Tattoo pigments contain all kinds of rotten constituents that you'd generally expect to see in automotive paints. They make these in prisons all the time you can use a paper clip for the needle instead just sharpen the end. The construction of your machine is top notch, but please remember, people will do this, and will spread disease. What you may not realize is for 30 dollars you can buy a decent rotary tattoo machine from Hildbrandt tattoo supply. Also, like mentioned in the first reply, in addition to sterile needles, tips and grips can be bought that are disposable.
The speed of a reciprocating needle is VERY important to the quality of a tattoo, the trauma the skin suffers, stability of the machine, and the speed at which you tattoo the person with. Your best bet in this case is to tinker with resistors of variegated capacities and resistance. Like you, I run short stroke, cutback liners, but I've seen pros who demand huge dollars working with up to 11mm of throw with no negative effects on the skin. Many people do not realize that an open flame, bleach, nor boiling water come anywhere near sterilization.
Real inks, or pigments, are made using water, glycol, glycerine and sometimes alcohol(not the drinking kind) as binders. You might as well use something that deep to tattoo from one side of a persons arm and have the ink show up on the other.
So, you may end up with a blur, that in a year may look like a permanent bruise, or more likely, it will quickly fade to nothing, except a scar where the skin suffered the effect of a combined chainsaw-scalpel-blade.
Personally, my lining machine is run on low voltage- around 5.5volts, high output coils, stiff springs, and has a hard hitting throw of only 3mm. That said- the depth of the needle you are using is very easily tunable to whatever desired depth by simply moving the bar closer or further from the center. In fact, I am going to use this instruct able to build an exact replica using sterile equipment, not even the resistors.
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