How-to: drilling and tapping holes

[Three se7ens]

I tend to take this stuff for granted, because its part of what I do every day.  But its far from common knowledge, and a few keys make the difference between broken taps and curse words and a job going smoothly.

Unless you have a machine shop with a particular need to have a tap of every size on hand, or you just like spending absurd amounts of money on tools you won't use, don't buy tap sets or full drill sets.  Quality is of the utmost importance when tapping holes, especially in difficult materials like stainless steel.  Quality taps are expensive, and worth every penny.  Just buy the sizes you need, and the corresponding drills.  

Looking up the recommended tap drill size is usually the first mistake.  Unless it specifies otherwise, the chart is for 75% thread engagement.  This is good for aluminum.  If you try it on stainless, you will break taps all day long.  Steel and stainless steel (and most hard materials) only require about 60% thread engagement for the tapped hole to be stronger than the fastener.  At my work, 60-65% engagement is a standard for stainless.  Anything more is practically guaranteed to break taps in a CNC machine.  I use this a lot for picking the correct drill size:

http://theoreticalmachinist.com/TapDrillSizeCalculators  

Once you know the size drill to use, you have choices in drill bits.  Id recommend at least HSS, but I prefer cobalt for stainless.  Carbon steel should be avoided, as its very easy to overheat and kill the temper.  If that happens, it will never hold an edge again.  HSS and stainless can tolerate far more heat without losing their temper.

I also prefer to use screw machine length drills, unless I need the extra depth of a standard length drill bit.  The shorter screw machine length bits wander less and are stronger.  Eg. http://www.mcmaster.com/#standard-drill-bits/=125okp2

Using a center punch or center drill is a good idea to keep the drill from wandering as it starts in the hole.  Its more important with standard length drill bits, the screw machine length bits tend to tolerate starting a hole a little better.  

Now onto taps.  First off, stick to the big names.  Dont be tempted by that $20 tap set at harbor freight.  Id even recommend avoiding Irwin unless you are in a pinch.  Greenfield, YG1, and widia are very good, and OSG and Emuge are better.  Just like the drill bits, you will want to avoid carbon steel taps.  HSS and variations of it are the way to go.  

There are 3 different kinds of lead.  Taper taps have a lead in of 5-7 threads and are good for through holes in thinner stuff, and for starting tapped holes.  Plug taps have 3-5 threads of lead, and are your general purpose tap.  Bottoming taps are designed to cut full threads as close to the bottom of the hole as possible.  They are difficult to start, so when they are needed, its recommended to start with a taper of plug tap before finishing with the bottoming tap.  

In addition, you have 2 more options.  Straight flute taps are generally considered hand taps.  For best results, these need to be backed off regularly through the tapping process to break the chip, and keep it from jamming up in the flutes.  

Next you have spiral point, or gun taps.  These have straight flutes, but have an angle ground into the start.  This angle causes the chips to curl up ahead of the tap.  These are best for through holes, as the chips will pile up in the bottom of blind holes.  But they perform better than straight flute taps, and do not need to be backed off like straight flute taps.  

http://www.mcmaster.com/#standard-taps/=125osvz  The taps for through hole are spiral point, while the closed end hole taps are spiral flute.

Spiral flute taps should be considered special purpose taps.  They are weaker than straight flute and spiral point taps, but they do pull the chips out the top of the hole.  Best for finishing blind holes, but must be used with care when tapping by hand.  Pretty much an industry standard for CNC machines though.

One last thing:  use cutting oil when tapping holes!  It makes a big difference.  Lots of good choices out there, I like Tap-Magic, Anchorlube, and especially Union Butterfield tapping oil.  

Mcmaster Carr has a mind boggling selection of taps and drills, but they do have pretty good descriptions of their uses.  And everything they sell is good quality.  Most is name brand, but they dont typically show the brand on their website.  

I hope this helps, and if anyone has any questions, feel free to ask.  

[Marcq]

I have drill and tap many many times and still learned a lot from your post

Thanks a bunch 

Marc..

[DaBigOno]

Man, I hope this gets stickied!

