Lathe chuck question

I think you're right. I have the same size spindle on my lathe and I wouldn't hang that much weight off of it.

Are you planning work that requires an 8" chuck?
 
Does your lathe have a spec on how heavy a workpiece it can handle?

I would expect lathe spindle bearings to have plenty of capacity to handle a heavier chuck when we consider how heavy a work piece they are designed to handle.
 
There's another way to look at this. Even iif your bearings can handle an oversize chuck, you may not be able to make practical use of it's size. Put a tool in your toolholder and back out the cross slide as far as it goes. Then run the carriage to the left till it's even with the spindle and measure from the tip of the tool to your spindle centerline. Twice that number is the largest practical piece of stock you can turn in your lathe. If it's less than 6", that 8" chuck won't do you much good.

I ran into that same problem with my lathe a while back. I have 5" 4 jaw on my 10" lathe. I put a piece of 5" square aluminum in to be faced off and bored. While the stock went into the chuck, there wasn't enough cross slide travel
to face off the part. I had to cut the corners off the part to make it work. And my chuck isn't oversize for the lathe.
 
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If I'm not mistaken, a Craftsman Commercial lathe is a 12" lathe and it can handle an 8" 4jaw chuck easily. The spindle is relatively small for a 12" lathe at 1-1/2" but I have no idea of the size or capacity of the spindle bearings.

I look at it this way. A spindle assembly can handle a fair amount of weight. My Emco 11" lathe with a 2" OD precision spindle and precision tapered roller bearings has a work capacity of 99# without tailstock support. The 4jaw I use the most weighs somewhere around 15# or so, so the spindle can take about 100# of weight without live center support. If my 4jaw were to weigh 50#, that leaves me with a max work weight of 50#. Beyond that, I would be concerned about the ability of the bearings to do sustained work.

Personally, I would look for a 4jaw that isn't so heavy. The reason Logan recommended the lightest, smallest chuck that could do the work was to limit the wear on the bearings. That makes sense to me.
 
I'm probably not going to take a chance on using the chuck, I don't want to take a chance on damaging the bearings.
Thanks for all the reply's.
John.
 
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