Thread: Centre Finder Gauge !

Hi Guys,

Based on some very helpful advice and comments I have decided to design and make a Centre Finder Gauge based on the wobbler design which works surprisingly well considering that it does not require a dial gauge in order to use it to accurately centre work in a four jaw lathe chuck.

I've done a drawing with the dimensions on it. I've used colours to identify the major parts. Apart from the 1/4" diameter steel rod with points turned on each end, one to go into the punch mark on the workpiece and the other used as a pointer.

Since this device is intended to be held in the lathe tool post, the black bar is just there to provide a means of support. Obviously it should be at or close to the lathe centre height. The far end of the rod will cease to wobble about when the work is accurately centred.

In use, since this device has a three to one times amplification, one thou of movement of the workpiece will display a three thou movement at the other end. By altering the ratio between the workpiece end and the pointer end the amount of deflection can be changed. In this device the overall length is 12 inches.

Centre Gauge.png
Note that the pivot is a very loose fitting, the rod has to be able to move in all directions. The scale is intended to be used as a guide when adjusting the work in the chuck.

I've used one of these a fair bit and I really don't think you need the scale, just bring a centre close in the tailstock and point the needle at it when adjusting. Its a nice touch if you want the challenge of making it but otherwise not needed

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What advantages does this have over what I call a "wobble bar" a length of rod turned to a point at one end and a small centre drilled in the other - the pic shows it in the 3 jaw - I didn't feel like mounting the 4 jaw just for the photo - I've been using the wobble bar for years but maybe an alternate centre finder will give better results.
20211123_100038.jpg

Mostly that it does not require an indicator and is very cheap. The part does need a centre pop or centre drilled hole though.

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Originally Posted by BaronJ

Note that the pivot is a very loose fitting, the rod has to be able to move in all directions.

I wonder how much error that introduces though John?
I use the method familyguy uses. It's very accurate when you want to centre a critical feature like a crankshaft bore in a model engine casing, for example.
I think your device would be great for quickly centring square stock in the 4 jaw before turning.

Hi Guys,

Thankyou for your comments, I appreciate them ! I'll try to answer your questions.

I did a comparison between using the wobble method shown by "FamilyGuy" and one shown by "Shed" some time ago. I found that using the wobble method you needed a dial indicator mounted somewhere so that you could get a reading from the vertical displacement of the rod then you turned the chuck by 90 degrees to adjust the workpiece. That method works very well, however the dial gauge will not provide an accurate indication of how far off centre you actually are, though it will be close.

Using a long rod pivoted a short distance away from the workpiece so that the distance from the pivot point to the tailstock end is three times greater than the workpiece to pivot point, creates a mechanical amplifier, so for a 1 thou movement at the workpiece you get a 3 thou movement at the far end. This is quite visible !

The idea of putting a scale on the end is simply as a guide to the amount that you need to adjust the workpiece in the chuck. It also gives an indication of direction. In the drawing I used an arbitrary scale.

I couldn't find a small enough ball end joint so I counter sunk the 3 mm pivot hole so the rod could move a little vertically.

I guess it is courses for horses - use what ever method works for you - the 'wobble bar' I use was shown to me by a tool maker I once knew - there is an error with this method which increases as you move the DTI away from the chuck towards the tail stock - the error is however very small, I did at one time do the trig calculations to check this, if the DTI is set on the bar (approx 125mm long) a distance of approx 12mm from the workpiece and you are able to centre it so the max shown runout is .01mm (1 division on the DTI) then actual runout is approx .011mm, a longer the bar reduces the error as does moving the DTI closer to the workpiece.

Does it matter what the actual off-centre error is? Aren't you just trying to make it zero?

It shouldn't matter but I feel it's always better to have full knowledge of the system/method you are working with. I've seen some pretty woeful attempts at centering in a 4 jaw with operators using lengths of pipe on the chuck key and being satisfied when they get the indicated runout down to 1.5 thou (around .04mm).

Originally Posted by jack620

Does it matter what the actual off-centre error is? Aren't you just trying to make it zero?

Hi Chris,

Yes you are ! When first setting up its useful knowing how far to move and in what direction is made easier when you have a scale to refer to. The actual value of the scale doesn't matter.

Quote "familyguy"
It shouldn't matter but I feel it's always better to have full knowledge of the system/method you are working with. I've seen some pretty woeful attempts at centering in a 4 jaw with operators using lengths of pipe on the chuck key.

I agree ! Like you I've seen the same. There should be absolutely no need no need to use such brute force to tighten a chuck, any chuck. Once I've got the workpiece centred, I just go round tightening the jaws by hand. Checking that its still centred. I've seen work marred by over tightened jaws, particularly on large chucks.

I keep threatening to make a baby chuck key to use on the back side when centring in the 4 jaw.

Originally Posted by jack620

I keep threatening to make a baby chuck key to use on the back side when centring in the 4 jaw.

Hi Chris,

That was one of the first things that I made after discovering that I couldn't turn the normal key at the rear because it fouled the headstock.