Finding Center

E

epj

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I manufacture sight pushers for auto pistols. Most of my designs require a series of holes drilled on the centerline of 1-1/2" bar stock. In reality, the same sizes of steel and aluminum are several thousandths different in width. When mating the two, the holes really need to be centered.
Anyway, I have a Grizzly 1007 equipped with a DRO Pros glass scale 2 axis DRO. A friend who is a professional machinist helped me set it up. We tramed the table and the vise and found it to be durn near perfect. Out less the than .001 for 360*.
When I use the DRO to find center I use an electronic edge finder mounted in a collet. When I drill a hole, I also use a collet. I'm still ending up 10-15 thousandths off center.
Any idea what I'm doing wrong?
 
Do you start the hole with a center drill? They are very short and thus stiff, don't wander about before digging in, which a drill will do.
 
I've tried it both ways. I'm using very short stiff drill bits 3/8". Can't imagine they wander that much. If I use the center drill I have to use the chuck that came with the machine. I'm sure the collet is much more accurate. I need a better chuck, but in looking at what's available, even the high dollar domestic stuff shows several thousandths run out. I'm not building space ship parts, but sure would be nice to get within a couple of thou.
 
+1 what Tom said. In addition to that, have you checked your center finder to make sure it is accurate and repeatable? Try finding the center from both sides of the part and see if the center is in the same place. Also, check your DRO against a dial indicator and see if they agree. You might also try a mechanical edge finder and see if the accuracy improves.

EDIT: One thing that I do is to zero off of the fixed jaw of the vice, that way I can do multiple parts without having re-zero.
 
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The way to achieve the best average results is to measure the stock you have at random, find the one nearest the mean average of all the variations, put it in the vise if that is what you are using, and ball off both sides. Zero the first side and then ball off the other side. Never mind what the actual reading is, just half it. You will be as close to center of all your stock as you can expect to be, unless you cut it up and segregate it into classes that fit your desired target zone. You still need to uses an edge finder of some sort to hit both sides, not just one. Let the edge finder measure the stock, thus eliminating any error it may have.
 
I always use an indicator in the collet or chuck to find center. But if using an edge finder I always find edge on both sides and then figure center.
 
Yesterday, I suspected the DRO was giving an erroneous reading. The math didn't seem to add up. Today I placed a 1.5" parallel in the vise and ran the edge finder tom both sides. The DRO read 1.3000, just as it should have. The half function divided correctly, .650. I dunno, I can see (and accept) being off .002-.003, but not 10-15 thousandths. The material I'm using is 1-1/2X 3/8 cold finished 1018. The other parts are 1-1/2"X 1/2" extruded aluminum. Most of the time the steel is 1.496. The aluminum can be anywhere from 1.490-1.510. On average the aluminum is closer to 1.500 than the steel is. So long as I can keep my holes on center, everything works pretty well. If it's more than a couple or three thou off, lots of fitting is required to make the parts work. That's why I bought the DRO in the first place, once it became obvious that the hand wheel scales were not that accurate. Actually though, when we first installed the DRO, the Y axis hand wheel was dead on with the DRO.
I'll try again with the center drill. I'll have to use my drill chuck. Chinese origin, quality suspect. Can anyone recommend a good quality chuck that won't require a second mortgage. Any other suggestions are welcome!
 
Tony Wells said:
put it in the vise if that is what you are using, and ball off both sides. Zero the first side and then ball off the other side.
Click to expand...

Tony, thanks for all the great advise you have given us in the past, but, I have to chastise you here lol. Please, for us beginners, what the ----- does "ball off " mean? thanks, JR49
 
I wondered who would be the first to ask, JR.

It's a slangish term for, originally, using the ball from a wiggler type edge finder. There are times when you want to avoid the very edge of a workpiece, and the straight stem of the cylindrical edge finder won't do that. It finds the outermost point only. I think the term "ball off" meant literally to run the work up to the wiggler until the ball runs off, thus indicating that the spindle centerline is half the ball diameter away from the point (not the line) that the ball contacts the work.
 
To accurately find the center of a round feature, I use an edge finder. The edge finder centers itself to the true spindle axis so runout on a chuck or collet is immaterial. Set the y axis as close to center as you can. Zero the y readout. Come in on the x axis until the edge finder jumps. Zero the x axis. Back off and find the edge again slooowly. I usually come in at about 1/10,000th/sec. Repeat until you have a consistent reading. Rezero the x axis and move to the other side of the piece at the same y and z coordinates. come in to the piece with the edge finder as before. Note the contact point and repeat as above. Use your DRO half function to locate the x axis center. Move the edge finder to some minus y position, same z position and x = 0. Perform the same center finding operation for the y axis.

While this may be good enough, I always go back and check the x and y centering again. If your initial estimate of the y axis center was off, you definitely should recheck the centers. This process gives me a reproducible center location to .0001". This may seem like a lot of work but once you are used to it, it is very fast. When I back off, I use my finger nail or a pencil to nudge the edge finder into running true. That way, I only have to back off about a thousandth. Coming back in is then a matter of 12 -15 seconds. On using the edge finder, the surfaces of the edge finder and the edge you are finding should be clean. Use a low to medium spindle speed and keep the speed constant throughout the process ( I generally use between 350 and 1000 rpm).

I would definitely use a center drill to start the hole. You can also use a small end mill (less than the diameter of your hole). It will cut a hole centered on your spindle axis without regard for any runout. A drill will then follow the pilot hole and due to its flexibility, it will be true to the center.
 
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