Turning Using Tailstock Center For Support Is P***ing Me Off

tomw

tomw

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Dear All,

I am having a frustrating problem. I can't turn between centers without getting a taper.

Here is the set-up:
1) Sherline lathe 4400, purchased in February 2015. This lathe was sent back to Sherline for adjustment of the tailstock (which was off a bit). Sherline reports the tailstock is now very well aligned.

2) I want to make a shaft that is 2" long, with each end being .19" in diameter for .19". The intervening diameter (between the two .19 diameter ends) should be approx. .09" in dia. This is thin enough and long enough that I need to turn this using a center in the tailstock.

3) I am mounting a 3/8" dia brass rod in a three jaw juck. The rod is center drilled on the outboard end (drilled with the stock snugged up short in the chuck) and extends from the chuck about 2.25 inches. A live center is mounted in the tailstock, and this is snugged up against the outboard end of the stock and locked down.
Set Up.jpg
4) I then attempt to reduce the stock to .19 diameter over most of this length. I am using HSS, turning at 2500 rpm. I am getting no chatter. My final pass is .005 DOC.

The problem:

When I turn a bit of 3/8" brass rod stock between centers to bring to down to .19, I am getting a taper of .06 over the 2" that I am turning. So, if the end of my shaft in near the chuck is .190, the other end will be .196. The tolerance on the part, for the ends, is .002. So, I'm sort of screwed. Yes, I could swap the thing around and machine both ends close the chuck, but then there is no guarantee that they will be concentric, which will be important for later machining operations. Plus, this seems wrong.

In the below photo I am showing the diameters at points 1.5" apart to correspond to the next test (see below)
Between centers.jpg

To see if the headstock or chuck are misaligned, I took a piece of .75" dia aluminum, approximately 2" long, chucked it in the lathe, turned it down down to around .5". Again, my final pass was .005 DOC.

Over 1.5" I am getting 0.000" of taper.
Unsupported.jpg

My request:

Please look over the photos of my set up and tell me what I am doing wrong.

Thanks,

Tom
 
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Try what Michael said. If still no good, you may have a little play in your live center.
 
The tailstock on a Sherline is not adjustable. They are fitted at the factory. According to the tech I spoke with, over a 10" test bar the tailstock/headstock offset of MY LATHE was less than .0015. Thus, over 2", it should be far less than .001.
 
You gave a good report of your problem. If I am remembering correctly, the tailstock is not adjustable? It’s either the tailstock or the live center is out of whack. I would not be looking at the headstock or carriage adjustment. You could turn a center in the headstock and line it up with that live center and visually try and see how close they are in alignment…Good Luck, Dave.
 
Does the spindle have a Morse taper in it? (many do) could you put a dead center in the spindle (with the chuck removed) and see how that alligns with the live center? Use a loupe (10x magnification) to verify alignment, or lack thereof. If you don't have a Morse taper, try what Chips (above) said, turn a point on a piece of scrap and bring the live center in to not quite touch.
 
Everything very well may have checked out with the lathe while at sherline.

Now it is home. Maybe something shifted in transit, the temperature is different, the lathe is supported on a different base, or they just plain screwed the pooch. Really doesn't matter, the tailstock is likely out of alignment and needs to be adjusted in its current location to correct whatever issue is wrong.

They also sell adjustable live centers. I would guess to make it easier to adjust. I'm not familiar with the tailstock adjustment on this particular lathe.

You might want to look into using a test bar to aid in the adjustment and for future verification of alignment.

Good luck getting it going. There are some very experienced here to assist.
 
I went to their website, http://www.sherline.com/latheins.htm, tapers are cut but moving the headstock. I am beginning to think your tool is pushing the stock out the .003. The larger stock is to thick to be pushed. Double check the clearances on the tool bit that it is not rubbing instead of cutting. Use a black marker and mark all the cutting edges. The only other thing would be to take the tailstock off and be sure there are no chips trapped under it.

I wish I could be more helpful.
 
Try indicating the bar from the chuck to the center before you machine it.

What are your tolerances on the finished part? A two place decimal dimension isn't usually held to great accuracy.

Machining slender stock like that requires dead on-center cutting. A hair high or low will give you crazies at the tool tip.
 
Dear All,

Thank you for your advice. I will try all of the above stuff and report back.

I have made sure the taper and chuck are clean, and I did a visual check with a dead center in the tailstock and headstock. However, I did not indicate it.

Thanks,

Tom
 
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