Tool height accuracy

[fizzy]

I have been getting a consistent 2 thou taper on test bars when turning. I thought my headstock was out. Bed level and no twist measured with a very accurate Wyler level.
Checked again today and less than 1 thou out. Only difference is that I spent ages getting the tool height spot on. I wanted a really nice face on the part.
Will tool height make such a difference?
Or perhaps it is a lot warmer today and the lathe has expanded

 

[Munkul]

Iirc its an old machine? It'll just be a worn bed. 2 thou is about right for a worn bed.

 

[brightspark]

an easy way to check tool height is use a 12 inch rule. place it against the round bar you are turning and run the lathe tool up to it and press against it if the tool is set to exact height the ruler will be a perfect 90 decree to the bed if it isnt the rule will be off at an angle

 

[fizzy]

Iirc its an old machine? It'll just be a worn bed. 2 thou is about right for a worn bed.

It is very consistent though hence why I was going to adjust the head. Now not so sure as rising the tool slightly has improved it. Less than 1 thou out on about 8" isn't bad for the old girl

 

[fizzy]

an easy way to check tool height is use a 12 inch rule. place it against the round bar you are turning and run the lathe tool up to it and press against it if the tool is set to exact height the ruler will be a perfect 90 decree to the bed if it isnt the rule will be off at an angle

Never actually tried that. Will give it a go.

 

[Rrobson]

If it’s not a tool room lathe is two thou really that big a deal? If your wanting consistent accuracy below a thou and it’s not a million pound cnc machine your onto cylindrical grinding...

 

[fizzy]

If it’s not a tool room lathe is two thou really that big a deal? If your wanting consistent accuracy below a thou and it’s not a million pound cnc machine your onto cylindrical grinding...

Ahh but it has to be spot on

 

[fizzy]

I mean it is only 55 years old - not much cop these old Colchesters

 

[Rrobson]

I thought that would be the case
eliminate bed wear by machining a diameter further down the bed, the first foot is going to take the most hammer, so make use of your dialled in tailstock and get some stick-out!!
all tool height will do is change the effective rake angle, so provided it wasn’t crazy high or low it shouldn’t make a difference to the dia accuracy. If it was miles too high it’ll rub and if it’s miles too low it’ll have to work harder...

 

[Munkul]

It is very consistent though hence why I was going to adjust the head. Now not so sure as rising the tool slightly has improved it. Less than 1 thou out on about 8" isn't bad for the old girl

That's not shabby at all.

What tool are you using? I was told if you're cutting a test bar, you need something razor sharp.

Also, how are you holding the test bar? I was getting over half a thou taper over 12" on my reground lathe until i turned down the test bar and gripped it better. It was deflecting slightly in the chuck.

My Harrison M400

I would imagine they could be ordered in Imperial or Metric.

www.mig-welding.co.uk

 

[Brad93]

2thou on an old lathe with an old chuck is nothing. Put a new chuck on you may eliminate 1/2thou of wobble. Is your tailstock definitely centred.

I mean deflection of material even something like 50mm diameter can be a good few tenths.

 

[My Old Landy]

Less than 1 thou out on about 8" isn't bad for the old girl

I dream of that sort of accuracy on my 1940's Smart & Brown

 

[Other Daniel]

I've never even measured mine.

 

[My Old Landy]

I've never even measured mine.

Do you mean you have not checked a diameter on both ends of what you have turned? Whatever I have turned, I tend to check both ends (and then emery them to be parallel )

 

[Other Daniel]

Do you mean you have not checked a diameter on both ends of what you have turned? Whatever I have turned, I tend to check both ends (and then emery them to be parallel )

I do measure both ends of a turned bar, if I make one, yes.
But, the precision required of the piece is relative to the project at hand.
I've never felt the need to do a test piece and measure it, though.
That there's a rabbit hole you can fall down, probably resulting in loss
of sleep and continued disappointment.

 

[My Old Landy]

I do measure both ends of a turned bar, if I make one, yes.
But, the precision required of the piece is relative to the project at hand.
I've never felt the need to do a test piece and measure it, though.
That there's a rabbit hole you can fall down, probably resulting in loss
of sleep and continued disappointment.

I have not turned a specific test bar either (I think the cost of a 50mm diameter aluminium bar prohibits that in my hobby shop) and I agree it is all relative
My lathe does what it does to the best of its and my abilities. I too don't wish to jump down the rabbit hole
This is the last thing I made before shutting the workshop down for the move. I used emery to get it near parallel as it is a thread. Probably not worth the effort on 11tpi but hey its all for fun!

 

[Other Daniel]

I have not turned a specific test bar either (I think the cost of a 50mm diameter aluminium bar prohibits that in my hobby shop) and I agree it is all relative
My lathe does what it does to the best of its and my abilities. I too don't wish to jump down the rabbit hole
This is the last thing I made before shutting the workshop down for the move. I used emery to get it near parallel as it is a thread. Probably not worth the effort on 11tpi but hey its all for fun!
View attachment 300223

That's a very nice looking thread.

 

[My Old Landy]

That's a very nice looking thread.

Thank you the only problem with it is 11tpi is 5/8th BSW and I really wanted 14tpi 5/8th BSF I should have checked the Zeus book and not my memory!

 

[fizzy]

I have not turned a specific test bar either (I think the cost of a 50mm diameter aluminium bar prohibits that in my hobby shop) and I agree it is all relative
My lathe does what it does to the best of its and my abilities. I too don't wish to jump down the rabbit hole
This is the last thing I made before shutting the workshop down for the move. I used emery to get it near parallel as it is a thread. Probably not worth the effort on 11tpi but hey its all for fun!
View attachment 300223

That looks beautifully made.
Yes I fell down that rabbit hole a long time ago.
Love my old Triumph. I had it from a retired toolmaker. He said it was a good un.
The only real problem was a tailstock 12 thou too low. Fixed that now it's spot on. Even a tenths indicator doesn't move on it.
Now the machine is right it is down to me!

 

[My Old Landy]

@Pete. made the thread dial gear to enable me to do this. Without his superb work, my lathe would just be turning pins