quality bends

8/8/2019 Quality Bends

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BURR TOOLAND GAUGE COMPANY, INC

Quality Bending on the Hairpin Bender

To get a good quality bend, it is important that the Hairpin Bender is set up correctly. There arefour basic adjustments that can be made; mandrel position, wiper pressure, clamp pressure andamount of lubrication.

1. Mandrel PositionThe position of the mandrel tips, in relation to the wiper block and the centerline of the bend

head, is shown in figure 1. The position varies with tube diameter, wall thickness and bendcenter. A good starting position for 3/8 dia. x 1.00 bend center is with the back of the mandrel

ball flush to the front of the wiper block with the mandrel cylinder extended. Note: When using

smaller diameter tubing or tighter bend centers, do not adjust the mandrels as far out from

the wiper block or mandrel damage may occur. The form block should be removed to set theinitial positions. Swivel ball mandrel tips do not require special radial positioning. The mandrel

tip position can be moved forward or back by changing the position of the lock nuts on the back

end of the mandrel rods.

If the machine has the mandrel rod micro adjustment option (see figure 2), then rotate the

adjustment screws as needed. Note: When the Strip Assist option is being used, it is

important to set all mandrels to the same length. Wrinkles may develop if mandrels are

adjusted too far out or in. Rotating the adjustment screws one complete turn will provide 0.05"of mandrel travel. Rotating one flat at a time will provide 0.008 of mandrel travel. A clockwise

direction (when viewed from the back of the bender) will move the mandrel away from the bend.

A counter-clockwise direction will move the mandrel into the bend. After the adjustment is


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made, tighten down the mandrel rod supports and/or the screw clamp at the adjustment.

The MASTER or EMERGENCY STOP push button must be depressed to turn the machine off

before making any adjustments. When making mandrel adjustments, keep in mind that movingthe mandrel into the bend has a tendency to make the bottom leg grow. However, if the mandrel

is moved too much into the bend, a bump will occur and the hairpin may break. If the mandrel is

moved away from the bend (towards the back of the machine), the bottom leg will shorten.

Moving the mandrel too much away from the bend will cause the hairpin to become flat andwrinkled.

1a. Mandrel Diameter

To reduce the amount of wrinkle, it is important to keep the internal clearance between the

mandrel and the wall thickness to a minimum. We at Burr Oak Tool and Gauge have established0.008" of clearance as our standard. Use the following equation to calculate the mandrel

diameter.

Mandrel dia. = Tube O.D. - 2 X Wall - 0.008

2. Wiper Pressure

Wiping pressure is an important factor in producing quality bends. Pressure must be adequate

but not excessive. This action is controlled by a hardened Cam (2400C, see figure 1). The

position of this Cam can be changed slightly through the use of cap screws and set screws which

are accessible only when the bend head is in the bent position. Depress the MASTER orEMERGENCY STOP button before attempting any adjustments.

Wiper pressure is increased by first loosening the two socket head cap screws slightly andtightening the two set screws. Then re-tighten the cap screws. Change the set screws only about

1/4 turn each time and be certain both screws are moved the same amount (the cam/wedge must

remain parallel to the face of the wiper block).Wiper pressure is decreased by loosening the setscrews and tightening the cap screws.


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3. Clamp Pressure

If the tube slips in the clamps in the process of being bent, flatness and/or wrinkling will occur.

There should be a definite clamp impression left on the tubing to indicate positive clamping

force. If there is not, it may be an indication of clamp slippage. Check the clamp pressure and

adjust as needed. Use 750 psi as a reference pressure. This will vary with tube wall thickness,tube diameter and material type. Keep in mind that too much clamp pressure will deform the

tubing. Also check the clamp equalizer pad for wear. This is done by inspecting the pad for

indentations or elongated holes. Replace it if needed.

4. Amount of Lubrication

Lubrication is required at the bend area to allow the tubing to bend freely around the mandrel.

The oil is distributed evenly as an air/oil mist through a rosette type oiler adapter. It can be

adjusted through a mixing valve attached to the bottom of this adapter at the rear of the hairpin

bender (see figure 3). A good starting point for mixing valve is 1/4 1/2 turn out. The oil isturned on when the mandrels IN solenoid valve is energized.

5. Form Block Tie Rods (Non-adjustment)

On tight bend centers like 0.807 and smaller, tie rods are placed through the form block to keep itfrom flexing while clamping the tubing (see figure 4). These should only be lightly tightened. If

these tie rods are not used, the tubing will slip in the clamps. Some older models did not have

the holes for the tie rods to pass through. These holes will have to be added.


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6. Tube Diameter

If the tube diameter is undersize, it may slip in the clamps and cause flat bends. Check the tubediameter as it is fed out from the cutoff. Adjust the sizing rolls and/or the cross axis sizing rolls

to obtain the correct tube diameter.

7. Tube Hardness / Temper

If the tubing is too hard, there will be more wrinkles and possibly cracks during forming. Mostof the hardness problems occur directly from the tube mill. If all the previous steps fail to

produce a quality bend, then contact your tube mill and discuss hardness. Sometimes the tubing

may be worked too much in the straightening rolls when straightening tubing. This causes thetube to be skin-hardened. Adjust the rolls so there is minimum pressure on the tubing, but

produces a straight hairpin.

quality bends

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