Thread: lathe test bar

So it won't pass the razor blade test?

Rob

By the sounds of it, using a razor blade could be down right dangerous!

Ew

Originally Posted by morrisman

After watching tubalcaine's U tube video. I decided to make a lathe test bar , I followed his method to the letter ,buying a 400mm length of ground 3/4" stainless bar . After checking the bar on the carbatec granite plate, I accurately centred the bar each end and it turned out quite OK .

using the Verdict dti along the 400 bar between centres , The initial test on my old Sheraton showed the tailstock off set ( I did this adjustment some time ago) to be spot on

Moving the dti along the top of the bar 400mm , the bar drops .015" down to the tailstock

Bringing the tailstock up the the headstock where a centre is held in the 4 jaw chuck, I can see the tailstock centre is noticeably lower . Shims needed I think MIKE

Mike how did you mark out and drill the centers..
aaron

Originally Posted by azzrock

Mike how did you mark out and drill the centers..
aaron

hi Aaron

I did it this way. Using a 3 jaw chuck , I roughly drilled the bar end with a centre drill , removing 90% of the 60 degree metal, I then drilled a approx. 4-5mm hole down the centre , to about 6mm past the roughed out section.

I changed to a 4 jaw chuck and as best as i could manage , I used the dti and centred the bar to nil runout . Then you use a small boring bar or similar tool , set your top slide to 30 degrees and carefully bore the hole

this is the video MACHINE SHOP TIPS #95 Making a Test Bar pt 1 tubalcain - YouTube

Thanks for that link, I will watch that video when I get a chance.

Before anyone rushes out and spends a whole lot of time and money making a test bar, first consider what it will be used for. They sometimes make life a little easier, and you pretty much need one for headstock alignment (with a taper to suit) but if tailstock alignment is all you want to do, where it's simply been offset and you need to accurately re-centre it, I'd suggest there are many other methods of aligning the tailstock that don't require a test bar yet will be equally accurate.

Pete

Have you examined the bar to check how good it is....

Measuring it firstly for how cylindrical it is...

Secondly putting it in precision V blocks on the surface plate and checking it for straightness...

Putting it between centres on a lathe and checking for runout...

Originally Posted by .RC.

Have you examined the bar to check how good it is....

Measuring it firstly for how cylindrical it is...

Secondly putting it in precision V blocks on the surface plate and checking it for straightness...

Putting it between centres on a lathe and checking for runout...

He covers some of that in his video, but relies heavily on using precision ground drill rod for cylindricity and straightness... I think when you consider the cost of a precision test bar it's a reasonable approach, he does spot check with a micrometer for diameter, but you asked specifically about cylindricity measurements, and that require a bit more specialized metrology than a micrometer...

Ray

Originally Posted by morrisman

hi Aaron

I did it this way. Using a 3 jaw chuck , I roughly drilled the bar end with a centre drill , removing 90% of the 60 degree metal, I then drilled a approx. 4-5mm hole down the centre , to about 6mm past the roughed out section.

I changed to a 4 jaw chuck and as best as i could manage , I used the dti and centred the bar to nil runout . Then you use a small boring bar or similar tool , set your top slide to 30 degrees and carefully bore the hole

this is the video MACHINE SHOP TIPS #95 Making a Test Bar pt 1 tubalcain - YouTube

cooli was wondering about the accurate center holes. boring them out is a kuta way to do it

As i found out when looking at rolls for my sine plate, the ground silver steel is not that great on dia and cylindricity (Ray used it so it must be a word) Better than .001", maybe +/- .0005". The ground 4140 i have bought is actually better, probably within +/-.0003". I'm guessing all the ground bar is fed through a centerless grinder at a pretty decent rate, and not really precision ground.

Ew

How far out was the tailstock quill? maybe thats the problem?
Not sure why you have the center in the 4 jaw but as long as its spinning true its all good.
Mark sure you DTI is set up rigidly.

Originally Posted by RayG

and that require a bit more specialized metrology than a micrometer...

But if it checks out between centers and is the same size both ends it has to be right.
No need to vee blocks etc........right?

Of course if it doesn't check out you wont be able to tell what the error is.

Stuart

Originally Posted by Stustoys

How far out was the tailstock quill? maybe thats the problem?
Not sure why you have the center in the 4 jaw but as long as its spinning true its all good.
Mark sure you DTI is set up rigidly.


But if it checks out between centers and is the same size both ends it has to be right.
No need to vee blocks etc........right?

Of course if it doesn't check out you wont be able to tell what the error is.

Stuart

I think that's right, measuring along the length with a dti and rotating, should tell the whole story.. I can't see that V blocks are needed either..

If it's not straight, then running a DTI along the length would pick that up, also that would pickup taper if combined with run-out, run-out, picks up circularity.. conbine circularity with straightness and parallell... that covers cylindricity... ( I think )

Ray

Originally Posted by RayG

also that would pickup taper if combined

Only if the centers are lined up..otherwise taper in the bar could be hidden by taper in the lathe* and VV, but damn you'd have to be lucky lol

Stuart

* I assume we are talking about centers on a lathe

Ideally you'd put the bar in a bench centre and measure it at different points while rotating. You could for example have a perfect test bar but centres that aren't drilled accurately, in which case it's a boat anchor. Conversely you could simply mic each end and they could measure the same, yet the bar is lobed, in which case it joins its mate fishing. Dropping things in vee blocks and measuring, where you're looking for potential multiple errors sometimes sounds easier than it is in practice, especially relatively long parts like this.

Originally Posted by Pete F

Ideally you'd put the bar in a bench centre and measure it at different points while rotating. You could for example have a perfect test bar but centres that aren't drilled accurately, in which case it's a boat anchor. Conversely you could simply mic each end and they could measure the same, yet the bar is lobed, in which case it joins its mate fishing. Dropping things in vee blocks and measuring, where you're looking for potential multiple errors sometimes sounds easier than it is in practice, especially relatively long parts like this.

If my memory serves me, from back when I was Centreless Grinding, roundness tests are carried out with 60deg Vees. When the work rest is set low as it often is when roughing, there is a tendency for the grinder to grind a tri-lobal cross section. A 90deg Vee can hide this, but a 60deg will accentuate it.

Re the boat anchors. Would they work for your boat? Based on Mikes description of the process, I would think if the centres are not drilled accurately then they are just a WIP waiting for the next step.

I am interested by your comment "I'd suggest there are many other methods of aligning the tailstock that don't require a test bar yet will be equally accurate." I have yet to make adjustments to or even check my tailstock and I am interested in any method to do this simply and quickly. It seems to me that if you have a test bar and the required centres this would be pretty quick. What other methods do you have in mind? The quicker and easier it is to put the tailstock back in line, the more likely it is that I will be tempted to move it in the first place. Having said that I am not sure at this point why I would want to move it anyway.

Dean