The Brakes: Press Brakes and You -- Preventing common mistakes on press brakes

WWW.THEFABRICATOR.COM AUGUST 2001

August 16, 2001

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Most common mistakes made while operating press brakes can be avoided with a little diligence and awareness of where others have gone astray.

Press brakes—they're wonderful machines that, unfortunately, present many opportunities to make mistakes. The good news is that many mistakes are so common that a nice file of prevention tactics has stacked up against them.

The following miscues and mishaps are commonplace—and preventable.

All Press Brakes

Quite a few mistakes can be made on all types of press brakes.

Dirty Machines. Perhaps the most common mistake we make in the average shop is not keeping our machines clean. Allowing dirt and grime to accumulate causes excessive wear to gibs and tooling. Dirt can scratch parts as they are formed, and metal dust that is allowed to enter the control panel can cause electrical problems and machine breakdown.

It should be firm company policy that machines and tooling be wiped down every day before the first setup is made. Any oil residue should be removed to prevent shop dust from being attracted to the machine. The practice of keeping the machine clean prolongs the life of the machine and also makes setup faster and easier.

Ram Upset. Ram upset is perhaps the most misunderstood term in press brake technology. All press brakes are designed with a deflection limit under full tonnage loading. In this case, deflection refers to the tendency of the ram to bend upward in the center and the bed to bend downward under full tonnage loading. When the load is removed, the bed and ram return to normal stability.

However, if the bending load is too concentrated and the bed and/or ram overdeflects, they take on a permanent upset in the center of the machine. When this happens, you see a larger angle in the center of long parts than at the ends. This can be corrected only by remachining the bed and/or ram.

This condition can be prevented by placing bending loads carefully and using only a sufficient amount of tonnage to make the part—provided you have adjustable tonnage capability on your machine.

Improper Lubrication. Proper lubrication is another serious and frequent oversight. All moving parts should be lubricated according to the recommendations of the manufacturer.

Ram gibs are a critical area and should be kept lubricated, if required. Some machines have nonlubricated gibways, some have grease fittings, and some have manual or automatic oiling systems. If the gibs require lubrication, failing to do it may cause them to freeze and/or become so badly worn that costly replacement is necessary.

Unlevel Machines. Ram gibs must be parallel so that the ram does not operate with a twisting motion. An out-of-level ram forms imperfect parts, and if the angle is bad enough, the gibs act as a brake, keeping the ram from returning to the top of stroke without increasing the return pressure.

Excessive Gib Clearance. Gibs should have the clearance recommended by the manufacturer. For some, this could be 0.001 to 0.002 inch but is seldom more than 0.006 to 0.008 inch. If they have too little clearance, the gibs hold the ram as a brake, and if too much clearance is allowed, the ram floats during the bending process. This floating has an adverse effect on the flange width and the consistency of the bend.

Worn Tooling. One of the major causes of imperfect parts is bad tooling.

The nose of the punch and shoulders of the die are subject to wear; if much work is done at the same place on the bed, the resulting wear causes erratic bends. Inconsistent bends and crooked flanges usually are caused by worn tooling. Ask your tooling supplier for proper tolerances and check your tooling for those tolerances at least once each week. Tooling can be remachined to new standards for less than the cost of new tooling. Also, tooling (new or remachined) should be as hard as the manufacturer can provide and, if possible, in the range of 40 to 42 on the Rockwell C scale.

Wrong Tooling. The specifications of the part determine the tooling to be used, provided that the press brake is compatible with the tooling.

For example, for a bend radius larger than metal thickness and an angle tolerance between 1 and 2 degrees, you can air-bend with almost any brake that is in good condition. This type of bend requires the least bending load and helps to protect the brake from overload. However, if the radius is about equal to metal thickness, or the angle is close to 1/2 degree, you must bottom the part with four times the tonnage of air bending.

Even a CNC brake with an accurate ram repeatability must bottom-bend for small inside radii. The secret is to understand fully the type of brake you have and use tooling that is compatible with that style of brake. Using the wrong style of tooling results in bad parts and possibly damage to the brake.

Unaligned Tooling. It is critical that the punch and die are properly aligned to produce accurate parts. The center of the punch and the center of the die should be in the same plane for the entire length of the machine.

The flange width of the part also is affected by misalignment. The backgauge is positioned from the center of the die, while the final flange width is determined by the center of the punch. The punch position is fixed (provided the gibs are tight), while the die position is adjustable to the centerline of the punch. Misalignment can occur when the die or the die rail moves on the bed.

Failure to Determine Required Bending Load. It is important to know the load required to make a bend. The load is based on the thickness and type of metal, the length of the bend, and the tooling used. By knowing the bending load, you can distribute the load on the bed and avoid damage to the bed, ram, and tooling. Never apply full tonnage to small parts.

Remember, in bottoming, you can exceed the required tonnage many times. This cannot happen during air bending, because only the tonnage required to bend the part is developed by the machine. This is why it is always better to air-bend when you can produce your part that way. Also remember that all tooling has a load limit and that tool breakage will occur if that limit is exceeded. You should consult with your tool supplier to determine the limits of your tooling.

Hydraulic Press Brakes

Ram Drifting. Sometimes on two-cylinder hydraulic brakes, the ram drifts down on one or both ends when the machine is stopped with the ram raised and left under pressure with the power off. This condition is caused by an internal oil leak in the cylinder. If only one cylinder is affected, the ram will drift far enough on that side so that it becomes out-of-parallel with the bed.

The oil leak can be corrected by replacing the internal oil seal, and the out-of-level problem can be controlled by always placing a block under the ram when the machine is shut down. If the internal oil leak is allowed to continue, it will get worse and eventually decrease the bending pressure in the affected cylinder. Also, if oil is leaking down the piston, it means that the external seals are going bad and will have to be replaced soon.

Failure to Adjust Ram Tonnage. Not all hydraulic press brakes have an adjustable tonnage feature, but if yours does, by all means take advantage of it. Using the minimum tonnage required to bend a part is good for the tooling and the machine. You can avoid overconcentrating the load and reduce the tendency to upset the ram or break the tooling. If your machine does not have adjustable tonnage control, you might check with the manufacturer to find out if a retrofit unit is available.

Failure to Change Hydraulic Oil. Changing hydraulic oil is an often-overlooked procedure that can have serious consequences if it isn't done. Oil can become contaminated with debris, water, or air. It also can break down if the machine is run at high temperatures for too long. All of these conditions can damage valves, cylinders, and manifold blocks.

Major repairs may need to be done if poor oil conditions are allowed to continue. It is a good idea to have the oil checked by a lubrication specialist once or twice a year, depending on how much the machine is used. Manufacturers recommend a change of oil on a regular basis, but if the machine is not used a great deal, it may be OK to put this off if approved by the lubrication specialist.

Mechanical Press Brakes

Unlike hydraulic press brakes, mechanical brakes can produce loads that are considerably higher than the rated capacity at the bottom of the stroke. This excess loading can be as much as two or three times the rated tonnage!

The rated tonnage is generated at a point near bottom, and the excess tonnage comes from the last movement of the ram to bottom. Take care to position work so that the load required for bending is not above the rated tonnage. Then, when the ram goes through a complete cycle, there will be no overload at the bottom of the stroke.



Bob Butchart

Contributing Writer
Press Brake & Shear Clinic
254 Somers Loop
Reidsville, NC 27320
Phone: 336-342-4393
Fax: 336-342-4129

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