Run out on bar

Hi all,

I’ve got a question regarding different length bar in my Chuck! Regardless of the length or diameter of bar, I seem to have run out with my dti gauge at the end of the bar. I’ve just fitted a brand new Chuck and backplate as I thought the old Chuck was worn !
I was under the impression , with a new Chuck and solid bar, it should spin with no run out??

sorry if this is a stupid question! Complete novice starting out!

thanks in advance.

Many more details required before anyone can provide a satisfactory explanation/excuse for that failure to meet your expectations.

Some photos of the lathe would probably help

… and before you ask : **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=103028&p=1

MichaelG.

Thanks for link! 1971 ml7! As I’ve never used a lathe before, I wasn’t sure what to expect! I assumed, a new Chuck with a straight bar, it wouldn’t have any runout. As I said, complete novice! Just want to learn.

Thank you

Sorry to have to give you the ‘reality check’ but 3-Jaw chucks cannot normally be expected to hold work really true.

That’s why we often resort to 4-Jaw chucks or to Collets.

… There will doubtless be a deluge of advice forthcoming !

MichaelG.

1: First check the runout of the chuck register and also the interior spindle bore.
Meticulously clean the spindle and chuck threads.

2: Try swapping the jaws around into different slots, see which position gives least runout.

Even if you get it close with that diameter bar it may differ at other diameters.

3: get a 4 jaw chuck for critical work.

A 3 jaw scroll chuck may be better or worse, no guarantees.
A 4 jaw can be dialed in.

To those who use them regularly it is well known that if you take something out of a 3 jaw chuck and replace it it is nearly impossible to get it back in the same position without some fiddling about. The issue here is that there is friction between the chuck jaws and the bar, the chuck wouldn't be much good without it. At a small angle from the centre line the friction forces will be higher than the forces trying to line up the bar along the spindle centre line. You will find examples on line of people mounting a bearing in the tool post to push on bars to line them up, you will also find examples of people tapping then into line after clamping. If you put the bar in the chuck but not fully tighten it you can position a tool in the tool post and move it close to the bar and than turn the chuck by hand to see where it needs moving back towards the centre. Light taps with a suitable mallet, copper drift or machinists hammer can be used to improve the runout. Once it is in the best position you can tighten the chuck fully.

Martin C

Have a look at this video about 3 minutes in. Its in a collet but the principle is the same. Some adjustment is required to achieve a low runout.

Edited By Martin Connelly on 11/11/2021 08:52:28

Thank you all very much! Just a big learning curve for me. I believe I also have a 4 jaw Chuck. I was under the impression, if you clamp something in a Chuck, it would be true! Clearly not 😂

Will watch the video later .

much appreciated

FYI, a dti is a comparison instrument, not a an absolute measuring device.

What was the run-out? I would expect up to about 0.1mm close to the chuck (that will increase further away from the chuck, of course) for a decent quality item.

Details of how you fitted the chuck on its backplate and to the lathe would be good. Complete novices are known to make mistakes.

Apart from chuck jaw aberrations, the spiral which drives those jaws is unlikely to be perfect along its multiple turns, so run-out may be different with different diameters of stock. This is also reliant on the stock being perfectly circular, of course.

As above, the general rule is to complete all machining before removal from the chuck if concentricity is required.

The ideal mounting of work is between centres, where the work can be removed and replaced between those reliable points, to maintain concentricity. Even then, the drive-end centre would be one turned on the lathe as a fixture for that particular job.

Collets can be good for smaller diameters, the 4 jaw independent chuck for larger items. The self-centring chucks are way down the list…

I have seen a couple of YouTube movies where people have taken ordinary 3 jaw chucks and converted them to "GripTru" by moving the backplate screws out to the front face, turning down the backplate for a sloppy fit and adding four grub screws, in from the sides, to take up the slack with perfect alignment.

Do we have an opinion on such reckless behaviour?

I see 3-jaw chucks as a speedy convenience rather than an accurate way of holding work with low run-out. Don't expect too much of them! Chucks get worse over time due to wear and tear. Jaws can be replaced or reground, but damaged slots, scrolls and registers are hard to fix.

