Intro: Using Taps to Change Thread Size... Oh, the Possibilities!
I have a 1986 VW Vanagon Syncro... the coolest part of which is the locking differential. I decided that the stock knob with which the differential is engaged was lacking in personality.
Step 1: 1. Measure Your Threads.
Retrieve the thread gauge from your Tap and Die kit. The thread gauge looks like a little keychain of tiny saws with different size teeth.
We want to figure out the thread measurement that we're going to recreate on our new part, and the way to do that is to try fitting different gauges until one matches perfectly. You will feel this instantly, and there won't be any question if it's the correct match- it's a powerful feeling of rightness, like a double rainbow over an ice cream cone of satisfaction. With sprinkles.
There are metric and english thread sizes, and if you can't find teeth that match your threads on one gauge, you're probably on the other measurement system. Metric threads are measured from peak to peak, if you look at them sideways. English threads are measured by number of peaks or threads per inch.
The gauge will have the thread information printed on it, allowing us to select the proper tap for the job.
Step 2: 2. Set Up Your Tap.
Taps are for threading the insides of existing holes, and Dies are for threading outsides of posts or shafts. Taps are long and skinny with threads on the outside, and dies are round with threads on the inside. They aren't actually threads, they're special little teeth, and we want to take good care of them in both cases.
Every Tap and Die kit will come with a handle for holding taps and a handle for holding dies. The photo demonstrates the set up.
Step 3: 3. Set Up Your Work Space
The most critical thing about setting threads is keeping your angles lined up very precisely. If you are re-threading a hole and you start at an angle, several things can happen: the teeth of the tap will cross the threads and pull them out, the tap will jam and break (becoming stuck in the hole you surely were planning to use for something other than the trash), etc.
So, to avoid this: think carefully about your set up. You want to maintain even physical pressure at 90 degrees to the tap. It depends on the shape of your parts, but in this case it was easiest to vice up the nice square tap handle and turn the knob by hand.
Step 4: 4. EnThread!
Start tapping by lining everything up very carefully and making gently but firm turns onto the tap. It will feel wobbly at first, and you'll be fighting to keep everything straight. Use enough pressure to get some threads going right away. Not enough pressure will cut out too much material at the starting point, so there isn't anything for the tap to bite into.
Every few turns, use a little tap oil or coolant from the machine shop. This is double extra important if you aren't changing thread sizes, but are tapping a drilled hole.
You need to clear the chips out of the teeth of the tap as you go, or it won't be able to cut and may shear off. I turn backwards halfway for every full turn, this is more than enough. Be very gentle when changing directions: you want to do everything you can to continue the threads you've started to set, and you can easily ruin them by too much wobbling or forcing.
Stay zen and work your way down: the further you get the more chips you'll have to clear. The bonus of this upside down setup is the chips want to fall out, but if you're going the other way they'll want to pack down into the threads you're trying to make.
Don't forget you will hit bottom at some point, in this case.
NEVER FORCE! If it feels wrong, it probably is.
Step 5: 5. Replace Your Custom Part:
Hooray! So much better with a little more personality in the cockpit!