There are a growing number of materials and methods used to build bicycle frames. This instructable is specifically about building steel bicycles frames. MIG, TIG, Lugged and Fillet Brazing are all common methods for joining steel bicycle tubing. The steps leading up to the actual joining of the tubing are very similar for these methods so that is what I will be covering in this instructable. It would be very difficult and beyond my abilities as a builder to try and describe how to effectively weld or braze. I recommend you learn to weld or braze from someone face to face and then practice (a lot). I personally learned to build bicycles through the United Bicycle Institute frame building class. Most of the information I give in this instructable I learned through the class and the literature they provided. They offer a really great way to learn frame building, I highly recommend you take one of their classes.

Here are some common acronyms I will likely use in this instructable:

HT - Head Tube
ST - Seat Tube
TT - Top Tube
DT - Down Tube
BB - Bottom Bracket
CS - Chain Stay
SS - Seat Stay

MTB - Mountain Bike

Step 1: Design

The first step in building a frame is to design the frame you intend to build. There are lots of different opinions, techniques and algorithms to sizing a bike frame to a specific rider. I will not go in to detail about bike fit.
There are many ways to draw your frame. You can use any old CAD software, bike specific CAD or a drafting table. Doing it on a drafting table is convenient because you get a full scale 2-D model of the frame before you build it. You can use this model to lay actual tubing down on to see how things are shaping up.
However you decide to do it, you want an accurate drawing of your bike frame. You will be using this drawing to take dimensions and angles from so make sure it is accurate!

I will be breaking most of the parameters used in frame design in to two categories fit and feel.
The fit parameters are determined based on the body of the individual the frame will be for. The feel parameters influence how the bike will feel or perform. If you want to learn more than I discuss about how these effect the feel of a bike take the UBI class or do some research.

Fit parameters (basically, what size bike are you building):
ST length (measured center of BB to where the center line of the TT intersects the centerline of the ST)
TT Length (measured from the ST TT centerlines intersection to the TT HT centerlines intersection)

Feel parameters (affects the comfort vs. efficiency of the bike):
BB drop (how far the center of the BB is below the axle line); influences frame stiffness, less drop = stiffer but less comfortable.
ST angle (measured clockwise from horizontal); affects weight distribution, shallow angle = more comfort but less efficiency.
HT angle (measured clockwise from horizontal); influences steering quickness and shock absorption, steeper angle = faster handling at the cost of shock absorption
CS length; also affects shock absorption as well as tire/fender clearance, longer stays means more shock absorption and more clearance for bigger tires at the cost of efficiency.
Rear axle over lock dimension (how wide the rear axle is going to be)

The best way for someone new to frame building to determine these fit and feel parameters (unless you have a fit bike at your disposal) is to take measurements of the bike you ride that fits you the best. Because you have ridden it you know what feels good about it and what you may want to change, starting with your current bikes geometry is a good way to go.
Another option is to look up different bike geometries online. Most bicycle frame manufacturers give frame dimensions on their website.

Other parameters important to your drawing/design
Wheel size; What size wheels are you going to use? (the important quantity is the bead seat diameter (BSD) or the diameter of the bead of the tire)
Common BSDs:
26” (MTB) 559mm
700c (road) 622mm
27” (old road) 630mm
29” (29er) 622mm
Tire Profile (this is the diameter of the tire profile i.e. the distance the tire extends beyond the BSD)
This quantity is usually given the by the tire manufacturer. For example 700 x 23 tires have a tire profile of 23mm.
Front tire clearance (how much room the front tire will have, bead seat to the bottom of fork crown)
Lower headset stack (how much room do you need for the lower headset bearings default dimension is 13mm)
Fork height (I am not going in to how to build a fork so you will get this number from the mfg of the fork you are going to be using. It is the distance from the center of the front axle to the base of the crown race seat (also called “axle-to-crown”).

Now time to pick your tubing dimensions:
Bicycle tubing can be bought from a variety of distributers. I personally have bought and built with True Temper tubing from Henry James. A great way to go for your first frame is to buy a kit that includes all the tubing for a frame. UBI sells Kasai tubing kits here (click the steel tubing tab). The numbers refer to the alloy type. Oversize and Standard refer to common tubing diameters.

