# Backing plate for 4 jaw chuck



## SimonM (Feb 7, 2022)

Just got a cheap 8” Amazon 4 jaw chuck delivered today and will need to make a 2 1/4”-8 backing plate. 

Does the backing plate need to be the same size as the chuck? 

I have some 5” durabar that could be used. 

Here’s a picture of the back of the chuck.


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## PeterT (Feb 7, 2022)

Durabar, lucky you LOL. You mean because your material is smaller diameter it would only extend like so? That's a good question. I thought they typically matched full diameter to contact the outer lip too. But now I have to go look at the backside of mine. Could have sworn I can see some of the cavities but it might be an integrated D1-4? 

BTW, cant' recall where posted but there have been some issues with balancing some of the imports due to casting inconsistencies. Might be good to remove the jaws & put on some kind of roller apparatus to see how it looks from that perspective.

Have you done this kind of threading / spindle nose fit-up before? Not questioning your skill, its just that it might pay todo a dry run on aluminum scrap or something & keep notes. Seems like tehre can be some minor variations based on lathe model.


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## Susquatch (Feb 7, 2022)

SimonM said:


> Does the backing plate need to be the same size as the chuck?



I don't think the backplate needs to be any bigger OD than required to support the bolt circle. The important thing is to be a tight fit to the indexing ring in the center. 

I agree with @PeterT. You should check the balance before you go to much trouble mounting it.


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## Dabbler (Feb 7, 2022)

5" should be fine. 

Judging by your question, you should be aware that the non threaded section should be a tight slip fit. You should be honing it with vary fine wet/dry...  Scoring marks in your backing plate won't matter:  you are looking for 90% contact, and when the 'screwing on' is happening the bore should be dragging on the spindle shoulder for best repeatability.


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## ducdon (Feb 7, 2022)

Susquatch said:


> I don't think the backplate needs to be any bigger OD than required to support the bolt circle. The important thing is to be a tight fit to the indexing ring in the center.
> 
> I agree with @PeterT. You should check the balance before you go to much trouble mounting it.


I agree. Get the register right at the center and if you get out to the edge of that inner circle where your red line is you should be golden.


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## SimonM (Feb 7, 2022)

Thanks for all the good info, hopefully the kids don’t skip the nap and I can give it a try tomorrow afternoon. 

@PeterT You have every right to question my skills, I am a find producer of scrap metal. 

I’m in the process of making a faceplate and managed to successfully make an adapter to bolt on a piece of mild steel plate. It fits perfectly but my order of operations wasn’t great and I had very little space to stop before hitting the chuck. It was my first time internal threading and 8tpi made it interesting in a slightly scary way. 





@Dabbler I thought that only the flat surface that mates on the spindle and threads mattered but what you are suggesting makes a lot of sense.


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## Degen (Feb 8, 2022)

I believe that plate needs to be same size as the chuck be it registers on the rim.  This means you cut slight recess onto to rim so that chuck just slips over it and self centers.  This happens as the lip is cut on the lathe. After that mount hole location while important become less critical.  Additionally it provides support as on a 4 jaw masses can be off centre and keeps the chuck centered not relying on only the bolts to do this.

This not like a D mount chuck.









						8" 2-1/4" X 8 TPI Back Plate at Grizzly.com
					

<h1>SB1390 8" 2-1/4" X 8 TPI Back Plate</h1> <p>For lathes with 1-1/2” x 8” and 2-1/4” x 8” threaded spindles, the SB1390 8" Blank Plates allow you to make modifications to fit your lathe chuck.</p> <p>Simply mount to your lathe, true up the face and turn down the ledge to fit your chuck.</p>...




					www.grizzly.com


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## Dabbler (Feb 8, 2022)

Degen said:


> it registers on the rim



@dgen  that is what the inner boss or step is for.  I'm sure on some chucks the outer ring is used, but not on any of my 4 jaw chucks with backing plates (3).


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## PeterT (Feb 8, 2022)

Degen said:


> I believe that plate needs to be same size as the chuck be it registers on the rim.  This means you cut slight recess onto to rim so that chuck just slips over it and self centers.  This happens as the lip is cut on the lathe. After that mount hole location while important become less critical.  Additionally it provides support as on a 4 jaw masses can be off centre and keeps the chuck centered not relying on only the bolts to do this.
> 
> This not like a D mount chuck.



