How to Bump and Move Sheetmetal the Right Way

Bumping is the heavy lifting in most autobody repair work. Its tools and techniques also have application to many fabrication projects. To be sure, bumping is not always the heavy lifting in body work. There are repair jobs that begin on frame machines, or with body-pulling posts even heavier lifting than bumping that are then followed by bumping operations. And some body-metal fabrications never employ bumping. Those that do may have their heavy lifting beginnings in sheetmetal brakes, or on slip rolls, before any bumping techniques are applied to them.

 


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The fact remains, bumping describes the group of actions that move metal with the likes of hammers and dollies, to push things pretty much into final shape. This does not mean that bumping operations, by their nature, require heavy hammering or prying with large tools. That may be the case but subtle approaches are also possible.

 

Bumping means to shape metal to final contours. This rust patch is being shaped and bumped into final position so that welding it in can be finished. Note the use of a square-faced hammer to work up against an edge.

Bumping means to shape metal to final contours. This rust patch is being shaped and bumped into final position so that welding it in can be finished. Note the use of a square-faced hammer to work up against an edge.

 

These pneumatic tools illustrate the gap between subtle work and violent assault on sheetmetal. The fender gun (top left) is more likely to do harm than good. The percussion hammer (bottom right) can slowly persuade small areas of metal to move a few thousandths of an inch.

These pneumatic tools illustrate the gap between subtle work and violent assault on sheetmetal. The fender gun (top left) is more likely to do harm than good. The percussion hammer (bottom right) can slowly persuade small areas of metal to move a few thousandths of an inch.


 

In this regard, let’s consider a large area of damage to a low- to medium-crown panel; say, a single impact at one point that deforms 70 percent of the area of a relatively flat door or quarter panel. It may look terrible because so much metal is displaced. However, to a seasoned metal worker, it may be possible to return 95 percent or more of the damaged area to its original contours with a couple of very undramatic and uncomplicated moves. Sometimes, most of this kind of damage can be pulled out with a suction device, or driven out with one, or a few, well-aimed blows with a rubber mallet.

Unfortunately, it is just as possible that attempting to remove this kind of damage with those approaches will be unsuccessful. It may even inflict further and more serious damage to a panel by creating ridges, V-channels, and minor upsets. The outcome depends on the skill and judgment of the metal worker.

In this case, the original shape, thickness, and hardness of the panel; the depth and configuration of the damage to it; any supporting structure behind it; and the specific nature of the tools and maneuvers employed to remove the damage determine whether the action fails or succeeds. In other words, simple approaches to bumping metal require keen judgments. A rubber hammer, used inappropriately, can cause almost as much damage as a metal hammer.

 

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One scheme for damage removal that never fails is described and illustrated in Chapter 3. It involves analyzing how damage occurred, and removing it in the reverse order of that sequence. What never works is simply banging against what seems to be high metal, and hoping that by doing so, everything will come out alright. That approach will produce additional and severe damage, in the form of additional deformations with upsets and stretches, galore.

 

At first glance, working with what almost amounts to a blunt pick hammer, over a shot bag, seems to be metal finishing. However, the hammer is being used to change a contour in the metal, not just a surface irregularity. That, by definition, is bumping.

At first glance, working with what almost amounts to a blunt pick hammer, over a shot bag, seems to be metal finishing. However, the hammer is being used to change a contour in the metal, not just a surface irregularity. That, by definition, is bumping.

 

This combination, a soft rubber mallet and a corrugated cardboard backing, is about as mild as it gets. You won’t knock down any ridges this way, but you may correct a small defect without creating a bigger one.

This combination, a soft rubber mallet and a corrugated cardboard backing, is about as mild as it gets. You won’t knock down any ridges this way, but you may correct a small defect without creating a bigger one.


 

Another bumping secret is to work firmly but gently. Usually, using the least possible force and impact, and applying it incrementally, is the best approach. If you swing a hammer too hard, or pry metal too far, you will be on the road to creating problems that are larger than the ones you started with. Think before you strike. Always consider possible alternatives to what may seem to be the obvious approach to removing damage.

As you fight the tendency to just flail at damaged metal with big, nasty tools, it is often important to remember to back up what you are hitting or prying, whether your tool is a hammer, dolly, or pry. Whatever you are hammering is attached to other metal. As you drive it, you are driving that other metal. This is how bad body workers add unwanted upsets and stretches to initial damage. It is also critical to hit metal at the right angle. A ridge can be hammered or pushed up or down from many angles. When you stop to think about it, some are much more corrective and less destructive than others. It pays to take the time to think about this before you swing into action.

Backing up metal as you hammer and/or pry on it usually saves you considerable grief. Hammering off-dolly, or with a dolly, or other backing surface supporting the area around where you are hammering, is usually desirable, unless you intend to stretch metal. Using soft backings often works well. These include rubber-clad dollies, dollies temporarily faced with a soft material like corrugated cardboard, hand-held shot bags, and blocks of soft wood.

