Replacing a Lathe Chuck

I bought one of the CME $299 specials, and the back plate is small, and likely too thin to use without adding a register for the adjusters. I haven't yet figured out what to do. I did some calculations, and I think I can get enough edge distance (1.5 times the major thread diameter from the hole centerline to the edge of the back plate) if I use 3/8" bolts instead of 10mm. That gives about the same "wall thickness" as a standard 3/8" nut if I reduce the bolt circle diameter by about 0.020. Using 10mm bolts, the wall thickness is less than 3/32".

I'm putting it on a 13x40 lathe, so the longer hang-out of the chuck design isn't as much of a problem.

The back plate that is included is 6.346 in diameter and a little over an inch thick.
 
That's very consistent with what Doug told me. Thanks for the info. Please keep posting as you work on it. At first I was concerned about making the added register "perfectly" flat and later realized that it doesn't matter. The chuck doesn't rest on the surface of the register; it only touches the sides. For anyone that hasn't seen pictures of this chuck, it looks like the adjusting screws don't directly touch the back plate. It appears that there is a center piece that registers on the nose of the back plate and the adjusting screws press on that center piece. Provincial, can you confirm or correct my impressions? If it is as it seems, I'm not sure why the depth of the nose is a factor.

CME 6 inch Adjustable Back View.JPG
 
When you guys talk about adjust tru or adjustable structure chucks, exactly what is that feature. I thought a three jaw chuck was all there were out there
 
David2011 said:
That's very consistent with what Doug told me. Thanks for the info. Please keep posting as you work on it. At first I was concerned about making the added register "perfectly" flat and later realized that it doesn't matter. The chuck doesn't rest on the surface of the register; it only touches the sides. For anyone that hasn't seen pictures of this chuck, it looks like the adjusting screws don't directly touch the back plate. It appears that there is a center piece that registers on the nose of the back plate and the adjusting screws press on that center piece. Provincial, can you confirm or correct my impressions? If it is as it seems, I'm not sure why the depth of the nose is a factor.

View attachment 486423
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Just like a Buck the four adjusting screws bear on a projection in the center of the back plate. In your photo above, two of the screws are visible in that register, and the internal hex of the head of a third is visible on the outside diameter of the chuck near the back corner. The projection of the back plate needs to be long enough for the adjusting screws to bear completely on it. Ideally, it should only leave a gap of about .020 between the back plate projection and the rear face of the register in the chuck. The chuck is held to the back plate by the three screws you see coming out of the rear face, and adjusted (just like a 4-jaw) by the four screws that bear against the projection of the back plate. There is enough flex in the mounting screws that you can jack the chuck around with the adjusting screws. Unless you run out of adjustment, the chuck itself never touches the projection of the back plate, only the tips of the adjusting screws. Thus, in theory, the projection doesn't need to be perfectly concentric with either the back plate or the chuck, but it would be more likely to cause vibration if it is wobbling around, an reduce the amount of adjustment available in one direction.

If adding the projection to the back plate, it is wise to have it recessed at least 1/8" into the back plate with a light press fit so that side loads will be transmitted from it directly to the back plate instead of relying on fasteners to transmit those loads.
 
Here is an image of an adjustable chuck back plate with the boss for the adjusting screws to bear upon.

t23771-7f75e3ea818e16cee850816ab9bd35da.jpg
 
Provincial said:
I bought one of the CME $299 specials, and the back plate is small, and likely too thin to use without adding a register for the adjusters. I haven't yet figured out what to do. I did some calculations, and I think I can get enough edge distance (1.5 times the major thread diameter from the hole centerline to the edge of the back plate) if I use 3/8" bolts instead of 10mm. That gives about the same "wall thickness" as a standard 3/8" nut if I reduce the bolt circle diameter by about 0.020. Using 10mm bolts, the wall thickness is less than 3/32".

I'm putting it on a 13x40 lathe, so the longer hang-out of the chuck design isn't as much of a problem.

The back plate that is included is 6.346 in diameter and a little over an inch thick.
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I have fit this exact chuck to two different adapter plates (not D series) in the past week and I'm not sure that will work...maybe, but there's at least one thing to consider. I used 3/8" bolts because I didn't have an M10 tap handy and that certainly gives you a bit more wiggle room, but I'd be concerned about how the bolt heads center in the chuck. The countersink for the bolt heads in the chuck isn't flat/square on the bottom so if you offset the adapter plate bolt holes the bolts are going to be on an angle. My thought is they normally want the bolt to self-center in the middle of the hole as much as possible. Putting the adapter holes offset you're going to have the bolts cocked in/out so you won't get contact all the way around the bolt head. If nothing else, it's something you might want to look at before you start drilling holes.
 
Provincial said:
Ideally, it should only leave a gap of about .020 between the back plate projection and the rear face of the register in the chuck.
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This seems to be a topic of great debate, in both directions. Several manufacturers specify a total of .008" which really only gives .004" in any direction, and seems quite lot to me. Others have a range .007 to .012" which is more reasonable. Others call for .020" and if you listen to some of the CNC folks they run upwards of 1/8" clearance. The last couple I've done I went with around .020" and it's been fine.

The one thing people need to check is that the clearance doesn't exceed the available travel of the adjusting screws. The adjusting screws and the boss on the adapter plate aren't really doing anything once the chuck is secured to the backing plate, so the clearance between the chuck and the boss shouldn't really matter.

Edit to add: I just came in from the shop and curiosity got to me. I have an 8" Buck True Adjust and matching Buck adapter plate (pictured below) that isn't being used so I measured the ID/OD on them. The OD of the hub is exactly .030" smaller than the ID of the chuck recess. I have no idea if it came that way from Buck or not, but thought it was worth checking.
 
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Provincial, thanks for the clarification. It looked to me like the OD screws might be different from the ID screws. Good to know that isn’t the case. I’m thinking about getting a Shars 8” D1-4 back plate to mate to the CME chuck. It will make for a lot of cast iron chips but should work as intended and I won’t have to kludge anything together. The Shars back plate is almost 1-1/2” thick.
 
Pevehouse said:
When you guys talk about adjust tru or adjustable structure chucks, exactly what is that feature. I thought a three jaw chuck was all there were out there
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Imagine a 3-jaw chuck that has an internal 4-jaw chuck in the back that latches on to the adapter plate. You loosely bolt the chuck body to the adapter plate with the mounting bolts just barely snug. Then you put something like a ground rod in the 3-jaw chuck jaws, put an indicator on that and adjust the four set screws on the back of the 3-jaw body to zero the runout of the whole assembly. When you've got it zeroed, tighten the bolts mounting the chuck to the adapter plate.

It's not perfect as the zero is really only for the diameter ground rod you just used, but it tends to be very close at other diameters as well...like a couple of thousandths. If needed, you can zero the chuck to a different diameter. I tend to zero mine for something around 1" as that's a good middle ground for most of my work.

This is an 8" Buck True Adjust chuck with a matching Buck adapter plate next to it...you can see how they fit together. The critical thing is that there's a gap between the inside recess of the chuck and the hub on the adapter plate and that the nose of the hub doesn't touch the back of the chuck...you want the chuck essentially floating on the face of the adapter only.

fullsizeoutput_5db.jpeg
 
A trick I learned as an apprentice was to mount a 3 jaw chuck into a four jaw. Thus the 3 jaw was fully tuneable. I found that when using the same size stock the 3 jaw was quite repeatable. But when changing to a different size stock the 4 jaw had to be adjusted for the 3 jaw to be true, but then it remained true at that size.

I have yet to do this on my new lathe, because I need to make a new backing plate for my 3 jaw, and I haven't got around to it yet.
 
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