Learning lessons from the past

February 14, 2002
By: Bob Butchart

For several years my work as a press brake consultant has taken me into many different fabricating shops. My job is to seek out problems and try to find solutions that make press brakes more productive. I have encountered many different situations, and I thought perhaps sharing some of the problems I've run across and their solutions might prove helpful.

Four major factors need to be addressed to form good parts efficiently on press brakes: the machine, the material, the drawings, and the operator. If any one of these factors is neglected, the results will not be satisfactory. However, many of my clients over the years have forgotten one or more of these standard items and have paid the price—some more than others. The following are just some samples.

Scenario 1: Air Bending

One of my clients had several press brakes. One operator of a large hydraulic machine summed up this particular shop's chief complaint (I'll paraphrase):

"I am making a small [8-inch-square] assembly made of two formed parts that must fit together so that the holes in both parts are aligned. Approximately 50 percent of the assemblies are rejected because the holes are out of line."

I found that bottoming dies were being used because of the part's required angular accuracy; however, the machine was not accurate enough to form the parts in an air bend. With this tooling, the estimated required tonnage to make the two parts was 42 tons, but the tonnage being applied to the parts was more than 10 times that. To make matters worse, the parts were being formed in the middle of the machine. As a result, the bed and ram of the machine were being overdeflected, and the parts were being coined with no two parts being the same.

This is a common problem when forming small parts on large machines. I suggested that the machine tonnage be adjusted to 50 tons and that the parts be formed off-center to prevent the overdeflection in the middle, which even 50 tons of force can create. By using only the tonnage required to do the job and doing the work off-center, the operator was able to produce each part with the same tolerance. And guess what? The rejection rate was greatly reduced.

Scenario 2: Flanges

Another client pointed out that he could not get consistent flange widths, even though the backgauge fingers were set accurately at a given distance from the center of the bed. The flange width was to tolerance on the left-hand side but was about 0.060 in. short on the right-hand side. The ram seemed to align correctly with the bed when the tools were brought together with no material in place, but the right-hand flange was short during forming.

I checked the ram gibs and found that on the right-hand side, they were open (out of adjustment) by 0.060 in. to the front. With material in place, the ram clamped the part 0.060 in. to the rear and pulled the material to the front as the bend progressed. At the same time, the ram closed the gib opening as the ram aligned with the bed, forming a short flange. We adjusted the gib to get it back to normal, and the problem went away.

A problem that has come up several times is erratic forming of wide flanges on long parts (10 feet or longer) with flange widths of 12 in. or more. The flanges turn out wavy or curved, even though the tooling seems to be straight. This error generally is caused by ram deflection or ram upset and, in some cases, by out-of-tolerance tooling. In a complaint like this, I check the tooling and then the ram straightness with a wire to determine the degree of ram upset. The solution is to re-machine the ram if the upset is 0.015 in or more and to replace or re-machine the tooling.

Scenario 3: Machine Controls

One client called and asked if I would come and tell him why he could not form parts to his customer's satisfaction.

I asked, "What machine do you have?"

"A CNC machine that is less than a year old," he said.

"What angular tolerance does your customer require?" I said.

"About plus or minus 1 degree," he said.

It was obvious that he had a problem, because a CNC brake certainly could produce that tolerance with new tooling (which he had).

I visited the shop a couple of days later, and before we even got close to the machine, I could see the problem. He wanted and thought he had a CNC machine, but he actually had a retrofit. This was a standard hydraulic press brake with +/- 0.002-in. ram repeatability with a retrofit CNC control for the backgauge and ram. While the retrofit enabled him to control the ram position, it did not change the accuracy of the ram. Someone sold him the wrong machine, and the brake he had could not form to his customer's accuracy.

There was nothing we could do to improve the accuracy of the brake, so I suggested that we visit his customer to make sure the part accuracy that was specified on the drawings was really necessary.

We had a long meeting with the customer and determined that some of the specified angular tolerances indeed could be opened, and that made it possible for the parts to be made on my client's brake. We did suggest that the shop purchase some new tooling, because the tooling it had was damaged from attempts to bottom-bend the parts. With the new specifications and tooling, the shop could make parts with air bending.

This problem alludes to a common problem in many shops - - bad drawings. The final part can only be as good as the drawing by which it is made. It does not matter how good the brake is or how consistent the material is or how good the operator is. If the drawings are not complete or are just plain wrong, the parts will be bad.

I see many drawings on which the inside radius is not shown or the angular tolerance is not specified. The angle itself sometimes is omitted! All of this information is important to help an operator know what brake to use, what tooling is best for the application, and where on the brake the tooling should be installed. For example, the blank size is affected by the inside radius, which is determined by the forming method (air, bottom, or coin) and the tooling used.

Scenario 4: Not-so-smooth Operator

I was invited to visit a client and was being given a shop tour. I noticed that two men were installing tooling in a mechanical press brake, and my host interrupted them to introduce me to one of them. When my host explained who I was, the man said, "Press brakes ... I have been working with press brakes for 20 years and I know all there is to know about brakes." He was the foreman of the sheet metal department, and we chatted for a bit. Then he excused himself to finish helping his brake operator install the tooling. He had turned on the brake to lower the ram and set the upper tool; all that remained was to position the backgauge fingers for the width of the part to be formed.

The brake, a mechanical unit, was running, and the foreman placed both arms between the upper and lower tools to move the backgauge fingers into position from the front of the machine. He was lucky that time, because nothing happened. But something could have happened, and the result would have been terrible. After 20 years, he still did not know enough to keep his arms or fingers away from a running machine.

Scenario 5: Bad Gears

I once did an evaluation for a client who ran 18 press brakes. One large (350-ton) machine was out of service because the gears werestripped in the gear box and the machine was being repaired. I looked at the ram and found a depression 12 in. wide and 0.060 in. deep in the center of the ram. On the tooling shelf, I found a flattening die that fit into that depression, so I asked my host, "How do you use this machine?"

"We use it to flatten bevel washers by putting several under the die and hitting them hard," he said.

Now this was a two-speed brake that developed 350 tons at high speed; but in low speed, at the bottom of stroke, the torque of the drive system was 2,750 tons when the flattening die hit the washers! That tonnage coming to a sudden stop caused the gears to backlash and strip. No one in the plant realized the real reason for the gear damage (they know now).

Scenario 6: Didn't Know Their Strength

Another client had several mechanical and some hydraulic brakes. While we walked through the shop, the foreman showed me where the gearboxes on the mechanical brakes were broken and had been welded. He could not understand why the mechanical machines were breaking and the hydraulic machines were not giving him any trouble.

I explained that the mechanical brakes were capable of generating much more than rated tonnage at stroke bottom and that this tonnage caused the brakes' drive shafts to deflect. Because the main gears were mounted on the end of the shaft, the deflection in the shaft caused the gears to misalign, creating stress inside the gearbox and eventually causing the cast box to crack. I told him that as long as the shop continued to overload the machines at stroke bottom, the breakage would occur again and again.

If these snippets show you anything, it should be to point out that running and maintaining press brakes requires a lot of care and respect for the power of these machines. Taking care of them and making sure you operate them in a safe and sustainable way will yield more profit than you may realize today.

Bob Butchart

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