The Helve Hammer - Theory, Tuning, and Practice

Helve Hammer Basics

The Helve Hammer is a simple tool that mechanizes the swing of an arm and a hammer. The arm has a hammerhead at one end, and a pivot at the other. The arm has a spring mounted on it, usually a leaf spring. The other end of the spring is connected to a linkage, usually a rod adjustable in length.  The linkage is connected to an eccentric, which is connected to a motor, often by a belt and speed reduction pulleys. The motor drives the eccentric, which drives the linkage, which drives the spring, which drives the arm, which drives the hammer head. The diagram is somewhat oversimplified, but allows you to see the key parts.

The Helve Hammer can be used for most purposes that a hand-held hammer can be, especially shrinking and stretching. By changing the anvil and hammerhead, the helve can perform a variety of tasks. For example, by using a domed hammerhead and a soft anvil or beater bag, the helve can stretch, similar to using a mallet with a beater bag. The same hammerhead can also be used to shrink, using a hard plastic anvil.  The helve will hammer down tucks, or ruffles, similar to tuck shrinking by hand

The Helve Hammer is a true hammer,  as are a Yoder sheet metal power hammer or a blacksmith power hammer. That is, the hammer head moves several inches (10 cm or so) for each blow, and the travel of the hammer head is not hard limited, due to the spring involved. A nibbler, such as a Pullmax, has a fixed travel, generally a fraction of an inch (a few mm), and is not a true hammer.

It is not necessary to have a machine shop or a lot of money to build a helve hammer. If you have a lot of junk lying around, you may not need to buy much.  This makes it particularly attractive to homebuilders. I built mine with a cutting torch, an angle grinder, a cutoff saw, a stick welder, and a drill press. Most of the steel was free.  I am sure that one could be built with less.

Safety

As you might imagine, a powered hammer such as the Helve hammer is inherently a very dangerous tool. For example, if you put your finger between the hammer head and the anvil, it will be crushed. The travel of the Helve arm means that you could be struck in the head by the arm as it goes up. If something gets loose, you could be hit. Most Helve hammers do not have safety guards, although they could be advantageous. The tool needs to be used with a great deal of caution, keeping body parts away from the arm and hammer head.


The Infinite Variety of Helve Hammers

Most Helve hammers are home built by individuals to their own design, often using available material, so you can find an amazing variety of designs. I will discuss here some of the common variations.

One variation relates to where the drive linkage and eccentric is placed relative to the arm pivot. In some cases, the drive is in front of the arm pivot, in other cases, it is behind the arm pivot. This decision is generally a function of the motor and drive system, and personal taste. I put my linkage behind the arm pivot, because I wanted the mechanism as far away from the operator as possible, behind the supports for the arm pivot. Other people put the linkage in front of the arm pivot, which gives the longest arm for a given footprint, and may make the design more compact.

The eccentric also varies from design to design.  Some have adjustable eccentric offset, while others go with a fixed offset, often around half an inch (12 mm).

Another variation is whether the spring is below or above the arm. In most cases, if the linkage is in front of the arm pivot, the spring will be below the arm. If the linkage is behind the arm pivot, the spring could be in either position.

The power for the helve hammer seems to be whatever people can get cheaply. An air drill motor can drive the eccentric directly, eliminating the need for a speed reduction mechanism. Air drills or other air motors do not need clutches, since they are inherently variable speed, controlled by an air valve.  They also stop quickly when the air is turned off. Industrial sewing machine motors are popular because they often have clutches built in. A standard electric motor, with speed reduction from belts, gears, or some other mechanism, can drive a helve hammer. Electric motors are typically in the range of one quarter to 1 horsepower, with 1/2 horsepower (400 watts) a common size.  Most users prefer to have a clutch to provide better control of the hammer. Some users have no clutch, and have a foot pedal on-off control. Virtually all helve hammers use a foot pedal control, connected in such a way that the further the pedal is pushed down, the faster the hammer runs, and the harder it hits.  Some helve hammers can make a single hit by stomping on the pedal.

The speed of helve hammers varies, but is generally between 100 to 400 hits per minute.  Most helves run on the slow end of this range, but the higher hit rates are believed to be better for planishing.

The length of the arm is driven by the space available, the size of the work being done, and how hard the designer wants the hammer to hit. A longer arm may have a greater hammer travel distance, hence may hit harder.  A typical arm length is around 3 feet (one meter).  Arms may be made of wood or metal.

What does the spring do?

The spring in a helve hammer does two things. First, it may prevent damage if the hammer is adjusted down too far or otherwise used incorrectly. Second, it lets the hammer hit harder by storing more energy. If the eccentric is rotated slowly, the hammer head will move up and down a certain distance. This is the basic travel of the hammer. As the eccentric rotates faster and faster, the spring comes into play, and the hammer will move up and down further. The difference between the two travel distances is the overtravel. A typical helve may have a basic travel of three inches, but a maximum travel of seven inches or more, with an overtravel of four inches.  Some helves have shorter travel.  The further the hammer travels, the higher the head velocity, and the harder it hits.

