June 18, 2001
Problems with your press brake imperil what could be the most valuable machine you have. Take pains to ensure the health of this valuable piece of equipment.
The press brake is perhaps the most valuable machine in the average fabricating shop. If the press brake is hard to set up and/or produces inaccurate parts, you are losing money every time you use the machine--excessive setup time, time wasted making bad parts, and scrap parts all cost you a lot of money.
However, you can do several things to reduce these costs and make your press brake more efficient. Let's take a look at some of them.
The word level is used loosely, but in fact, it refers to parallel gibways on the side frames.
The gibways control the ram, and if the gibways are unparallel, the ram is in a twist. To check the parallelism of the gibways, place a precision level on the back or front of each gibway and make sure that the two have the same degree of vertical. If there is a difference in the vertical reading, the gibways act as a brake against the ram in the cycle. In the worst case, the braking action of the gibways prevents the ram from returning to the top of the stroke.
Unparallel gibways cause the ram to twist, which affects the alignment of the tooling. The misalignment makes accurate setup difficult and tends to produce inaccurate parts.
Another factor affecting gib alignment is lubrication. Some press brakes require gib lubrication and some do not. You should check your service manual to determine whether your machine requires lubrication. Improperly lubricated gibs can cause gib damage and even cause the machine to freeze.
Oil can become contaminated with metal particles, dust, grit, condensed water, and heat. Most hydraulic press brakes have oil filters that must be changed regularly.
A dirty filter contaminates oil and causes overheating. When the oil is dirty or hotter than 140 degrees, the machine will not run properly. It cannot be set up properly and does not produce accurate parts. Check the oil filter regularly and at least once a year (every six months in a dirty shop) by drawing off a pint or so of the oil and having it checked by a lubrication specialist. Keeping the oil clean keeps the brake operating properly and greatly reduces downtime.
Proper tooling selection is critical for both quick setup and accurate parts. Ideally, the parts should be designed within the following parameters, if at all possible:
If your parts fall within these parameters, you can use good conventional tooling and always air- bend using the least amount of tonnage to produce parts to specifications. The advantage of air bending is that you cannot develop sufficient tonnage to damage the machine or the tooling. Setup time also is reduced because you don't have to bend-test parts--the setup procedure for air bending is fixed.
If you do not have the ideal situation (e.g., flange is smaller, angle tolerance is less, or inside radius is smaller), you may have to bottom the parts to achieve your specifications, and that increases your tonnage and setup time. Look at the following reasons for this adjustment, which may help you to understand better why part parameters are so important:
1. The inside radius of the bend in air bending is directly related to the die opening at the ratio of 0.157 times the die opening. This means that if the die opening is eight times the metal thickness (ideal for air bending), the inside radius will be 1.25 times the metal thickness. For a smaller radius, it is necessary to use bottoming tooling, resulting a large increase in the tonnage.
An alternate approach uses a smaller die opening, which also requires more tonnage. The increase in tonnage for bottoming is about four times the tonnage required for air bending.
2. The flange width in either bottom or air bending is equal to 0.7 times the die opening. To achieve a smaller flange width, you have to use a die opening that is smaller than eight times the metal thickness, again resulting in greater tonnage.
3. The accuracy of the bend (for bends less than +/- 2 degrees) is related to the repeatable accuracy of the ram, the consistency of metal thickness, and the accuracy of your tooling. Therefore, you can see that for tight angles, it may be necessary to bottom, which increases tonnage and setup time.
Many times you'll find that considering these variables can lead you to change your parts specifications so that air bending (with good tooling) will suffice, allowing you to enjoy reduced setup times and more accurate parts.
For our purposes, "good tooling" means tooling that is not worn excessively and is within the manufacturer's specifications. It is suggested that you use a precision micrometer to measure your tooling from shoulder to shoulder on the dies and from nose to shoulder on the punches. If the measure is out-of-parallel more than +/- 0.001 inch per foot or +/- 0.005 inch overall length, the tooling should be remachined. Bad tooling is perhaps the biggest cause of bad parts and increased setup time.
The most overlooked cause of bad parts and slow setup is concentrated loading. Remember this rule: Never apply full tonnage over less than 60 percent of the distance between the side frames.
This means that for a brake with 10 feet between the side frames, the minimum width to apply full machine tonnage is 72 inches. Applying full tonnage over a width that is less than 72 inches long (in our example) will overdeflect the ram and bed and may cause a permanent upset.
Once the bed or ram is upset, the resulting parts have more angle in the center than at the ends. This adds considerable setup time and makes it very hard to produce good parts.
Of course, concentrated loading can happen only during bottom-bending, because it is not unusual to see a set-up that utilizes full tonnage when bottoming over a part 12 inches or so in width! In bottoming, the tonnage limit on a hydraulic machine is the machine capacity, whereas in air bending, the tonnage used is equal to the tonnage required to bend the part. To protect the machine, it is always better to air bend whenever possible.
When a ram or bed is upset excessively, your only recourse is to have it remachined to remove the upset.