ByronF
02-13-2004, 09:50 AM
I learned quite a lot about drilling out trunion rivets last night. It was my first try at it, so naturally I was on the steepest part of the learning curve.
I recently bought one of those cheap Chinese benchtop drill presses from a friend who wasn't satisfied with it. It's the kind Sportsman's Guide sells for about $50 including a bit set. It's not much of a drill press but plenty good for building AK's. It's well worth the $50 if you only have a hand drill. I can't imagine drilling the rear trunion rivets with a hand drill. One tip for drill press use: adjust the table up toward the bit so you don't have to crank the handle way down to meet the part. Any spindle runout will be amplified, and your chuck will be way below the spindle bearing where it's not located as firmly.
I ground the rivets to near flush with my dremel. I then used a fairly fine tooth file to make them perfectly flush with the side of the trunion. The trunion is way harder than the rivet so you know you've got it flush when the file stops biting quite so much. You'll rub the finish off the side of the trunion but you won't take much metal off, if any. Just keep the file flat so you don't hit the trunion with the edge. It's all hidden anyhow once the trunion is mounted.
Center punch the rivets, aiming for the center of the TRUNION HOLE, NOT THE CENTER OF THE RIVET! The rivet probably upset sideways to some extent, so the center of the rivet isn't necessarily the center of the chamfered hole The front top rivets of the front trunion weren't chamfered on my AMD kit, BTW. The bottom rear holes and the rear trunion holes were chamfered. I used the front of an old throwing dart since my center punch is as sharp as a tent stake. In hindsight I should have punched it with the dart THEN repunched it with a real punch. The dart doesn't make a very large target for aiming a drill bit.
Some have mentioned fabricating some sort of stand-offs to level the irregular front trunion sides when drilling. Not necessary with this cheapo press. Instead, lay the trunion along the edge of the table so only the flat part of the trunion is on the table. The irregular high parts (the part of the trunion that sticks up above the sheet metal) hang over the edge. Now loosen the table clamp and swing it to the side to center the bit over the rivet. I could have clamped the trunion to the edge of the table but I held it by hand instead. I broke a couple bits but I don't know if that's the reason.
Start out with a small bit and just barely kiss the punch mark. Verify that the drill hit center. If it didn't, chuck up a stout bit that won't wander very much and kiss the center. Go very slow when re-establishing center because even stout bits will wander to the misplaced punch mark if you put too much pressure on it. Just barely kiss it with the large bit to make a crater, then go back to the tiny bit.
For the rear trunion rivets drill only as deep as about 2X or 3X the diameter of the bit you are using, then shake/pick out the shavings from the hole. If your bit starts to grab counterbore the top of your hole with a larger bit. This keeps the flutes from grabbing the sharp corner at the top of the hole and helps prevent broken bits. Drill about 1/4 of the way through the rivet with the tiny bit. If you go much deeper you'll probably break the bit. Now work your way up to the size bit that comes close to the edge of the hole. Remember, the chamfered holes are probably smaller than the head of the rivet since the rivet likely expanded in the chamfered area! Step down a couple of sizes and drill half way through the rivet. Then step back up the larger bits. Repeat on the other side. Tap the edge of the rivet from one side. It'll probably push right out. NOTE: If your hole isn't centered STOP increasing bit sizes as soon as the hole gets close to the edge of the rivet. You've relieved a lot of the compression on the rivet so it'll knock out just fine without egging out your trunion hole.
Drilling the bottom rear rivets of the front trunion probably isn't necessary but do it anyhow. Drill a small hole, then a larger hole, then just drive it out with a pin punch. No need to get dangerously close to the edge of the hole. Take most of the meat out of the rivet then knock it out. Rivets are rather soft.
You've read a lot of posts about drilling into the barrel when drilling out the front rivets. Totally unnecessary if you have a drill press with an adjustable stop. You can see how thick the trunion is by simply looking at the front of the trunion! Set your depth stop so the bit can't reach the full depth of the trunion. Better back it out just a shade so you can approach the bottom carefully. Set the stop, but don't rely on it. Note that I've not drilled completely through the front rivets yet so we'll see how that goes tonight.
