What's acceptable for accuracy?

epanzella

epanzella

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I'm going to be building my first rifle this winter and I'm just trying to get the best degree of accuracy that's possible from my Grizzly G4003G. I have a ground 12 inch piece of 1 inch od drill rod so I thought I would start by getting that spinning true just by impersonating a machinist. I stuck it into a 5C collet and center drilled it. Then I put the drilled end into a live center and stuck the dead end about one inch into the 5c collet. I got my DTI needle going from .000 to .001 on one end and about (-).001 to .000 on the other. I don't know if I should call this .001 or .0005 runout (on each end). Wanting to do better, I parted off the center drilled end and chucked it in a 4 jaw. After 20 minutes of doing it over and over I got the DTI reading about .0002/.0003 at the 4 jaw end. (interpolating between the .001 ticks). I center drilled it again. Putting the drilled end in a live center and about 1 inch into the 4 jaw at the dead end I got it dialed in (4 jaw) to just about nothing. Stuck the DTI on the live center end and it was STILL pulsing .000 to .001??? I put the DTI onto the live center and it was dead nutz. So, is this as good as it gets or am I doing something wrong? Could it be the Jacobs chuck that is holding the center drill? I want to fine tune my tailstock which is off about a thou or two but how can I trust readings that are hopping around? Any thoughts would be appreciated.
Ed P
 
I would be pleased with .001" run out. That's only .0005" off center. But to answer your question you've got a couple of things working against you. One; you're drilling the center hole which is not absolutely accurate as the drill can walk off center. It's not you rather the nature of the process. You might get it a bit closer by machining the center hole after drilling. Two; a live center has inherent run out. We're talking a couple of tenths because the live center bearings have a miniscule amount of clearance between the balls and race. If you want near zero run out then I suggest using oversize material and machining between centers. If your goal is zero run out then you will need the services of someone with an OD grinder.

Hope this helps.

Tom S
 
Try that test with a dead center in the tailstock. That will tell you that it is the centerdrilled hole, and as Tom mentioned, you can machine them after drilling to improve the runout.
 
I always shoot for as perfect as I can - I do not believe in "close enough" or "good enough" - but if things happen and "perfection" is lost, one does need to know what is acceptable.

I don't know anything about gunsmithing, but I can't imagine a thou of runout is going to make anything explode abnormally! One thing to point out - actually machining to accuracies measured in only a few tenths of a thou on a standard lathe is quite the feat, and certainly not a tolerance that older and most likely even newer guns are made to.

Now, like TomS has said, live centers do have some runout - though typically miniscule if it's not worn and in good shape. If you need greater accuracy in a center support, opt for a dead center; then it's just down to the tailstock alignment. Just make sure you put a healthy amount of oil on it.

And also reiterating TomS, using a small boring bar to bore the 30-degree center hole would provide the best accuracy. That may be impossible or impractical though depending on the maximum size of the hole...

As well, drill chucks aren't perfect. Although on a lathe things can sometimes self-center, it won't necessarily be perfect. Most drill chucks have a couple of thou of runout at minimum.

For fine-tuning your tailstock, here is what I would do, my friend. Purchase some decent size material in reasonable length - say 3/4 or 1" diameter, length to suit your needs for precision - that is known to be true and round, such as drill rod.

Chuck in in a four-jaw chuck with minimal or no protrusion, indicate to your desired closeness - keep in mind that it takes one heck of a DTI to truly be precise to under a half-thou. Center-drill it with a decent size center drill, large enough so that you can then proceed to bore out the center angle. Check it with a center gage (also called a fishtail or thread flag) if your compound is not exactly true.

Flip the stock and repeat. Then, mount it between centers - preferably a dead center in the tail stock. Check the headstock center for runout as well.

Then mount your indicator to the cross slide or compound and move it along the material, and see what you come up with. Adjust the tailstock as needed!

That's more or less how I do it, and how I've done it. On my 6x18 lathe, I have my tailstock aligned within .001 over about eight inches. I used chrome-plated hydraulic shaft for a test bar, so really, I add +/- a thou.
 
I like the way zmotorsports described it in his PM1340GT review. Take a piece of rod an inch or larger in diameter and mount it in your chuck with 6 or 7 inches sticking out. Cut a few thou off and mic it. If the headstock is inline with the bed it will mic the same next to the chuck and out on the end. Next take a piece of rod and center drill both ends. Mount your face plate and put a dead center in the spindle and one in the tailstock. Mount the rod between centers with a lathe dog hooked in a slot on the face plate to drive it. Turn a few thousands off. Mic both ends. Adjust the tail stock center until they are equal. Now your alignment between head and tail stock should be accurate.

To check your cross slide, mount a disk in your chuck and make a face cut. Check for flatness using a straight edge.

Your best accuracy will be with your dead center in most cases.

Dave
 
if you are machining the whole part it doesn't really matter how off you start at. you will be machining it true.
 
Dave, if you want to check squareness of a face, cut a disc, and then run a test indicator across it. Should read zero across the front, since that was the cutter path, but past center it will show any deviation from square. Actually it will show 2x, but ideally, it will be zero from front edge to back edge. No need for a straightedge. It will show a cone if the cross slide is out of square with the ways and or the headstock.
 
Lots of good info here. I didn't want to turn the 1 inch ground drill rod as it's dead nutz when rolled on a surface plate on so I'll have to scare up some stock to turn between centers so I can fine tune my tailstock.
Thanks all.
Ed P
 
Drill rod looks accurate,but it is not as accurate as you might think. It tends to be "lobed".rather than perfectly round. I can't recall how lobed it is,but it is not a standard of accuracy that you should expect to be perfect.
 
george wilson said:
Drill rod looks accurate,but it is not as accurate as you might think. It tends to be "lobed".rather than perfectly round. I can't recall how lobed it is,but it is not a standard of accuracy that you should expect to be perfect.
Click to expand...
I picked up a hunk of 1 3/4 whoknowsium from the scrap yard today. I'll turn some rings on it when I get a chance. Thanks, George.
Ed P
 
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