Tapping, quick and cheap
Posted: Thu Jul 15, 2010 12:56 pm
When making things, I love tapped holes, as there is no nut to lose, but until I learned this trick, I hated the actual tapping process itself.
I'm sure most of the folks here have had experience with a dull "True Value" or Chinese tap and breaking little ones off in the hole, sometimes ruining the whole workpiece, which is both expensive and frustrating.
So, here's how I do it without tears, expensive gear and a lot of time wasted.
Step one:
Get the right sized hole! This will vary with material and use of course, the little guide on the back of the hardware store taps doesn't get all the information you need there. Neither does the attached file, but it's a great place to begin. I have one of those 115 piece drill bit sets that is used only for precise holes for things like this, and don't use it for knock around the shop kinds of things -- that alone is a pretty serious help. Some materials "upset" or flow into the space between the tap fins more than others, and those materials need a larger hole in the first place if tapping is to be easy. Some just cut clean, so you can use a smaller hole for those. In other words, for "goopy" things like copper or plastic, use a drill size for "60%" threads, but if cutting holes in something that machines like 12L14 steel, use the right drill for 75% or more tapping (eg a smaller hole).
Step two:
Make a special loose tap holder. I took a hardware store one, removed the cross bar, and ground three flats on it so I could chuck into my Makita or Ryobi battery drills. You want a little play here, so you won't as easily break off the tap if you hold the drill a little crooked. You can then see how to hold to get a straight tapping job as it goes along.
Step three:
Tap the hole, using the drill's screw driving torque limiter to prevent twisting the tap off it if jams. On metals, I use Moly-Dee lube (better than hardware store tapping oil), on other things, I just do it dry. For deep blind holes, nothing beats backing the tap out a time or two and cleaning both the hole and the tap with compressed air, something that's worth it for many shops to have. If you don't, I suppose a can of it would do as well, at more expense on a per-hole basis.
As luck would have it, my 1/2" Makita (which I use in first gear for this) has a clutch setting that matches most taps just right, eg use "4" for 4-40, and "10" for 10-32 and you'll never break a tap that way. All this is a little touchy-feely -- you don't hold the drill real tight or run it real fast, especially as you learn this. Let the tap decide how to guide the drill, and let your hands have a little give in case the tap jams. At that point, you've not broken it yet, so back it back out, clean things off, and go again.
Attached find a file of drill sizes correlated to taps, and a picture of the "special" tap holder that makes this all work. Since the taps have a square end, I don't tighten them into the tap holder all that tight -- you can usually pull the drill/holder right off a stuck tap with no damage to anything. A little play is what makes this trick work. That is a 10-32 TiNC coated tap there -- don't use a TiN coated tap on Al, it will bind -- use the carbo-nitride or plain tool steel for grabby stuff like aluminum.
Always get really good taps at McMaster-Carr (or other trustworthy vendor in this business, I like McMaster fine) -- they will save you both money and frustration over those in the stores. They are often offered in sets of 3 for long taper, normal, and blind hole tapping to close to the bottom.
This isn't as good as, but is faster and cheaper than, a tapping head on a milling machine, for example, and for nearly everything -- plenty good enough.
I'm sure most of the folks here have had experience with a dull "True Value" or Chinese tap and breaking little ones off in the hole, sometimes ruining the whole workpiece, which is both expensive and frustrating.
So, here's how I do it without tears, expensive gear and a lot of time wasted.
Step one:
Get the right sized hole! This will vary with material and use of course, the little guide on the back of the hardware store taps doesn't get all the information you need there. Neither does the attached file, but it's a great place to begin. I have one of those 115 piece drill bit sets that is used only for precise holes for things like this, and don't use it for knock around the shop kinds of things -- that alone is a pretty serious help. Some materials "upset" or flow into the space between the tap fins more than others, and those materials need a larger hole in the first place if tapping is to be easy. Some just cut clean, so you can use a smaller hole for those. In other words, for "goopy" things like copper or plastic, use a drill size for "60%" threads, but if cutting holes in something that machines like 12L14 steel, use the right drill for 75% or more tapping (eg a smaller hole).
Step two:
Make a special loose tap holder. I took a hardware store one, removed the cross bar, and ground three flats on it so I could chuck into my Makita or Ryobi battery drills. You want a little play here, so you won't as easily break off the tap if you hold the drill a little crooked. You can then see how to hold to get a straight tapping job as it goes along.
Step three:
Tap the hole, using the drill's screw driving torque limiter to prevent twisting the tap off it if jams. On metals, I use Moly-Dee lube (better than hardware store tapping oil), on other things, I just do it dry. For deep blind holes, nothing beats backing the tap out a time or two and cleaning both the hole and the tap with compressed air, something that's worth it for many shops to have. If you don't, I suppose a can of it would do as well, at more expense on a per-hole basis.
As luck would have it, my 1/2" Makita (which I use in first gear for this) has a clutch setting that matches most taps just right, eg use "4" for 4-40, and "10" for 10-32 and you'll never break a tap that way. All this is a little touchy-feely -- you don't hold the drill real tight or run it real fast, especially as you learn this. Let the tap decide how to guide the drill, and let your hands have a little give in case the tap jams. At that point, you've not broken it yet, so back it back out, clean things off, and go again.
Attached find a file of drill sizes correlated to taps, and a picture of the "special" tap holder that makes this all work. Since the taps have a square end, I don't tighten them into the tap holder all that tight -- you can usually pull the drill/holder right off a stuck tap with no damage to anything. A little play is what makes this trick work. That is a 10-32 TiNC coated tap there -- don't use a TiN coated tap on Al, it will bind -- use the carbo-nitride or plain tool steel for grabby stuff like aluminum.
Always get really good taps at McMaster-Carr (or other trustworthy vendor in this business, I like McMaster fine) -- they will save you both money and frustration over those in the stores. They are often offered in sets of 3 for long taper, normal, and blind hole tapping to close to the bottom.
This isn't as good as, but is faster and cheaper than, a tapping head on a milling machine, for example, and for nearly everything -- plenty good enough.