These comb inserts are designed to be mounted on special brackets and help eliminate wrinkles in a variety of crankshaft applications.
A client comes to you with a 90-degree pipe forming job. This application requires 2″ tubing. Outer Diameter (OD), 0.065 in. Wall thickness, 4 inches. Centerline Radius (CLR). The customer needs 200 pieces per week for a year.
Die requirements: bending dies, clamping dies, press dies, mandrels and cleaning dies. no problem. It looks like all the necessary tools for bending some of the prototypes are in the store and ready to go. After setting up the machine program, the operator loads the pipe and makes a trial bend to make sure the machine needs to be adjusted. Turn one came off the car and it was perfect. Thus, the manufacturer sends several samples of bent pipes to the customer, who then concludes a contract, which will surely lead to a regular profitable business. Everything seems to be in order in the world.
Months passed, and the same customer wanted to cut material costs. This new application requires 2″ OD x 0.035″ diameter tubing. wall thickness and 3 inches. CLR. Tools from another application are held internally by the company, so the workshop can immediately produce prototypes. The operator loads all the tools on the press brake and tries to check the bend. The first bend came away from the machine with creases inside the bend. Why? This is due to a component of the tool that is especially important for bending pipes with thin walls and small radii: the wiper die.
In the process of bending a rotating draft tube, two things happen: the outer wall of the tube collapses and becomes thinner, while the inside of the tube shrinks and collapses. The minimum requirements for pipe bending tools with rotary arms are a bending die around which the pipe is bent and a clamping die to hold the pipe in place as it is bent around the bending die.
The clamping die helps maintain a constant pressure on the pipe at the tangent where the bend occurs. This provides the reaction force that creates the bend. The length of the die depends on the curvature of the part and the radius of the center line.
The application itself will determine the tools that you need. In some cases, only bending dies, clamping dies and press dies are required. If your job has thick walls that produce large radii, you may not need a wiper die or mandrel. Other applications require a complete set of tools, including a grinding die, mandrel, and (on some machines) a collet to help guide the pipe and bend the plane of rotation during the bending process (see Figure 1).
Squeegee dies help maintain and eliminate wrinkles on the inner radius of the bend. They also minimize out-of-pipe deformation. Wrinkles occur when the mandrel inside the pipe can no longer provide sufficient reactive force.
When bending, the wiper is always used with a mandrel inserted into the pipe. The main job of the mandrel is to control the shape of the outer radius of the bend. Mandrels also support internal radii, although they only provide full support for applications involving a limited range of certain D-bends and wall ratios. Bend D is the bend CLR divided by the outside diameter of the pipe, and the wall factor is the outside diameter of the pipe divided by the wall thickness of the pipe (see Figure 2).
Wiper dies are used when the mandrel can no longer provide adequate control or support for the inside radius. As a general rule, a stripping die is required to bend any thin-walled mandrel. (Thin-walled mandrels are sometimes referred to as fine pitch mandrels, and the pitch is the distance between the balls on the mandrel.) Mandrel and die selection depends on the pipe OD, pipe wall thickness, and bend radius.
Proper grinding die settings become especially important when applications require thinner walls or smaller radii. Consider again the example at the beginning of this article. What works for 4 inches. CLR may not fit 3 inches. The material changes required by CLR and customers to save money are accompanied by the higher precision needed to tune the matrix.
Figure 1 The main components of a rotary pipe bender are clamping, bending and clamping dies. Some installations may require a mandrel to be inserted into the tube, while others require the use of a mandrel doctor head. The collet (not named here, but will be in the center where you will insert the tube) helps guide the tube during the bending process. The distance between the tangent (the point where the bend occurs) and the tip of the wiper is called the theoretical wiper offset.
Selecting the correct scraper die, providing proper support from the bending die, die and mandrel, and finding the correct wiper die position to eliminate gaps that cause wrinkling and warping are the keys to producing high quality, tight bends. Typically, the comb tip position should be between 0.060 and 0.300 inches from tangent (see theoretical comb deflection shown in Figure 1), depending on tube size and radius. Please check with your tool supplier for exact dimensions.
Make sure the tip of the wiper die is flush with the tube groove and that there is no gap (or “bulge”) between the wiper tip and the tube groove. Also check your mold pressure settings. If the comb is in the correct position with respect to the tube groove, apply slight pressure to the pressure matrix to push the tube into the bend matrix and help smooth out the wrinkles.
