A wiggler gives a quick visual indication of how centered a punch mark is in the lathe chuck.

Basically a small circular movement at the chuck side is transmitted via the nut to a much longer rod on the tailstock side and if the end pointer wiggles around in a circle then your punch mark isn't centered.

The rod lengths Ive used give an 1:8 magnification.

This was something I saw on Youtube center finder and had the pieces to make my own.

Supplies

Tools:

Drill
Tap and die
Hacksaw
Grinder

Parts:

Round steel rod 4mm dia x 45mm long
Round aluminium rod5mm dia x360mm long
Steel rod 10mm square
Large nut, 19mm

Step 1: Parts

I drilled out the threads of a 19mm nut, heated it up with a blowtorch and dropped it into dirty car oil to get the blackened appearance. It was drilled and tapped on 2 opposing faces.

I sharpened the points by chucking the rod into a drill and running it against a grinder, it will still work well even if the tip isn't symmetrical.

The steel rod was sharpened to a point on both ends and one end threaded to screw into the nut.

The aluminium rod was threaded on one side and sharpened to a point on the other.

I used square rod to fit my 10mm tool post holder and bent it to a 45 degree angle in the hydraulic press, it too was heated and dropped into oil.

It was then fastened in the tool post and marked with the tailstock center, drilled with a 2mm dia bit and the bottom of the hole was center punched. This is because the tip of the drill bit leaves a tiny flat on the bottom of the hole.

Step 2: Using the Wiggler.

I start by mounting the pivot in the tool post and hold the rod point inside the nut, up against the pivot.

Then I get the device moved onto the marked work in the chuck by winding the cross slide and compound rest.

After that I simply rotate the chuck by hand to do the rough centering, then the lathe is powered on to see if the wiggler end is stationary, if so, job done.

Accuracy isn't critical and even a bent rod will still indicate well.