Standard Road/Track Frame Oversized Road Frame

TT 25.4 mm 28.6 mm
DT 28.6 mm 31.8 mm
ST 28.6 mm (27.2mm seat post) 28.6 mm (27.2mm seat post)

Head tube diameter is determined by the size of steerer you want. 1" threaded steerers are the older standard, 1 1/8" threadless steerers are the more common modern standard. The steerer you choose influences the internal diameter of the head tube but the outer diameter is determined by the wall thickness of the head tube. They make thin walled head tubes for bikes being built with the added support of lugs thicker ones for luggless bikes.

It is worth noting at this point there are many options for BB shells. If you know all about bottom brackets, great, buy the shell you need. If you don't know much about bottom brackets get the most common 68mm bb shell.

Most bike tubing is butted. The wall thickness varies across the tube. The end of each tube has a thicker wall (stronger joints) while the center is thin (lightweight). For your first build I recommend you stick with thicker tubing like 1/.7/1 for example which has 1 mm thick butts and a .7mm thick center. (.9/.6/.9 is also a good option)
Tubing comes in different lengths and each piece can have different length butts. All this information is given by distributors and is important when selecting cutting your tubing. You want to make sure you don’t cut the butt off of one end of the tubing...

After you have selected what diameter tubing you will be using and have come up with all the necessary dimensions it is time to draw your frame. Compile the necessary parameters in to an easy to use list. This example is from the UBI frame building handbook given to me in the class.
Road bike:
Rim BSD: 622mm
Tire profile: 25mm
Tire diameter (BSD + 2 X Tire profile) 672mm
HT: 73°
ST: 73°
BB Drop: 75mm
Fork Length: 370mm
Chain stay length: 410mm
ST length: ?
TT Length: ?
Fork dimensions

Take your time and get it right. Draw a side profile and a top down view of the chain stays.
Precision is important.

Step 2: Tube Mitering

Once you have a accurate drawing of the bike you intend to build, it is time to start cutting your tubing. Well, first you need to order it. Get tubing based on the dimensions you get from your drawing. Make sure there will be plenty of butt length for each joint.

Frame builders call notching tubes to fit tightly together mitering. Tube mitering can be done a variety of ways. The cheap and tedious method is with tin snips and hand files. The expensive but somewhat simpler method is with a milling machine. Regardless of how you do it, the goal is for your tubing to fit together snugly at all the joints. It need to fit snugly with the tubing at all the right angles based on your design. Having a bicycle frame building jig is really helpful for this step. (See my instructable on how to build one here.) Jigs are helpful because they hold all the tubing at the right angles, you can miter a tube and throw it in the jig to test the fit. If it needs work you can pull it out, file it a bit and repeat.

When mitering a tube you need a series of miter dimensions from your drawing. The first miter I do is the ST BB shell miter. To do this with a mill you secure the ST in the mill vise (using v-blocks), install a hole saw that is the diameter of your BB shell (probably 1.5") in to the mill chuck, make a 90° cut off the end of the ST (make sure it is the right end, most STs are externally butted on the end that seat post is meant to slide in to). (Before you make the cut make sure you are perfectly center on the tube) This will produce a notch that should fit your BB shell perfectly. Throw the BB shell and the ST in to your jig. Throw the head tube in there too. The next miter I do is the TT ST miter. Following the same idea as the BB miter, I clamp the TT in the mill vise. Install the appropriate hole saw (the hole saw diameter should match the diameter of the tube you will be joining to). In my case the ST diameter is 28.6mm (1 1/8") so I installed a 1 1/8" hole saw. If you are building a bike with a horizontal TT and a 73° ST then the hole saw needs to pass through the end of the TT at 73° (you can measure this angle directly from your precise drawing). After you make this miter things get a little trickier. We now have to miter the other end of the TT to fit the head tube. It is really important that this miter be in exactly the same plane as the first TT miter. Take your time setting up the mill to ensure that this second miter is in plane with the first. We now not only need a miter angle but also a distance. Use your drawing to measure how far away the inside edge of the next miter needs to be from the inside edge of the first miter. Get the miter angle from your drawing as well. Make your miter. Use the same process for the DT. The part that gets complicated is the ST, DT, BB joint. The DT needs to be mitered three times. Once for the HT joint but Twice on the other end. Once at 90° for the BB notch but a little bit needs to be removed for the ST to fit. See my pictures.

