The Planishing Hammer: A classic metalworking tool with staying power

April 1, 2004

Airframe Technology

A classic metalworking tool with staying power
By Ron Fournier

During the 1930s and ’40s a useful, air-powered metal working tool called a planishing hammer came into common use and became quite popular. Once adopted by craftsmen, it quickly become a mainstay and has experienced a resurgence of interest in recent years.

A planishing hammer is a good tool, but it might be one of the most commonly misunderstood metalworking tools on the market today. It was never intended to be a primary metal shaping tool. It is essential to understand what planishing means before we can understand the tool and the function it performs. Let’s clarify. Planish means to flatten, smooth, or polish (metal) by rolling or hammering. The process of planishing uses many light blows to smooth metal which has already been formed by some other means. So always keep in mind when using a planishing hammer, the part you are making is first hammered out, roughed, or blocked out close to its general shape.

One of the many ways to accomplish the task of planishing a metal part is to use a planishing hammer. In my experience, the most effective way to use a planishing hammer is in combination with a shot bag. Quite simply, they were made for each other. You can use a variety of hammers, wood slappers, and mallets to pre-shape or “block” the metal pieces and then smooth them using a planishing hammer.

Sizes and styles
Planishing hammers come in several sizes and styles, and of course, a range of prices. Some are bench mounted and others are mounted on a heavy stand. An important measurement to consider is the throat depth. This measurement indicates the space the metal can be inserted into the hammer. I have seen them 12 inches deep up to 36 inches deep. Of course, the larger the projects you wish to fabricate, the larger the throat depth you will need. Planishing hammers usually come with (or have available as options) various styles and sizes of forming dies for planishing, stretching, and curving.

When selecting a planishing hammer for your shop consider not only the size of the projects you will be working on, but also how many and what type of forming dies are included with your purchase. The planishing hammer that appears to be the most cost-effective can suddenly become quite expensive once “optional” tooling is added on.

While working with a planishing hammer, take the time to choose a die that is the proper fit for your project. For example, the lower die should be nearly flat for planishing a flat slat surface and should be curved similarly to a curved surface.

I’m using a Mittler Brothers 24-inch throat planishing hammer. The pressure valve is at the top. Use low pressure for best smoothing results. Be sure to smooth the entire surface, going over the part several times. Use very low air pressure to avoid stretching.

Use low pressure
Another key to a successful smoothing/planishing operation is to always use low air pressure. Too much air pressure can stretch the part past the desired shape. Remember, a planishing hammer works at smoothing a preshaped metal part. It does this by hitting the metal often, not hard. A regulator on the end of the hammer controls the striking force you’re using. The higher the air pressure, the harder the head hits. Just as metal shouldn’t be hit too hard when hand planishing, it shouldn’t be hit too hard with a planishing hammer. The same things happen: work hardening and additional stretching.

It is always best to get the feel of any new tool before setting out on your next important project. Be sure to experiment with several settings on some scrap steel and aluminum to get an idea for what the planishing hammer can do before starting on the actual piece you are making.

It’s important to mark the metal where it needs to be stretched, as well as where it doesn’t need to be stretched. This will help keep you on track while working on your project. It is also helpful to use a template to monitor the amount of stretching.
Most serious hobbyists and full-time professionals will, at one time or another, consider adding a planishing hammer to their tool arsenal. They are a powerful tool that can greatly increase metal shaping efficiency.