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11/22/2017 10:05:29 PM
Posted: 10/27/2004 5:11:33 PM EST
[Last Edit: 10/27/2004 5:12:31 PM EST by AtlantaFireman]
Long winded:
Maybe its just the cloud hanging over my head. I came home today from the fire department to find ANOTHER [DARN] LEAK...the same one I fixed 36 hours previous. Whoever did the plumbing and wiring in my house should be drawn and quartered.

I found that using a #21 drill for a 10-32NF tap does not completely remove the rivets on the rear trunnion. They won't spin for me, and they won't come out. So, I went to the next larger size: 6mmx1 tap. (It may not be the next larger size, but it works for me.)

I tapped one rear trunnion with a 6mmx1 thread. I bought good drill bits (cobalt): went right through at 650RPM with Kroil (all the way through). I drilled using a #10 drill (.1935"). This size drill bit removes ALL the rivet. The chart at the fastener store said to use a #10, but the tap package said to use a 5mm (.1968"). I don't know if a difference of .0033" makes that much of a difference. When tapping the second hole, I heard a nasty chatter. Guess what? I broke off a leading edge of my teeth on the plug tap. So, I finished the hole with the bottom tap. After this, I was only able to get about four threads before that tap would go no further! On the third hole, I heard a nasty chatter from the bottom tap; teeth broke off it too. The teeth that broke are very near the end of the tap, which lends me to believe that the hole was not wide enough. I was cutting 1/8 turn, and backing off 1/4-1/2 turn to clear it.

I looked at a chart, www.engineersedge.com/tap_drill_chart.htm. It lists a 5mm (.1968") drill for a 6mmx1 tap, and a 5.5mm (.2165")drill for a 6mmx.5 tap. I figure that using the 5.5mm drill gives me .0230" more, which is that much less the tap has to cut away. I do not see why there is a .5mm difference between a 6mmx1 and a 6mmx.5 tap.

Do you think that the larger drill bit (5.5mm) will ruin the next holes? They will be filled with red loctite or Devcon Steel Epoxy Putty anyway when finally assembled.

I have two more holes to drill on my other rear trunnion. I would have had four, but there is a tap lodged in one (broken)(10-32NF), and I want the holes to match on both sides: TIG rosette welds come to mind.

I don't know if the trunnion steel is that tough or my drill holes were too small. I can't figure it out. I am an A&P mechanic and used to design and build prototype rockets when the Cold War was hot.
Link Posted: 10/27/2004 7:21:53 PM EST
You are far more qualified to explane it then me... but... I drilled mine out with an undersized cobalt and then the #21 bit. I then used a punch to work at the rivet until I got the remainer out. It was a real bugger but mine did come out.

I have also snapped three taps in those rear trunions when I thought I would ignore that little bit of rivet remaining! I am now anxious to try rivet builds!
Link Posted: 10/27/2004 10:03:06 PM EST
i snaped my drill off in 1 of the holes buddy of mine got it out tho
i have no clue about the taps but i do know that there is 2 diff sized holes in the rear block
Link Posted: 10/27/2004 11:16:28 PM EST
The difference in drill sizes is becuase the threads themselves are smaller in the .5 pitch thread therefore the starting hole should be bigger. By using a larger hole than is recomended before tapping the threads won't be as sharp. If the bit is just VERY slightly larger than specified then the threads will still work and yes it will be easier to tap. The larger the drill the duller (ie less thread and therefore weaker and easier to strip) the threads become. I have used slightly (.005" or less for fine threads more on course threads depending on the required strength of application and how the forces are applied) oversized bits in the past when when working with hard materials. Look at it this way. If the trunion material is that hard and the screws themselves are hard and the reciever is hard and realistically there isn't a whole lot of force on the rear trunion then you should be fine especially after loc-tite.

The primary forces on the rear trunion are puting the screws in shear, not tension.
Link Posted: 10/28/2004 2:15:11 AM EST
[Last Edit: 10/28/2004 2:25:35 AM EST by AtlantaFireman]
That was my thought too: shear more than tention. Thus, Loctite or Devcon Steel Putty would do the job.

A weld or two in the right place may also do the job.

I will also be using a recoil buffer.
Link Posted: 10/28/2004 7:29:00 PM EST
I bought a 5.3mm cobalt bit and a 6mmx1 taper tap today; my other two taps had missing teeth from these trunnions.

The 5.3mm bit is the way to go: leaves enough meat for the machine screw and easier to tap WITHOUT ruining the tap.
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