brazing tolerances for practice work
My plan is to but some 1 1/8" 4130 with an I.D. of 1.009". I'd also get some 1" tubing. I'll be practicing brazing with 56% safety silv. I plan to cut rings of varying widths from the 1 1/8" tubing to braze onto the 1" to simulate lug work.
From my reading, lugs will be press fit with only a couple of thousandths tolerance. Will a nine thousandths tolerance be acceptable for practice work?
That would be fine, .058" wall is commonly used for sleeves and is what I assume you are talking about. Make sure you buy your one inch stuff with .035 or .028 wall to better simulate a real joint. use AO cloth to clean the dark coating from the tube before you start so that you can "see" the heat.
I didn't know it was .058" wall but looking at my supply list that is wall dimension. I like the idea of brazing two pieces of 1" together with a sleeve. A slightly different dynamic for a second way to practice.
Originally Posted by KNEEL
Thank you for the quick response.
I`d like to practice some mockup lugs too. Is that .009 in the ballpark of the normal gap between lugs and tubes?
Painthawg is practicing with 56%. Would it be silly to practice with 45% and try 56% for just a few before using it on a frame? Better to use 56% for practice AND on bikes? Any issues with 45% on bikes?
.009" is roughly double the clearance you should have when brazing with silver so you might want to experiment with brass. Tons of frames build with brass so no issues going that way other than it's easier to overheat the tubes if you are sloppy. 45% will also work but it doesn't flow as easily as 56% so it needs a little more clearance. Most of the available lugs these days are overly tight and you will have to grind on them a bit to get the clearance necessary for these less fluid brazing alloys.
Over the holidays I talked my brother-in-law into giving me a bit of guidance. On his recommendation I got a combined oxygen and MAPP torch. It is able to produce a lot more heat than I need but should be more versatile if I want to try any brass brazing. We cut some rings from the 1.125" as planned. My b-i-l is a tool and die man and had pretty serious reservations about "silver soldering" a bike together. He brazed a ring on and we ground off a couple of flat spots to verify that the silver was penetrating the entire joint. Then he watched as I did several more rings. After each one we would grind off parts of the ring/lug simulator and check for penetration.
When he saw how much penetration we were getting he began to get a bit excited about strength. As a test we placed the tube, on end, against the ground and my b-i-l used a 5/16 punch and a 24oz ball peen hammer to try and knock one of the rings off where we had ground through it to check the braze. After five solid hits there was no sign that the ring had moved at all. No hairline cracks in the silver joint. Nada. After that he blessed my endeavor. All of the folks that have gone down this path before me should be relieved. :-)
Here is a shot of a couple of joints after being ground through as a check. I have 15 more rings cut out to practice with. Maybe I'll be ready to order a frame kit by February.
I also need to add more hours to my study of geometry. Since my goal is to build a bike and not change the world with some revolutionary build, I will likey copy one of the frames I am already riding.