Using the right impact tools, tools with the right contacting surfaces, is another way to move metal without damaging it. Rubber, plastic, and rawhide hammers have a definite place in metal bumping operations. If a surface will yield to these tools, it is often a good idea to use them. If not, then harder tools are required. When hammering a crowned surface, a hammer with a fairly low crown probably works best for most purposes because it distributes hammering force over a wide area, and minimizes denting damage. However, dead-flat hammer faces have limited application to this work. Each different situation requires its own, appropriate, hammer crown. There is no single hammer that does it all.

 

Even from the back, you can see that this hammer has more crown than the reverse crown on the metal that it is hitting. This is necessary, or the hammer face’s outer edges would deform the metal while its center would fail to reach it.

Even from the back, you can see that this hammer has more crown than the reverse crown on the metal that it is hitting. This is necessary, or the hammer face’s outer edges would deform the metal while its center would fail to reach it.

 

Sometimes, you can make special tools that greatly speed work and improve uniformity and quality for specific bumping jobs. This little die is used to back up hammering the end of a rib into a proper and consistent round or spear shape.

Sometimes, you can make special tools that greatly speed work and improve uniformity and quality for specific bumping jobs. This little die is used to back up hammering the end of a rib into a proper and consistent round or spear shape.


 

When working on reverse-crown surfaces, a hammer must have more crown than the surface on which it is used, or its edges will imprint the metal, without actually contacting it at the hammer’s center. Square-faced hammers are great for working up to edges, but should not be used in other situations because their corners can cause damage to reverse-crown configurations.

When you hammer against a reverse-crown area, both on-dolly and off-dolly, or against any other unyielding backup surface, the crown of the support surface should be greater than that of the metal. If the backing is held against a convex area, its crown should be chosen for the purpose intended. For example, if you want to stretch metal, you select more crown in your backing than if you do not want to stretch metal. In the case of dollies, these tools usually offer many different crowns in a single tool. With about half a dozen different dollies, you should be equipped with just about every crown and configuration that you will ever need. On some occasions, you may want to create a custom shape to hammer against for a particular purpose or situation. Cutting such a shape out of wood, or making it out of metal is often useful.

 

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The other major category of bumping tools is prying bars and spoons. These helpful devices come in an almost limitless variety of sizes and shapes, and have specific and general uses. Prying can be difficult to control and is used primarily where access problems prevent direct hammering approaches. For example, when metal that you need to work on is directly behind substructure. Some pries are twisted to move metal, while others are levered or hammered.

When you hammer on a broad pry, you are really using it as a body spoon. Spoons, and pries used as spoons, are employed to address areas where access is a problem, or where it is desirable to spread hammering force over a wide area of metal. That last case is very important. In situations that require the minor movement of a broad area of metal, a spoon will distribute a hammer’s force to accomplish that kind of application. Hammering on spoons is often overlooked. It is a very clean and effective approach to the problem of moving large amounts of surface over small, incremental distances.

 

Hammering against either of these spoons greatly lessens the chance of a stray hammer blow damaging the panel. Placing a spoon or pry between your hammer and the metal spreads its force and averages its blow onto a broader surface area.

Hammering against either of these spoons greatly lessens the chance of a stray hammer blow damaging the panel. Placing a spoon or pry between your hammer and the metal spreads its force and averages its blow onto a broader surface area.

 

Dolly-off-dolly is one way to make a swing in very limited space. The dimple removal that is the object of this exercise works, because the hitting dolly’s face has more crown than the panel’s inside.

Dolly-off-dolly is one way to make a swing in very limited space. The dimple removal that is the object of this exercise works, because the hitting dolly’s face has more crown than the panel’s inside.


 

Beyond the rules covering the best ways to move metal (without unacceptably upsetting or stretching it), and the rules of sequence for attacking complex damage, each metal worker finds his or her own comfort level in the choices of procedures and tools to get jobs done. For example, if a situation requires more hammering pressure than can be generated in a small space, it makes perfect sense to use a dolly as your hammer. That might be the case when hammering out the side of a fender from behind, when it has been pushed in, and where there is insufficient room to swing a hammer.

Let’s take that case a bit further. If you are working inside a fender to hammer down a ridge, and using a dolly as your impact tool, you run the enormous risk of missing the exact spot that you are trying to hammer out, or of hitting it hard enough to further damage it. You just don’t have the control over a dolly used that way that you would have over a hammer.

Here are two solutions to that problem: One is to position a spoon on the apex of the ridge, and then drive the spoon against it with a dolly. This would allow more accurate positioning of the force than would be possible with a direct dolly hit. It would also spread the force along the ridge, which would improve the odds on a favorable outcome. The second solution, which takes skill and practice, is to use two dollies, and hammer carefully with them on opposite sides of the panel, in opposite directions. Of course, they must be offset from each other for this to work.

In any specific situation, there are almost always numerous ways to approach a bumping task or job. As you gain experience, you learn which ones work best for you, and you will probably create a few new ones, as you go along.

The next step in working metal is metal finishing, the fine adjustment of surfaces to near-dimensional perfection. It is very important that bumping operations return or bring surfaces very close to correct positions before metal finishing measures are applied. Good metal workers know that a little time spent getting bumping right can save significant time in metal finishing.

 

Written by Matt Joseph and Posted with Permission of CarTechBooks

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