BICYCLE FRAME CHOPPING TUTORIAL
This basic tutorial will guide you through the process of cutting up a typical diamond shaped bicycle frame into its individual tubes and components so that you can re-use these parts in your own designs. A diamond frame is made up of a front and rear triangle, forming a diamond shape. The round steel tubing on a bicycle frame ranges in diameter from 3/4 inch to over 2 inches in diameter depending on the style of the frame.
Three different types of grinder discs will be used when cutting up the steel tube bicycle frame: a standard coarse grinding disc for rough metal cleanup (center); a thin zip disc for cutting the tubing; and a sanding disc for final cleanup of the tubing and removal of paint. All grinder discs should be the proper size for your grinder as indicated in your owner's manual. Along with an angle grinder, you will need the usual safety gear: eye protection, hearing protection, and work gloves. A full face shield in front of your safety glasses will give you optimal protection when grinding metal.
The cutting disc (also known as a "zip disc") is a very thin grinder disc that can quickly cut through hollow steel tubing of various diameters. Round tubing as well as square tubing can easily be cut by the zip disc, although a hacksaw can also be used if you don't mind the added effort. The zip disc used in this tutorial is a 5 inch diameter disc with a 3/64 inch thickness.
A sanding disc is also known as a "flap disc" because it is made up of many overlapping flaps of sandpaper, and can remove metal as well as paint and stickers from bicycle tubing. A flap disc is usually used for final cleanup or when you are grinding a weld flush to the surround material. Because a flap disc does not remove metal as quickly as a rough grinder disc, it is much better for grinding an area where only a small amount of material should be removed.
Even if you are planning to build your own frame completely from scratch, you will still need to salvage the head tube and bottom bracket from a steel tube bicycle frame. Some of our plans such as the Warrior Racing Trike use only square tubing and require a pair of head tubes along with a bottom bracket. For this reason, even a bent up or damaged steel bicycle frame can be a good source of reusable tubing and components, so never pass up a free or good deal on an old bicycle. Also, aluminum frames are not used in any of our plans, so ensure that you are acquiring a steel tube bicycle frame when looking for salvageable parts.
The head tube is another round steel tube that will have an outer diameter somewhere between 1.25 inches and 1.5 inches. The head tube carries a set of press fit bearing cups, which in turn carry the fork bearings and supporting bearing hardware. Since there are varying diameters of head tubes, you should keep the bearing cups and head tube together as a set to ensure that you have a matched set.
The bottom bracket is a threaded round steel tube that must carry the crankset, as well as the weight of the rider. The bottom bracket also forms a junction for many frame tubes, so it is by far the thickest wall tubing on a bicycle frame. There are two common types of steel frame bottom brackets: a threaded type for a 3 piece crankset and a non-threaded type for a single piece crankset. The threaded bottom bracket is by far the more useful of the two, as there will be a large assortment of quality 3 piece cranksets available. Single piece cranksets are normally found on very inexpensive bicycles and kids bikes. If you have a choice, always choose a frame with a threaded bottom bracket.
The threaded bottom bracket has a set of threaded bearing cups that carry the crank axle bearings and supporting crank hardware. The bearing cup on the left side of the frame is the easiest to remove. It has standard threads, so removal is done by turning the cup in the counterclockwise rotation. The cup in the right side of the bike is much more difficult to remove as it has only a small flat ridge to grip and is a reversed thread. It is usually best to just leave the right side bearing cup installed on the frame as this will help protect the shape of the bottom bracket when the extreme stresses of welding heat are applied. If for some reason you do want to remove the right side cup, turn it out in the clockwise rotation using a large crescent wrench to grip the flat ridges.
Unless you need to include some of the steel frame tubing with the head tube or bottom bracket, cutting is normally done as close to the surface as possible in order to minimize later cleanup. The down tube is the lower tube in the front triangle that joins the head tube to the bottom bracket. This tube will be cut first. The cut will be made as close to the bottom bracket shell as possible using the grinder disc or hacksaw.
Throughout this tutorial, I will mention "frame collapse", which can be a problem when using a thin cutting disc in your angle grinder. Because the welding process puts a great deal of contraction stress on a welded joint, a triangle formed by 3 welded steel tubes will want to collapse on itself when opening the first joint. When a frame collapse occurs, the two tubes that were just cut create a vice as they attempt to collapse together. This pressure on the grinder disc can cause a stall, making the grinder jump and pull from your hands. Always be aware of frame collapse when cutting into any frame and hold the angle grinder tightly at all times with two hands.
