Possible problem.

[Highpower]

This is a recent video from Joe Pieczynski that provides more food for thought.

https://www.youtube.com/watch?v=uOjao04axGo

[Dr Stan]

I only watched about a minute, but could tell he was on target.  It demonstrates exactly what I was talking about.

BTW, it doesn't just happen with small dia material.  While in the Navy I'd stub twisted pump shafts to put them back in service.  They were about 3 1/2" in diameter in the middle and roughly 3ft long.  One had to dial in the diameter in the chuck and the end hanging out in the middle of the air.  We then would use a steady rest to keep it aligned while making the repairs.  Same was true when we would simply repair the bearing diameters by welding or electro plating.  The bearing diameters were about 18 or 20 inches apart and had to have no more than .001" TIR.  More than that put the bearings in too much strain due to misalignment.

[TomG]

It looks as though the axis of the chuck/part may not be on axis with the lathe spindle. Remove the chuck and check the mounting surface for dirt or burrs. The face of the chuck should also run true if it is mounted correctly.

Tom

[Dr Stan]

(12-22-2019, 02:05 PM)TomG Wrote: It looks as though the axis of the chuck/part may not be on axis with the lathe spindle. Remove the chuck and check the mounting surface for dirt or burrs. The face of the chuck should also run true if it is mounted correctly.

Tom

Good point.  A single chip between the spindle nose and the back of the chuck will give you fits when trying to indicate in a part.

[Swarf_Newbie]

(12-21-2019, 09:53 PM)Highpower Wrote:

(12-21-2019, 07:18 PM)Swarf_Newbie Wrote:  Now I would not think that 10mm silver steel 300mm long would flex that much under the pressure of a dti? but I could be wrong!   

Cheers.

Roy.

Actually a small diameter bar of that length will flex quite a bit under it's own weight when held at one end. What I was trying to ask is how are you adjusting the far end to have zero run out? Are you smacking the tail end of the bar to move it, or are you moving the chuck jaws to do it?  Moving chuck jaws.

Stan suggested checking your stock for straightness which is a good place to start. Just because a piece of stock is ground at a factory doesn't necessarily mean it will still be perfectly round and perfectly straight by the time it gets to your door. Fair enough, unluckily at this moment got nowt to check it on.( Dr Stan your post has been noted and filed! :-)  )

You might try drilling a 60° center into one end of your stock (up close to the chuck jaws). Then put the stock back in the lathe the way you have it now, but support the far end with a center in your tail stock. Zero the run out at the chuck end, and then check the run out at several places along the bar and see what you get.
Will do!

Thanks for the time and patience guys.

Cheers.

Roy.

[Swarf_Newbie]

Got a few things  to check thanks to the good folk on here.

But, Just took the chuck off and found  a quite large cob of swarf hiding in-between the rear of the chuck and the front of the spindle flange!
Will have to wait till after Xmas, cos s.w.m.b.o is breathing down my neck whilst whining about wrapping stuff up!

Cheers.


Roy.

[Swarf_Newbie]

Long story short...............got fed up with messing about.

Bought an ER40 collet chuck and collets, jesus, what a difference, same 10mm silver steel round bar, tir less than 0.01mm at headstock, tir 100mm from headstock still less than 0.01mm, and this is a chinese lathe!! 4 jaw chuck is  naff! need a new one😢.

Thanks for the input guys 👍

Roy.

[Dr Stan]

Install your 3 jaw chuck on the lathe and chuck up a piece of 50mm or so stock and take a clean up cut to make sure it is running true.  Mount the 4 jaw on the cleaned up portion of the material with the back of the chuck facing away from the headstock.  Indicate the outside of the 4 jaw so it is running true then use your DTI to inspect the back of the chuck. If it is not running true then take clean up cuts to true it up.  You have not said what type of spindle nose you have but if it is a camlock (D type) make sure you inspect and true up as necessary the angled portion.  Make sure you cut the angle (taper) correctly and also face off the back of the chuck to make sure it seats properly.

[Swarf_Newbie]

(10-18-2020, 08:51 AM)Dr Stan Wrote: Install your 3 jaw chuck on the lathe and chuck up a piece of 50mm or so stock and take a clean up cut to make sure it is running true.  Mount the 4 jaw on the cleaned up portion of the material with the back of the chuck facing away from the headstock.  Indicate the outside of the 4 jaw so it is running true then use your DTI to inspect the back of the chuck. If it is not running true then take clean up cuts to true it up.  You have not said what type of spindle nose you have but if it is a camlock (D type) make sure you inspect and true up as necessary the angled portion.  Make sure you cut the angle (taper) correctly and also face off the back of the chuck to make sure it seats properly.

Will try that before I bin the 4 jaw, you learn summat new every day!

ATB.

Roy.

[Dr Stan]

This is essentially the same method I used to convert a Pratt & Whitney 12" four jaw with a L type mount to a D-1. Before I mounted the 3 jaw I used the lathe spindle nose to set the angle on the compound rest.