Machining A Chuck Adapter

Hi all,

My 9A lathe restoration project is almost done, but one accessory I wanted to add before the lathe can be considered complete is a good 3-jaw chuck. My lathe came with a very nice Skinner 6” 4-jaw chuck, which will get a lot of use, but a decent 3-jaw can be used for a lot of the less-demanding jobs, or those which can be finished in a single setup.

My first mistake was to purchase a used chuck. I DO NOT RECOMMEND THIS. You will likely wind up with the target of many years’ worth of abuse by previous owners. (By the way, if anyone’s interested in a decent anchor for their fishing boat, with a nifty built-in rope gripper, contact me off-line.) So with that lesson learned, I decided to purchase a new Bison 5” (125mm) 3-jaw chuck, with an unmachined, threaded adapter plate. The Bison is a Polish-made chuck, of decent quality and a good value. Not the best, but certainly not the worst, and just fine for occasional home-shop use.

I decided against the “direct mount” type because I wanted the chuck to run as true as possible on my lathe. Given the inherent inaccuracies of a scroll-type chuck, I figured it could use every break it could get. So, I purchased the bare chuck along with a rough-machined adapter plate, pre-threaded 1-1/2"-8 to fit the spindle nose of my lathe. This means that the seating face and shoulder which locate the chuck will be machined on the actual lathe that the chuck is to be mounted. This way, any irregularities of my particular spindle nose will be compensated for.

I tried not to include too many pictures, since I know how they can affect the loading time for those without high-speed connections. The first one is a cross-section that I did on the CAD system, to arrive at the desired mounting configuration. The lathe’s spindle nose is shown in red, a cross-section of the chuck body is shown in green, and the machined adapter plate is shown in blue. The original, unmachined outline of the of the adapter plate is shown in brown:



One of my goals was to reduce the overhang of the chuck to an absolute minimum. My theory is that every millimeter that the chuck sticks out past the end of the spindle bearing amplifies the effective bearing clearance, increasing the possibility of deflection and chatter at the workpiece. Therefore, you will notice that I tried to get the spindle nose to fit as close to the chuck as possible, even to the point of needing to counterbore the scroll retaining plate slightly to clear the tip of the spindle thread. One drawback to this configuration is that it slightly reduces the amount of workpiece clearance behind the jaws, but I figure the overhang issue trumps it. Another drawback is the amount of material that needs to be removed from the adapter plate’s hub! I toyed with the idea of setting this up on the saw somehow, but later decided that this is what automatic cross-feed is good for.

Since I was going to be modifying the hub, and this operation would affect the alignment of the plate to the spindle nose, it needed to be done before machining the registration surfaces for the chuck. The first step is to thread the adapter plate onto the spindle and take a skim cut on the face and O.D., to get these surfaces in alignment with the spindle thread:



NOTE: The swarf from machining cast iron is some nasty, gritty stuff. So I took steps, where practical, to catch the chips as they were produced. Also, after each phase of the machining process, I took the time to clean up and re-lubricate the lathe.

After machining the face and O.D. of the plate, I removed it from the spindle and mouned the 4-jaw chuck. The adapter plate was carefully centered in the 4-jaw so that the thread’s counterbore was in alignment with the lathe axis. Now I was ready to begin machining the hub:



I had to make quite a few chips getting the hub down to size! But even so, it didn’t seem to take that long. After facing the hub to the correct length, I used a stubby boring bit to counterbore the threads to the required I.D. and depth, along with a 30 degree lead-in. Also, since there was visible runout of the rear face of the adapter plate, I decided to take light, truing cut. This is not a functional surface, but I didn’t want to have to see that surface wobbling around every time I used the chuck. Here is what the completed hub-side of the plate looked like, removed from the spindle:



The next step was to remove the 4-jaw and thread the adapter plate back on the spindle:



Notice that the chamfered end of the spindle protrudes slightly from the face of the plate, just as I had planned. I am still utilizing practically all of the available spindle thread, while reducing overhang to a minimum. The chuck’s scroll retainer plate will be counterbored slightly to clear this protrusion.

