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.

What exactly is happening when the motor rotates the cam, which moves the yoke, which makes the needle go up and down.

Any rotary tattoo machine that you can buy has an offset cam.

It is how the motor turns rotational movement into linear movement.

It is how far the shaft of the motor is offset from the center of the cam.

I have also shaded areas in the background, blue in the middle and red on top and bottom.

For the sake of this explanation we can say that the motor is spinning at a constant rate all the way around its movement.

That means that if you separate the cams 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.

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.

The top quarter and the bottom quarter of the stroke, together make a half.

Ive 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.

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.

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.

Almost all of us who have ran rotaries have experienced that sensation where the needles seem to snag in the skin.

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 isnt, then you are going get that snag sensation.

Rotary Tattoo Machine Diagram