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Trouble Drilling Receiver

Anhur

Plastic
Joined
Nov 11, 2017
Hey all. I have an old bolt action rifle that had a scope installed with a mount bolted to the top of the receiver. Unfortunately it was drilled and bolted crooked originally and I wanted to repair it. I TIG welded the old mounting holes and smoothed out the receiver. I then wrote a program to place and drill the new holes in the repaired surface. I am attempting to drill two 0.166" holes in this receiver that is 0.170" thick mild steel. I used a 3/8" carbide spot drill down to 0.050" to start the hole. I then used a HSS .0166" twist drill to complete the hole, or well I tried that. The spot drill worked flawlessly, but the twist drill didn't even make a dent before shattering. I was running 1000rpm at .5ipm which I thought to be fairly conservative. I loaded up another bit and reduced the feed by 70% and it did not drill either. It just loaded up and I backed it off before breaking. The tip was completely wiped out. I then grabbed a 1/8" carbide end mill and plunged the feed manually 0.0001" at a time and managed to break through. After that I decided to write a program that would helix the 1/8" carbide to depth in 0.170" diameter. This worked on the first hole, but broke the carbide end mill on the second hole. How the heck am I going to get both holes to spec? I already have a bit of a mess as the hole was drifting slightly when the twist drills were breaking. The plunged end mill holes are straight and square, but are still too small. Any ideas?
Thanks in advance.
 
You could use a carbide drill designed for hard steel.

The end mill should work to open the hole if you spiral down.

Once you have the holes, how do you plan to tap them?
 
Maybe he's going to use bolts like the last guy:D.

At this point I am about ready to use JB Weld and some hose clamps.

All kidding aside I ran out to the local machine tool store and picked up a few items I may or may not try on Monday. I found a 0.166" carbide twist drill. I also picked up a 5/32" flat end mill and a 4mm flat end mill. I figure between all of that I should be able to get the hole within tapping range. Tapping on the other hand I hadn't really put a lot of thought into yet. I typically thread everything on the machine with thread forming taps, however I don't think that would be a great decision here as the outcome would most likely be a broken off carbide tap in the receiver. I will probably thread them by hand with a cutting tap and a bunch of patience. That is unless anyone else has a better idea...
 
I never weld to repair incorrectly drilled holes, due to difficulty in tapping a weld that's as hard as peckerwood...

I tin a screw to fit the existing hole, then screw it in (being careful not to go too deep) and solder it in with a small butane torch. This assures a very small HAZ that doesn't affect receiver temper. Cut the screw off slightly above flush- then peen it in place and file flush.

Locate, drill and tap the hole(s) in the correct location. Drilling/tapping partially into the old hole/screw is now easily done.
 
I never weld to repair incorrectly drilled holes, due to difficulty in tapping a weld that's as hard as peckerwood...

I tin a screw to fit the existing hole, then screw it in (being careful not to go too deep) and solder it in with a small butane torch. This assures a very small HAZ that doesn't affect receiver temper. Cut the screw off slightly above flush- then peen it in place and file flush.

Locate, drill and tap the hole(s) in the correct location. Drilling/tapping partially into the old hole/screw is now easily done.

Agree. Any welding on the receiver will not only create dead hard weld area, but will affect the temper of the steel. Many years ago I used to shorten or lengthen (out of two receiver making one long) receivers with tig welding, but then re-heat-treated the whole.
To plug old holes I sometimes use a similar to above technique, except that I machine a custom screw with a slight taper. This binds in the hole and once cut (slightly proud), lightly hammered and filed becomes almost invisible.
For drilling I use only carbide drills and I find the PCB type drills effective and inexpensive. Often no pilot hole is needed as the drills have a two facet point.
Some old receivers are very had and even carbide will not touch it. One way is to spot temper the area to be drilled. This was done by an arc created with a pointed graphite rod. I now use a plasma needle welder (1/16 diameter flame at 18000F) that can very fast heat the spot without affecting the surrounding area.
 
It might have been easier to plot out the holes then make a custom mount to fit the existing holes. I have built 3 custom one piece Weaver mounts and it is not really that hard.
If this idea has merit it will not help you now but may give you another option next time.
 