For many jobs 3-jaw inaccuracy doesn't matter because the lathe cuts relative to the spindle axis. Relatively short jobs held close to the chuck come out OK. However, don't move jobs in and out of the chuck because it's extremely difficult to position work on the spindle axis exactly as it was before.

For low-run out, and if work has to be moved, it's necessary to dump the 3-jaw in favour of a 4-jaw chuck or collets.

4-jaw chucks can be adjusted with the aid of a DTI to almost completely eliminate run-out, but it takes time to do. Nonetheless, there is a faction who never use anything but 4-jaw chucks - speed comes with practice.

Collets are fast and accurate but pricey because lots of different sizes are needed and they suit some types of work more than others.

A rarely used method these days is to set the job between centres in the spindle and tailstock, and drive it with a dog. No chuck of any sort required, and it's accurate, but setting up is quite slow. Also possible to bolt work to a faceplate, but setting up is very slow. Faceplates are good for turning jobs too big to fit in a chuck, and for boring super-accurate holes with buttons.

I've yet to use my faceplate for anything, and don't own any lathe-dogs.

Dave

PS sometimes new owners start by looking for defects in their pride and joy by measuring stuff, or - even worse - stripping machines down. Unfortunately measuring down to 0.02mm and reassembly both require skill and it's all too easy to get the wrong end of the stick. Even experts cock-up sometimes. I suggest it's better to start by using the machine because nothing highlights genuine faults like cutting metal. If the machine misbehaves in action, then look for faults. Bad cuts provide many clues as to what's wrong, if anything. Only when there's evidence to work from start measuring and taking stuff apart to isolate the fault. Bear in mind before assuming the machine is wrong there's a lot to learn about materials and technique. Owning a Stradivarius doesn't make one a violinist.

Use tailstock support on a hobby lathe

Compared to most on this forum I am a novice with a 1950s Denham DL lathe. I used to use a three jaw but I now normally only use the four jaw. I have found with practice I can get the bar to run pretty true within about 40 seconds. The 4 jaw has the advantage that it can take square or rectangular bar. I do use the 3 jaw for production of several identical items.

Whilst I can take out the eccentricity with a 4 jaw, run out at a distance remains a problem. I am now starting to machine between centres with some success.

I thought that the experience of a relative novice might be helpful.

John

I would not follow JC's advice unless the runout is in excess of a mm or so. Three jaw chucks usually have the jaws numbered 1,2,3 which fit into their respective numbers on the chuck body.

Also, I have never understood why, but I usually tighten the jaws onto the workpiece and if I need the bar stock to be held really tight, I rotate the chuck around to the next jaw and tighten again - Perhaps its just habit !

Bob

I would not follow JC's advice unless the runout is in excess of a mm or so. Three jaw chucks usually have the jaws numbered 1,2,3 which fit into their respective numbers on the chuck body.

Also, I have never understood why, but I usually tighten the jaws onto the workpiece and if I need the bar stock to be held really tight, I rotate the chuck around to the next jaw and tighten again - Perhaps its just habit !

Bob

At technical school 70 odd years back we only used 4 jaw chucks. There were 3 jaw chucks there but we never used them

Roy

Round bright bar is neither very round nor very straight, the chuck may be fine.

A new 3 jaw may have a 3thou wobble built in, some may be perfect.

Rotate the bar bit in the jaws and re tighten it may be better.

My first question would be did you machine the backplate in-situe on the lathe spindle? and if so how good a fit is the register in the chuck recess?

What is the parentage of the chuck and did it give a spec for likely runout from where you bought it?

Are you measuring a ground bar of just a bit of what was laying around?

What is the actual reading you are getting?

Edited By JasonB on 11/11/2021 12:58:58

As with Speedy Builder5, I too "open with one, tighten with three", lathe or drill chuck.

Why ? no idea, except I was told to many moons ago.

As regards 3 jaw chucks, I used to think they were "Dead on" ! Wishful thinking, which is why I now use my 4 jaw, even if it takes a bit longer to set up. Not that much really, once you get used to it.

I was taught this many moons ago when I did my contractors plant and maintenance course was told it helps grip the drill bit or whatever you was holding in the chuck