BEND TOOLING INC.:  Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools

Step 3

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Step #3:  Wiper Tip Rake

 

The wiper's job is to prevent a wrinkle from forming at the end of the bend.  Check whether or not there is a wrinkle at the terminal end of an otherwise smooth inside radius.  If not, you do not need a wiper, and this step is completed.  If so, the wiper tip needs to be positioned for optimal tool life. 

 

The wiper fills the gap behind the line of tangency between the inside line of the tube and the curve of the bend die cavity [see figure to right].  If the tube wall is not sufficiently rigid it will bulge outward to fill this gap.  With enough direct pressure die pressure this bulge will flatten out between the bend die cavity and the mandrel nose as the tube is drawn through the line of tangency.  But at the terminus of the bend the bulge is not drawn through and flattened, and it can then form a wrinkle, or small series of wrinkles, if the bulge extends far enough to exceed the elasticity of the tubing material.  Properly set, the tip of the wiper will catch the top of this terminal bulge before it sets into that wrinkle.  Note that because all tubing materials have some elasticity, the bulge will to a certain extent flatten itself out once pressure is relieved from the point of bend. 

 

Therefore, it is not necessary for the wiper tip to contain the entire height of the terminal bulge.  The wiper tip needs to obstruct only that marginal bulging which exceeds the material's elasticity and would set the entire bulge into a permanent wrinkle.  This is why a wiper can be raked away from the line of tangency.  The value of raking the wiper is that doing so extends its life.  The key to this step in the "Forward Mandrel, Low Pressure" set-up is finding the natural resting position of the wiper at zero rake and then determining the maximum rake that can be set for the application. 

 

To do this, hold a straightedge at the bottom of the grip section of the bend die cavity.  With the wiper loosely mounted on the wiper post, bring in the wiper so that the bottom of its cavity also lines up with the straightedge.  The wiper is now at zero rake.   To find its natural resting position, gently slide the wiper along the straightedge towards the line of tangency until resistance is met.  Check if the feathered edge of the wiper tip is in complete contact with the bend die cavity.  If so, you have found the natural fit.  If not, apply slightly more force until the feathered edge is securely backed by the bend die cavity. 

 

If you must use considerable force to find a "fit", most likely the wiper is improperly cut or you are trying to get the tip too close to the line of tangency.  The latter is a common problem because to the eye it looks better when the wiper is at the line of tangency; however, most wipers are not cut to permit such a setting, nor is it necessary if the purpose of the wiper is limited to containing the terminal bulge. 

 

Once you have found the natural zero rake position, rake the wiper by rotating the tip along the bend die cavity away from the line of tangency.  You can increase rake in this manner so long as the terminal bulge does not set into a wrinkle.  Once you have set the rake, you can measure the linear distance along the bend die cavity from the wiper tip to the line of tangency.  Provided that replacement wiper tips are of the same design and manufacture, you can short-cut this step in the set-up by setting a new wiper at this distance.

 

Note that raking the wiper applies only to low-pressure bending applications.  Also, if the wiper is raked, the feathered edge should be cut to "simple-sweep" geometry.  This is the most common geometrical form.  For high-pressure jobs, the wiper should be kept a zero-rake and the feathered edge should have offset geometry to accommodate this.  next page


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