Here’s a Harbor Freight machine....
Here’s a Harbor Freight machine. It’s good for smoothing small panels, and enables you to do power planishing on a shoestring budget.
Pneumatic planishing hammers are a real workhorse for both professional and amateur metalworkers. In this article, we’ll look at new hammers available at different price points, plus some vintage planishing hammers. We’ll discuss the relative merits of each machine, and show some of the varied tasks these machines excel at.
Let’s start by defining the term planishing, which means to smooth by light hammering. Metalworkers have been planishing sheetmetal parts ever since hammers were invented. The basic idea is that hammering with relatively light blows will tend to smooth the metal, and as long as you use the proper technique, the metal won’t become over-stretched.
Body hammers are fairly light, and if you just allow a hammer to fall from several inches above the metal, the light hit doesn’t cause any change in the metal. You need to swing the hammer with enough force so that inertia causes the hammer to do some useful work. The moving parts of most pneumatic planishing hammers weigh just a few ounces, so it requires a lot of inertia for the lightweight die to move the metal. This is accomplished by oscillating that small weight rapidly.
Handheld hammers can sometimes...
Handheld hammers can sometimes be used to take dents out of a panel without removing it from a car. This is a vintage Chicago Pneumatic (CP) machine.
Now, let’s compare planishing hammers with some of the more powerful metalshaping machines, such as an electric power hammer. A power hammer will most likely have a moving upper die and holder that can weigh upwards of 10 or 20 pounds, and it’s operated by an eccentric crank that lifts this die and then literally throws it down against the lower die. Most power hammers have some sort of elastic material (usually leaf springs and fabric webbing) between the crank and the moving die, which allows the mechanism to store and release energy, something like “cracking a whip.” This causes the dies to strike together with enormous force, and this will make the metal move in a hurry. Planishing is a much more delicate operation, better suited for smoothing rather than for big-time shaping.
There are many other machines that are used to move metal; a partial list includes Helve Hammers, English Wheels, and Universal or fixed-stroke reciprocating machines.
This vintage CP handheld machine...
This vintage CP handheld machine is mounted on a shop-built pedestal. It’s operated by a foot pedal that operates the control valve with a cable. This makes a great machine.
The traditional way to planish a piece of metal is to work against a dolly block that has a shape close to what you want for the finished panel. You place a rough-formed piece of metal on top of the dolly, and hammer the metal so that with each blow, a small portion of the metal gets squeezed tightly between the hammer and dolly. Even if the metal is initially lumpy or wavy, the metal will smooth out with enough hammering, assuming the hammer and dolly have smooth surfaces, and as long as enough force is applied to exceed the elastic limit of the metal.
If you’ve done much hand planishing, you know that it’s a slow process at best, and even a proficient metalworker is sometimes challenged to get everything aligned just perfectly, so the hammer hits the metal at exactly the point where the dolly is touching on the back side. If you miss this contact patch by even 1/4 inch, the process doesn’t work well at all.
This tool can work a panel...
This tool can work a panel in place, sometimes saving hours on a repair job.
A key benefit of using a pneumatic planishing hammer is that the “hammer and dolly” (actually the upper and lower dies of the machine) are always precisely aligned, presuming the machine was set up properly to begin with. Further, many planishing hammers work at about 2,500 hits per minute (a few go up to around 10,000) so they are much faster than hand hammering, and the quality of smoothness obtained on the finished part is usually much better than what you’d get with hand hammering. Working as fast as I can while maintaining control, I can only manage about four blows per second, (240 hits a minute) so a planishing hammer can be at least 10 times faster than hand hammering.
One of the handiest features...
One of the handiest features of planishing hammers is their ability to crush welds completely flat, and to correct the distortion caused by welding.
After about a minute’s work,...
After about a minute’s work, the dent is virtually gone.
This panel warped noticeably...
This panel warped noticeably when it was welded, but after a little planishing the weld is completely flattened, and the contours of the panel are nicely restored.
This is the Mittler Brothers...
This is the Mittler Brothers planishing hammer. It uses a needle scaler as the power source, so it’s more powerful than some other machines.
This machine is made by C....
This machine is made by C. Cook Enterprises. It’s very similar in concept to the vintage CP machines, and the dies are sized the same.
Planishing hammers have the...
Planishing hammers have the ability to work close to flanges and beads. Here, some dents on a model T door are being smoothed right next to the bead.
Another task that planishing...
Another task that planishing hammers do well is turning flanges. That’s how this flange was formed on the front of a transmission cover.
The construction of a planishing hammer is fairly simple. The power unit (typically an air motor or some sort) is the “heart” of the machine. This can be as simple as an air chisel on some entry-level machines, or it could be a needle scaler, which will typically have more power. It could also be a riveting gun, or an air motor designed specifically for this application. Using a purpose-built air motor or a rivet gun brings a lot more controllability to the process, allowing you to “feather the throttle” a great deal, adding a valuable degree of control over the process. Air chisels and needle scalers usually have limited speed adjustment.
