Terry_Cowan (16K)

Tips & Tricks From the Pros

This column is available for professionals in the metalshaping community to share some of their knowledge.
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Ron Covell

Tips from the Pros

For the Metalshapers Website
© 2003 by Ron Covell

Photos by Terry Cowan

 

 

 

 


Q. What’s the secret for getting metal smooth using a hammer and dolly? Sometimes when I try it, it gets worse instead of better.

A. Hammers and dollies are simple looking, but it takes a good measure of skill to use them properly. Most everyone has some experience using a hammer, since the nail-driving style is so widely used for household maintenance. Unfortunately, driving nails is not the same as smoothing lumpy metal.

Let’s discuss hammers. The face is usually the ‘business’ end of a body hammer, and it’s usually round, but sometimes square, and occasionally you might find an oval-faced hammer. Probably the most important characteristic of the hammer is the curvature and diameter of the face. Diameters can range from 1 1/4" to 2", with around 1 5/8" being my favorite. The face of most body hammers looks relatively flat, but closer examination shows that they have a certain amount of curvature.

This hammer has between a 12 and 24 inch radius

 In fact, a hammer with a truly flat face would be nearly impossible to use. The problem is, if you struck something with the face at even a slight angle, the edge of the hammer would leave a ‘smile’ mark in the metal that could only be removed by sanding, filing, or filling with lead or plastic body filler. Most low-crown hammers have a face that with about a 20” radius, but a few hammers are made with much more curvature than this. The curvature of the hammer face has a lot to do with how well it works, and it’s important to understand why. For general-purpose work, a low-crown hammer face is the one used most often, but you may wonder why some hammers have higher crown faces? There can be a variety of reasons. When you strike metal with a hammer, the contact area of a low-crown face might be around 5/16” across. This means that small area absorbs the entire force of the hammer blow. If you use a medium crown hammer (6” to 10” radius), and hit with the same force, the contact patch might be only ¼” across – or about 2/3 the area of the low-crown contact patch. A hammer with a high crown face (2” to 4” radius) might have a contact area only 3/16” wide, or about 1/3 the surface area as the low-crown hammer. If you put the same force into a smaller area, the effects of thinning and stretching will be exaggerated. On the rare occasions when you need to stretch the metal a great amount, these medium and high-crown hammers will certainly speed the work, at the expense of leaving a more ‘textured’ surface (recall the look of a hand-hammered copper vase to help visualize this). A more common reason to use a hammer with a medium or high-crown face is to work into a concave area where a lower-crown hammer face would only hit on its edges.

Body hammers come in a wide range of weights, from 12 oz. to 18 oz., or even more. I prefer the lightest hammers for delicate work, but the heavier ones may better suited for doing heavy collision repair. The handles are usually made of wood, but fiberglass handles have been on the market for many years, and a few people prefer the slightly more flexible ‘feel’ of the fiberglass-handled hammers. I think the fiberglass handles may be more damage-resistant than wooden handles, as well. Although it’s rare for a wooden handle to break, it can happen, especially if you are abusive with the hammer. My personal preference is for the traditional wood handles, but you might want to try them both.

Body hammers are often named for the shape of the head that is opposite the face. Some common types are Pick, Chisel, and Cross Peen.  

While it’s nice to have a selection of hammer styles, I only occasionally use the end opposite the face. A pick hammer can be used to raise small low spots for metalfinishing, sometimes called ‘pick & file’ work. A cross-chisel or cross-peen hammer can be used to re-establish a peak, ridge, or crevice that has been damaged.

 

 

 

 

While we are on the subject of hammers, we should probably address so-called ‘shrinking’ hammers. There are two styles of these – the ‘meat tenderizing’ style, and the spiral-faced cam-action style. 

 
Snap-On pyramid style


British made Sykes-Pickavant flat style

The ‘meat tenderizing’ style has a gridwork of grooves embossed into the face, creating rows of sharply-pointed pyramids. While a hammer of this style may in fact ‘tighten up’ metal that is ‘loose’, it does so at the expense of putting a waffle-like texture in the metal.

If you plan to use plastic filler over the worked area, this texture won’t be any problem, but if your goal is to metalfinish or polish the panel, putting a waffle pattern on it is a step in the wrong direction. As for the spiral-faced hammers with a cam that rotates the head as you strike, I’ve tested them in every way I can think of, and my measured opinion is that they don’t shrink metal, at least not any better that a smooth-faced hammer would. Again, they put texture into the metal, which may be undesirable. (I normally do everything possible to keep the metal smooth.)

All of the US made hammers I’ve seen are high quality, although the surface finish will vary a bit from one manufacturer to another, but there are huge differences in price. Surprisingly, some of the super high-end (and very expensive) ‘Tool Truck’ hammers do not have the best surface finish, even though they are forged from good steel. There are many imported hammers on the market now, and the ones I’ve seen that come from Germany and England are excellent, but the ones that come from China aren’t so hot. They are made of low-quality metal, and the ‘balance’ doesn’t feel right to me. As much as I dislike these cheap tools, you could probably make a case for buying a $20 set of 3 hammers and 3 dollies to find out what sizes, shapes, and weights of hammers and dollies you like, and then buy some ‘good’ tools once you’ve discovered your preferences. Don’t rule out swap meet and Internet auction-site tools – there are some great bargains to be had if you know what to look for.

There are probably even more varieties of dollies available than hammers! The ‘good’ ones weigh 2 to 3 ½ pounds, although there are certainly some that weigh more and less. A dolly lighter than 2 pounds doesn’t really provide adequate inertia to work with a medium or heavy hammer blow, but a light, thin dolly just might fit into a ‘tight spot’ that no other dolly can. My heaviest dolly is the 4 ½ pound Martin “Heavyweight” dolly, and I generally only use it in the rare instance that its shape happens to fit a particular panel better than any other dolly I have. It’s quite a strain for me to hold that dolly at arm’s length for more than a minute or two!