Very go info here

[josa1]

Just great information, stated like a true professional.  Thanks a lot for sharing.

josa1

[FatTuna]

Thanks a lot Adam. Your experience is appreciated.

[Three se7ens]

And now some pictures:

First one: the tap on the left is a Widia 2 flute spiral point plug tap.  The tap on the right is a YG1 3 flute spiral flute semi-bottoming tap. I use the Widia for through holes and starting blind holes, and finish blind holes with the YG1.

The semi-bottoming tap is a special type that's used primarily on CNC machines.  The lead is a little longer than bottoming taps by just enough to make it start well in a CNC machine.  It is not a good choice for starting a tapped hole by hand, but does make a nice tap for finishing a blind hole very close to the bottom.

The second picture shows the difference between standard length and screw machine length drill bits.

[Three se7ens]

Another picture.  This is a set of straight flute hand taps showing the 3 different leads.  On the left is a taper tap, in the middle is a plug tap, and on the right is a bottoming tap.

McMaster Carr sells 3 tap sets like this in most, if not all, sizes.  They are a good choice and are not terribly expensive. Personally, I prefer the spiral point and spiral flute taps.  Those are considered high performance taps, and it's because they work better.  

Most of the lower cost taps McMaster Carr sells are Widia branded.  YG1 is pretty easy to find on eBay for good prices.  I use eBay a fair amount for buying taps and end mills.  You can get good quality for slightly better than market prices on most, but you do have to be vigilant about brands and a reasonable price range for said item.  If it's too cheap, it may not be real.  

[Tiddlerbasher]

Thank you Adam an excellent article - should be stickied.
The 60-65% for stainless work probably explains why I've broken a few taps in the past I' always used 75% Doh!
I typically use cobalt hss drills to the nearest .5mm then drill to final hole size with hss drills to (to the nearest .1mm).
Can the same guide be used when using a die to make a threaded rod e.g.:
A 5.9mm major diameter for a 6mm threaded rod (assuming 1mm pitch)? If that's correct then I suppose a simple rule would be:
Thread diameter (6mm in my eg) pitch 1mm therefore major diameter of rod should be thread size minus 1/10 of pitch=5.9mm

[sdlehr]

should be stickied.

It was 

[Cone]

Good post Adam. I usually use 75% and 50%. I printed this chart and laminated it.
http://littlemachineshop.com/Reference/tapdrill.php
Bob

[mhc]

Good stuff Adam, thanks for sharing.
Mike

[Marlinmate]

This thread relates to your thread.

http://alantani.com/index.php?topic=1829.msg9025#msg9025

[Three se7ens]

Thank you Adam an excellent article - should be stickied.
The 60-65% for stainless work probably explains why I've broken a few taps in the past I' always used 75% Doh!
I typically use cobalt hss drills to the nearest .5mm then drill to final hole size with hss drills to (to the nearest .1mm).
Can the same guide be used when using a die to make a threaded rod e.g.:
A 5.9mm major diameter for a 6mm threaded rod (assuming 1mm pitch)? If that's correct then I suppose a simple rule would be:
Thread diameter (6mm in my eg) pitch 1mm therefore major diameter of rod should be thread size minus 1/10 of pitch=5.9mm

Its standard practice when cutting threads on a lathe to turn the OD a little smaller than nominal.  For threads under an inch or so, I typically turn the OD to 0.003"-0.005" smaller than nominal before threading.  Above that, 0.005"-0.010" works well.  What this does is "top" the threads.  Theoretical threads do not come to a perfect point at either the major diameter or minor diameter.  Special inserts called topping inserts can be used on a lathe that cuts the top of the thread to the proper height, but at my work, they are rarely used.  Instead, we just size the OD for outside threads, or the ID for inside threads to compensate. 

[boon]

Repeat after me:

Half in, quarter back

Half in, quarter back.

Do it enough times and you will have a nice clean thread.

Mostly with regards to tapping aluminium and other soft metals by hand and ensuring that you clear the debris/swarf correctly so you didn't end up with a flat thread.

Obviously much less sophisticated than the degree of knowledge being distributed in this thread!!

[Cor]

Very usefull stuff!   I never knew that so much was involved.