Live center accuracy, what's acceptable

[jumps4]

i'm not a machinist and have no training but i would think that for the tolerences that are being talked about here a live center is a needle in a hay stack and some adjustment of a tailstock is going to be required to acheive the tolerances being disgused anyway? a good machine has a heavy headstock with precision bearing then on the other end of the lathe there is the tail stock, much smaller at no where the precision of the headstock. the greater the distance from the head stock and pressure put on the cutter i cant see the precision of the live center being any more of a factor than bed rigidity and tailstock bore fit. am i missing something in my thinking. or wouldnt it be better to turn the part around in a 4jaw whenever possible and work at the headstock end and work towards the center for real precision at the levels you are talking about.
I have one indicator that reads in .0001 increments but i cant figure out how to use it and not breath on it at the same time:lmao:
steve

 

[Ray C]

Thank you! In the home shop enviroment, they are fine.

It's a valid question for the good folks who are learning this for the first time. Money is at a premium for all of us and we want our tool purchase to give the best bang for the buck and we want to make the right tool choices.

Yes, modern live centers (even the cheap ones these day) hold tolerances that I cannot measure and they are heavily used and holding-up fine. And if one of them goes bad, I will thank it for it's good service then, soul-lessly throw it in the scrap pile. -Take that back... I'll probably cut the MT3 tail off and do something else with it.


Ray

i'm not a machinist and have no training but i would think that for the tolerences that are being talked about here a live center is a needle in a hay stack and some adjustment of a tailstock is going to be required to acheive the tolerances being disgused anyway? a good machine has a heavy headstock with precision bearing then on the other end of the lathe there is the tail stock, much smaller at no where the precision of the headstock. the greater the distance from the head stock and pressure put on the cutter i cant see the precision of the live center being any more of a factor than bed rigidity and tailstock bore fit. am i missing something in my thinking. or wouldnt it be better to turn the part around in a 4jaw whenever possible and work at the headstock end and work towards the center for real precision at the levels you are talking about.
I have one indicator that reads in .0001 increments but i cant figure out how to use it and not breath on it at the same time:lmao:
steve

 

[Ray C]

Well, it just so happens, I was re-testing the lathe on the 1-week anniversary of doing a precision leveling adjustment. After that, I was checking the tailstock with an extended shaft and decided to test with a live and dead center. Here's what the setup looked like. You'll have to take my word for it but, the 2 collars read identically using both a live or dead center. Measurements were taken with a micrometer reading tenths.

Not saying all live centers will perform well but, given their generally good service they give me, I never questioned that it made much difference -at least for the stuff I do. -Darn it! I just remembered that I have one with a long nose and I forgot to test it. Oh well, some day...

Ray

 

[Ray C]

Hey Rich,

I'd love to see this... to check all the centers I have (live or not).


Ray

When I was an apprentice many moons ago a Journeyman machinist showed me how to check a dead center. He took a ground V block and a angle block, he laid a piece of paper on a surface plate, then set the angle block on the paper and shim it up so the ball hits the center of the ball, then put the V block on the paper next to the block. Then he had a precision ball bearing (single ball) that he put a dab of grease on it and pressed the ball into the female center at the other end of the center point. Then laid the center in the V block with the ball resting on the angle block. Then put a height gage with a .0001" indicator on the paper and rested the indicator on the pointed center surface, zeroed the indicator and carefully turned the center. The paper kept everything from sliding. Takes some practice but it works good. I never thought of it until now, I suppose you could check a live centers concentricity with the shank that way too.

 

[kd4gij]

Thank you! In the home shop enviroment, they are fine.

It's a valid question for the good folks who are learning this for the first time. Money is at a premium for all of us and we want our tool purchase to give the best bang for the buck and we want to make the right tool choices.

Yes, modern live centers (even the cheap ones these day) hold tolerances that I cannot measure and they are heavily used and holding-up fine. And if one of them goes bad, I will thank it for it's good service then, soul-lessly throw it in the scrap pile. -Take that back... I'll probably cut the MT3 tail off and do something else with it.


Ray

If your live center gives up the goast wouldn't that make it a DEAD center.:rofl:

 

[Dranreb]

Well, it's arrived, bigger in the hand that it looks in the photo, and seems well made.

Being Chinese made translations can vary, the included specs say the 0.01mm accuracy, D412 here> http://www.zhengzhoutools.com/en/cp_show.asp?id=336

I make 0.0008" = 0.02032 mm.

Can I be bothered to quiz the seller on this?.....Ho hum..

Bernard

 

[george wilson]

If things get really critical,the most accurate center is a dead center,preferably carbide. Just keep track how tight the rotating metal gets on it. The metal gets longer as you heat it up working on it. A carbon center is perfectly capable of getting too hot and friction welding to the work you are doing,and shearing right off into your metal.

I managed to buy the very last can of white lead they had at the paint store way back before I had money to buy a live center. Messy stuff,though. Hard to keep it from getting on your hands.

 

[Bill C.]

Well, it just so happens, I was re-testing the lathe on the 1-week anniversary of doing a precision leveling adjustment. After that, I was checking the tailstock with an extended shaft and decided to test with a live and dead center. Here's what the setup looked like. You'll have to take my word for it but, the 2 collars read identically using both a live or dead center. Measurements were taken with a micrometer reading tenths.

Not saying all live centers will perform well but, given their generally good service they give me, I never questioned that it made much difference -at least for the stuff I do. -Darn it! I just remembered that I have one with a long nose and I forgot to test it. Oh well, some day...

Ray


View attachment 48953

Ray I like your setup. The shops I worked in we never checked the centers. Never cut long tapers either. If I owned a lathe I would double check the centers with a test mandrel like yours. By-the-way most of my work was chuck mounted.

 

[Ray C]

Hi Bill... As you probably know, with chuck-mounting, you can't test tailstock positioning worth a darn. On the face plate is a balanced type of dog setup. Looks a little weird but works OK. If you look over in the Machine Rebuilding section, as well as a thread called YALB Yet Another Lathe Bench. You'll see a ton of information about how to check lathe alignment. I learned a lot in the process of this. My last lathe was very old and worn out and it couln't really cut straight enough to make a difference. My very first lathe was a monster and was properly setup when I started using it. This is the 1st time I'm going though my own setup really although I did learn a lot over a few years trying to make an antiquated and worn lathe run well.


Ray

Ray I like your setup. The shops I worked in we never checked the centers. Never cut long tapers either. If I owned a lathe I would double check the centers with a test mandrel like yours. By-the-way most of my work was chuck mounted.

 

[refinery Mike]

I like the live center because i do not have to worry about getting some swarf in the greese. then when you put it back in the lathe it chews up your work and your center. (Been there done that) But runout is not the only thing to think about when buying a live center. The cheep Chinese live centers may only last a year and then get so loose the bit chatters like crazy. (Been there done that). Might be better to go for a middle of the road live center, for a hobby lathe. Does anyone else remember when a live center was a plain center that was actually hardened, and a dead center was left soft. (for the head stock)