Now it was time to begin machining the shoulder that would align the adapter plate to the chuck. One needs to work carefully here, as the fit between the shoulder diameter and the I.D. of the chuck is important. Once you get close, take only light cuts to “sneak up” on the required diameter. Ideally, you want a zero clearance fit -- not a press-fit, but something closer than a slip-fit. I guaged the last couple of cuts by offering up the actual chuck to the machined shoulder.

After machining the shoulder to the correct diameter and length, turn the O.D. of the plate to match the chuck’s O.D. This is not an absolute necessity, but it does look more professional. Also, try to get the chamfer on the adapter to match the chamfer on the back of the chuck. As a last step, chamfer the back side of the plate. Here’s the finished adapter plate, still in place on the spindle nose:



The only thing left to do (other than drilling and counterboring for the mounting screws), was to re-mount the 4-jaw and counterbore the back side of the 3-jaw chuck's scroll retainer to clear the end of the spindle thread. Here’s the finished job (notice the brass shims between the jaws and chuck body, and the paper towel stuffed into the center hole to keep chips out of the scroll):



With that operation completed, I decided I wanted to see what the new chuck would look like mounted on the lathe spindle. While I still needed to drill and counterbore for the mounting screws, the fit between the adapter plate and chuck was tight enough that it would stay in place while I snapped a picture (Just keep that hand away from the motor switch!):



Since I don’t currently have a good, accurate way of locating and drilling the mounting holes, I will take this job to my friendly local machine shop. I’m sure they can take care of it in a jiffy.

Well, one more step completed toward finishing an almost year-long restoration job! I hope some of you found this useful or interesting.

Paula

Great Job Paula, I'm surprised you actually plan to use that lathe, it looks like a museum piece.

I'm thinking of purchasing the direct mount Bison chuck, what I was planning to do was to grind the contact surfaces on the jaws using the toolpost grinder. I will loose a couple of thou on the minimum and maximum clamping diameter, but I can live with that. What do you think of that idea, am I overlooking something?

paco

I think that will work, Paco, but do you have a way to pre-load the jaws prior to grinding? For best results, you need something to tighten the jaws down against, while still being able to grind the inside faces. Here's one idea I've seen, though there are others:

http://www.loganact.com/tips/chuck-jaw.htm

As far as my lathe being too nice to use, yes I kinda have that feeling. I think it will be much like the experience of owning a brand new car. At first, one is very careful about parking too close to other vehicles, wiping bird doo and squished bugs from the finish several times a day, etc. But eventually, as the little scratches and blemishes multiply, a more resigned and tolerant attitude settles in, where one is content to use the machine as it was intended, while still keeping up with maintenance and repairs.

Paula

This is just my thought on jaw grinding, so i don't know if it is right or wrong. I would think that you would use a large bored piece of metal to go on the jaw OD and tighten them up against it, like grabbing a piece of pipe by the ID. Then grind the jaws ID its so they don't flop around when grinding. Just my thought...Bob

Ah, you left out the tricky bits!

1) how did you get the register diameter to
be the nice, wring-on fit to your spindle?

2) how did you undo the backplate from the
spindle, after it had locked on after all the
hogging out you were doing?

I've had to resort go clamping a bar on the
face of backplates to unwind them after
cutting them. On in particular, for my 8"
adjust true chuck, had me cutting the OD down
into some previously drilled holes, causing
an interuppted cut. Really locked on tight.

Once the backplate is 'done' I like to reverse
it on the spindle (with a spacer, to account
for the register diameter) and take one final
cut on the backside, as you say, just for
appearences sake.

The first time I fitted a plain backplate to
a lathe, I actually removed the spindle and
did the job on another machine. That's the
easiest way to get the threading done, and
the register diameter bored just so.

My hat is, as usual, off to you - certainly for
the work, but also for the near-publication
quality photos and descriptions.

Jim

Hi Jim,

With regard to the register diameter of the spindle nose, I didn't go to any great pains to get a precise fit -- I simply measured the spindle nose in advance, and bored the plate a thou or so over that dimension. Now, I may get flamed or hooted down for saying this, but I don't believe that the register diameter is all that important for centering threaded accessories on the spindle -- certainly not like the register diameter between the adapter plate and the chuck.

