Potential Source of Error in Chuck Backplate?

I'm a VERY novice machinist, I have been setting up my 9A lathe which is my first ever machine. It came with a 3 jaw chuck, but it's got bellmouthed jaws that can't hold anything and has 0.011" of runout. I bought a NOS 6" 4 jaw chuck without a backplate. Since I didn't have a currently working chuck or thread dial, I opted to buy a pre-threaded backplate so I could machine it on the lathe, which I did.

I've been trying to get it level. I have a 3/4" piece of O-1 drill rod that I have been using as a test piece, so it's very round and straight. When I chuck it up, I can get it to 0 runout right at the chuck jaws, but I noticed that if you look further down it looks as if the bar is oscillating, and the TIR increases even if I re-zero the indicator. To me, that seems to indicate that the chuck might be sitting at an angle on the spindle-it does seat against the register. I realize this could be from the bar drooping slightly, but it seems way too excessive and is more than just a couple thou.

However, I realized that I did not machine the surface on the backplate that seats against the register, only the OD and chuck seat. Could this be my source of error, or is what i'm describing a symptom of not having a level bed yet? Theoretically, I should just be able to take the backplate off, put it back on the spindle backwards, and true up the register seating face? Or will I need to redo the chuck register step at the same time as well?

Typical O1 is neither round or straight....and 3/4" is way to flexible for your purpose.

And to be blunt cheap 4j chucks suck balls...you can dial one in at the jaws but who knows what it's going to do 6" out from the chuck....truly frustrating if you are doing an offset.
What is your NOS chuck??
If you want to see if the chuck is holding true get some TGP shafting.

I don't remember the make, but it's something nicer. Not a cushman, but still a reputable brand. Assuming I have a rod that is straight and the problem persists, would my assumption of the chuck surface that mates against the spindle register be correct?

I agree with 10k for the most part. My best chuck is a 5” from Enco. It turns .003 at 6”, about the best you can to with an inexpensive chuck.

I think your problem is partially in the backplate. The backplate register for the chuck should be fit to the backplate face and the step. I like to taper the step slightly so When the mounting bolts are tightened the chuck will be drawn tightly on center.

On the backplate mount side the register is the inside diameter and the back face, not the threads. Threads serve to hold the chuck on but do not center it on the lathe spindle. There was never a standard between manufacturers regarding these registrars where they come in contact with the spindle. So a backplate from say, a Logan lathe will not necessarily fit a Southbend lathe. Pre machined backplates spindle registrars are usually oversized so To fit on all spindles thus the fit to some lathes will be loose. Fitting the backplate to the lathe spindle is every bit as important as fitting the chuck to the backplate.

There should be little expectation that a 4-jaw chuck will hold long work perfectly true to the spindle by default. I struggled with the same thing before learning this. I get about the same results as mach2 describes. You have to dial in both ends and bump the TS end around or preferably hold the TS end with a center in the TS, or more preferably both ends in centers.

I've at one point repeatedly chucked the same "test piece" and tried tracking the orientation of the runout vs jaws and found it didn't necessarily repeat, and thus point to one jaw, or set of jaws needing work. I thought I had at first but I was wrong. If yours does repeat you can switch around the position of the jaws and see if it helps, but I'd just change your expectations and technique accordingly.

specifics preferably with pics are your best bet, many times the solution is some simple but easily overlooked thing.

"... I realized that I did not machine the surface on the backplate that seats against the register, only the OD and chuck seat. Could this be my source of error, or is what i'm describing a symptom of not having a level bed yet? Theoretically, I should just be able to take the backplate off, put it back on the spindle backwards, and true up the register seating face?"

Probability there's some thing wrong with the lathe bed: 0.01 percent
Probability there's an issue with the way the new backplate is mounted: 99 percent.

You can't just flip the backplate. You need to make a ring up so the front surface of the chuck (remember this
is backwards, the front is bearing on the spindle) can seat to the flat surface on the spindle. The threads
don't go all the way down as you correctly identified. Be sure the ring has parallel sides.

Take a skim cut on the spigot so it seats neatly. Be sure the threads are generously sized so the
backplate threads don't bind on the spindle threads. Ideally the register diameter in the chuck spigot
bore will be a very snug fit on the spindle's register diameter.

Once this is all in place again take *another* facing skim cut on the surface the chuck mounts to.

There may be a sharp corner inside the chuck mounting bore. Be sure chuck is not hanging on that corner,
be sure you see no daylight on the joint where the two faces come together.

jim rozen said:

"... I realized that I did not machine the surface on the backplate that seats against the register, only the OD and chuck seat. Could this be my source of error, or is what i'm describing a symptom of not having a level bed yet? Theoretically, I should just be able to take the backplate off, put it back on the spindle backwards, and true up the register seating face?"

Probability there's some thing wrong with the lathe bed: 0.01 percent
Probability there's an issue with the way the new backplate is mounted: 99 percent.

You can't just flip the backplate. You need to make a ring up so the front surface of the chuck (remember this
is backwards, the front is bearing on the spindle) can seat to the flat surface on the spindle. The threads
don't go all the way down as you correctly identified. Be sure the ring has parallel sides.

Take a skim cut on the spigot so it seats neatly. Be sure the threads are generously sized so the
backplate threads don't bind on the spindle threads. Ideally the register diameter in the chuck spigot
bore will be a very snug fit on the spindle's register diameter.

Once this is all in place again take *another* facing skim cut on the surface the chuck mounts to.

There may be a sharp corner inside the chuck mounting bore. Be sure chuck is not hanging on that corner,
be sure you see no daylight on the joint where the two faces come together.

Click to expand...

Thank you Jim! Tremendous help! I feared that i'd always end up chasing a ghost, but it sounds like i'll be able to take care of the issue with this procedure!

Absolutely have to take a skim cut across the face of the backplate.
Once you've done that, stick an indicator on the backplate near the outside of the diameter- sans chuck- and spin it...should be near zero runout, unless your spindle is effed.

If the backplate isn't machined precisely at 90 degrees to the spindle's axis of rotation, how can you expect the chuck to be true?

Specification - True Back Plate

Once the bore is close I use blue dye and a scraper to fine tune the fit.

all good stuff.
add to that before machining to fit the chuck, spin the backplate on and off a few times until it repeats, then machine for the chuck.

Some closure to wrap up this thread. The chuck is a Skinner, virtually new when I got it, very little to no wear whatsoever!

I turned a precision ring to fit over the spindle to space the backplate off the register, so I could seat it squarely when the backplate was mounted backwards. It turned out good, (a few tenths of variation in thickness, which seemed adequate). Just wish I had remembered to deburr the dang thing before parting it off! This was my first "functional" part made on the lathe, so I was very happy.

Cleaned up the spindle threads and register really good, seated the spacer, and mounted the backplate. Took a skim cut off the back, cleaned the backplate of the cast iron grit, and remounted it to turn the chuck register.

When all was said and done, I indicated a random aluminum bar to 0.001" runout by the chuck, and it had just a tiny bit more than that at approximatelu 8-10 inches away from the chuck. So i'm pretty pleased! Now to finally get the twist taken out of the bed! Thanks everyone for your help and knowledge!