Resonant systems theory as applied to tuning helve hammers

Any combination of a weight and a spring has a natural frequency.  This is basic physics.  Hang a weight on a rubber band or a long soft spring, pull the weight down, and watch the weight go up and down.  No matter how far the weight travels, the number of times per minute that the weight goes up and down remains the same.

A correctly adjusted helve hammer is a resonant system, consisting of the arm, the hammer head, the spring, and the drive system.  If you drive a system with a force at the system's natural frequency, the magnitude of the variation (travel) will be maximized.  If you drive a system at a frequency far from it's natural frequency, you will not get much for your efforts. For example, if the hammer/arm/spring system has a natural frequency of 50 cycles per minute, and you drive it with 100 revolutions (cycles) per minute, the hammer head will hardly move at all!  However, if you cut the length of the leaf spring in half, that will double the stiffness of the spring, and double the natural frequency of the hammer/arm/spring system to 100 cycles per minute.  If you take a 100 cycle resonant frequency hammer/arm/spring system, and drive it with an eccentric turning at 100 RPM, you will get the maximum travel and maximum hitting force.  Alternately, you could change the gearing or pulley to make the eccentric run at 50 RPM.  Either way, you are matching the natural frequency of the hammer/arm/spring system to the eccentric speed.  When the system natural frequency is the same as the eccentric RPM, the system is in resonance, which will maximize hammer travel and force.

The goal of tuning a helve hammer is to make the system resonant;  to match the frequency of the hammer/arm/spring system to the eccentric RPM.  This will maximize how hard the hammer hits.  You can change the tuning of a helve hammer by changing one or more of the following variables:  spring, hammer weight, arm length or weight, and eccentric RPM.

Step by step tuning process

Do you need to understand all this theory about resonant frequencies to tune your helve? An understanding of the theory will make it easier to tune the helve, but you can also tune it by trial and error. It doesn’t need to be tuned perfectly to work well, but a poorly tuned helve will not live up to its potential.

Before you start tuning, you need to measure the basic travel of the hammer head. Adjust the hammer head up such that the hammer in the rest (down) position is several inches above the anvil, such that the hammer can swing freely. Rotate the eccentric slowly, hold a ruler next to the head, and measure the travel distance, which is the basic travel of your hammer.

If you have a clutch or other way of varying the speed of the eccentric, gradually increase the speed, measuring the hammer head travel as you go. If the maximum head travel does not occur at maximum speed, then your spring is too soft, and you need to shorten it. You want the maximum travel to occur at maximum speed. If the maximum head travel occurs at maximum speed, you are probably close, but your spring may be too stiff. Lengthen the spring to make it softer, and repeat the test. To be sure that it is tuned correctly, you will need to make the spring too soft (too long), and notice when maximum head travel occurs a little below maximum RPM. When this happens, shorten the spring a little until maximum head travel occurs at maximum RPM. At that point, you are finished tuning your helve; it will hit as hard as it can, given the design and motor.

If you do not have a way to vary the hammer speed, just an on-off switch, you can still tune your hammer by trial and error. Start by trying three different effective spring lengths, short, medium and long. Measure and write down the travel for each spring length. Try a spring length between the two positions that produced the longest travel, and then measure and write down the new spring length and travel. You simply try different positions until you figure out which spring position produces the longest travel. Once you are satisfied that you have located the optimum position, or close enough, your hammer is tuned.

What do you do if the optimum spring length after tuning is very short or very long? If that happens, you probably need to find a different spring, or change the speed of the eccentric. If the optimum spring length is very long, your spring may be too stiff, look for a narrower or thinner leaf spring. Alternately, you might be able to speed up the eccentric by changing a pulley. If the optimum spring length is very short, your spring may not be stiff enough, look for a wider or thicker leaf spring. Alternately, you might be able to slow down the eccentric by changing a pulley.

Once you are satisfied that your hammer is tuned, you should adjust the hammer head down to where it is just above the anvil when the hammer is stopped. That will make it easy to slip your part in before you start hammering, and allows the hammer to hit the anvil at about maximum speed.

Adjusting basic travel

Generally, if the hammer has more basic travel, it should have more maximum travel.  So more basic travel means a more powerful hammer, and less basic travel means a less powerful hammer.  If you have too much travel and a weak motor, the motor will not be able to keep up and do the job.  If you do not have enough travel, your helve may not have the power you need.  However, many users never need to adjust the basic travel.

If you want to adjust your basic travel, and you have a variable eccentric offset, simply increase or decrease the offset.