In my years of experience with threaded spindle noses, what I've found is that the actual centering is a combination of the V-type thread pulling against a solid shoulder. As further evidence of this, I have had numerous threaded spindle attachments over the years, whose register I.D.'s were way over the spindle's register diameter, and they centered with perfect repeatability. I think it's desirable to get a close fit here, but I don't think it's as important as it's crapped up to be. Not like the short taper on a D-type spindle.

I believe that South Bend produced their 1-1/2"-8 spindle noses with a 1.509" nominal register diameter -- and mine measures between 1.5085-1.5090 -- but, for example, the 4-jaw chuck (made for SB by Skinner) that came with the lathe has a register I.D. of 1.523", and it seems to center with excellent repeatability. I think if the register I.D. is too close a fit on the spindle, then it may give rise to a situation where you're over-defining the center -- such that the register, the threads, and the shoulder are working at cross purposes.

Of course, I could be all wet, too.

With regard to your second point, I *almost* wasn't able to get the adapter plate loose from the spindle with my bare hands. I had to use a shop rag and both hands to break it loose. I might have had to resort to your clamped-on breaker bar method if it was much tighter.

Thanks for the nice comments!

Paula

Hi Paula,

I notice you are using a KRF Omni-post. What series did you use (900?) and how do you like it? I have a KRF on my Rivett and so far so good. Concerning you 're original post, great work as usual. You might consider rigging up a shop vac hose when cutting iron, it sucks up 95% of the chips right away, really cuts down on the clean up.

Hi JK,

Yes, I'm using the 900 series tool post. I used one of these for over ten years on my last lathe, and got quite comfortable with it. Excellent rigidity and repeatability. I think the Aloris type tool posts are great, but they seem a bit over-kill and obtrusive on lathes of this size. Just my opinion, since I know many users of small lathes love them.

One thing I noticed when I got my new Omni-Post is that they increased the overhang of the tool bit slot on the standard holder. In other words, it's farther from the post. Not sure if I like this yet, or not. So far I've only got one standard holder and a boring bar holder that takes 3/4" dia. bars, with a sleeve to use 1/2" dia. bars. I intend to get some additional holders.

The shop vac idea sounds good. That's one "tool" I've still yet to purchase since once again having a workshop.

Paula

I only use a shop vac during a cut when cutting iron. The little gritty swarf goes everywhere. As long as you fab a holder to get the hose withing a couple of inches of the cut the swarft just gets sucked right up. You can cut a lot of iron and not even have a spoon full of that dust on you're machine.

One great thing about the KRF holders is they are dead simple to make, so if the factory offerings don't fit the bill you can always hack a few out yourself. The holders KRF sells sure are nice however, great finish and nice material and heat treat, tough stuff.

Paula,

I've been lurking around this site for the past year and admiring your work- it's great. Thanks for the elaboration on the backplate machining; done this a number of times myself with good results but never really gave much thought to the overhang issue- a good point to consider. Probably due in part to machining backplates for larger machines with roller bearing spindles where the additional overhang may not be quite the concern it would on a plain bearing spindle. That said, it's something to keep in mind, regardless of machine design.

Regarding three jaw chucks and regrinding of jaws: Back in the time when I worked in real-life machine shop, we typically used a shop made device we called a "spider". Basically, a big "spring"- if you will- made from steel strap bent in such a fashion that each jaw nested in a bend in the strap. Unfortunately, my description is lacking and I don't have the computer smarts to generate a pictorial representation. Imagine a steel ring formed from strap and then placed in the chuck. As you tighten the chuck, it deforms the ring leaving three "inside" angles and three "outside" angles. Of course this forming was done on the bench but maybe this better illustrates my poor description.

A variety of these spiders were kept on hand in varying diametrical ranges (there's an inherent overlap in diametrical range due to the ability to compress the "spider" with the chuck).

