Bull Nose Live Center

ddickey

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So the MT would hold fine? Any concern about heavy cutting?
 

Ray C

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So the MT would hold fine? Any concern about heavy cutting?

As long as the workpiece is being supported by the live (or dead) center in the tailstock and there is sufficient pressure from the tailstock ram to hold the piece securely then, yes that MT shaft will stay put in the spindle.

The point is, a stand-alone MT device stuffed into the mouth of a spindle will not stay put by itself. It needs some kind of force to hold it in place. That force can come from the tailstock or, as one person suggested, a drawbar thru the spindle can hold it. Personally, I've never seen a setup where a drawbar holds a taper piece in the headstock but, sure, it's possible.

Ray
 

ddickey

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First things first, lets talk about turning a shaft. Boring a hole is pretty much the same thing (but in reverse).

1.310 - 1.1250 = 0.1850 (EDIT: I had a typo here and changed 1.1255 to 1.1250).

On my machine, when I dial-in 30 thou, it reduces the diameter by 60 thou (some lathes are calibrated for diameter reduction and not tool position). Since .1850 / 0.60 is 3.0833 we know I must take 3 passes at 30 thou and an oddball pass at 2.5 thou (.0833 x .06 / 2 is 0.0025).

Here’s the rub… What you dial in and how much comes off are two different things most of the time. Unless there is a gross F&S error or tool alignment issue, it will almost always take off less than what you desire. First, just go ahead, dial in the 2.5 and remove it. An amount that small should come off with no problem.

Ray C.
Ray, I think you meant 0.06 not 0.60.
What if you miss your .0025 and end up taking say .002 off. Do you just start from there or?
My machine was taking more off than less so my percent number was over 100.
 

Ray C

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Ray, I think you meant 0.06 not 0.60.
What if you miss your .0025 and end up taking say .002 off. Do you just start from there or?
My machine was taking more off than less so my percent number was over 100.

If you under shoot by a small amount, emery cloth or if necessary a good file followed by emery cloth.

You will likely get more than 100% if you dial-in and increase from your last dial setting vs, scratching-off and re-establishing your next starting point.

For example. Take your first cut and dial is on 0.015. When you take your next cut, you just add the new amount to your old 0.015 and go directly to say 0.030. In this case, you might take off more than 0.015.

In contrast:

Take your first cut and the dial is on 0.015. When you go to take your next cut, you back the tool out then move in until you see a faint scratch (this is called scratching off). The scratch off happens at say, 0.012 so for your second cut of 0.015, you turn the dial to 0.027. You will never get more than 100% this way.

The technique works either way and you have to solve for a ratio either way.

Ray
 

ddickey

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That's exactly what I was doing. Okay. I'll try again perhaps tomorrow.
 

Ray C

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That's exactly what I was doing. Okay. I'll try again perhaps tomorrow.

Carbide? HSS? Are you below center? Is the piece supported properly? If you're scratching off, followed by a dial-in, the amount removed should never be more than twice the dial in assuming your crossfeed indicates distance and not diameter reduction. (The crossfeed dial on some lathes is calibrated to show diameter reduction).

If for some reason, you very lightly scratch off and the overall diameter is reduced by more than 2x the dial-in amount, something else is going wrong.

E.g. if you dial in 30 thou, your part should theoretically be reduced by 60 thou. In reality, it will be reduced by say 55 thou.

So, you dialed in 30 and effectively got 27.5. Lets say you need to reduce the diameter by 58 thou on the next cut. Here's what you do:

To reduce dia by 58 thou, you theoretically need to dial-in 58/2 = 29 thou. In reality, you need to dial-in a little more than the theoretical amount.

27.5/30 = 29/X. Solve for X: 31.636 (which I would round to 31.6).

Go to your lathe, crank the dial until it just barely scratches, move the tool away then dial in 31.6 thou. That should do it.


Ray
 

ddickey

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Oh, I was meaning that I was taking a cut and returning then feeding in another .03" on the dial rather than re-establishing my next start point That explains why I was getting a positive removal rate.
 

Ray C

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Oh, I was meaning that I was taking a cut and returning then feeding in another .03" on the dial rather than re-establishing my next start point That explains why I was getting a positive removal rate.

Ahhh, ok. Either way, it works. This is nothing more than linear extrapolation. As long as you don't try to extrapolate beyond about 10% of your previous data points, the system works quite well.

FWIW, when I'm doing a critical cut and the two or three cuts before it, I do a scratch-off. For bulk removal, I just stack the cuts from the last value.

Ray
 

Ray C

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Quick status report here... The Bullnose Live Center was used for the first time today and it performed perfectly. It was a fun project executed only to see if I could pull it off. LOL: If I had to do it all over again, I'd pry a couple hundred out of my wallet and buy one. But... this one does the job nicely and now I'll see how long it holds up.



IMG_20180211_152157.jpg

Later

Ray
 

SLK001

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OH MY GOD!!! THAT'S 10-20 MILLIONTHS OF RUN-OUT! I THOUGHT WE WERE DOING PRECISION WORK HERE!;)

Did you test at different angles in the setup? I do love it when a 1/10's indicator barely wiggles!
 
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