It might help if you pour boiling water over the chuck first then with a bar ACROSS the jaws give it a smart tap, best of luck!

Sorry John H my dividing head and indeed the one shown in the link have a threaded back plate [mine has a Myford 7 thread] so it’s just a matter of unscrewing said chuck/backplate assembly. Not an ideal method of attachment for milling! The black grooved collar shown below covers the thread.

Tony

These should help:

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MichaelG.

This guy seems to have had the same problem with a similar head.

I is probably held by a combination of old oil having gummed up and possibly the common lathe problem of a hot chuck put onto a cold mandrel and forming a bit of a shrink fit.
Unbolt the chuck so you don’t have that mass to bother with, revealing the backplate and the end of the spindle. Decide on and make ready your method of applying the releasing forces.
Make a wooden or thick very thick cardboard disc to place over the spindle end and fill the hole with a big bolt or MT2 taper (but don’t get that stuck too) to add thermal mass. Now you can quickly heat the front of the backplate with a paint stripper blower hoping not to heat the spindle (and its bearings and their grease). When just too hot to touch (40C) unscrew it. A sharp tap method is the best if possible.
If it doesn’t cooperate the first time cool it in the freezer for a day and try again.

The Vevor 160mm six jawed scroll chuck that I bought had had the inside jaws ground after assembly without being cleaned afterwards.The chuck was dismantled and washed before use and has prooved to be outstandingly good at all diameters whichever the jaw tops are facing, I was very lucky as the name has very mixed reviews.

I don’t believe in subjecting any chuck to hard shock loads and heavy stress directions there not designed to take. Chuck bodies and the jaw slots are easier to spring into permanent deformation by heavy side loading the jaws or even over tightening than some think. Better I think would be to remove the chuck from the back plate, use a couple of well fitting bolts and nuts through the back plate bolt holes, and use those with a bar to remove the back plate off the dividing head spindle.

However you did get it off. There’s an old machinist trick for mostly preventing the same from happening again on any threaded spindle including lathes. Unless there’s contamination or corrosion also involved, threaded chucks get hard to remove because of how the spindle and back plates are designed to work. As the chuck gets tightened on the thread, there’s a slight amount of elastic thread stretching involved and no different than any threaded connection. But the majority of the issue is caused by the metal to metal frictional contact between the spindle chuck register face and the internal face in the back plate recess. With very well finished surfaces and heavy tightening, I’m not so sure there isn’t a bit of Van Der Waals forces being at least part of the cause for seized or very hard to remove chucks as well. Cold welding between close fitting parts is a real effect, and even more so for anything tightened and joined face to face over long periods of time.

Cut a thin (.010″-.020″) cardboard washer shape slightly larger for the hole diameter than the plain un-threaded portion of the spindle nose behind the thread. And its outside diameter slightly smaller than the back plates recess. Single thickness paper and cardboard is extremely consistent for thickness and parallelism because of how its manufactured. Soak that cardboard washer in some light oil for a minute or two, blot off any excess and use it between the back plates face and the spindle registers face. It will compress slightly as the back plate face tightens up against the spindle face.

The oil helps a bit with chuck removal, but also prevents the cardboard from absorbing any humidity and possibly causing rust. There’s also just enough give in the cardboard to prevent a seized chuck, and it will be much easier to remove the next time. Those cardboard washers are also not that durable, so make a few spares. It was the very first thing I did after getting the chuck off my own dividing head. Frankly I think threaded spindles and especially on any milling accessory should have been changed to something better, and at least 80 years ago.