Slight taper turning between centers

[Technical Ted]

How level is your lathe? What did you use to level it?

Ted

 

[Alcap]

Is it possible that the live center with .004 runout cause issues if it didn’t rotate constantly with the shaft ? Maybe mark the shaft and center to verify it’s not slipping.

 

[tq60]

Start fresh.

Get some round stock, 2 inch or so diameter, a few inches long.

Maybe another, 1 inch diameter a foot long.

Short piece 1/2 inch diameter.

Long one needs to fit through spindle so whatever size that is.

Start by facing the ends of everything but the short 1/2 round.

Make a good center drill in each end. De-burr after drilling.

Place the large round I chuck and support with tail and remove material from center leaving maybe an inch or 1/2 inch at each end.

Reduce center by about a third, not critical, just make it look good.

You now have a "spool"

Place short in chuck and turn a center.

Place center in tail

Put spool between centers, no dog needed.

With a marker paint the part as it spins.

Take very light cuts, prefered with power feed on finest setting.

Only cut enough to remove the ink on both ends.

Measure both ends and now you see offset in difference in diameter.

Place dial indicator so you can measure tail shift and move it 1/2 the difference.

When it gets close often just tightening one side is enough, if it moves it was loose...

Repeat until both ends same.

Ts now aligned.

New tool made.

Now you do same but use dial indicator in tool post

Put long round in and make a cut at the ends.

Position cutter just past end and insert stock between centers, make cut then remove stock and move tool to opposite end and repeat.

Od should come out same.

Wear in bed pay cause some difference, this is same as first tool but not wasting the material and a different part of the bed.

Ours looks like this.

Sent from my SM-G781V using Tapatalk

 

[ddickey]

Flip your piece around and take a spring pass. The result may be interesting.

 

[Lo-Fi]

A live centre is not the tool to use for accurate work unless you've got some serious faith in its lack of runout and repeatability. If it's got 4 thou runout unloaded, I'd not be surprised if it's rocking over under tool pressure, which explains your success measuring with the test bar but not with a cut.
I learned this the hard way making a precision spindle recently and ended up going old school with a dead centre.

 

[pdentrem]

I do cylindrical grinding on a Landis machine and always between dead centers. When trying to hold sub 0.0003” it is the only way.
As for live center, there are accurate ones and as the accuracy increases so does the price. Even a cheap one holds better than .004”. Many can be serviced. The back cap is either screwed in or pressed on. Under the the cap is often a nut and spring with the support bearing. A little servicing may reduce the looseness.
Pierre

 

[Janderso]

Tail stock- bed wear?

 

[Beckerkumm]

I've been messing with the same issues and this is what I believe happens. The tailstock is usually sitting on the part of the ways with the least wear as is the stock nearest the chuck. As you move the tailstock for different lengths, it is moving up or down relative to the chuck center. The the cutter moves from less to more wear, the tool tip moves down and away as the wear increases and closer and higher as it approaches less wear. That movement also changes where the tip hits the bar. As it moves down and away, it moves off center and takes less DOC ( assuming you start at center or below ), opposite as it moves to less wear. You can adjust taper by moving the tailstock or even with slight adjustments of the tooling up or down. The problem is it is a pain if moving from 6" to 12" as you have to readjust the tailstock and seldom want to mess much with cutter height. I do set up a dial and zero it against my cutter tip that works best for non tailstock turning and know that a couple thou adjustment works better when the live center is out a certain length. Some of this isn't necessary most of the time but I do a lot of experimenting with a machine to learn its quirks or give it a chance to learn mine.

I've also had poor luck with cheap live centers, and by cheap I mean 100+. I have a NOS Bison with a short nose and a Royal long nose that have run out less than .001 but have a new center that came .002 out of the box and the seller ignored my attempts to return. In fairness I let it sit untested for a few months so test them immediately as even the $125 ones can be crummy. Dave

 

[graham-xrf]

Tail stock- bed wear?

There is a considerable psychological resistance to ruthless discovery and acknowledgement of this possibility. The good thing to know is that if you know it's there, you can work around it, compensating for it when you happen to need to work on a long piece. I dislike it so much that I suppose it's the one thing that would have me either scrape it out, or seek to change the whole machine!

 

[graham-xrf]

I've also had poor luck with cheap live centers, and by cheap I mean 100+. I have a NOS Bison with a short nose and a Royal long nose that have run out less than .001 but have a new center that came .002 out of the box and the seller ignored my attempts to return. In fairness I let it sit untested for a few months so test them immediately as even the $125 ones can be crummy. Dave

Dave - I so agree about the not-so-good new stuff that has a three digit+ price. Though I have not yet tried it myself, I am tempted by the Joe Pieczynski video. Making your own is not such a bad idea. He used cheap bearings, and did not turn a 60 degree point, but if one wants to just expand the idea a bit, then using other bearing types, or making a taper back end, whatever, seems reasonably possible.

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