While somewhat unusual, some very effective planishing hammers have been built that are completely mechanical, and don’t require compressed air to operate. Check out the photo of the Mechammer in this article.
This motorcycle gas tank side...
This motorcycle gas tank side was roughed-out with a mallet and bag, then smoothed with the planishing hammer in about two minutes.
The power head is fastened to a (usually) C-shaped frame, with a receiver placed directly below it to hold the bottom die. As you’ll see, there can be a number of different lower dies used, depending on what you’re trying to accomplish with the machine. Many machines have a foot pedal that starts and stops the hammering action, although some machines use a hand-operated valve.
It’s important to be able to change the gap between the top and bottom dies. One reason is to simplify die changes, and to accommodate the use of dies with different heights. Another is to allow panels with flanges, beads, or other obstructions to be easily fed into place between the dies. The tightness of the dies (the gap between them as you’re hammering) has some effect on the way the machine works, too, and some machines are set up so the same foot pedal that starts and stops the hammering action also adjusts the gap between the dies. While not essential, this is a desirable feature.
There is a special linear...
There is a special linear stretch die that’s ideal for curving flanges. After turning the flange, this panel “opened up” a bit, so I regained the proper contour with the linear stretch die.
Some machines are pedestal-mounted, so you can position them wherever you want, while other machines are bench-mounted, and a few machines can be handheld, as you’ll see.
Let’s take a look at several different styles of planishing hammers, keeping an eye out for the unique features of each type. At the end, we’ll show examples of a few machines that are often confused with planishing hammers, but are fundamentally different.
These are just a few of the...
These are just a few of the dies available for the C. Cook machine.
Here’s a more traditional...
Here’s a more traditional steel die, used for general planishing. Usually you will select a lower die with the same contour that you want on the finished part.
Here’s a very heavy-duty machine...
Here’s a very heavy-duty machine made by TM Technologies.
The foot pedal for this machine...
The foot pedal for this machine can be scooted wherever you want it. This is very helpful when working on large, awkward panels.
This dome was roughed-out...
This dome was roughed-out with the leather top die, and planished smooth with steel dies. The total working time was just a few minutes.
The TM Technologies machine...
The TM Technologies machine can be fitted with special dies made from different hardness materials. This is a leather top die used for doming metal, much like working with a mallet and sandbag. There other dies that can even do shrinking on most metals.
If you’ve ever done beading...
If you’ve ever done beading on panels, you know it often gives the metal an unstable twist.
The planishing hammer offers...
The planishing hammer offers a great remedy for this situation. You can pre-stretch the panel where the bead will go, which makes all the difference in the world.
Here you can see how flat...
Here you can see how flat and relaxed this panel is after having a bead rolled into the pre-stretched area.
This process is most efficient...
This process is most efficient with a machine that uses a foot pedal to move the dies together, and then start the hammering. It helps to use a support table to hold the panels level.
The planishing hammer is great...
The planishing hammer is great for riveting, too. This is a 3/16-inch dome-headed rivet. I’ll use a number of these to attach a blister to hood side.
Look at what a great job the...
Look at what a great job the planishing hammer does setting the rivets. Details like this can add a lot of interest to your car.
These are special riveting...
These are special riveting dies; the bottom die has a recess that matches the head of the rivet, and the top one has a similar pocket, which forms the back side of the rivet.
Some people build their own...
Some people build their own planishing hammers. Here’s a great machine built by Joe McGlynn.
I’m using linear stretch dies...
I’m using linear stretch dies in Joe’s machine to create a challenging, “potato chip” shape with four continuous reverse-curves that blend together smoothly.
There are several machines...
There are several machines similar to planishing hammers, but different in construction and use, such as this shop-built Helve hammer. These are great for roughing-out panels, but not ideal for smoothing.
The linear stretch dies must...
The linear stretch dies must be fixed so they can’t rotate. Here’s a simple sheetmetal holder for the die, fastened with a hose clamp.
Here’s a Universal (fixed-stroke...
Here’s a Universal (fixed-stroke reciprocating) machine made by Dake. This is an extremely versatile tool, but in a different class than a planishing hammer—it works by putting a powerful squeezing force on the metal.
If you’re building a machine,...
If you’re building a machine, consider making the arms removable. Here’s a horizontal lower arm, great for rings or cylinders, which often isn’t possible with C-shaped lower arms.
Rick Scott builds this Mechammer...
Rick Scott builds this Mechammer (mechanical planishing hammer), which is driven by an electric motor. Note the two fully adjustable lower arms, one straight and one angled.
Here’s Craig Naff with his...
Here’s Craig Naff with his Yoder power hammer; a heavy, powerful (and expensive) machine great for doing big-time stretching and shrinking of metal.
Ron Covell has made a DVD...
Ron Covell has made a DVD to accompany this article, showing each of the machines and processes in much more detail. Planishing hammers are used by both amateur and professional metalworkers, and some are quite affordable. This DVD has a wealth of information for anyone interested in learning more about these versatile machines.