Domestic and European dollies are usually made from forged tool steel, and are quite tough. All the Chinese dollies I’ve seen are made from cast iron, and they dent much more easily than the ‘good’ ones. If you do much work on aluminum you might consider having a special dolly or two that are never used on steel, since any little nicks or gouges in the dolly would leave marks in soft metals.

The advantage of having several dolly shapes is that the closer a dolly’s contour matches the panel your are working on, the easier it is to get the panel smooth by hammering. For a start, I’d suggest getting a dolly with a large, low-crown face, such as either the Martin ‘Loaf’ or ‘Egg’ dolly, and a dolly with a totally flat surface, and one edge that’s perpendicular to it. The low-crown dolly is the one I use for most of the work I do, and the flat dolly with a right-angle edge is helpful when you are working on flat panels, or when you need to create or straighten a flange on the edge of a panel.

Now let’s look at how hammers and dollies are used. Hammers are generally used to tap high spots down. Whenever you tackle a metalworking project, you need to have a plan of attack before you start. If you just start hammering away with no strategy, things are likely to get out of hand!

The greater the crown of a panel, the more strength it has. On medium or high-crown panels, you can sometimes tap high spots down without using a dolly underneath. It won’t hurt anything to try this, since either the hammer will move the metal, or if not it will simply ‘pop back’. This doesn’t work very well on low-crown (or flat) panels, because they don’t offer enough resistance, so you’ll probably have to use a dolly behind the metal on these panels.

 

 

 

So how are a hammer and dolly used together? There are two fundamentally different techniques – hammer ‘on-dolly’, and hammer ‘off dolly’. It’s crucial that you understand the difference between these.

When working on-dolly, the metal being hammered is compressed between the hammer face and the dolly, which makes it microscopically thinner, and therefore ‘stretches’ it. A good way to visualize what’s going on here is to think of the metal as a sheet of pizza dough. As you ‘roll out’ pizza dough with a rolling pin, it gets longer and wider as it gets thinner. This is exactly what hammering on-dolly does to metal. Since the width of the hammered area is increasing, it has to go somewhere, and it will ‘dome up’. 

 

Actually, this is not always the case. The truth is that it will dome more in the direction it is already domed. The vast majority of the time when I hammer on a panel, I’m hammering on a convex surface, and if I hammer on-dolly, the surface will rise. If I am hammering in a ‘cupped’ ‘dipped’ or ‘hollow’ area, the metal will actually go DOWN as I hammer on dolly! Another consideration is that it is crucial to press up on the dolly with a lot of force when you are hammering on-dolly. If you don’t use enough pressure, the process doesn’t work well, and the part you are hammering on may go down instead of up. So in summary, the result you should see if you are hammering on-dolly correctly is that the area you are hitting moves up, while the metal next to the hammered area stays just where it was.

Hammering off-dolly is an entirely different kettle of fish! In this case, the dolly is held so that it touches any low spots on the metal, and you hammer on the nearby high spots. The hammer makes the high spots go down, and the rebounding action of the dolly moves the low spots up! The contact areas of the hammer and dolly have to be pretty close together for this to work– if your dolly is more than about an inch away from the hammer, you probably won’t get much rebound action from it. The hammer may make the high spot go down, but the low spot probably won’t be raised.

And this brings us to the crux of the matter – how do you know where the hammer and dolly are actually touching the metal? Well, let’s start with the hammer. If you are working on clean metal in good lighting, you can often see a small shiny mark left by the hammer blow. If your first blow or two is slightly off-target, you can make a correction on the fly - this is the easy part! What’s far more difficult is to know EXACTLY where the dolly is touching on the back side! With a low-crown dolly, changing its angle even a couple of degrees can make the contact patch move ½” or so! The truth is that it takes a good deal of practice and persistence to learn to control where the dolly touches the metal, but it is exactly this skill that determines your ability to smooth lumpy metal with a hammer and dolly.

Most beginning metalworkers feel challenged by gaining precise control over where the dolly touches the metal. Here’s a little trick that can be a huge help when you’re learning, or when you ‘get lost’. (Hey, I’ve been doing this work for nearly 40 years, and I still ‘get lost’ occasionally!) If you can’t tell exactly where the dolly is touching, hold it still, and tap around with the hammer in a spiral pattern until you ‘find’ the dolly. How do you know when you’re on it? By the sound! When you are tapping on unsupported metal, the hammer blows make a dull ‘thud’ sound. When the dolly is EXACTLY under where you are hitting, the hammer blow will make a sharp ‘ringing’ sound. Anybody can hear the difference, even when using earplugs! Once you’ve ‘found’ the dolly contact patch, continue hammering (lightly) and move BOTH hands together until you’ve brought the dolly’s contact point to the exact position you want. Then, you can hammer either on-dolly or off-dolly, as the situation requires.

A great way to practice these techniques is to get a junk fender, pound some dents into it, then straighten them. As you build your skill on small, easy dents, go on to larger, more complicated ones. The simple hammer on-dolly and off-dolly techniques described here should enable you fix most damage. There may be times when you can’t get high spots go down and stay down, and in these cases you may have to use heat to ‘shrink’ the metal, but that’s a story for another time.

I encourage each of you to practice using a hammer and dolly until it becomes second nature. Even if you plan to get an English wheel, Pullmax, Planishing Hammer, Helve Hammer, or Power Hammer, you still need to learn to master the simple hand tools, and the knowledge you gain will be a great benefit to you as you tackle the learning curve of the ‘big tools’.



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