Wiper arrays come in a variety of shapes and sizes. You can purchase rectangular/square wiper dies for rectangular and square pipes, and you can also use contour/shaped wipers to fit specific shapes and support unique features.
The two most common styles are the one-piece square-back wiper matrix and the bladed wiper holder. Square back wiper dies (see Figure 3) are used for thin walled products, narrow D-bends (typically 1.25D or less), aerospace, high aesthetic applications, and small to medium batch production.
For curves less than 2D, you can start with a square-backed wiper die, streamlining the process. For example, you can start with a 2D square back curved scraper with a wall factor of 150. Alternatively, you can use a scraper holder with a blade for less aggressive applications such as 2D curves with a wall factor of 25.
Square back wiper plates provide maximum support for the inside radius. They can also be cut after tip wear, but you will have to adjust the machine to accommodate the shorter wiper die after cutting.
Another common type of scraper blade holder is cheaper and more cost effective in making bends (see Figure 4). They can be used for moderate to tight D bends, as well as for bending various pipes with the same outside diameter and CLR. As soon as you notice tip wear, you can replace it. When you do this, you will notice that the tip is automatically set to the same position as the previous blade, meaning you don’t have to adjust the wiper arm mounting. Note, however, that the configuration and location of the blade key on the cleaner matrix holder is different, so you need to make sure that the blade design matches the brush holder design.
Wiper holders with inserts reduce setting time but are not recommended for small radii. They also do not work with rectangular or square tubes or profiles. Both square back wiper combs and insert wiper arms can be produced in close proximity. Non-contact wiper dies are designed to minimize pipe waste, allowing for shorter working lengths by extending the attachment behind the wiper and allowing the collet (tube guide block) to be positioned closer to the bending die (see Figure 5).
The goal is to shorten the required pipe length, thereby saving material for the correct application. While these touchless wipers reduce waste, they provide less support than standard square rear wipers or standard wiper mounts with brushes.
Make sure you are using the best possible scraper die material. Aluminum bronze should be used when bending hard materials such as stainless steel, titanium and INCONEL alloys. When bending softer materials such as mild steel, copper and aluminum, use a steel or chrome steel wiper (see fig. 6).
Figure 2 In general, less aggressive applications do not require a cleaning chip. To read this chart, see the keys above.
When using a knife handle with a blade, the handle is usually made of steel, but in some cases both handle and tip may be required to be aluminum bronze.
Whether you use a comb or a brush holder with blades, you will be using the same machine setup. While holding the tube in a fully clamped position, place the scraper over the bend and back of the tube. The wiper tip will snap into place by hitting the back of the wiper array with a rubber mallet.
If you cannot use this method, use your eye and a ruler (ruler) to install the wiper matrix or wiper blade holder. Be careful and use your finger or eyeball to make sure the tip is straight. Make sure the tip is not too forward. You want a smooth transition as the tube passes the tip of the wiper matrix. Repeat the process as needed to achieve a good quality bend.
The rake angle is the angle of the squeegee in relation to the matrix. Some professional applications in aerospace and other fields use wipers designed with little to no rakes. But for most applications, the tilt angle is usually set between 1 and 2 degrees, as shown in fig. 1 to provide enough clearance to reduce drag. You will need to determine the exact slope during setup and test turns, although you can sometimes set it on the first turn.
Using a standard wiper matrix, set the wiper tip slightly back behind the tangent. This leaves room for the operator to move the cleaner tip forward as it wears. However, never mount the wiper matrix tip tangentially or beyond; this will damage the cleaner matrix tip.
When bending softer materials, you can use as many rakes as you need. However, if you are bending harder materials such as stainless steel or titanium, try to keep the scraping die at a minimum slope. Use a harder material to make the scraper as straight as possible, this will help clean out the creases in the curves and the straights after the curves. Such a setup should also include a tight-fitting mandrel.
For the best bend quality, a mandrel and a scraper die should be used to support the inside of the bend and control out-of-roundness. If your application calls for a squeegee and a mandrel, use both and you won’t regret it.
Returning to the earlier dilemma, try to win the next contract for thinner walls and denser CLR. With the wiper mold in place, the tube came off the machine flawlessly with no wrinkling. This represents the quality the industry wants, and quality is what the industry deserves.
FABRICATOR is North America’s leading steel fabrication and forming magazine. The magazine publishes news, technical articles and success stories that enable manufacturers to do their job more efficiently. FABRICATOR has been in the industry since 1970.
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