by clamping the top tube in the block vertically and "square to the world" by holding the head tube with centerlines against the finished miter at the head tube, and place the seat tube in the miter at the top and sight down to the head tube and check parallel. You will find that it is very easy to see if it is perfect or if the miter needs to twist slightly one direction or another. The centerlines and miters all work together to allow you to check everything as you go. When you are done the miter will be at the correct length, proper angle, fit perfectly and be in plane. You can now walk over to your drawing and place the mitered tube over your plans and it will fit EXACTLY within the lines of your drawing. BTW, for precise scribing marks using investment cast lugs as patterns (pressed lugs work too but IC are easier to use), use the #11 Exact-o knife blade as a scribe. Nothing else will get in there close enough.

I think that does it. If you have any more questions let me know.

Brian Baylis
La Mesa, CA