Grinding the Jaws

**thehomeengineer** · Apr 6, 2020, 02:02 AM

Hi HUMARIA
I agree with your comment totally. As I stated I understand that we all do things differently and I do apologise to all who have read my posts and bouboulas if my comments are offensive in anyway, this was not my intention more constructive on how the chuck could have been made concentric rather than grinding. Like you say it would only be fair to ask the reasons rather than how and why.
please again accept my apology
The Home Engineer

**olderdan** · Apr 6, 2020, 05:00 AM

Grinding chuck jaws should be a last resort after all other checks have been explored. I did post a while back on doing this but in my case it was on 60 year old chuck jaws that had worn bell mouthed, you do not need to step the front of the jaws. It would be prudent for someone embarking on any project they are not sure of to use the search bar on HMT first.
It is surprising how many posts have already covered what you are looking for.

Chuck jaw truing tool

**bouboulas** · Apr 7, 2020, 12:48 PM

Hi first things first I am sorry my English are very bad .
The chuck I bought it 3 years ago. It is a cheap Chinese chuck the cost is 110€.
Before I put it re machined the back plate used a test bar and clocked is 0.08mm.
Two months ago I made a worm gear . The taper tap slipped and hit the jaws
For these reasons I grinded the jaws !

**MrMetal** · Apr 8, 2020, 11:17 AM

I acquired a well used 3 jaw chuck and I need to make a new backing plate to fit a small Atlas lathe with a headstock spindle of 1" 10. I plan to machine the new plate on that machine. Any words of advice? I tried to provide a photo but the image would not attach.

**thehomeengineer** · Apr 8, 2020, 02:36 PM

Hi MrMetal
If making the backplate from scratch I would start with screwcutting a gauge to suit an existing backplate you use. The reason for this you can then screw cut the new backplate and check the fit without removing from the chuck also I take it you will be using the lathe the backplate is for, to make the new plate. Once the back of the plate is machined and screw cut to the desired fit to gauge, it can then be screwed to the nose of the lathe. Chucks normally have a register diameter in the back of the chuck. I measure this and then machine the back plate 0.005/0.008" smaller to allow for final trueing of the chuck on the backplate when bolted up. Also make the length a few thou shorter than the depth. Once the chuck slides over the register the back plate can the be set up to be drilled, to the chucks bolt hole arrangement. Some chucks bolt from the rear into threaded holes in the chuck so the backplate will need clearance holes and some chucks bolt all the way through the chuck body with socket head cap screws so the back plate will need tapping, Once the chuck is mounted to the back plate ideally a test bar (or a round ground piece of HSS can be used) to clock the concentricity of the chuck and any deviation at this point should be small and slight adjustment until running true can be obtained due to the spoggot of the backplate being 0.005"/0.008" smaller than the chuck register. Clamp up nice and tight and the chuck should run true with no problems. This is the method I use and have never had any issues with chucks in the past running out. Again I am sure there will be other methods people use that work just as well as this one.
I hope this helps.
The Home Engineer

**MrMetal** · Apr 9, 2020, 04:36 PM

> Greetings Home Engineer,
>
> Thanks for your good advice. I would not have thought of screwcutting a gauge. Seems like an advantage to have the backplate fit snugly in the register diameter of the chuck. Isn't that for the original concentricity when the chuck was made? What would you think of putting a test bar in the 3 jaw chuck while it off the lathe and then chuck the protruding end of the bar in a 4 jaw chuck on the lathe. That would allow a runout check of the register diameter relative to the jaws. I would be hesitant to mess with the register diameter because any error would probably be in the scroll and may not be consistent throughout the range. That being said, I would like to try a snug fit of the new backplate and if the chuck runout is not acceptable then use your method with a slight clearance. I obviously do not have your experience or expertise.
>
> This chuck has the backplate screw holes threaded into the chuck. I have attached 3 pictures for reference. As you can see from the current backplate, the chuck was much oversized for the very small lathe it was on. I do not see a brand name on the chuck. Is that common or does it mean I have a cheap no-name. Thanks again for sharing your expertise.

**thehomeengineer** · Apr 9, 2020, 04:55 PM

Hi MrMetal
You can try and make the registers of the chuck a good fit on the backplate and you might get lucky (I would be very interested in the result). I would definitely machine the backplate on the nose of the lathe after screw cutting to the gauge as this is then going to remove several other errors that can be introduce by holding on the test bar in another chuck. Also that could be a lot of unsupported weight on the test bar and at a distance from the headstock. But remember there is no right or wrong way, if you end up with the correct result it doesn’t matter how you get there as long as it is safe.
let me know how you go
The Home Engineer