The type of angle grinder typically used in this hobby will be the 4/5 or 5 inch type, which is more than enough to cut, grind, and sand the round or square tubing used in these projects. The term "angle grinder" describes the angle of the disc in relation to the grinder body, which in this case is 90 degrees. Throughout the tutorials on our site, the grinder is shown with the safety guard removed, but this was only done to allow clearer photos and videos. You should always use the recommended safety guard that came with your grinder, which will help avoid high speed debris in the rare case of a disc breakage.
Your grinder will specify a diameter of discs that will fit, which will likely be either 4.5 inch or 5 inch diameter discs. It is not advisable to run larger discs than recommended in the user's manual as the motor may become overworked and the safety guard will probably not fit correctly. Also, check the rotation speed (RPM) of your grinder and ensure that the maximum recommend RPM on the discs you plan to use are within specification.
The grinder disc is held on by a removable cap that threads on to the grinder shaft. Usually, one side of the cap has a rubber gasket to cushion the disc, but your grinder may differ somewhat. To remove the cap, the grinder may have a locking mechanism that holds the shaft from turning while you rotate the disc, or it may require you to grip the disc with a small supplied wrench. Follow the included disc installation instructions and never mix parts from different model grinders together.
When cutting across a round bicycle tube, try to keep the disc or hacksaw blade perpendicular to the rubbing and avoid changing the angle as you cut. If you keep the disc or saw blade running at the same angle through the entire cut, there will be a lot less friction and heat generated. Set the part to be cut in a sturdy vice in such a way that you can clearly see the work. If using a grinder disc, keep your face out of the line of fire and direct all hot sparks into a safe area that does not contain materials that could ignite.
While holding the angle of the cutting disc stable, push into the work and move the edge of the disc along the tube slowly, as if sawing a small line. This helps open the cut and will reduce friction where the sides of the tubing wall are hitting the face of the cutting disc. Only a moderate pressure is needed with a zip disc as it will easily slice into the thing walled steel tubing.
Always place the work in such a way that it is held securely and so that you can work the grinder in a comfortable and safe position. It is best to have the bottom side of the disc cutting the metal since the grinder will want to pull away from you rather than push towards you if the disc becomes stuck. Also, remember that this is the first tube in the triangle, so there will be frame collapse. For this reason, the last quarter inch of the tube will not be cut with the grinder but finished with a hacksaw instead.
The last small section of the cut on the other side of the down tube has been left so that it can be safely completed with a hacksaw. The front triangle will certainly be under a lot of tension due to the contraction of the welds, so it is not advisable to make the entire cut with a grinder disc. The frame collapse could easily stall your grinder, making it run out of your hands. This danger is compounded by the use of a locking switch, which should not be used when making tubing cuts.
This is the small section of the down tube joint that has not been cut using the zip disc. The risk of a frame collapse on the initial cut is too risky, so the last small section of cutting will be done using a hacksaw. Of course, all of your cutting could be done completely with a hacksaw, but it will certainly take a lot longer.
Using a hacksaw is easy - just make long straight strokes while pushing the blade into the work. If you are cutting the entire tube using a hacksaw, take your time and cut at about 1 stroke per second, keeping the blade running at the same angle throughout the entire cut. Of course, blade teeth need to face forward when installing the blade on the saw.
The hacksaw makes a much thinner cut than the zip disc. You can see this where the tubing has collapsed back onto itself. The hacksaw will probably also become stuck in the joint, but it is only a matter of prying the frame apart to release it. Note the tension on the frame as you open the cut area to release your blade. If you want to see for yourself how much pressure there would have been on the zip disc, remove it from your grinder and stick it in the hole, letting the tubes collapse on the disc surfaces. Usually, there is more than enough friction to stall your grinder.
Your hacksaw blade is probably stuck around the frame tubing now that it has passed through the final section of the down tube. To release the blade, grip the down tube and pry it away from the bottom bracket to open the joint. Notice how the frame tubing easily moves about a quarter inch before becoming stiff. This is one of the qualities of steel tubing that determine the stiffness of a steel frame bicycle. Steel tubing can flex somewhat when necessary, yet form a very rigid shape at the same time.
With the first tube cut, the frame loses a lot of its rigidity to the point where you can flex the down tube almost an inch from its original location with only the press of a thumb. A triangle is the strongest of all geometrical shapes, and since a bicycle frame is made up of two of them, it is extremely rigid. Some of our single boom projects like The Marauder Lowracer incorporates frame flex as part of the suspension system, making for a smoother ride.
A cutting disc is made to hit the work on its edge, so when a frame collapse occurs, the friction of the two tubes pressing on the face of the disc becomes great enough to stall the grinder motor or pull the grinder out of your hand. Always be aware of frame collapse when cutting the first tube in a frame, and keep a good grip on the grinder, especially when completing the last part of a tubing cut.
