make sure your dowel holes are on location!

Most spotting drills have a 135 deg. split point, which is supposed to allow you to drill shallow holes without using a center drill. I don't trust them! Where hole positions are not that critical, spotting drills are acceptable.

Also, when using center drills, they needs to be running as true as you can get it to run.

Drill chuck run out is one of the major causes of holes not being in true position. That's why you need to keep your drill chucks in good shape. The jaw don't last for ever. They need to be changed out when they start showing signs of wear. All of my drill chucks have fairly good jaws in them.

Also your reamer needs to be running dead true too. Any reamer run out will cause a hole to ream oversized and can put it out of true position, too, even though in theory generally follows the drilled hole.

Ken
 
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Using a spotting drill for hole locations @+- .001 is subject to the 3 d's Dont Do Dat. Always use a center drill. As in some other replies, for locations that close you must bore the holes. Bore to about -.005 and use a good quality reamer. As for the hole Dia. You should always "characterize" your reamers. In other words make practice holes.
If you have ever achived that + - .001 tolerance using the procedure you have described that woud be a good day to buy a lottery ticket. As peviously posted that's what jig borers and jig grinders are for.
OK so you screwed up. Don't knock yourself out. Learn from it. I can remenber my first job as a toolmaker. My father in law got it for me and the very first job I was given making a bunch of parts for a commercial vibration tester. I cut all the pieces too short. I can still see that shop owner jumping up and down on his cigar. That was over 58 years ago and I can still draw you a good sketch of that part. Anyway I never made that mistake again and I went on to become the lead toolmaker in that shop and later on a member of the board of directors for the company. So take heart and learn.
Ron
 
thanks again everybody, and ronzo, i love that visual:roflmao:
Anyway i thought i share what one of my senior workmates told me when i was first starting out and had been making some BIG mistakes.
the dialogue went sort of like this:

Dan: **** happens.
Me: I know but why does it always seem to happen to me?
Dan: You only know what you've done. You have no idea what I've f$%&ed up over the past thirty years.

I'll never forget that
 
I completely understand what's going on here but, I'll show my naivete and tell you what I'm really wondering... How often does it arise and how realistic is it to locate a hole with such precision? I certainly understand the need for precise diameter holes but, if the overall structure or part was not measured and produced at a specific temperature, expansion of the overall part could easily blow such a tight tolerance.

Please don't misinterpret... I'm not speaking in the offensive. Just really curious how often these situations arise as it seems to me, the part could possibly benefit from redesign to eliminate or mitigate such vulnerabilities...

Ray



Skip said:
Many times in my toolmaking years I was called on to do exactly what you tried. I would first spot the place to put the first hole. Second I would center drill, drill and ream the hole for a slight press fit, (I would go back and ream for the slip fit after the next hole was finished.) Third I would put a dowel into the press fit hole and then indicate the pin true to the quill. Then I would step off to drill and ream the next holes. This sounds like it takes a lot of time and it does, but remember, the tolerance was put there for a reason. You may or may not be making the mating part or where the part you are making may be going into the next county or state. Precision does not come cheap.

Skip
Retired and loving it.
Click to expand...
 
It's not as uncommon as you might think, Ray. In the last oilfield machine shop I worked in (other than mine) I can think easily of maybe 40-50 part numbers that had location tolerances that tight, accompanied by size tolerances of 0.0002 total. Granted, the sizes of the features was usually small, so it's not like holding a 24" diameter bore on location to 0.001, it's more like 0.5000 or less on the parts I refer to. Almost always there were dowel pins involved, so there were two parts that were to be assembled with precision alignment. Sometimes it was also to assure the proper assembly of a mating pair of multi-pin electrical connectors similar in construction to a DB-9, 15, or 25. Or some proprietary connector. The pins were often so small and delicate that it would be extremely easy to damage them during assembly, and a lot of these connectors are located deep in small diameter bores where you can't see to align them anyway. So the dowels pressed in, and close fitting clearance holes engaged first, and ensured the connectors' alignment. At times, to complicate things, there were spatial relationships between features on opposite ends of a part several feet long that were toleranced to just a couple of thousandths.

Other reasons include orientation of a series of sub-assemblies in a tool housing. Much of the instrumentation used in MWD/LWD relies on accelerometer alignment, and other sensors. A small error will accumulate to possibly hundreds of feet when working remotely at the depths of some well bores. Some of the accuracy of the directional drilling is quite impressive, but it is only possible because of the precision of the components and their assemblies.
 
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