Byron
I recently bought one of those cheap Chinese benchtop drill presses from a friend who wasn't satisfied with it. It's the kind Sportsman's Guide sells for about $50 including a bit set. It's not much of a drill press but plenty good for building AK's. It's well worth the $50 if you only have a hand drill. I can't imagine drilling the rear trunion rivets with a hand drill. One tip for drill press use: adjust the table up toward the bit so you don't have to crank the handle way down to meet the part. Any spindle runout will be amplified, and your chuck will be way below the spindle bearing where it's not located as firmly.
I ground the rivets to near flush with my dremel. I then used a fairly fine tooth file to make them perfectly flush with the side of the trunion. The trunion is way harder than the rivet so you know you've got it flush when the file stops biting quite so much. You'll rub the finish off the side of the trunion but you won't take much metal off, if any. Just keep the file flat so you don't hit the trunion with the edge. It's all hidden anyhow once the trunion is mounted.
Center punch the rivets, aiming for the center of the TRUNION HOLE, NOT THE CENTER OF THE RIVET! The rivet probably upset sideways to some extent, so the center of the rivet isn't necessarily the center of the chamfered hole The front top rivets of the front trunion weren't chamfered on my AMD kit, BTW. The bottom rear holes and the rear trunion holes were chamfered. I used the front of an old throwing dart since my center punch is as sharp as a tent stake. In hindsight I should have punched it with the dart THEN repunched it with a real punch. The dart doesn't make a very large target for aiming a drill bit.
Some have mentioned fabricating some sort of stand-offs to level the irregular front trunion sides when drilling. Not necessary with this cheapo press. Instead, lay the trunion along the edge of the table so only the flat part of the trunion is on the table. The irregular high parts (the part of the trunion that sticks up above the sheet metal) hang over the edge. Now loosen the table clamp and swing it to the side to center the bit over the rivet. I could have clamped the trunion to the edge of the table but I held it by hand instead. I broke a couple bits but I don't know if that's the reason.
Start out with a small bit and just barely kiss the punch mark. Verify that the drill hit center. If it didn't, chuck up a stout bit that won't wander very much and kiss the center. Go very slow when re-establishing center because even stout bits will wander to the misplaced punch mark if you put too much pressure on it. Just barely kiss it with the large bit to make a crater, then go back to the tiny bit.
For the rear trunion rivets drill only as deep as about 2X or 3X the diameter of the bit you are using, then shake/pick out the shavings from the hole. If your bit starts to grab counterbore the top of your hole with a larger bit. This keeps the flutes from grabbing the sharp corner at the top of the hole and helps prevent broken bits. Drill about 1/4 of the way through the rivet with the tiny bit. If you go much deeper you'll probably break the bit. Now work your way up to the size bit that comes close to the edge of the hole. Remember, the chamfered holes are probably smaller than the head of the rivet since the rivet likely expanded in the chamfered area! Step down a couple of sizes and drill half way through the rivet. Then step back up the larger bits. Repeat on the other side. Tap the edge of the rivet from one side. It'll probably push right out. NOTE: If your hole isn't centered STOP increasing bit sizes as soon as the hole gets close to the edge of the rivet. You've relieved a lot of the compression on the rivet so it'll knock out just fine without egging out your trunion hole.
Drilling the bottom rear rivets of the front trunion probably isn't necessary but do it anyhow. Drill a small hole, then a larger hole, then just drive it out with a pin punch. No need to get dangerously close to the edge of the hole. Take most of the meat out of the rivet then knock it out. Rivets are rather soft.
You've read a lot of posts about drilling into the barrel when drilling out the front rivets. Totally unnecessary if you have a drill press with an adjustable stop. You can see how thick the trunion is by simply looking at the front of the trunion! Set your depth stop so the bit can't reach the full depth of the trunion. Better back it out just a shade so you can approach the bottom carefully. Set the stop, but don't rely on it. Note that I've not drilled completely through the front rivets yet so we'll see how that goes tonight.
Byron