Tapping drilled holes for a scope base

I have seen a number of Youtube videos showing the tapping of a drilled hole, usually for a scope base but others too. In some cases the so called gunsmith uses a milling machine to drill the holes but them removes the item, puts in in a bench vise and proceeds to tap the hole! That is stupid! Getting the tap perfectly square with the hole is not going to happen! I'm guessing the hole was tapped in the mill then moved to the bench vise. Even if a drill press is used the spindle should be used to tap the hole! There is a simple tapping tool that is available on ebay and other supply stores cheap and works great! It simply is put in the drill chuck, the other end has either a center point or hole that is spring loaded into the shank put in the chuck. Put the tap in the hole with the wrench then the other end supported by the tool mentioned above. Bring the spindle down and compress the spring. Turn the tap with the wrench. If anyone is familiar with the Starrett 91 series tap wrenches (I have all 4 of them) but have an exact copy with no name on it that is only 3" long at best! Even that small it will easily break a tap below #8! I make a device I seen on Joe Pi videos. !" round aluminum with outside knurled. Hole drilled in the center large enough to fit most small taps and a set screw to tighten onto the tap. I tighten the set screw on the round part of the shank. I have found that strong enough to cut the threads but will slip before the tap breaks even on taps as small as #2.
 Of course being a machine shop I have better, a torque limiting tap holder that is factory preset for the torque limit of each tap size, I use it to power tap holes as small as #4 and never broke a tap with it! They too are available on ebay, cheap because they no longer are favored by industry, modern CNC machines will thread mil a threaded hole.
 Worst case is you have to tap a hole and can't use the spindle, drill a hole so that the tap just fits in a flat piece on metal, hold it down on the item you intend to tap, the hole will now hold the tap square with the hole to be tapped!!
 If anyone wants photos let me know.

I'm neither a gunsmith nor machinist and I've often used the drill press to start the tap, merely by hand turning the chuck, but I finish the tapping with the hand held wrench.

I've also broken a lot of small taps before I learned to clear out the chips, use plenty of oil and proceed delicately for smaller holes (#4).  I built an EDM to remove some of those taps, but have used it more for broken bolts and playing than for broken taps.  I even used it to punch a hole in some T1 steel for a backstop, but it took all day.  For that, I used a .38 case for the electrode.

I also finish many tapped holes by hand after starting the process with the spindle. Tap will go in 2 threads or so. That is why I like the knurled fat washers. I also use the torque limiting tap driver by hand especially on #6 taps. Not sure why but most other machinist consider the #6 tap the most likely to break. Of all the taps I have broken the #6 was the most until I got the torque limiting driver.
 Broken bolts or screws I use left hand drills. Get on center as best I can, drill small hole then continue increasing size. As everyone probably knows drill a slightly bigger hole the drill tends to screw in. Being it next size the drill will cut before it breaks but as the hole size increases the broken screw will weaken, After a few increases in size due to the drill being left hand the screw will back out. Of course I put kroil on the screw and let it sit before drilling. My guess is that 95% on the broken screws back out.

All taps work great as long as the drilled and reamed hole is the proper size. Additionally, it's a good idea to check the tap under a stereo microscope and make sure the relief on the tap is ground correctly. Don't ask how I figured this out: not all tap manufacturers have QC.
I used a Tig welder for 43 years at work to remove broken taps/drills and Kroil (God's oil).
Hope this helps.

I don't use cheap taps, Irwin and others don't work well. In tough tool steel I use expensive taps like Guhring or OSG the type needed for the material being taped. Doing shallow depth holes like a .22 Nelson barrel the hole is .150" deep. He is using 6-32 screws to hold on the scope mount. His holes are flat bottomed so I us ea 7/64" end mil to flat bottom a #36 drilled hole. (I used 6-40 in my homemade conversion) I then modify a plug tap by grinding off the point (used as a center for grinding the tap) to start the hole inserted in my torque limiting  tap driver as far in that the tap will go. I then put the spring loaded tap guide in the spindle and continue tapping with a bottoming tap. When torque needed to turn the tap has me concerned about breakage I remove the barrel from my Jig Borer and complete the tapping with the torque limiting driver or the knurled fat washer I mentioned (shown in the photo 7/8" diameter) by hand.
 I seen a gunsmith on Youtube tapping by hand with out any type of tap guide and a rather large tap wrench tapping a 8-36 hole in a gun barrel starting with a bottoming tap after drilling the hole in a milling machine! If the tap is not started square (bottoming tap easily will start at an angle) the hole will get harder to tap as the depth increases. Probably the tap will want to correct itself but that will make the first few threads oversize! Being full strength for threads is acquired by 3 threads the strength is weakened by the first few threads being oversize and because the depth is limited in a gun barrel. It's probably why the smith is using a 8-36 tap for a 8-40 screw! Screw will get tight but strength is not at max.
 The fat knurled washer is great to clean-out tapped holes, I use it in on my shoes. Easy to spin the tap in the fingers not to mention that it makes a great tap wrench for small taps!
 BTW Mark Novak on Anvil videos calls Kroil Angle Piss   

On the subject of tapping, I used to use a product called Tap Magic that was really magic!  Smelled a little like almond extract and was a very thin liquid. Now off the market, probably caused cancer in crickets in California. Anyone found a substitute product that works?

'Chlorinated' cutting oils. Great stuff ...
I got a 5 gallon pail of water soluble right before they 'banned' it. And now its haz-mat to dispose of ...
We had some heavy draw lube for forming cups. Basically, big bullet jackets made from steel, or Kovar. I machined thousands of those things to length on a lathe. The smoke coming off of them was *unpleasant*, and rusted the machine everywhere it drifted across.

Ah, the 'good old days' ... that maybe I'm paying for now?

I have several 'fat washer' tap drivers like Frank's. Hugely handy. Maybe file that away as a future 'product'.

Yeah too many shops were using large amount of Tap Magic the fumes created problems all over not just in CA where everything causes problems.
 When I need to tap tough material Moly Dee is what I use, it was bought by Castrol but they kept the name. Kinda expensive and messy but works. General tapping I use High Sulfur oil.

Yeah, Moly-d is filthy ... most of the chlorinated stuff is ok until you're generating enough heat to make smoke... gag!!!

Was carbon-tet' a thing? Or is that the HP 'chlorine' additive?

I have some Hangsterfers ??? tapping lube that works pretty well. But small holes, pretty much anything 'wet' will do the trick.

I remember people suggesting you could make your own by mixing some pool chlorine in the with the oil.

Ummmm ... no, that's not how it works ...

Thanks for the input. I’ll try some Moly-D.

I use my current chambering oil for tapping these days on. normal stuff.. Vipers Venom.  

Still have quite a bit of the original formula Tap Magic as i used it for chambering back in the 80s