Original Post: December 18th, 2017
In this latest installment I am elaborating on rotary stroke characteristics. What exactly is happening when the motor rotates the cam, which moves the yoke, which makes the needle go up and down. It may seem simple, but there are some characteristics to this movement which you may not realize but knowing these characteristics will help you chose a more appropriate machine and may even help you tattoo better. Below is a diagram I drew of an offset cam. Any rotary tattoo machine that you can buy has an offset cam. It is how the motor turns rotational movement into linear movement. The offset is what the stroke is often referred to. It is how far the shaft of the motor is offset from the center of the cam.
In this diagram I have separated the cam into 4 equal parts, shown here as arrows around a circle, the circle representing the cam. I have also shaded areas in the background, blue in the middle and red on top and bottom. I will get to the shaded areas shortly.
As the motor spins it is spinning a cam. For the sake of this explanation we can say that the motor is spinning at a constant rate all the way around it’s movement. That means that if you separate the cam’s path into 4 equal parts, as I have here, then the cam spends an equal amount of time in each of the 4 quadrants as it goes around. Let’s now look at the shaded areas in the background. The shaded areas represent the vertical movement of the cam, or the needle movement in this case. As we look at the shaded area we can see that the vertical movement is shorter in the bottom and top sections, the shorter areas are shaded red. As I said before, the needles are spending just as much time in this shorter area as the larger blue area. What does this mean? This means the needles are slowing down at the bottom and top of the stroke, and speeding through the movement around the crest of the top and bottom quadrants.
This seems like a no brainer, of course the movement has to slow down before it reverses direction right? But this isn’t just showing that the needle movement is slowing down, it shows that it is slowing down on half of the stroke. The top quarter and the bottom quarter of the stroke, together make a half.
Now that we got the complicated bit out of the way we can talk about how this translates actual tattooing. I’ve already talked about how a larger cam offset translates in to a faster needle speed in my previous posts, lets now talk about how the cam offset affects this “lag” at the top and bottom of the stroke.
As you can see in the shaded diagram the red sections are where the needles are slowing down in their up and down movement. This area of lag grows as the offset grows, and shortens as the offset shortens, but the ratios always stay the same. The needle will always be slowing down through half of the entire stroke.
This lag is beneficial on the bottom of the stroke. We want the needles to hang in the skin a bit on the down stroke, that allows our hand movement to open the skin and deposit ink in the cavity that forms behind the needle. In turn we also like the needles to speed down to the skin, that gives us the penetrating power to break the skin and deposit the ink without causing a lot of undo trauma to the skin. It is the top area of lag which is the most troublesome. Almost all of us who have ran rotaries have experienced that sensation where the needles seem to snag in the skin. The operator, thinking the machine is running too slow, or not hard enough will put more voltage to the machine speeding it up which just makes the needles come down with too much force, and come out of the skin much too fast. Running a tattoo machine too fast, rotary or coil results in skin that is beat up and undersaturated. That snagging sensation is actually just the needles slowing down at the top of the stroke. If the stroke is too short then the needles will actually start slowing down before they retract fully in to the tube. If the needles are slowing down at the top of the stroke, but your hand isn’t, then you are going get that “snag” sensation.
I like to make sure that the stroke on my rotary machines is long enough where the whole top quarter of the cam rotation happens inside the tube. This turns this lag in to a benefit, as it slows down in the ink reservoir picking up as much ink as possible before racing down to skin. That means if you are running tube to the skin the needles are coming out of the tube at max velocity, slowing down at the bottom, and racing back up to the tube and your hand doesn’t feel the lag at all. The image at the bottom shows how this looks at the needle end. The short stroke shows the needles slowing down before retracting in to the tube.
The longer stroke shows the needle coming back from the bottom lag and entering the tube at it’s maximum speed.
I had mentioned the needles retracting fully in to the ink reservoir and taking advantage of the top lag of the stroke. I want to explain something else that is happening while the needles are moving up and down. For this image I’ve used a shader but the concept holds true with liners as well. Most tubes have a separated ink reservoir and a flat area for the needles to ride on. The tattoo needles have a solder lug holding the individual needles together. This solder lug acts as a lid to the ink reservoir. In a longer stroke machine the needles are allowed to move up enough for ink to spill in to the needle slide area. On a shorter stroke machine the lug may never leave the top of the reservoir keeping the ink from spilling down to the skin. Many tattooers get around this by bending their needle bar, or bending the solder lug to allow the ink to flow under the needles but this is often not the best solution as the needles flatten out when tension is put on the bar from a rubber band. And bending isn’t a practical option for cartridges. A shorter stroke or a faster cycling needle will also cause turbulence in the ink reservoir and will actually push ink away the needles and back up the tube.
So what if you prefer a smaller cam offset? Some people prefer a shorter cam offset, they feel it makes their tattoos look smoother and the movement doesn’t feel as slappy, or harsh. If we think about the needle travel on a smaller offset rotary this makes sense. The red shaded area at the top of the diagram, the area of lag, is closer to the tip of tube, and often even happening outside the tube. The needles are easing in to the skin rather than entering at their peak velocity. That makes the movement feel softer. And as the needles are coming out of the skin they are slowing down before retracting fully into the tube. As the hand is moving the needles are slowing down, usually at the top couple millimeters of the stroke, right off the tip of the tube. As the hand is moving and the needles are slowing down and scraping across the surface of the skin they are making superficial marks on the surface of the skin. The needles aren’t depositing this ink into the skin deep enough for it to stay, but it does have the appearance of “smoothing” things out. Either black and gray or color, these superficial marks give the tattoo a well blended appearance but look at the result only a year or two later and much of the color, or grays will have fallen out. Ink has to be deposited in to the layer of retention or it will fall out prematurely, there are no shortcuts to this. Going over areas multiple times doesn’t push ink further in to the skin, it only makes a more saturated superficial tattoo. Good for a photo but not for longevity.
I feel it’s important to know your tattoo machine and how it is moving. If you prefer a shorter stroke, just make sure the needles are fully in the tube the whole top quarter of the cam rotation. If you feel the snag sensation it’s best not to turn the machine up but rather be aware of what you’re actually feeling. If a longer cam offset feels too punchy or abrasive just slow it down and give it a try. When you turn a rotary down, try keeping your hand speed the same as before. You want the needles to move slightly slower than your hand, turning rotaries down, or slowing them down is actually the most efficient way to use them and often speeds the work up. I try to run my rotaries at the lowest speed possible without slowing my hand down.
Thanks again for reading, hope this adds a bit of knowledge or at least gives a bit more familiarity to you and your machine.