Tattoo guns are machines that rely on the completion of a circuit to work. They are easy to work with and repair, if you understand their anatomy. It's always a good idea to have a couple spares, so you can complete a tattoo in a timely manner if problems arise with your machine.
When your tattoo machine quits, the first thing to check is the contacts. These are the points where your machine connects to your power pack. With the constant vibration of tattooing, these contacts can easily become loose or shift during use. Adjust your connection and try again. If the gun still doesn't work, clean the contacts with paper or use a fine grit sandpaper to renew the surface. Ensure that all grit and debris are clear before trying to use your machine again. (See Reference 2)
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Those cylinders in the middle of your tattoo machine are the coils. They have a significant role in the electromagnetic field created by your tattoo machine. These coils can go bad and require replacement. This is another reason the tattoo machine might quit in the middle of a tattoo. Keep some extra coils on hand for an easy repair and solution to this problem. (See Reference 2)
To find out if there's a problem with your power pack when the tattoo machine quits, hook up a different machine for testing. If a different machine doesn't work, then your power pack is the problem. If your power pack has gone bad, it is likely time to purchase a new one. (See Reference 2)
The armature bar bounces the needle up and down. This bar can become jammed or be situated against the coil so that the coil no longer moves the needle. Loosen the contact screw on the top of the machine, and reposition the needle and armature so that it works properly. Test it out before continuing to tattoo. (See Reference 1)
The bar that holds the needle must be snug on the armature. If it is not and is moving, insert a new grommet on the armature. This will prevent unnecessary movement and make for a better working tattoo machine. (See Reference 1)
Make sure your needle is set appropriately. If you are used to thin ink and switch to thick ink that flows slower, there is a tendency to set the needle too far out. This is not necessary. Ink thickness does not affect the flow in such a way that you must adjust the needle to account for it. (See Ref. 1)