I also use my cranker right around 6 -6.4v w/about a 3.8 stoke length. Crankers don't have a lot of presence off of the tip so you definitely need to apply enough pressure to take up the "slop" and get enough torque behind the stroke. This is easier to do on more delicate spots when you turn the machine down a bit, especially at a 4mm throw.

Hopefully it was just a bad needle, but still a good idea to push the the bottom end of the usable voltage range and then work back up to a comfortable voltage. I feel like finding the right volts for a particular tattoo (needle configuration/skin type) is sometimes key. I like my rotaries to be at a voltage where my motor begins to bog down slightly under the right amount of pressure. Almost like the machine is talking back to you.

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