I have deliberately reduced the registers of several of my chucks to give them at least 0.010" clearance, for fine adjustment of any size work that will fit. If you are worried about the chuck moving when in use, there is plenty of room on the back of a chuck to drill and tap more fixing holes. Of course, the bolt holes in the backplate have to be slightly oversize, or they would impede the adjustment.

A tight register means there is no way that variations in manufacturing tolerances can be overcome, some sizes of work may run well, but not all.

If it’s less than a thou undersize just get and use it, don’t bother with loctite etc.

Martin,

I always make my back plate register undersize by around 5 thou. If you make the holding screw holes oversize, then you have a cheap Griptru chuck that you can adjust to get zero run out for any work diameter you want.

Andrew.

If you are thinking about a glue approach, remember that chucks need to be dismantled and cleaned occasionally.

This debate comes up on all the forums on a regular basis, those who must have a tight register have obviously never tried any other approach. Then they tell everyone that a self centring chuck can never be relied on for accuracy.

If you have the register within a quarter to half a thou, that should be good enough for a precise location (short of size/size fit )

The precision with which a 3 jaw chuck holds work probably varies with the size of work being held, anyway. Doubt if many chucks will hold work to a thou. 3 -5 would be a good one, in my book.

If absolute concentricity is essential, either use a 4 jaw and clock it, or turn two or more diameters without disturbing the work in the 3 jaw.

Howard.

fat fingers again!

Edited By Howard Lewis on 24/08/2019 08:18:18

Several points worth flagging up on this saga:

1) "1/4 to 1/2 thou undersize" for what is probably about a 4" register, that is pretty fine tolerances anyway.

2) Another piece of received wisdom is that lathe tools have to be at centre height. If you are trying to take wee shaving cuts and the tool is actually a shade OVER center height, then the tool won't cut. So you put on a bit more and try again. Then, the tool finally gets "in" and takes off a load more than you bargained for. Moral – keep tools a shade BELOW centre height – especially on fine finishing work.

3) Funny stuff is cast iron. The finish straight off the tool (HSS at any rate) will be "whiskery", which gives a false over size reading on a mic or calipers. Before measureing take the whiskers off with a bit of wet & dry paper.

4) Another way to rescue such a situation is to let in and loctite 4 plugs, say about 20 mm diameter for your example, equispaced at 90 degrees and straddling the register. Then machine the plugs back to (hopefully) finish at size. Assemble it and hope you are long gone by the time someone else finds out your rescue bodge. Seen it done in my working days to rescue a very urgent part.

5) I agree with the others – no need to resort to bodges in this case. Crack on and well done.

Hope that helps,

Martin

I am just in the process of fitting a new 160mm three jaw chuck to a backplate which had another 160mm on it. I had found that the backplate was undersize for the original chuck, which had a 142mm pcd for the 8mm bolts. I had drilled and tapped for six 6mm bolts at the same pcd, giving me a little more meat on the od of the plate.

The new chuck, by pure luck has six 10mm threaded holes at a pcd of 140mm. I have a supply of new 8/10mm titanium studs originally fitted to Westland Lynx helicopters to use for mounting the chuck. Using the rotary table on the mill, I plunged a solid carbide 8mm end cutting four flute end mill through the original 6mm holes in the backplate at the new pcd of 140mm. The register had to be reduced and now is 0.010" smaller than the chuck register. I will use 8mm nylock nuts, and a little low strength thread lock on the stud-chuck threads. I prefer six bolts to the more common three when mounting scroll chucks.

The second chuck was cheap on ebay, and has both sets of jaws, which the original did not. The jaws are not interchangeable between bodies, the new one has slightly larger jaws. It is either Chinese or Indian, the only marks on it are 3000 rpm, the serial number and 160.

On my Sherline lathe i mounted an Emco Compact 8 er25 collet chuck, i mounted it onto the Sherline face plate with no register at all between the face plate & the ER25 chuck. I simply faced the face plate off & mounted the chuck so as i could tap the chuck into true, this worked very well for what i wanted. Depending on what collet i have in the chuck run out is .0002" – .0003", as you go up in collet sizes its around .0003" – .0005". I could spend more time tapping it truer but you end up chasing your own tail + i am happy with the run out i have got, if i was mounting a chuck on a larger lathe i would def use the same method where the register would not be a tight fit in the chuck recess so as to be able to tap the chuck true.