With one tube in the front triangle completely cut all the way through, the risk of a frame collapse is eliminated, so you can make a complete cut thought the next tube. The top tube will be cut next, where it meets the top of the seat tube. The action of the cutting disc will be the same here - a slow back and forth motion while pushing the cut through the tube at 90 degrees.
The grinder will be easiest to control when using the lower section of the disc for cutting as the force will try to pull the grinder away from you rather than pushing it towards you. This position also makes it easy to control the hot cutting sparks and keep your face out of the line of fire when cutting.
Although a frame collapse is no longer an issue, there will still be some friction on the face of the disc when the frame finally comes apart, causing the tubing to fall like a tree being cut down. If you start at the top of a tube and let the other half fall away, then this is not an issue as the joint will open up rather than fold onto itself.
On most bicycle frames, the top tube will be one of the larger diameter tubes, along with the down tube. The seat tube will likely be the smallest diameter tube of the three front frame tubes. Be careful when handling a newly cut tube as the edges on the inside and outside of the cut will have razor sharp edges. Handle a newly cut tube with gloves until the sharp edges have been filed off.
The front triangle has now been removed, leaving a complete rear triangle that includes the bottom bracket, seat tube, seat stays, and chain stays. Depending on your project, you may need a complete rear triangle or just the section of the front triangle, but we will be cutting this frame right down to the individual tubes to show how it is done. An example of a complete rear triangle used on a trike can be seen on our StreetFox Tadpole Trike, although in this case it is a suspension rear triangle.
All of the newly cut tubing will include a razor sharp outer edge and a few burrs on the inside of the tubing. A hacksaw cut may not create as much of a razor edge as a grinder disc cut, but you should always take a file around the tubing just to ensure that you don't accidentally puck it up without gloves at some later time. Cut tubing is about as sharp as the edge of a cut open pop can.
Much like the bottom bracket, the head tube is a key component in almost any bike or trike design as it creates a very strong and light steering system. Even a four wheeled bike, like our StreetFighter QuadCycle, will make use of a standard bicycle head tube, and in fact will use four of them. The head tube will be the first component to free from this frame, and that will happen by cutting away the top tube and down tube at the head tube shell.
Before you can do any cutting near the head tube shell, the two bearing cups must be removed from the inside of the head tube by banging them out. The bearing cups are press fit into the slightly tapered bottom bracket shell and can easily be tapped out from the inside using a solid rod or a chisel and hammer. Always strike the cups from the inside of the head tube, not from their outer rings. Since they are hardened steel, they could be chipped if struck on the outer ring. I use an 8 or 10 inch length of 3/4 inch solid rod to tap out the bearing cups.
When you have released both head tube bearing cups, place them flat on your workbench and take note of the two different sizes. The bearing cup that was removed from the underside of the head tube will usually be slightly taller than the bearing cup from the top of the head tube. This is not always the case, but if there is a larger cup, it is definitely from the bottom of the head tube and needs to be reinstalled that way.
The head tube will now be completely removed from the front section of the frame, by cutting the top tube and down tube as close to the head tube shell as possible. Since there is no other frame tubing in the way and no risk of a tubing collapse, cutting can be done without stopping using a zip disc. The closer you can get to the head tube shell, the better as there will be less cleanup to do with the sanding disc later on.
Cutting around the welded area has the advantage of leaving less material left over for later cleanup. With a smaller diameter cutting disc, you probably won't be able to cut the inside of the joint on the first tube because the other tube will prevent you from inserting the disc. A hacksaw can be used to cut the last small segment of the tubing joint.
The freshly liberated head tube ready for cleanup and installation into another frame. Just about every one of our Downloadable Plans will make use of one or more head tubes, so this is one of those staple components that you should always salvage from a discarded bicycle frame and store for later use.
The newly removed head tube will need to have some cleanup done in order to remove the left over weld metal and frame tubing. For this work, you will need two other types of grinder discs in addition to the zip disc: a rough grinding disc and a sanding disc. The rough grinding disc is the heaviest of all discs and will probably last much longer than the other two types of grinding discs. Rough grinding is done on the face of the disc, unlike zip cutting, which is done on the edge of the disc. It is worth purchasing quality grinder discs, as cheap department store discs may not be well balanced, causing the grinder to vibrate in your hand.
When using the rough grinding disc, press down on the grinder so that the face of the disc is pressing into the work, using the outer face of the disc. Try to keep the grinder focused on the material to remove and avoid digging into the head tube shell. Final cleanup will be done using a sanding disc, so leave a slight bit of material around the joint.
