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Engineer's Toolbox:
How to design a new kind of soft-touch tattoo machine

An offset weight attached to a DC motor shaft causes the shaft to oscillate, driving the needle.

The type of nuanced imagery that defines tattooing today requires more than just precision. Subtle shading may involve delivering ink to different levels of the skin in multiple passes, and doing so in a way that minimizes tissue trauma. Although dual-coil tattoo machines provide ideal ink application, they can be heavy and noisy, as well as generating significant vibration and skin damage. To address those drawbacks, designers have tried alternatives ranging from pneumatics to cammed servo motors. Ultimately, they realized that those formats have drawbacks as well, leaving room for innovation.

Mike Snijders, president of Centri Tattoo, decided to step into the gap and fill the need. By combining an innovative design enabled by small DC motors from MICROMO, he developed a novel tattoo machine that marries the nuanced performance of the dual-coil design with the ease of use, minimal maintenance, and long lifetime of DC motors.

Figure 1: The Centri Tattoo machine uses a small DC motor to drive the needle with the same type of soft performance as found in a dual-coil machine. [Photo courtesy: Centri Tattoo Machines]





For nearly a century, the dual-coil tattoo machine has been the gold standard in tattooing. The effect tends to be cushioned, allowing artists to vary needle pressure and penetration depth to achieve a variety of results. They can hold onto the needle to gauge its force or even slow it, without damaging the device. The design minimizes the amount of time the needle spends in the skin, which reduces tissue trauma and speeds healing. The problem is that the traditional machines vibrate and tend to be quite heavy, with a very painful and traumatic experience for the client. Coupled with the long hours required to produce many designs, the machines expose artists to the risk of repetitive-strain injuries. The guns cannot be easily sterilized and require regular maintenance.

Pneumatic tattoo machines, which use air pressure and a rotating cam to drive the needle, can be sterilized but don't allow users to modify needle pressure. The circular cam produces symmetric needle motion, leaving the needle in the skin 50 percent of the time, which intensifies tissue damage. That limits the number of passes that the artist can make over the same patch of skin and lengthens the healing process. In addition, while the handpieces themselves may be light, the systems require bulky air compressors to operate.

More recently, designers have developed servo-motor-based tattoo machines. These versions also suffer from the 50-50 duty-cycle problem, and the needle penetrates the skin with a solid, direct force. "It's like a jackhammer, almost," says Snijders. "After one or two passes, you start chewing up the skin." Dissatisfied with the other options, he decided it was time to develop something better. "People have been trying for years to have a rotary machine that has give like a magnetic version. I thought I'd try to break the barrier."

Snijders knew the type of performance he wanted to achieve with the system. He just didn't know how to get it. When inspiration struck, it came from an unlikely source. "I went down to Radio Shack and bought one of every motor they had and just started tinkering with them," he says. "By the end of the night, there was just this little tiny one sitting on the table."

The motor was designed for pagers and included an offset weight attached to the shaft that caused the motor to vibrate when the shaft rotated. Snijders looked at it, picked it up by the wires to attach them to the battery, and made the connection -- in more ways than one.

Figure 2: Pinning the motor on two sides and applying an offset weight causes it to pivot back in forth. [Photo courtesy: Centri Tattoo Machines]



"I hooked it up, and when it spun I noticed that the whole motor was tipping back and forth," he says. "When I grabbed the motor, the weight would continue to spin -- but I wouldn't damage the motor. Then I would let go, and the whole motor would start shaking again. And I thought, 'Wait a minute, there's the whole idea.'"

Like the dual-coil tattoo machines, the design can tolerate the user holding the needle to slow or even stop it without damage to the motor. Perhaps more important, the motion has the softness that he sought. "You can press up into the stroke and it will start to slow or give. If you were to push really hard with my machine, you will not break the full dermal layer -- it's very hard to blow out [inject ink beneath the dermal layer, where it spreads.]"

He started by developing a housing for the motor that features mounts on either side (see figure 2). These mounts connect to the outer housing via ball bearings that contact to the leads on either side of the motor. Pinned crosswise by the mount/ball bearing combination and driven into oscillation by the offset weight, the motor rocks back and forth at a repetition rate proportional to the rotational speed. That rocking is what drives the needle. Because of the small diameter of the motor and the limited distance of travel, the assembly essentially drives the needle back and forth in one dimension.

Once he'd developed the concept, Snijders had to find a way to make it practical. First, he needed a reliable motor that could supply high-speed, high-torque performance in a compact form factor. He found it with MICROMO. "Basically, they're the only ones who can manufacture a motor that will fit in the machine that I'm making and create the centrifugal force that I want," he says. "Their motor is exactly what I need."

Perhaps the biggest challenge was finding a way to deliver power to the device. During operation, the motor pivots back and forth at 3,000 rpm, which would cause any wire directly connected to the leads to fail within minutes. The solution was to solder the wires directly to the ball bearings (see figure 3).

Figure 3: Soldering the leads (top and bottom) to the bearings that connect the motor housing to the subassembly powers the motor without danger of early failure. Plastic insulators (black caps) electrically isolate the device. [Photo courtesy: Centri Tattoo Machines]





Current passes through the ball bearings to the motor housing and then to the leads so that the motor can move freely with an uninterrupted flow of electricity. Plastic caps over the bearings insulate the device so that current can't travel to the outer housing. The Centri operates on 5 V to 7 V, depending on the desired effect on the skin.

The next step was to develop an external housing to connect the needle module to the motor subassembly. It consists of a 3.6-in.-long, CNC-machined anodized aluminum housing that weighs just 4.5 oz (see figure 4).

Figure 4: The external housing for the tattoo machine features an aperture through which the tattoo needle passes. The eye ring at the distal end of the needle passes over the motor shaft. The thumbscrew (right) is used to attach the tube and grip. [Photo courtesy: Centri Tattoo Machines]





The distal end of a tattoo needle bundle terminates in an eye ring. The tattoo artist passes the eye ring through the aperture in the housing and slips it over the shaft of the motor, securing it with an elastic band (see figure 5). The rocking of the motor causes the needle to oscillate. The artist can modify the stroke using the adjuster screw, or he can adjust both stroke and speed using the power supply.

Figure 5: The eye ring at the back of the needle bundle slips over the motor shaft; an elastic band secures it in place. [Photo courtesy: Centri Tattoo Machines]



One of the key benefits of the MICROMO motors is their consistent performance. Tattoo artists typically run different machines for different tasks -- lining, shading, etc. -- but they need them to operate as expected.

"It's very important that the motors are consistent," says Snijders. "If one of my clients buys more than one machine at a time, when he first plugs in both of those machines, he would expect for them to run identically."

In the few months since the tattoo machine has been on the market, Snijders has sold dozens to customers around the globe. They not only perform -- they last. "I'm pretty impressed by MICROMO's motors," he says. "They're pivoting back and forth at something like 3,000 rpm. It gets violent inside of the housing, and the motors are still surviving."


Published October 2013

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How to design a new kind of soft-touch tattoo machine]

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