Chuck and backplate Vs direct mount chuck

[JeepsAndGuns]

So I am wanting to buy a new chuck for my lathe. It has a D1-4 spindle. I currently have a direct mount 6 inch that came with the machine. Jaws are a little bell mouthed, but otherwise works fine. It does not have reverseable jaws (but has the 2nd set of jaws for gripping external stuff) its just a pain swapping jaws. Also the size has been a limiting factor on several things I have wanted or was trying to machine. I am wanting to upgrade to a 8 inch 3 jaw chuck. I am still keeping what I machine well within the limatations of my machine, I just would like a little more room to work with (chuck wise) and kinda like the reverseable jaws.
So shars chucks seem to be generally well spoken of here and seem like a reasonable "bang for the buck" since my budget is limited. I was wondering is there are any advantages or disadvantages to buying a plain back chuck and a seperate backplate, or buying the direct mount model? Pricewise, there is only a few dollars difference.
Specificly, buying these:
http://www.shars.com/products/view/962/8quot_3_jaw_Self_Centering_Scroll_Lathe_Chucks_2_Piece_Jaws
http://www.shars.com/products/view/20189/8quot_D14_Fully_Machined_Lathe_Chuck_Adapter_Plate

Or buying this:
http://www.shars.com/products/view/...entering_Scroll_Lathe_Chucks_D14_2_Piece_Jaws

If I have seen some threads on here correctly, is it my understanding you can, or have to, take a backplate, mount it to your spindle and "true" it up? If so, I suppose I can see a slight advantage to the seperate chuck and backplate. Being able to true it up before mounting the chuck I could see as being a potiential of more accuracy. So far that is the only advantage I see for seperate chuck/backplate Vs the direct mount.
I know on my current chuck, if I take it off (to run my 4 jaw) and then go to put it back on, I have to get it back in the exact same set of holes (I have chuck and spindle marked for this reason) otherwise it has more runout. I suppose thats one disadvantage to a direct mount. You have only 3 chances to get a good/acceptable ammount of runout. If none are good, could it even be adjusted?
Thanks for any input or advise you can give me.

 

[Ray C]

I personally wouldn't touch an integral type chuck with a 10 foot pole. First, you fit the backplate to the spindle then you fit the chuck to the backplate. I try to avoid cutting shoulders on backplates to fit tightly with the chuck. For normal chucks, you do not need a shoulder at all and that buys you latitude to fine-tune it later. Also, once a chuck is mounted to a backplate and properly adjusted, leave it there until you have a real good reason to mess with it. Each chuck should be mated to it's own backplate. I usually put index marks so I can put the pieces back together the same way but, that's no guarantee it will run perfectly true and after disassembly, it will need a little tweaking to get it dead-on again.

Please have a look at the following threads that give some background and instruction on the topic.

http://www.hobby-machinist.com/showthread.php?t=24596&highlight=fitting

http://www.hobby-machinist.com/showthread.php?t=10651&page=2&highlight=fitting

Finally, folks should be aware that each time you remove a D1-type chuck from the spindle and remount it, it could be off a little. I mark my spindle with ink and always put the chuck on in the same orientation. -And even still, sometimes it's necessary to unlock and re-tighten the camlocks for it seat the same way. The tiniest bit of swarf can (and will) throw things off slightly.


Ray

 

[JeepsAndGuns]

Good info. So seperate backplate and chuck it is. Hopefully I will be able to order them in the next couple weeks.
The only time I remove the current chuck is to install a 4 jaw. I have only had to do that twice.

 

[darkzero]

Direct mount chucks, I don't trust them at all even if it's a quality name brand. I hear on occasionthat they have low run out but the mfgd does not guaranteee it. I choose to stay away from them. I do have one direct mount chuck but it's a 4 jaw. It's nice cause it has less overhang but other than that 4 jaw I prefer to fit & dial in myself with a backplate..