How to choose and use wiper dies
September 25, 2003
A wiper die is a piece of tooling used in tube bending that helps keep the bend from wrinkling. While there are many reasons and ways to use a wiper die in a tube bending maching, you should also know what types are available, their differences, and how to choose the right one for your application.
Here's a quick quiz for you to take:
What "wipes" the wrinkles from the bend in a tube and is one of the five pieces of tooling in a die set?
If you guessed wiper die, you're right.
A piece of tooling used to keep the inside bend of a tube from wrinkling, a wiper die works with the bend die, clamp die, pressure die, and mandrel in a tube bending die set.
Not all tube bending applications require a wiper die.
Generally, the tighter the bend radius and the thinner the tube wall, the more likely it is that you'll find a wiper die is a key component for a successful tube bend. When a wiper die is necessary, it's critical to choose the correct style and material. In most cases, a wiper die is used with a mandrel.
Solid, or Square-back. A solid, or square-back, wiper die (see Figure 1) can be used in many tube benders, and it is suitable for precision bending, such as that required in aerospace applications.
This wiper also is used for small runs. Its biggest disadvantage is that it wears out and then needs to be recut or refurbished. This means the wiper must be taken off the machine and sent to a tooling manufacturer, so if you don't have a backup wiper readily available, this will cause some downtime.
Depending on the length of the square-back wiper when it's new, you usually can recut it two or three times. A refurbished wiper will be shorter, so the tube bending machine will have to be reset to accommodate the shorter length.
Inserted. An inserted wiper die (see Figure 2) is a more recent design and is made of two pieces of metal bolted together to work like a solid, or square-back, wiper.
This wiper type offers ease of repeatability and readily available replacement tips. It is suitable for high-production environments because, when necessary, an operator can remove the worn tip, put on a new one, and start bending again. In addition, little or no adjustment is necessary.
Depending on your application, you might be using several combinations of materials. Most people start with a steel holder and use aluminum bronze tips.
If you're bending mostly stainless steel to a tight radius, you should use an aluminum bronze holder and tip. It's also best to use aluminum bronze wipers and tips when bending INCONEL® alloys, titanium, and other exotics.
Generally, you should use a steel wiper and tip when bending most other materials, such as mild steel and most other steels. It's also helpful to use a steel-chromed wiper and tip when bending aluminum and copper for greater lubricity.
When choosing a wiper die, it's important to consider the type of tube bending machine you'll be using, in addition to your application. If the machine is a newer CNC bender, you might need to move the carriage nose closer to the bend die.
This brings us to one of the newest styles of inserted wipers: close-approach wiper dies (see Figure 3). This type of wiper allows the carriage to move closer to the bend die, thereby eliminating some tube waste.
Also, depending on the machine, the style of the holder can vary, but the tips generally are the same as for a standard-style inserted wiper.
One of the first things to check in a tooling setup is the wiper die's centerline radius. It always should match the centerline radius of the bend die.
If you're using inserts, you also should confirm that the tip and holder have the same radius. If the tip and holder have different centerline radii, they won't always match up, and this can affect the outcome of the bend.
When using interlocking tools, make sure that the interlock dimensions match. If they are different, the wiper die might not fit in the bend die properly.
In addition, inspect the feathered edge of the tip for nicks and gouges. The feathered edge rubs against the tube, so if the edges are jagged, it will scratch the tube. For bends in a cosmetic part, this is undesirable.
When mounting and setting up the wiper die, make sure you position the tip near the tangent point, which is where the bend starts and the point used to set up all the tooling (see Figure 4). Generally, it's not a good idea to go past the tangent, because this can rip off the wiper die tip. Chances are slim, however, that you can position the tip forward of tangent because the tool design will not allow it.
A good way to set up your wiper is to insert the tube over the mandrel, clamp up on the tube, and advance the wiper die manually as far forward into the bend die as you can. Then tap the back of the wiper to make sure you have advanced it as far as possible. The wiper then should be in its proper position.
Another concern is the rake setting of the wiper die, the angle that determines how much pressure is on the wiper die's tip. The higher the rake angle is, the more pressure there will be on the tip of the wiper. The lower the rake angle is, the more drag there is on the tube.
You also should consider the tube's wall factor when setting the rake: A high wall factor requires a low rake angle. Too much drag on a tube can cause heavy wrinkling and break the mandrel.
If wrinkling occurs throughout the bend and extends into the wiper die area, try advancing the wiper closer to tangent. If this doesn't work, try decreasing the rake on the wiper die. Also check to make sure that the wiper isn't worn too much—it may need to be replaced or refurbished.
If you see deep scratches throughout the bend and in the wiper die area, increase the rake by 1 or 2 degrees. If this doesn't prevent scratching, inspect the tube groove of the wiper for galling—buildup of material from similar materials rubbing against each other—which will require the wiper to be recut or replaced.
Inspecting your tooling for wear and confirming you have the right parts for the job can take a little extra time, but the payoff may be less downtime during production.
INCONEL® is a registered trademark of the Special Metals group of companies.