Doing a Screw Build: Drill Bits, Taps, Screws needed
I have now drilled and tapped four parts kits for screw builds and I have learned that doing a screw build sucks! I am going to do one more with photos as the frst coupe I did were not photographed as I was leanring and the last two were done for someone else that lacked the tools and space to drill and tap and I didn't want to take the time taking photos so I could get it done ASAP.
The Drilling the holes:
Tool List
5/32 Colbalt Drill Bit (I use Craftsman)
To Drill the rivets in the trunions you want to use a good Cobalt drill bit to start drilling out the rivets. I used a Craftsman 5/32 Cobalt drill bit as the diameter of this bit is 0.15625 which is just a little smaller then the .1590 size of the #21 drill bit that is the correct drill for a hole to be threaded 10-32.
To drill this hole I first took a dremmel with a reinforced cut-off-wheel and ground the rivets flush with the front and rear trunions. I then used a center punch and a heavy hammer to give me a nice starting point in the center of the rivet for the cobalt drill to start.
I drilled the rear trunion on the slowest speed my little drill press had with lots of standard 3-in-One oil for lubrication and I slowely fed the drill bit into the rivet. I drilled the rivet completely through the rear trunions which in hind sight might have been a mistake as if I had drilled about half way through the trunion it may have been possible for me to use a punch to knock the rest of the rivets out of the rear trunion holes?
I next drilled the front trunions and I used the same drill bit as the rear trunions but I drilled a little bit at a time before looking at my depth and comparing it to my estimate for how thick the trunion was. It wasn't a scientific method but small cuts with frequent inspection kept me from drilling too deap and drilling into my barrel chamber. I have seen WAY TO MANY PEOPLE DRILL TOO DEEP - TAKE YOUR TIME!
After the holes were drilled just a little undersized I use the correct #21 HSS drill bit to bring the hole to it's correct final size. I had some troubles with the rivet comming loose from the trunion holes and binding to the drill bit but that was actually a good thing as it made it easier to pull those rivets from the trunion holes. If the rivet material didn't come out with the drill bit, I used steel punches and a very small slotted screwdriver to work as much of the rivet material from the holes before tapping.
VERY IMPORTANT NOTE! When I started this little discussion I expressed discust over my problems with broken taps. My discussion above is based on my experience which tells me if you leave much of those f-ing little rivets in the holes that the rivet material WILL come loose during tapping and the tap will get stuck in the hole. When the tap gets stuck, you will not be able to go forward or back it out and the tap will break! It has happened to me FOUR TIMES in FOUR BUILDS and it is the reason I am moving on to do rivet builds for now on... I wish you luck but getting that rivet out of the way will save you some huge problems...
Tapping the Holes
Tool List
10-32 Taps as follows:
Taper Type Tap
Plug Type Tap
Bottoming Type Tap
The Rear Trunions
WARNING AGAIN!! When tapping the rear trunions it is easy to get greedy with your tapping operation and try to tap the hole much deeper than it needs to be. Trust me I know, as I have done that extra depth but here again those little 10-32 taps are quick to tell you when you ask too much of them and they will break with little to no warning!
To tap the rear trunions, use the standard taper style 10-32 tap and a small handle and slowely tap the hole with LOTS OF CUTTING FLUID! I use standard 3-in-One Oil and it works fine. After every half turn I go back a little bit to brake up any metal shavings removed by the cutters. Affter I make several turns on the tap, I also completely remove the tap and wash out the hole with an aerosol can of 3-in-one oil as well. I am not sure how deep to suggest going but there becomes some mystical point where the force applied to the tap will become more on the tap then it can handle and at this point the tap will snap... It happened to me with zero warning TWICE and it was a real MAJOR PITA to get the thing back out of the hole. For that reason, use discression as by this point you see how little rivet is left in the front trunion so obviously a half-inch threaded hole just isn't needed from both sides of the rear trunions much less anything deeper!
The Front Trunions
WARNING AGAIN: These holes look shallow. Don't let the shallow depth of these holes fool you as if you left lose rivet material in these holes it will also bind to the tap and you will snap the tap! Been there, trust me on it or don't and e-mail me when you get lucky or you brake the tap and want advise on how to get the thing out... (NOTE, I have no good advise on how to remove a broken tap except try to prevent it from happening!)
OK, I have now given my last warning and the process to tap the front trunions requires some special tools. To start these threads it is necessary to use a plug style tap and you need to be careful to tap only the depth of your hole to prevent stripping the threads. After your threads are started with the plug style tap, you can then use a bottoming style tap to produce sharp threads that do the entire depth of the holes. Remember to use plenty of cutting fluid with these holes as you did with the rear trunions and remove the taps to clean the holes and you shouldn't have any major problems.
Trigger Guard:
Tool List
8-32 Screw Taper Tap
Drilling and tapping the trigger guard is rather ant-climatic compared to drilling and tapping the front trunions because you can remove and work with the trigger guard and backing plate off the gun. To do these holes, simply drill them with a standard HSS #29 drill bit and tap these with a standard taper 8-32 tap.
Screws
By now, I would hope it is clear that the directons I have posted here are for the use of 10-32 button head screws for the front and rear trunions and 8-32 screws for the trigger guard. I go to my local Sears Hardware for these screws and they never seem to have the correct length for my needs so I am always cutting them to length. I cut the screws to length by first measuring the depth of each hole that I need a screw for and then add a nut to the screw so that the nut is on the inside of the screw at the point where I need to cut. I then use a dremmel tool with a cut-off-wheel to cut the screw and when I remove the nut from the screw it takes care of most issues with burs left by the cutting down of the screw. For my build I cut each screw to the length I needed and then test fit them into the trunions to test the fit.
To do the trigger guard, I simply purchased 8-32 nuts and used these on the inside of the receiver of my first build. I have thought about replacing this with a chunck of steel that I will drill and tap but I ahve not had the chance to do this as of this posting.
How to I get a broken tap out?
I warned you to be careful! The bad news is that really don't have great advise on this one. With the four taps I broke, one was broke in a front trunion and it was not very deep. This tap I managed to break apart by picking at the tap with a screwdriver and chipped the tap apart. Two of the taps that broke for me were because I tried to tap the rear trunions too deep. One of the two broke fairly deep and I was able to just leave the broken piece of the tap in my build. The other, requred extensive hamering with steel punches that resulted in me trashing my nice threads in that trunion. The fourth that broke was also a rear trunion and this happened on my first build when the rivet bound to my tap and with this tap I was able to slot the top of the tap that was not inside the hole and I heated the rear trunion with a MAPP/OXY torch to soften the rivet and I managed to unscrew the tap from the hole but that tap was not real deep. In hindsight, I got lucky with that one based on my other experiences that followed!
In conclusion:
Well, this post is now huge and I am too tired to go through and proof read all of this ramblings tonight. I am sure I missed some details but this is a fairly good start to describing the process as I experienced it. Like I said, I am going to try ONE MORE screw build for the good of this website as I will take a bunch of photos but from then on I am doing rivet builds and I will see if my own advise as described above can help me do another screw build without a broken tap. Note, one of the four kits I have done so far was so honored to date... yea I said 75% of my attempted screw build had at least one broken TAP
Hybrid Mode
