Question about turning between centers

[creedtown182]

How concentric should the center holes be in a shaft before it is placed between centers to be turned? I'm guessing, the closer the better, or doesn't it really matter as long as they are fairly closely aligned? What happens if the center holes are too far out of concentric? Thanks guys.
Brian

[DonT]

Brian,

First if you think of it as drawing a line between the center of the two holes it will give you a better idea of what I am saying.

If the material is large enough to get you the finished diameter you want then in theory it doesn't matter, the the following considerations.

Having the holes as close to center as possible allows you to use a smaller piece of stock and therefore requires less machining.

When the holes are not centered you will most likely end up, for a period of time, making and interupted cut on one end untill the stock is cut enough to make the holes on center. This can cause additional strain on the equipment and cutter and you have to watch to make sure the vibration from the interupted cut does not loosen things up. If the material is thin it can cause it to bend slightly so you want to take lighter cuts until you get past this point.

One thing to keep in mind is if the part is overly large in diameter turning it in the lathe when it is too far off center is like having a tire out of balance. The faster you turn it the more vibration you get till it gets to a dangerous point.

Finally it can take longer to bring the stock to size since you will be making a cut on one end till you reduce the diameter enough and the rest of the travel the cutter will not be cutting or only taking very shallow cuts.

On occassioin in the past when I had a hole on one end that was off center and I had enough stock length I would drill the end that was off out about 1/2 its diameter then make a (lets call it for lack of a better term) false center. This would fit snuggly into the hole and then I would use a center drill to redrill the hole closer to the center where it belonged. When done just cut the piece to length and discard it with the false center.

Good luck..
DonT

[creedtown182]

Thanks man that helps a good bit. I didn't know if I was being overly concerned about getting the center holes in the center of the shaft or if it didn't really matter.

My last job I had, we did no work between centers. I was working with either a 3 jaw universal chuck or 4 jaw independent chuck in combination with the tail stock revolving center . I did my best to get the shafts running as true as I could before center drilling one end for the tail stock live center; otherwise it seemed to me that the chuck would be fighting against the tail stock live center if the center hole were too far off center.

I guess it really doesn't matter though, since I've seen quite a few people take a lengthy bar, chuck it up short, and center drill the end; meanwhile, the opposite end of the bar that is back in the headstock spindle was running out of true a good bit.