The chain stays and seat stays will get mitered later. For now just focus on the front triangle.
Once you get the ST, TT, DT mitered and fit up nicely in the jig lets switch our attention to the chain stays.

Step 3: Chain Stays

The first thing to do to the Chains stays is install the dropouts. Socket style dropouts simply slip over the ends of the stays while tab style dropouts require a little more work. I will go through installing tab style dropouts.
You need to make the tab on the dropout fit snugly inside the CS. To do this, measure the inside diameter of the CS and modify the dropout to fit. I made 6mm tabs of the CS internal diameter on my dropouts and 6mm deep slots the width of my dropouts in my CSs. The slots need to be angled slightly. This angle can come from you drawing. The two inner dropout faces will be parallel and a certain distance apart (likely 130mm as this is the standard for rear hub spacing). The CSs angle inward from the dropouts to the BB shell. Once you have the stays slotted and the dropouts tabbed it is time to braze them in. To make sure everything is aligned and accurate, it is best to use a fixture to hold the stays and dropouts while you braze. You really just need to make sure the angles are correct. I made a quick fixture that holds the stays the right distance apart where they will meet the BB. You want the chains stays to be at least 5mm inset from each side of the BB shell to allow room for your weld or fillet. The dropouts are held securely by a dummy axle from the UBI store. With the dropouts held parallel at the correct distance apart and the chain stays held the right distance part where they will meet the BB shell, you braze the dropouts in to the stays.
Once your stays look good, set them aside and get back to the front triangle.

Step 4: Weld/Braze the Front Triangle

Now it is time to start making things look like a bike.
The first major joint to braze is the BB ST joint. Make the ST is lined up in the center of the BB shell and perpendicular to it (use a jig). Tack the fore and aft tips of the joint together in the jig. You will make all your tacks in the fore aft plane to prevent any alignment issues.
After the BB is tacked to the ST you can pull it out of the jig and fully attache them. Braze or weld the entire joint. If you are fillet brazing, keep the fillet small so there is room for the DT.
Once that joint is done you can put it back in the jig and do a dry fit with the rest of the tubing. Use a marker to trace each joint. This is so you can drill vent holes. Vent holes are important for cleaning flux out of the tubing and so that moisture can drain from the bicycle frame. Just drill a small hole inside your trace lines.
Once everything looks good you can tack all the joints together. Remember to keep your tacks in the plane of the frame. Tack each joint sequentially, obtuse angles first and acute angles second.
Once all joints are tacked you can remove the front triangle from the jig and get ready to fully braze (or weld) the joints. I do this in a bike repair stand as it is easy to rotate and move the frame as you work.

Step 5: Miter and Attach the Chain Stays

Using a mitering jig or careful alignment to miter the chain stays. Doing them both at one time as if they will mount to the bike is the easiest. You could miter each individually using angle and length information form your drawings. Make are they miters are perpendicular to the faces of the dropouts.
Before you mount the chain stays tab the other part of the dropouts for the seat stays. Follow the same procedure as before.
Put the front triangle back in the jig and mount the chains stays in the jig. Tack them like you did the other joints and pull the frame out of the jig for the complete weld/baze.
Now it is really starting to look like a bike frame.

Step 6: Attach the Seat Stays

Slot the seat stays to fit the dropouts. Use the jig or your drawings to make sure these angles are correct.
There is room for some artistic work at the heads of the seat stays. I file and cap the ends with small pieces of tubing.
Set the seat stays on the dropouts and mark/eyeball where you want the stays to attach to the ST. File a little groove in the ST for them to sit in then braze them on. Now throw the frame in the jig, get the dropouts seated well and braze them in.

Step 7: Braze Ons and Bridges

You will need to add braze ons to your frame based on its desired use. There will most likely be braze ons for cable routing, a binding bolt for the seat post, cantilever brake braze ons? Water bottle bosses? there are lots of options.
The binder boss for the seat post is straight forward, get it centered and braze it on. After it is brazed you need to notch a slit down the middle of it and the seat tube. This allows the binder to pull the tube tight around the seat post.
The cable stops will be placed based on how you will be routing the cables for brakes and shifters/derailleurs.
The brake bosses need to be placed a certain distance up the seat stays and a certain distance apart from one another. For 26" wheels they should be 251-256mm from the axle (280-285mm for 700c). They need to be 75-85mm apart.
You also need to add a bridge between the two seat stays. This adds rigidity and a fender mounting location.