The cavity and blocks don't always jive :-)

[Buckshot]

...............One of the board members here asked me if I could fix a mould someone had drilled for a HP pin and messed up. I could, within reason depending upon how bad the 'messed up' part was. I'd done it before a couple times and figured I had the routine down.

When I got the mould I looked at it and the hole was definately offset deeper into one block half then the other. Whoever had drilled it did a fine job of it. The hole was straight, clean and not angled or wallowed at either end. Looked like about a #40 bit. You can't use another drill bit to re-drill as it will want to start in the original hole and follow it.

I took a solid carbide 1/8" 4 flute centercutting endmill (short and very rigid) and after indicating in the cavity I ran the endmill in. Due to it's short length I got about a 1/4" deep hole started. I had a longer 1/8" carbide 2 flute centercutting endmill so used that to finish boring on through the blocks. Satisfied that I was going to be a hero, I pulled the blocks out, flushed the swarf out with some carb cleaner and checked it out.

DANG! The new hole was also offset. No way. I watched close to see if the endmill was going to flex over and try to follow the hole and it hadn't. The hole was as straight and clean as the original had been. It had to be something else.

I put a .0005" test indicator in a holder and set that and the blocks on a granite surface plate that is flat to 50 millionths. Laid on their backs, the faces of both blocks were flat and within a thou of being the same height. Nice. Then I found the problem. Running the TI's probe in one cavity half and then the other, I found the cavity was .015" deeper in one block half then the other.

Just shows what assuming does. Whoever had originally drilled the blocks had done a good job, and there was nothing wrong with their work. I was also very surprised that the small diameter bit used to drill the original hole hadn't tried to follow the blocks' parting line.

.................Buckshot

[dubber123]

I've run into quite a few of these offset bored moulds, mostly Lymans, (Thats mostly what I own). Some you can look at and visually see are off set. A royal pain to get bullets out of. There is no cure I can think of either.

[Linstrum]

That kind of out-of-tolerance problem is often caused by a chip getting into the vise or chuck and acting as a shim to re-locate the part being machined so it is out of whack like that. When I was a 17-year-old apprentice machinist I had a job of drilling a 1" diameter hole 18" deep into a 2.5" diameter bar of expensive 17-4 ph stainless steel. After I got the hole drilled I was supposed to turn the outside diameter to a certain size to true it up with the cylinder bore. Something that long of course needs a center to hold up the far end and I had center drilled the closed end of the bar before beginning the bore. I didn't see a chip that was stuck to the bottom of the carbide-tipped center with some white lead high temperature center grease and I chucked the bar up with the far end off center by about 0.010". The master machinist who I was apprenticed to came by after I had begun turning the outside and told me to stop and clear the center pocket with the center drill held by hand, his trained eye saw that the tailstock center was not concentric with the tool marks left on the end from when I had squared it up before drilling the center pocket. Being able to see a 0.010" radial offset (0.020" off the diameter) takes a good eye. Fortunately I had only made one 0.025" cut off the radius and had not ruined the piece of bar stock that in today's dollars would cost about $200 plus the time I had put into it, around another $50. That is part of learning to be a careful machinist and I now check the center pocket and tailstock center for chips before tightening things down in the lathe. Good thing the master machinist was a nice easy-going guy more interested in teaching me than yelling at me.

Working with a milling machine or surface grinder also requires the same exact care in checking the table and vise for chips before clamping the part down in preparation for chopping and grinding away at it to remove the metal that is hiding the part within.

There are lots of pitfalls like that and they can cause some really weird trouble. The next door neighbor I had back in 1975 had been a machinist during WW2 for Pratt and Whitney manufacturing fighter plane engines and he told me about one engine that came back from the test shop because when they test fired the synchronized machine gun through the prop it shot a hole through the test prop. They found that the problem was the synchronizer lobe on the cam shaft that operated the sear on the machine gun was machined a few degrees off. They figured it was from having a chip get into the chuck when the machinist chucked up the cam shaft in his profile cutter lathe and had then not used a dial indicator to check it for being in position in order to save a few minutes of time.

[Bret4207]

So how'd ya fix it Buck?

BTW- Somewhere in the central Adirondacks there resides a Savage 99 in 300 Sav that has 2 plugged and peened 6-48 screw holes in addition to the proper screw holes for the scope bases. This is the result of a chip getting between the base and arm of my Forster sight jig. That was the very first time I had anyone call me on the phone and swear at me. Quite traumatic for a young guy. I made it right and he apologized.

[Buckshot]

...........How'd I fix it? Well it didn't really need fixing. The original HP pin hole was on center to the cavities axis. I just did the same thing only made the hole bigger is all. Still on the cavities axis.

..............Buckshot