The sanding disc is basically a composite disc that contains many small flaps of sandpaper around the face, which is why it is also known as a "flap disc". Sanding discs are great for final weld cleanup, paint removal, and frame sticker removal because they do not gouge out the material in the same way as a rough grinder disc. You can certainly remove metal with a flap disc, but it can be done in a much more controlled manner.
Once you have the joint area ground flush to the head tube shell, run the sanding disc up and down the length of the head tube while rotating it so that you remove all of the paint and stickers from around the entire circumference. Burning paint and stickers give off toxic fumes, so they need to be remover prior to welding the head tube.
The head tube is now ready for installation into your own project, clean and ready for welding. If you plan on storing the head tube for later use, tie the bearing cups around the head tube using a zip tie or string so that you know they are a matched pair. This will save time if you begin to collect a lot of bicycle component inventory.
The bottom bracket is the kingpin in a diamond bicycle frame as it joins four different tubes together, creating the double diamond shape. The bottom bracket also carries the crank hardware, so it must endure an intense amount of stress. For this reason, the bottom bracket is the thickest walled tube out of all the other bicycle frame tubes. If you have been cutting your frame as per this tutorial, then three tubes must be removed in order to salvage the bottom bracket; the seat tube, and a pair of chain stay tubes. Also, remove any cable guides that may be installed on the underside of the bottom bracket as they will melt during the grinding process.
The goal is always to remove all components and tubes so that they are as long as possible for use in other projects. Stay tubing is just like any other round steel tubing, and can be used in many projects. Our HighRoller Short Wheelbase is an example of a frame that uses stays cut from a bicycle frame.
Almost all of our Bikes and Trikes include an adjustable bottom bracket, which allows the cranks to be moved along the frame to accommodate riders of just about any leg length. This adjustable bottom bracket is an assembly that includes a bottom bracket, two clamping plates, and a section of the seat tube welded to the bottom bracket. Some work can be saved by simply leaving a length of seat tube welded to the bottom bracket when making this adjustable bottom bracket assembly.
The cleaned up bottom bracket is now ready for transplant into your own creations. The bottom bracket may be the only bicycle tube that you would not attempt to fabricate due to the fine internal threads, so it is one of the most important components to salvage from a frame. Every one of our Human Powered Vehicles requires at least one bottom bracket in order to allow the installation of a set of cranks.
The salvaged and cleaned up head tube and bottom bracket ready to be reborn in another human powered vehicle. These two parts are the most important building blocks for just about any Bike, Trike, or QuadCycle so never toss out a frame without removing these key components. Even a badly damaged frame will probably have a salvageable head tube and bottom bracket as these parts are extremely robust.
Many times, the rear dropouts will be brazed (gas welded) to the frame using a bronze filler metal, and this must all be removed before you can weld over the area. Brazing will look like gold plating in the area where the stays used to meet the dropout material. If you try to arc weld over a brazed area, there will be a lot of spatter and the weld will not take to the metal.
If you intend to reuse any of the frame tubing, the area to be welded will need to have the paint removed at least 2 inches around the joint area. You don't have to remove all of the paint when recycling bicycle tubing, but all stickers and any paint near the area to be welded will have to be removed.
Stickers are easily removed with a sanding disc by turning the tubing slowly as you run the face of the disc back and forth over the area. I find that a well worn sanding disc is better for initial sticker removal as it will not get mucked up with the melting plastic as you grind. I take the stickers off with the worn out sanding disc and then finish the cleanup using a newer sanding disc.
Although burning paint will usually not interfere with your welding, it will certainly make you sick from the terrible smell and toxic fumes. The paint should be taken off a few inches around the area to be welded so that it will not begin to burn. If you still have some burning paint after welding, run a wire brush over the area and the paint will come off easily.
The bicycle frame has now been separated into nine reusable components: the head tube, bottom bracket, dropouts, brake studs, chain stays, seat stays, down tube, seat tube and top tube. Many of our Bike, Trike & Quadcycle Plans make use of one or more of these components, and when you are experimenting with your own designs, you can never have a large enough spare parts bin. It is rare that any bicycle frame would be so damaged that it would not offer some salvageable material, so never pass up a good deal or free bike for parts.
Components such as the head tube, bottom bracket and brake studs are important building blocks when creating any kind of human powered vehicle. If you have the time to cut up a few frames and clean up these mission critical parts, then you will be able to quickly move your designs from the sketchpad or computer drafting program right to the prototype stage in a hurry. Thanks for stopping by, and we hope this basic tutorial has "sparked" your interest in this great hobby!
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