Best way to match an existing diameter?

[zoltan]

I have a project I'm working on, and I'm not sure what the best way to perform this operation would be. Hopefully someone can tell me a better way to do it.

I have a long shaft who's end have a 1" long bearing surface which rides inside a bronze bearing. I have welded an extension onto the shaft, and have already turned the weld bead down to within 0.1mm of the bearing surface of the shaft. The question is, what is the best way to exactly match the diameter of the welded area to the shaft's bearing surface without reducing the diameter of the entire bearing surface?

I tried bluing the shaft and slowly moving the tool bit in until it just took the blue off, but at the point it was taking a slight bit of metal with it as well. Maybe I just need to do it even more carefully?

 

[Walsheng]

Pictures are always helpful.
Do you have it in a 3 jaw or 4 jaw chuck?
If it is in a 3 jaw it probably won't be close enough unless it is a set true where you have some adjustment.
If in a 4 jaw, and the weld has not "distorted" the part you are trying to turn you should be able to dial it in with an indicator.

John

 

[bazmak]

Do you need the weld to be part of the bearing area or can you just undercut slightly.You should be able to turn to within 1/2thou without a problem but its more likely that concentricity to less than that would be the problem.Unless its running true to less than 1/2 thou then you cant turn to match the dia within 1/2 thou.Take it down as close as you can and polish down with emery

 

[goldstar31]

If you have an independent 4 jaw, you will still need to clock it in at the chuck end but you have to clock it in at the welded end but using the non welded part. For that, you need a fixed steady.Once this is all running true- and not until:wall:, can you address your new bit and its weld. With it all running true, you can centre drill your new end bit and perhaps use a half centre- and then remove your fixed steady.

Now you have only half started. You have a weld( of sorts, you don't explain) and it will be probably have hard inclusions. Really, you should grind the 'land' left but you might get away with using a a decent and really sharp HSS tool to finish cut. Again, and ideally, you should use your top slide off set to cut 'tenths of a thou'. OK, I do 'Imperial' but you will have do your own trig if you 'Metric'


You should have a tool which will literally shave off barely perceptable swarf- so fine that you can crumble it in your fingers.

Not easy on some lathes, but best of luck

Norman


PS this was written BEFORE Barry's bit. Emery- really? I don't own any!

 

[Walsheng]

PS this was written BEFORE Barry's bit. Emery- really? I don't own any!

Norman: No emery! You should try it, tastes like chicken!

John

 

[goldstar31]

I'm a Brit and don't understand.
Do you mean 'It tastes foul'?

We Goldstars( Royal Air Force 31 Squadron) used to have a Latin Motto- Nil Illigitimi Carborundum. It went with 'Per Ardua Asbestos' which went with the more official Royal Air Farce motto. :hDe:

Ah, well! Back to the grind.

Cheers

Norm

 

[Blogwitch]

I was in the RAF, No.1 Schol of Helicopter Training, we trained helicopter and Harrier pilots, and WE did use emery, wet or dry AND crocus paper, and I still do.
A workshop isn't a workshop unless you have some in there.
I am sure though that you are so good, you can skim down to 0.0001" and come out with a mirror finish every time.

John

 

[goldstar31]

I moved to diamond whenever possible - I found it an improvement.

For wood- yes but I try to avoid it.

Off the record, I lived in one of these eras of 'old wives' and diamond and steel problems. Someone gave me a worn out diamond wheel used in making spectacle grinding- and that was me- convinced.

So that is my take. I can 'dust' on an old lathe. Arkansas stone or diamond paste. My father stropped using leather and a 'corroded' stick of lead.

Regards

Norman

 

[MachineTom]

Zoltan, Better information, yields better answers. Dealing with what you have given, it sounds like you have welded the piece onto the shaft already. Now you begin to turn it down, you find that the pieces will not be concentric to each other.

If the pieces will fit between the chuck and tailstock, extend the good piece out 10 diameters, support it if a steady rest. The shaft will now turn concentric, If you don't need heavy turning just cleanup the weld your good to go. But if you need to turn an area longer than an inch or so, you need to watch for taper in the work, and adjust the steady to correct. Also heavy cuts will not work at the steady, light cuts are the ticket.

I would have thought that your issue would be more with how far off the piece you added is from straight. That has always been a issue with addind length to a shaft.

 

[Brian Rupnow]

If you are working with a 3 jaw chuck, it will be impossible. All 3 jaw chucks have .001 to .003" of "runout". Unless you can put the piece with the welded on extension back into the chuck in exactly the same rotational position to one of the jaws as when it was originally machined, then the runout will bite you on the butt. Although you can turn the welded on extension to be exactly the correct diameter, the odds are about 1000:1 that the center will be in the same position as when the part was originally made. That means that by the time you get the newly welded portion down to the "exact" diameter, you will be cutting away one side of the original portion of shaft.

 

[goldstar31]

I agree with Brian which I why I framed my reply on the use of the 4 jaw independent chuck.

Incorrectly returned, it will work its way out of the 3 jaw chuck- and bugger the jaws and scroll as well.

Not a pretty sight.

I seem to recall the engineering term but it is probably acting as a sort of swash plate.

Regards

Norman

 

[kiwi2]

Hi,

You don't say what the diameter is. Is it small enough to use a collet? If so, problem solved. Otherwise you'll need to use a 4 jaw chuck.
You might find this link useful for getting a good finish on the shaft as you take the last few fine cuts off.
[ame]https://www.youtube.com/watch?v=_qZvUD48cZY[/ame]

Regards,
Alan C.

 

[bazmak]

First time i have come across the vertical shear tool.Looks interesting
Will have to give it a try.Thanks for the post

 

[goldstar31]

It's always difficult to 'moralise' on something where the object is miles away and has not been fully explained. There is a point when the first question should be 'Why?' Why is there a piece 'stuck on'?
Here the mind boggles for an answer.

Moving on, there is a piece 'stuck on' for whatever reason with a weld. Again, we don't know what the joining process was and how it was done. My thoughts are- recalling that I was a Certified Welder( whatever that is- and means) indicates that sufficient heat has been applied to release the inbuilt stresses of the original part.

My thoughts, and they are only thoughts are srious doubts that the original part might never 'clock' again.

All is not lost, it does raise two important points. The first is 'the abilty to better' the cut by off setting the top slide to work in 'tenths' and the second is ' the shear tool' mentioned by Bazmak. I came across the tool way back when L.C.Mason- the traction engine man raised it in Model Engineer. I have it 'somewhere' but the modern dissertation is by a Conrad Hoffman who described how he made up a simple jig to 'finish hone' his lathe tools. Oddly, it isn't new but the second Tubal Cain( dear old Tom Walshaw) described it in his book on Ornamental Turning. Brilliant book! Tom describes a Goniometer which goes back to Jacob Holzapffel . at the time of Maudsley who invented the slide rest.
I don't use either device as such because of owning four tool and cutter grinders. Unquestionably, knocking up something out of a bit of round as Mr Hoffman did will answer many difficulties.

So my coffee is getting cold and I have 'the wounded to tend'.

Regards

Norman

 

[zoltan]

Thanks for the help. Things turned out pretty well using emery cloth for the last little bit. The work was for this project:

http://benchtopmachineshop.blogspot.com/2015/01/lathe-z-axis-fine-adjustment.html