Ya, I was wondering that myself if screw-on were different for that reason alone. The inside edge of the lip of his particular chuck doesn't look finished or deep to support a backplate boss. But I guess the extra contact area might be beneficial?

I only have experience with D1. The 2nd picture shows a D1-4 direct mount where only the smaller inner face is in contact with spindle. The casting webs & outer rim are exposed.


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## Dabbler (Feb 8, 2022)

Perhaps the OP can clear this up.  I seee no registration lip on the outer rim as per the OP's photograph, but I *do* see and inner registration lip in the photograph.  That is all that is needed to support a 4 jaw chuck.


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## Susquatch (Feb 8, 2022)

Degen said:


> I believe that plate needs to be same size as the chuck be it registers on the rim.



I'm with @Dabbler on this. I am not a chuck manufacturer and it's not like I've seen thousands of chucks. But of the two dozen or so that I have seen, not even one registered on the outer rim.


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## Degen (Feb 9, 2022)

Sorry to say gentlemen, very first chuck I did was a 4 jaw had a similar rim to the one shown (mind you better casting and finish), the trick is you turn the plate flat and the cut a slight relief in the rim that it just allows the chuck to slip on yet not bottom out on the center flat, this insures that you achieve max bore dia, otherwise you constrict the bore size.  Again this was when I was starting out and I consulted with some old machinist (prior to info being available on the internet) and this what he advised.  One important note is once you start this final step do not remove the plate from the lathe, all fitting and work the plate remains mount until you are ready to locate the holes for bolting it on (btw I didn't remove it and located everything using a straight edge across the bed, horizontal reference, and a drafting square, best I had at the time.  A machine shop checked, drilled and counter sunk for me, did't have a drill press yet, only missed locating one hole by 0.0005")

Here is an example, see about 2/3 into the video.


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## SimonM (Feb 9, 2022)

No luck getting in the shop yesterday… Took the kids skiing in the morning and thought it was a perfect recipe for a great nap, too bad it turned into a short power nap in the car on the way back.

The chuck doesn’t register on the outside, I will give it a try with the 5” chunk that I have.


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## PeterT (Feb 9, 2022)

Degen said:


> Here is an example, see about 2/3 into the video.


The chuck in the Tony video has mounting holes in the rim (only) whereas the OP has holes in the inner face (only). I didn't hear the size of Tony's new chuck but suspect the main reason is the new chuck is substantially smaller, like maybe a 5" vs a 8 or 10?. So possibly the backplate contact diameter of small one might be comparable diameter to big one?


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## Degen (Feb 9, 2022)

I'm beginning to think this is one of those depends on the size of the backplate used.  Backplate matches chuck index of outer.  If it is smaller index of the center hole.


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## Susquatch (Feb 9, 2022)

Degen said:


> I'm beginning to think this is one of those depends on the size of the backplate used.  Backplate matches chuck index of outer.  If it is smaller index of the center hole.



Maybe, maybe not. The two that I saw most recently were both same size as the chuck. Prolly deliberately so to look good. Indexing was on the center hole.


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## SimonM (Feb 16, 2022)

Finally got some shop time Monday and yesterday and managed to finish the backplate. 

I’m quite pleased with the result. There is about .001” of runout on the outside surface of the chuck.


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## PeterT (Feb 16, 2022)

Nice. Don't forget to check balance.


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## SimonM (Feb 16, 2022)

@PeterT How would you do that without dedicated equipment?


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## PeterT (Feb 16, 2022)

Sure ask me the tough questions LOL. Can you rig up even a simple flat bar balancer like pic in post 51? (unfortunately unlike a 3J which self-centers you would have to pre-center grip a piece of nice round stock stock but could do that in the lathe with a dial indicator). I like John's idea more but involves bearings.









						Unbalanced 4 jaw on my 12x36
					

@Proxule I have an idea, one that will need a well fitting faceplate.  I'll do some calculations and we'll talk once you get your new one.




					canadianhobbymetalworkers.com


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## PeterT (Feb 16, 2022)

this pic, maybe about a simple as you could do, maybe with 2 parallels on a level bench/support so rod can rest on both


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