If you have a fixed eccentric offset, and you want to adjust your basic travel, you can change the eccentric attachment to the spring.  If the eccentric attachment to the spring is closer to the arm pivot, that will increase the basic travel.  If the eccentric attachment to the spring is further away from the arm pivot, that will decrease the basic travel.  This will change the angle of the linkage.  Generally, one wants the linkage more or less straight up and down, but a small angle is not likely to cause a problem.   To avoid changing the tuning of your hammer, do not change the spring length when you make this adjustment.

In the two drawings below, the effective length of the spring is the same.  The difference is in the relative location of the top of the linkage and the arm pivot. To put it another way, for more travel, tilt the linkage towards the pivot, for less travel, tilt the linkage away from the pivot.  Notice that the location of the clamp is different in the two drawings.

Hammer Travel - Longer is Not Always Better

All else being equal, the longer the hammer travel, the faster the hammer speed, and the harder it hits. If you are roughing out a piece, you may want hard hits. But once you start planishing or smoothing, the harder hits start to work against you. If you have a variable speed drive, then you could just back off the pedal some, slow down the hammer, and it would hit less hard. However, the reduced rate of hits means that it will planish more slowly, and it may not be easy to hold the pedal part way down to get a consistent speed.

Perhaps the best solution is the travel limiter. Travel limiters have an adjustable rubber bumper that limits the arm travel in the up direction. By limiting the travel, you limit the effectiveness of the spring, and how hard the hammer can hit.  The rubber bumper absorbs most of the energy, slowing down the hammer.  (Springs reflect almost all the energy when they are stretched or compressed, but rubber absorbs most of the energy.)  With a travel limiter, you can run the hammer at full speed, and planish to your heart’s content. The more powerful your hammer, the more likely that you will want a travel limiter.

Alternately, you could adjust the hammerhead above the anvil an inch or so (a few cm). That would also reduce how hard it hits, since the spring would slow down the head before it contacts the anvil.

Keeping Your Helve in place

A helve has a lot of mass and parts moving up and down. Hence it tends to shake, vibrate, and move around a lot. Most helve hammers are bolted down to concrete. If you don’t bolt it down, you are likely to find you need a surprising amount of weight in the base. I ended up with 125 pounds (55 kg) of weight in the base of a small helve hammer with an 18 inch (45cm) arm and a two pound (1 kg) head, just to keep the hammer base from jumping up and down. A standard size helve hammer would need much more weight in the base, if it is not bolted down to a concrete floor.

Stretching with the Helve

A helve hammer is a stretching fool. It will hit and stretch as long as you hold the pedal down.

A metal domed hammer head is generally used for stretching. On the bottom, you can use a beater bag or a rubber anvil.

Many experienced helve users prefer a beater bag. The helve works just as it does when using a hand held hammer on a beater bag. For example, it will form ruffles or tucks on the edges while you hit in the center.

Alternately, you can use a rubber anvil. The rubber anvil consists of one half to one inch (1 to 2 cm) of moderately hard rubber; 70 durometer neoprene is a good choice. The helve will stretch the area that the head hits, without greatly affecting the area around it. For example, you can stretch the center without tending to bend or ruffle the edges.

Note that some people are using a concave die on the bottom instead of a beater bag or a rubber anvil.

Shrinking with the Helve

Just as one would hammer down a tuck with a hand held hammer, you can hammer down a tuck with a helve hammer. Slow the hammer down or limit the travel, and use a hard plastic on the anvil, to avoid inadvertently stretching the area you wanted to shrink.

Heads and more Heads

There are an amazing variety of hammer heads that people have made for their helves. Doming heads and linear stretch heads are common, as are metal heads or heads with plastic surfaces.  Some people are designing and using dies on the anvil instead of rubber or plastic or a beater bag.  You are limited only by your imagination.

It is probably best to make all the heads the same weight, to avoid changing the tuning of your helve hammer.  However, if your hammer weights vary, tune your hammer with the heaviest head.  When you put on a lighter head, the hammer will be somewhat out of tune, which means it will not hit as hard.  But then, you probably didn't want to hit as hard with the lighter head anyway.

Summing Up

As you can see, there seem to be an infinite variety of helve hammers. Before building your own helve, do a little web searching and look at what other people have built. Consider your needs and the space available, and start looking for an appropriate motor.

This overview is intended to help people design, tune, and use their helve hammers. I do not claim to be an expert in helve hammers, but I have asked a lot of questions and studied the answers from the real experts. I want to thank the many people who have helped me design my helve and write this article. I hope that you find this overview of helve hammers to be useful.

Happy hammering!

Richard Ferguson
December 5, 2004
http://www.fergusonsculpture.com

Note:  Diagrams by Richard Ferguson, photos by Jim Bailie.