Unwanted taper produced - how do I correct?

[raynerd]

Sorry, another thread!  Is there another forum for stupid noob questions - I am cluttering your forums but desperate to ask!

I was turning a 8" piece of acrylic today and I noticed that I am producing an unwanted taper! Wider at the tailstock end and tapering inwards towards the headstock - infact, it was pretty bad.  Compound slide is set at 0 and adjuster left alone, I was using the powerfeed to drive the tool so it should not be producing any taper right?

Can anyone tell me what is wrong with my lathe (clarke cl300m 12x7) and how I can correct this?

Al the best
Chris

[John Hill]

I am sure John Bogstandard will be along soon with the complete story but in a nutshell it think it means either the headstock is not parallel with the ways or the tailstock is not set in line with the headstock.

If I understand correctly the tailstock usually has an easy adjustment for the very purpose of turning tapers.  The headstock is another matter but not impossible to correct.

John
aka The Artful Bodger

[ja2on]

This may help
http://www.gadgetbuilder.com/Lathe_Align.html

I expect the clarke manual is a bit poor so try this one
http://www.littlemachineshop.com/Info/MiniLatheUsersGuide.pdf
unfortunately it does not cover the tailstock adjustment

[bogstandard]

Chris,

There is most probably nothing wrong with your machine.

When you have a bar sticking out of the chuck that is unsupported by a centre in the tailstock, especially if it is a flexible material, what happens is the the bar will bend away from the cutter the further away from the chuck it is, so you will not get the full cut that was applied. As you get closer to the chuck, the bar will become more rigid, so you will get the cut that was applied. So less material is removed the further from the chuck the cut is. Hence your taper.

Anything say over 4 diameters sticking out of the chuck (if 1" diameter, then 4" max unsupported) then really you should be supporting it with a centre in the tailstock. That is only a ballpark figure, for a fairly flexible material, say like acrylic or nylon rod, that figure could be reduced to 2 or 3 diameters. Even then, if the rod is fairly long, it will flex in the centre, so you will get the centre part of the rod with a bulge in it. So then a travelling steady should be used, which supports the rod very close to where it is being cut, and stops it deflecting.

John

[John Hill]

Now didnt I say Bogs would be along with the complete story!

[bogstandard]

John,

Because Chris is a newbie to lathework, he will be trying allsorts in the beginning, and will be coming up with a lot of questions about why such wierd things are happening. We have all been there at one time or another.

People in this situation always assume it is the machine at fault and start to think all doom and gloom and expensive solutions, whereas with a little more experience, this problem would have been recognised.

I don't know if that will cure Chris' problem, as he might already be using a centre in the tailstock. Assuming his lack of experience, my solution seemed to be the most feasible explanation.

My only suggestion would be to Chris and others still on the steep learning curve, have a look at these couple of sites which I have put in the resources area.

http://www.jjjtrain.com/vms/library.html

http://www.nmri.go.jp/eng/khirata/metalwork/index_e.html

A couple of hours rooting thru there will give a basic insight into what machining is all about, and might solve the problem before you actually have one. Not everything is covered by them, you would be reading for a lifetime, but some of the basics that need to be known.

Bogs

[raynerd]

Thanks John

I have just been back to the lathe and had another look  -your right, it is so flexibile that it is bending at the tail stock end and therefore the tool isn`t cutting it as deep. As the tool moves down to the headstock, there is less flex in the material and so the cut is bigger.   Thanks, at least I don`t think there is as much wrong with my machine now...
Chris

[Brass_Machine]

Ja2on & Bogs...

Great links even for us not so newbies!

Eric

[SPiN Racing]

Great Post John,

And yes Chris.. Im on the other side of the pond.. but take time and read all the posts as you are... go back and read all the older posts too if you can. There is a ton of info out there that the collective here  has posted at various times.
I found reading a lot of the posts I read about things people experience and it directly relates to something I was running into.. and didnt understand.


BTW.. If you are spinning acrylic.. you might be tempted to spin it a lot faster to prevent flex.  Um Im sure the more seasoned members can give the good and bad.. as was already mentioned above.. but be careful.  REALLY BE CAREFUL.

My dumb a** put a 1.75" chunk of deltin in the lathe, thinking I would duplicate a shifter I had in Delrin FIRST.. before using Stainless. (This was like my 2nd week owning the lathe) I put in the dead center.. chucked that badboy in.. made sure it was as "close as I could get it" to centered on the tailstock side.. and proceeded to start the lathe.
It had a tiny wobble...
and when cutting to true it.. It really wasnt getting better.. just a tiny cut per time.
But I got all sorts of wierd results. Cutter wasnt set properly.. too low.. I was spinning slow, and cutting either too fast.. or too slow.

SO I got the bright idea to kick the speed up a bit.. like 2 pulleys faster.

I turned on the lathe.. it hummed up to speed.. and that slight imbalance had it buzzing like crazy. I then slowly put the tool into it.. and didnt realise I was off to the side a bit.
Moving the cross slide without pulling the tool back caused a really amazing spiral of delrin to stream up off the lathe.. the motor to slow a bit.. and then to my horror a sudden boom as the deadcenter..that was about 1/8 inch into the delrin, suddenly wasnt holding it.
It spun out violently, banged into the back splash and twirled a few times in the bed area.

SO after cleaning my underwear.. I decided to stop and get my head out of my ass before continuing.

The problem.. the softer plastic when slightly off balance with the dead center.. heated the tip.. plus the bite into the tool bit caused it to simply rip from the chuck. And fly into the back of the lathe.. thank god.
SOoo be careful spinning up things that are not ultra secure.

Months later, and a LOT of reading what others experienced.. and some very very careful learning.. Im starting to get a hint of a clue. I now see there are things out there I might be able to do.. if carefully approached.