Tribal Knowledge: More on Tapping Metal

Back in 2014, when I was putting together my EB-63A solid state amplifier, I had to learn how to tap aluminum so that I could attach my transistors to the heat sink.  I did short YouTube video on how I did this (with tales of woe!).  This video turned out to be amazingly popular -- more than 27,000 views!  Apparently there is a great thirst for this kind of knowledge out there.  Recently someone in Alaska came across my video and shared with us some more useful information on how to tap metal.  Here is some tribal knowledge from Paul KL7FLR:  

Hi Bill and Pete,

Stumbled over Bill’s YouTube video about tapping in Aluminum and I would like to add my experience doing such. I have probably tapped over a 1000 holes in various metals, cast iron, steel, aluminum, bronze and brass from 0-80 to ¾-20. Each metal poses its own problems and solutions. There was nothing in error with your video or the method you presented.

My comments:

What causes broken taps is the tap is not being perpendicular to the work. A hole not perpendicular to the work, a worn/dull tap (this applies to the drill bit too) and lack of lube.
Every hole to be drilled and tapped must be perpendicular to the metal. A hand drill electric or battery will not be perpendicular unless two other people can spot the drill operator. Not possible when you are alone in the shop.

Any serious homebrewer should have a drill press. A small bench drill is very inexpensive. Even the $69 Harbor Freight model is adequate for the average ham’s workshop.  I see these items at flea markets and on Craigslist all the time cheap. Estate sales not so cheap but sometimes on last day you might get a bargain. Adjustable from 500 to 3600 rpm will cover all the normal sizes of drills. An x-y table could be added ($$) for added accuracy if desired. (search x-y table for examples and sources)

The size of hole you drill makes a difference too. Most drill charts assume you are tapping at 75% of thread contact. This is amount of the male thread of the bolt/machine screw threads contact the female thread in the piece. This is about the max you can expect without some precision tooling that won’t be in the average home shop.  75% provides maximum strength of the fasteners.  So unless you’re working on something that requires strength like a tower support bracket or the wings of an airplane you don’t need 75% as 50% thread contact is more than adequate. So when the drill chart shows that a #43 you can instead use a #42 or a 3/32 drill. A little bit larger hole will be a lot easier to tap threads.

The type of tap will also influence the difficulty of tapping threads. The typical hardware store tap is a straight 3 or 4 flute taper tap. The chamfer of a taper tap is 9 threads making tapping much easier. This is the tap for hand tapping.  The plug tap has a chamfer of 5 threads and is best used in a tapping head or fixture. The bottom tap is for tapping threads in a blind hole, usually after started with a taper tap. That increases the pucker factor x10. The normal taper tap cuts as it is rotated in a clockwise direction or counter-clockwise for left hand threads. The cuttings will clog the tap and you must back the tap out frequently to clean the cuttings from the flutes preventing the tap from seizing and possibly breaking the tap. Breakage is easily done with the smaller taps but also will happen to a larger tap too.  There is a style of tap called a spiral point tap. It will push the cuttings up to the top of the hole and not clog the flutes as easily as the straight flute taps. I still back it out and clean it out of habit. These don’t cost any more than the hardware taps. A 4-40 is about $2.90 the last time I bought one.

Avoid “carbon steel” taps. These will break easier than “high speed tool steel” taps and they are usually a few cents cheaper too.

Lubrication is necessary. Bill’s use of 3-in-1 oil is a good lube but “Tap Magic 20016A Aluminum” fluid is better for tapping aluminum and regular “Tap Magic” for everything else. https://www.amazon.com/Tap-Magic-20016A-Aluminum-Yellow/dp/B00065VEUO

The metal you are tapping can be an exercise in patience. “Hardware” store aluminum comes in various grades from crappy to real crappy. They are of an alloy best suited for the extrusion process and are not wonderful for the tapping process. Aircraft grade aluminum is nice stuff to work with but not cheap. Comes in many grades from 1000 series to 8000 to match the application. Grade 2024 is communally found and is easy to work with while 7075 works like steel.  Since most of us won’t be buying a 21 foot stick of aluminum or a 5x8 foot sheet we must take the hardware store grade into account when tapping holes and lean towards the 50% thread engagement, plenty of lubrication and lots of patience. I frequent several aluminum boat building firms and they let me pick through their scrap piles and I’ve made frequent donations of a case of soft drinks or a can of coffee to the break room fund and have also paid the going scrap price for the scrounging privilege.

Worn taps:  These get dull with use and will break at the worst possible time and in an expensive  workpiece. Broken taps  can be removed but are time consuming and require some expertise in doing so. Best to retire them when worn. I used to keep a chart of how many holes a particular tap had been used and at 35 holes for the small taps they got retired. 50 holes for taps larger than ¼”.  I once spent over a week of evenings extracting a worn tap from a locomotive cylinder casting. Never again! As my mentor said many times, “dull tools cause accidents and ruined work”.

Clamp the work to be tapped stationary if possible. You don’t want it moving around thus inviting a broken tap. Use some method of a tap guide (as Bill did with a block of wood, not great but better than nothing). I use what is called a “Piloted tap wrench”, https://www.amazon.com/Set-Wrench-Piloted-Spindle-Capacity/dp/B077SVQD4S .

Picture is of my setup.
Step 1: Work clamped securely to table at desired hole location.
Step 2: Drill hole.
Step 3: Use piloted tap wrench to tape hole.
Step 4: Done.