What excess tonnage does to a press brake and tooling
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This month, I have a couple of questions related to accidents on the press brake. They’re not so much about keeping the operator safe, which is also very important. Instead, we’ll look at what happens to the press brake and its tooling. When an accident does occur, the first concern is, of course, the operator. Were they injured? Hopefully not. If not, do they need to change their underwear?
All humor aside, the press brake is a dangerous machine that can get seriously damaged in an accident. On that note, let’s have a look at some reader questions.
Q: Our operators were running some parts last night, and some 250-mm-tall tooling was pushed to the side of the machine. The program they were running required 175-mm-tall tooling, which was loaded into the machine. However, they did not see the 250-mm-tall tooling section pushed aside. The safety devices were disregarded, and they tried to form their first bend.
When the machine would not cycle through the bend, they retracted the ram and increased the tonnage. Then they tried their first bend again. They embedded the punch into the bed of the machine and broke the upper punch. Luckily no one got hurt. My question is, what damage can that have on the press brake? How can I verify what, if anything, was damaged? And what about ram upset?
A: I’m glad that no one got hurt except for the machine. But how was it damaged, and how do you know if it was? You stated that the tooling was off to the side of the press brake. You also noted that the tools became embedded into the bed of the machine.
This damage usually occurs on brakes with short beds or under-the-ram power sources, such as the hydraulic cylinders. Embedding generally happens under the power flow because there is no place for normal deflection to occur. Deflection is the usual flexing of the bed and ram under load. This does not mean damage can’t happen in the center of the ram, the point of greatest deflection. Trust me, it can and does happen. Also, short-bed-length press brakes, like those with 4-ft. beds, are more likely to embed rather than deflect in the center.
Once an embedding has occurred, there will be a weak spot on the bed and the ram, which may require shimming from then on. Run your hand down the bottom of the ram or the bed of the machine, and you will easily find any place the embedding has occurred. Other than shimming the die at the point of embedding, the only way to correct the problem is to have the entire length of the bed and ram machined flat again.
Depending on circumstances, you could have damaged the hydraulic cylinder in a way that lessens the available forming tonnage. If the press is still working, it can be hard to tell if the drive system was damaged. A good tell is if the ram now needs to have the ram tilt invoked all the time.
You can calculate the tonnage required to embed a tool—what’s known as the sinking tonnage limit—first by calculating its land area over 1 ft. of tool length by measuring a tool’s shoulder width, as shown in Figure 1, then doubling it and multiplying it by 12. Assuming a 15-ton shear load for the bed and ram of the press brake, you then multiply the land area value by 15 to get the maximum applied load before embedding occurs.
Land area = [(Shoulder width × 2) × 12]
FIGURE 1. To calculate the land area, you first need to measure the width of the die shoulders, as shown, then double the result.
Sinking tonnage limit = Land area × 15
So, if your tool has a shoulder width of 0.350 in., you’d calculate the sinking tonnage limit as follows:
Land area = [(0.350 × 2) × 12]
Land area = 8.50 in.2
Sinking tonnage limit = 8.50 × 15 = 127.5 tons
Note that your machine may have a different value for the shear load, and you may need to research that. Also, if you think the tonnage requirements might be getting close to the maximum tonnage, multiply the maximum tonnage by 0.8; that will knock 20% off the tonnage, giving you a safety margin and a chance to find a better way using less tonnage.
Q: I read your April 2018 article about checking press brake repeatability and accuracy, and I’m trying to understand where you put the mag base. Is it directly on the bed of the press brake, or do you use a level surface that is not a part of the press brake? We are actually using this article to set up a validation plan for our press brake area to support ISO 9001 requirements. Thanks in advance for your help!
A: To answer your question about where to put the mag base, I’ll start with some context. Before anything else, you need to determine the centerline load limit of your machine by contacting the manufacturer.
The average design limit for deflection is 0.0015 in. per ft. between the side frames. So, to calculate the deflection limit in the center, multiply 0.0015 in. by the number of feet between the side frames. If you have 10 ft. between the side frames, the equation would be: 0.00 15 x 10 = 0.015 defection at the center. Again, this is just an average design limit. Your press brake manufacturer should be able to give you the centerline load limit.
Let’s say you’re working with a 100-ton press brake with 10 ft. (120 in.) between the side frames (see Figure 2). To determine the maximum tonnage per inch at the centerline, you divide 100 by 60% of the distance between the side frames:
FIGURE 2. Most press brakes have a maximum allowable deflection in the ram and bed when a full-tonnage load is applied over 60% of the distance between the side frames.
Maximum tonnage per inch at centerline =Press brake tonnage rating/(Distance between side frames in inches × 0.60)
Maximum tonnage per inch at centerline =100/(120 × 0.60) = 1.3888 tons per inch, or 16.66 tons per foot
When you exceed that value, you run the risk of causing ram upset (see Figure 3). Ram upset is the permanent deflection of the ram and bed; in other words, the ram and bed will never return to flat again.
If you have ever been around an old mechanical press brake, it is common to see the operator place a pile of shims underneath the die in an attempt to level the bed. That is an extreme example caused by years of coining and bottoming; it does, however, give an excellent visual of what you are attempting to measure yourself.
The proper way to check for this kind of damage is to do the following:
You are looking for the indicator’s needle to remain at zero across the bed length. Note that the needle jiggles a little bit as it reads surface imperfections. This is normal and will even be seen on brand new machines.You do not want to see the indicator’s needle climb as you move toward the center and then descend again at the far end. This reading will indicate that the ram is upset and demonstrate just how bad the damage is.
You may see the indicator needle steadily moving up or down as you cross the bed. This kind of reading tells you the ram is out of parallel, which may or may not be an issue. If the press brake seems to perform OK when tilted down, it could indicate the hydraulics might be a little weak and warrant further investigation.
It also may be that the ram is deliberately tilted to execute a particular operation, in which case the tilt can be adjusted back to parallel through the controller.
If you find that the ram is upset, you may be able to adjust for it by moving the wedge in the top toolholders to make them taller—if, that is, you are using European-style tooling and holders. If you have traditional planer tooling, you can shim with paper.
If the upset is more significant than can be adjusted for, you need to repair the damage by having the bed machined back to flat. Not doing so will have your operators fighting with a weak spot in the center of the press brake, costing you time and money.
FIGURE 3. Ram upset can permanently deflect the bed and ram of the press brake.