As a beginner at lathe work I'd appreciate some advice.
I have a Boxford lathe which came without outside jaws for the 3 jaw chuck Burnerd.
I've got some secondhand jaws from ebay which look absolutely right. 1 & 2 fit nicely, a bit tighter than the inside set but easy to fit and remove by hand.
No 3 is tighter though - I can engage it with the scroll by hand but I need to tap it to remove it. I haven't tried winding it in.
To be absolutely correct, as Nick has said, the jaws need to be matched to the chuck, although all chuck manufactures do sell spare sets of jaws. If the jaws are "Burnerd" from the same model chuck as yours and are a mached set (1,2,3 with same serial number), you should be able to get them to work reasonably well. If not, or as I had once 3 jaws that were marked 1,2&3 but with different serial numbers, it's down to luck really. I'm no expert in chucks but have got some odd jaws to work but NOT if you are looking for "Buenerd" accuracy. I'm assuming that it is a self centering chuck.
If the jaws are going in with a push but not coming out look for a sharp square edge on the rear edges of the slots in the jaw and put a slight chamfer on the rear corner with a stone as it might just be picking up when you try to pull it out. With the mixed set I had although I eventually got them to go in and work they did not hold particularly concentric, but were good enough for rough work. I moved the tight jaw in and out a few times until I could see a polished area where the jaw/chuck body were rubbing, better still use some "engineers blue" if you have any. This area ON THE JAW I carefully stoned until the jaw slid reasonably easily in the body. Do not touch the chuck body and do not force in by winding with the scroll as it is realatively easy to damage and will render the chuck useless.
The position of each jaw when the chuck is tighted is controlled by the position of the "teeth" on the back of the jaw and the position of the scroll in the chuck body, if you are unlucky even if you get the jaw to slide in the slot these will be in the wrong place and the jaws will be a long way from concentric. In this case it might be worth trying the jaws in different slots and see if you can get a reasonable compromise. If you put the three jaws together and look at the "teeth" you will see that they are in different positions on each jaw. This difference should look similar between 1&2 and 2&3. I have sometimes had a set become usable by moving them round the chuck by a slot, ie 1 in 2, 2 in
3 and 3 in 1 etc.
If you have bought the jaws just to try in this chuck then I suppose that nothing is lost by slightly relieving the jaw to allow it to fit. If these are from another chuck which you use then obviously removing metal from one of the jaws will ruin the fit in the original chuck. Not sure if this helps really but do remember that this is a bit of a "bodge" and to retain the normal "Bernerd" accuracy you would need to get a set from them. If, like in my case, they were a worn odd set then it might be worth a try.
I stand to be corrected (I normally am!) but isn't it the just the jaws that need the same serial number. I remember buying some outside jaws for the Burnerd on my Myford and all I was told was to make sure that all the jaws had the same serial number. I did and they fitted with no problem.
Any set of matched jaws ought to 'fit' into any chuck of the correct type. i.e. they should work fine on the scroll without binding and into the slots without binding. What they might not do is hold work perfectly concentric as they won't have been ground in situ on that actual chuck. You can remedy that by refacing the jaws on a grinder.
I did mine many years ago, not because they were the wrong jaws for the chuck but because I wanted to improve the concentricity of an old chuck by that last few thou. I mounted a 6" grinding wheel in an arbor on my mill, (that matched the largest diameter the jaws would hold) clocked up a piece of bar in the chuck and tickled the jaws that were holding too high until they held true. I might have been pissed at the time though because I kept getting mixed up between the high jaws and the low ones and going over and over them again trying to get it right. Write on the board 100 times "when you've found the high point on the bar stock with a dial gauge it's the OPPOSITE jaw you want to grind". Hey ho.
By the time I'd finished I'd removed most of the cross slots in the jaws together with the bulk of the case hardening and they wouldn't grip anything smaller than about 8mm before butting together. The solution to that was to grind the chamfers down too on my mate's belt linisher to reduce the width of the gripping surface. All ended well but it was a rather ham fisted procedure.
It wasn't a complete cure all either because the concentricity of an old chuck also depends on the size of the bar stock being held and hence how much wear there is in the scroll at that point. You can often get round that by choosing which of the three key holes to tighten up with or by judiciously tapping the workpiece or the jaws before finally nipping them up tight.
Maybe there are more than just one statement that is true?
1.) It might be, that the jaws are selected for the best fit with the chuck's body.
2.) Then they get the same serial number as the chuck and are ground in that chuck for concentricy (as Dave described it).
If 1.) is not true (depending on the mfg-process) then you are right and I stand corrected. I can imagine, that both of us are right, depending on the (non-)quality of the chuck.
I buy Burnerd soft jaws for a five inch three jaw chuck and as supplied don`t fit.Burnerds policy(with soft jaws anyway)is to leave a grinding allowance on the slots so they can be individually fitted.Grinding instructions are supplied with them. regards,mark.
Thanks for all the replies - I've been doing more homework too (Burnerd and Rotagrip both very helpful even when they knew there was no sale involved) and jaws are supplied oversize (to allow them to be fitted to worn chucks) - normally need ing a little taking off the edge of the slots.
There is apparently a risk with buying secondhand jaws that they will be sloppy having been fitted to a less worn chuck than the one you have.
I'd be interested in the instructions on grinding jaws - any chance of a scan please?
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