I have a theory about backing plates on screw-threaded spindles. Common wisdom claims that it is the smooth cylinder between the spindle thread and the spindle collar that axially aligns the backing plate and therefore the chuck. I do not subscribe to that theory for a couple of reasons:
The class of fit required between the backing plate and the spindle is called a wringing (I call it damn tight!) fit and, if it is tight now, will loosen up in use. In fact I have never found a screw-on chuck with a tight fit at this point. My theory is that "therefore, that cannot/should not be the source of the alignment between the spindle and the backing plate."
The most powerful aligning element on the spindle is the thread. I believe the tread can be treated as a special class of taper and will quite happily align the spindle/plate given half a chance. The other aligning element is the spindle collar. When the backing plate is threaded onto the spindle, the plate can wobble until the butt end of the plate meets the spindle collar. At that point, the backing plate has no choice but to center itself on the backside of the thread cone as it comes to a stop.
Since spindles and backing plates are not (as far as I know) hardened, all thread-on backing plates are ultimately going to be somewhat loose
I've made backing plates with wringing fits and those with what Id call sloppy fits (3 to 5 thousandths or more clearance on the smooth cylinder back of the threads). As far as I can see, either way is viable. You can guess which one takes longer to make.
My last backing plate, for an eight inch, six-jaw adjust-tru type chuck was made without a wringing fit. After adjusting the chuck to run true to within 0.0005 TIR, I have not had any change in runout even though the chuck has been removed many times.
I also made a collet adaptor for my 6" lathe over two years ago and it still has a runout equal to the spindle runout.
Precisely measure your spindle nose. Diameter, length, length of thread, diameter of spindle collar. Use an accurate method to measure the threads such as a thread micrometer or a three-wire setup.
Make a dummy spindle nose. This will be quite useful for other applications so leave a little extra metal in back of the nose collar so you can mount this on other equipment. This Step is not absolutely necessary but unless you have an internal thread micrometer, it does make measuring the internal threads (using the dummy spindle as a tread gages) much easier.
Note - if you have a tap of the proper size, don't worry about this step at all. You can use the tap to finish the treads. However, a 2-1/2 X 6 tap for my lathe is pretty expensive. The 1 X 8 tap for my 6" Craftsman was about $75 three years ago. (I tried a cheaper Polish tap but it took three mess-cooks and 5 small boys to turn it. The U.S.-made Greenlee cuts with very little force.
If you have a tap, see step five.
Either weld up a plate and bar stock (works really well for smaller lathes) to make the backing plate blank or purchase a cast iron blank from Enco or MSC, or start with a billet of stock of the appropriate size and cut (a lot) it down.
(Ive done all three and I prefer the cast iron blank from Enco for speed and the dampening effects (in use) of cast iron.
Grab the chuck-side of the blank in a 3 or 4 jaw chuck (or clamp it onto a backing plate) and bore a hole front-to-back through the center of the blank at the spindle-thread root diameter (you can get this value from Machinerys Handbook)
Without removing the backing plate, cut the internal thread to specification, to a nice fit with the dummy spindle nose or by finishing the tread you first cut with the lathe, with a tap.
Bore the treads out to form the internal surface that will face the smooth portion of the spindle. This should be fairly close but dont make it too tight a fit.
Take a facing cut on the face of the backing plate that will bear against the spindle collar.
Take a facing cut on the large diameter plate that will ultimately bolt to the chucks back. This will true it up so you dont have an visually annoying out-of-true condition on this plate. Otherwise, it will not harm anything. Make this entire area look pretty because that is what people will see.
Take a deep breath and remove the nascent backing plate from the holding device and remove the holding device from the spindle (assuming you have only one lathe). Now thread the backing plate onto the spindle. It should go on smoothly and bottom out on the spindle collar.
Now take a truing cut on the face of the chuck plate, the face upon which you will mount the chuck.
Cut the required step in the face of the chuck plate to fit into the recess on your chuck. This must be a good fit or you will have a bit too much runout when you get done.
Note - What can go wrong here? Several things:
1. The chuck can be untrue. I bought an Enco 4-jaw that was made in India. The whole thing was pretty bad. I clamped the chuck onto a 1" mandrel, mounted the whole thing between centers and trued the back, back recess and outer circumference of the chuck. Now it is OK relative to truth.
2. You can cut the step too small and then the whole thing is gonna have too much runout. You cannot depend upon the bolts holding the chuck to the plate to hold everything true
If you mess this up and the backing plate is of steel, run a bead of weld around the step and cut it again. And maybe again.
Some chucks require that the backing plate not bear on certain areas of the chuck back. Cut out any relief that is required.
Cut the diameter of the backing plate down to meet the diameter of the chuck.
Place the chuck on the backing plate and use transfer punches to mark the hole locations for the hold-down bolts. You can also use a rotary table or a bolt-pattern coordinate set calculated with Machinerys Handbook. This is a noncritical area of the backing plate as long as the holes are pretty well aligned. Drill and tap the holes to match the supplied bolts.
Uniformly torque the bolts holding the chuck to the backing plate. Best to use a torque wrench. Otherwise you may cock the chuck on the backing plate or worse.
Put the newly mounted chuck on the lathe spindle, chuck a piece of good quality round bar stock in the chuck and measure the runout a few inches out from the front of the chuck jaws. Should run within 0.002 TIR at worst, probably 0.001 at best. If the runout is too large but the body of the chuck is not moving the same amount, it is time to return the chuck or, if you are brave, fix the chuck.
This is a workable method for mounting chucks. I made a collet adaptor for my Craftsman using this technique and its runout is about 0.0002 when measuring a piece of drill rod 2" out from the collet. I used ETR double taper collets because of their short length and wide clamping range compared to C5 or R-8 collets. And it left the spindle bore open too.
I think there is a reasonable way to make any chuck into an Adjust-Tru type. The backing plate has to be big enough in diameter for the chuck to sit down inside a bore in the plate and to be moved around by four fine-threaded, opposed bolts through the periphery of the plate and bearing upon the chuck. The adjusting bolts are manipulated until items place in the chuck jaws run true. Then the normal hold-down bolts are tightened and the chuck will then run true at that diameter.
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