Thread: Cylindrical Square

Originally Posted by .RC.

Or to put it another way..... If I chuck something in the three jaw held by the tailstock centre it will still turn it parallel... Three jaws never hold anything concentric...

Hi Richard
It will turn parallel only if the tailstock centre is concentric with the headstock spindle

Phil

Originally Posted by pipeclay

Does it really matter as long as the job is true.

Not at all. But this discussion helps people to get a better understanding of whats going on, and in this case might save someone time on their setups in future. If the result had gone the other way, it could have saved someone a lot of time trying to work out why they were ending up with a taper. Does that bother you, that people like to understand what's going on and improve their abilities as a result?

Originally Posted by Steamwhisperer

Hi Richard
It will turn parallel only if the tailstock centre is concentric with the headstock spindle

Phil

Yes, I was dead wrong with my original line of thought.... Lucky Stuart knew he was right and was persistent until I saw the light....

Dosent bother me at all,but I think its a bit of overkill,the way itas going it would take an apprentice 6 to 8 weeks to cover all the scenarios about turning between centres,and still not sure if they would understand what they have done.
But then again I am just a practical person and not overly involved with the theoretical side of pedantics.

Originally Posted by RayG

Turning between centers, means that the spindle axis can be skewed a little, but as long as the center point of rotation (not necessarily the "center itself") is exactly the same distance from the ways as the center point of the rotation at the tailstock end, the axis of rotation of the work will be parallel to the ways.

Hi ray

I always thought that if there was a center at one end and another center at the other end then you put something between them and machined it you would be turning between centers

With tongue in cheek regards,
Phil

Originally Posted by Steamwhisperer

Hi ray
With tongue in cheek regards,
Phil

Pedant.....

Regards
Ray

Originally Posted by pipeclay

But then again I am just a practical person and not overly involved with the theoretical side of pedantics.

I think you need to know the theory behind the practice.... Very helpful for problem solving..

Originally Posted by Jekyll and Hyde

If the headstock centre is offset, but tailstock is dead nuts, you will indeed turn a taper UNTIL you reach the point where your depth of cut at the tailstock end EXCEEDS the amount of offset at the headstock end. .

No, you will be turning parallel, turning off the runout.

Ray as always you have a way with words.

Stuart

Originally Posted by RayG

Pedant.....

LOL. Sorry Ray
I couldn't resist

Last night I turned up a cylinder square.
0.1mm taper. repeatable within 0.01mm with a 0.04mmdoc.
I then moved the center by 1mm.
Three cuts later(the dog wasnt up to much as far an intrupted cuts went).
0.1mm taper(yes the same way). repeatable withing 0.01mm with a 0.04mmdoc.

Must run

Stuart

Thanks for clearing that up Stuart...

Now lets put the tailstock out of alignment and see what happens...

Originally Posted by Stustoys

No, you will be turning parallel, turning off the runout.

Ray as always you have a way with words.

Stuart

My brain hurts when I try and visualise this setup for too long. Ending up with the parellel result once depth of cut exceeded headstock offset was easy to grasp, but I managed to confuse myself about what happens along the way.

Do I get partial credit if I say it's sort of a taper? A kind of squashed, lopsided.... ah, forget it.

As a newbie to metal turning, I'm watching this thread with interest and amuzement. It's not always easy sorting out the crap from the good stuff but it's still fun to read!

I have a metal turning book by Harold Hall and one of the projects in there are a set of cylindrical squares. He says not to use the tailstock because even the most accurate machines will have tailstock alignment problems which will be enough to affect the overall accuracy (parrallelism of the cylinder) of the job.

So I guess my question is, if you don't need to use the tailstock to turn up a set of cylindrical squares, why use it? Surely it just adds an extra unwanted potential complication? I'm am kinda assuming this discussion is purely for the sake of theorising...

Cheers,

Simon

I think that Harold Hall was pitching his books to a modest home shop/model engineering audience. Not many lathes available to the hobbyist had offset adjustable tailstocks, nor was there a ready supply of good yet cheap metrology tools.

You'd have a hard time turning a useful cylinder unsupported, and temperature growth and tool/work springiness would be a bigger potential error source I would think.

Making workshop standards and metrology aids is probably the most challenging task for a machinist if your goal is to end up with a tool that is ten times more accurate than the most accurate job it'll be called upon to measure.

Someone will chime in soon with the objection that this only applies in climate controlled, clean workshops. To which my reply is "Yes". Air conditioners and vacuum cleaners are cheaper than the other tools in the shop by far.