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I did end up spot annealing the areas around the holes. That made the steel workable and I was able to tap the holes. There was just no way to cut the holes with the steel being that hard. Thanks again.
 
I did end up spot annealing the areas around the holes. That made the steel workable and I was able to tap the holes. There was just no way to cut the holes with the steel being that hard. Thanks again.

How did you accomplish the spot annealing?
 
for future refence either map holes out and make custom mount as stated above, or if you are going to tig weld hole, be sure to preheat, reg oven to 300-400 deg f and post heat, or with propane torch, biggest is the post-heat- slow cool, welding on big cold receiver the small spots you weld the heat gets drawn away much to fast and hardens extremely hard.
 
How did you accomplish the spot annealing?

I used two pieces of stainless rod turned down .010" under the hole size. Took and heated them to just over bright red and inserted one in the hole. Left it in the hole for a few minutes until it was loosing color while I kept the other one hot. Swapped them out and heated the other and repeat. Did that about 6 times or so while reducing the heat every time. Worked great.
 
for future refence either map holes out and make custom mount as stated above, or if you are going to tig weld hole, be sure to preheat, reg oven to 300-400 deg f and post heat, or with propane torch, biggest is the post-heat- slow cool, welding on big cold receiver the small spots you weld the heat gets drawn away much to fast and hardens extremely hard.

A custom mount would have been difficult. The holes were not square to the top of the receiver and not in alignment with each other. Picture the beginnings of a radial engine. It was a hack job probably done with a power drill by someone doing a handstand. I think your analysis is probably spot on with the welding. The receiver was cold and most likely sucked all the heat away from the holes. Next time I won't make that mistake. Thanks again all.
 
The receiver was cold and most likely sucked all the heat away from the holes. Next time I won't make that mistake.

For welding 4140, you are supposed to pre-heat to 600°F before welding, which I think will ill affect the heat treatment of the receiver. While it sounds sort of half ass, I think the procedure you ended up following (TIG weld holes, spot anneal) would be better. Maybe preheating to 300°F would help but more than 400°F starts to affect the heat treatment.
 
Use a carbide drill to break through and finish with standard size drill bite. On 03 action most were only surface harden. I've drill mosins and krags there hard
 
The receiver was cold and most likely sucked all the heat away from the holes. Next time I won't make that mistake.

For welding 4140, you are supposed to pre-heat to 600°F before welding, which I think will ill affect the heat treatment of the receiver. While it sounds sort of half ass, I think the procedure you ended up following (TIG weld holes, spot anneal) would be better. Maybe preheating to 300°F would help but more than 400°F starts to affect the heat treatment.

Receivers made from 4140 or similar usually hardness test at about RC 35-40. The tempering temp to achieve this hardness is about 1000 F.
So, any temp below this is not going to affect receiver hardness. Spot annealing will have no effect even if heated to dull red because
the HAZ is confined to the surface and is not even close to the locking lugs.
 
Receivers made from 4140 or similar usually hardness test at about RC 35-40. The tempering temp to achieve this hardness is about 1000 F.
So, any temp below this is not going to affect receiver hardness. Spot annealing will have no effect even if heated to dull red because
the HAZ is confined to the surface and is not even close to the locking lugs.

Tig welding a cold receiver is what affects hardness. It is a job for someone who has the correct training, knowledge, and experience, which I do not.

Of course, the OP never told us what receiver, so we don't know what it is made of, or how much of the receiver was affected by the tig welding. He apparently did not know about tig welding receivers, so we don't know if it is safe to use. It most likely is, but we don't KNOW that it is.
 
I used two pieces of stainless rod turned down .010" under the hole size. Took and heated them to just over bright red and inserted one in the hole. Left it in the hole for a few minutes until it was loosing color while I kept the other one hot. Swapped them out and heated the other and repeat. Did that about 6 times or so while reducing the heat every time. Worked great.

I'm gonna steal that procedure. Never ran into this particular problem, closest I've gotten is an 03A3 receiver. They are case hardened, which gets to top "crust" wicked hard. But a carbide drill spotting it usually gets one past the bad part. Usually.
I don't drill many any more since I started only messing with my own property only. Less hassle.
 








 
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