We used chucks with two piece jaws and typically used soft, aluminum jaws that we'd bore- using a "spider"- to suit whatever diameter job we were dealing with. Additionally, if the job had some odd configuration that required clearance (a shoulder, for instance) this could be incorporated into the jaw configuration. You'd pick a specific key hole in the chuck and use only that hole through completion of the job. While not "perfect", it was a method that allowed a decent amount of precision with a relatively minor amount of set-up time.

Anyway, sorry for the long-winded description; simply food for thought.

Paula,

Make up a wrench with an L handle for your old 3 jaw chuck so you can open and close the chuck while it is laying flat on a bench. Makes a good rough duty vise for holding round parts while modifying the ends, such as cutting screw slots.

...never really gave much thought to the overhang issue...

Click to expand...

It's not such a big deal, I suppose -- just one of those things that bothers me. Here's a good example of what I'm trying to avoid:



Contrast that with the 5" 3-jaw that I had on my 10k:



It just seems to me, if (for example) you're facing off the end of a part that's sticking a couple of inches out of the jaws on that first lathe, you're nearly a foot away from the bearing. Just doesn't strike me as a very rigid setup.

Paula

I realize what you are trying to do to keep over hang to a minimum. But there is a potiential problem being caused, that is, You allowed the spindle thread to be exposed to the "Dirty" end of the chuck adapter. In other words, you get chips or swarps in the spindle thread, just takes one small little bity one to get in there and you will have bloody heck getting that chuck off the spindle.

Paula,

Locating and drilling the mounting holes is pretty darn simple. All you need to do is get a screw that’s threaded the same as the chucks mounting bolts, cut the head off, cut a straight slot in the bolt (like a screwdriver slot) then turn the slotted end into a point.

Screw this into one of the chucks threaded holes until 10-20 thous is left sticking up, (using the screwdriver slot you made) now mount your chuck on the backplate, a little tap to get a mark from your pointy screw, remove backplate and your first mounting hole is now marked.

Drill the hole and continue with the rest, just check screw hole alignment as you go by installing screws.

Easy

Mike

I think the common wisdom is that threaded
tooling like that needs to have a fairly loose
thread fitup, and very snug fit to the register
diameter.

By snug I tend to say, less than a thou on the
diameter, if possible. I like to do it such
that I wind up taking cuts of a half thou at a
time, and checking for fit each time when I
get close.

You avoid overdefining the fit by making the
thread a relaxed fit. A thread like that can
never provide a real accurate radial location.

So I rely on the face-to-face contact between
the backplate and the spindle register diameter
face to give axial location, and the cylinder
to cylinder fit of the backlate counterbore to
register diameter outer surface to provide
radial location.

Four jaw chucks are a bit different I would say,
extra slop on the register diameter isn't nearly
as bad, as long as the face-to-face contact is
clean. For a three jaw that won't repeat much
better than a thousanths anyway though, an extra
thou of uncertainty at the register diameter
won't be a terrible problem.

As far as the reduction in overhang issue, I
think this cannot be stressed enough. The
backplate I mentioned before for my Buck 8"
six-jaw was surplus, so it was way bigger than
what I needed.

I think I reduced the overall mass of that thing
by a factor of two or so, shortening it up as
much as possible.

Jim;
My backplate got stuck when I did mine, and no
matter how much forcing, I couldn't remove it.
Even with a strap wrench, I'm not that strong.
I came up with the Idea of; placing the 4 jaw
chuck upto the plate, then tightening the jaws
evenly onto the plate. I then placed a couple of
old phone books on the ways under the chuck,
then using the chuck key with a firm tug, it
loosened right up.
Paula;
Nice work.
Jamie

Good thinking.

It's amazing how handly a lathe chuck can be,
as a holding fixture.

Several times in the past I've used one or
the other chuck as a way to hold motorcycle
parts that can be held no other way (except
by factory fixtures) so fasteners could be
undone.

Jim

I think the common wisdom is that threaded tooling like that needs to have a fairly loose thread fitup, and very snug fit to the register diameter.

Click to expand...

There may be some wisdom in that idea, but I'm not sure how common it is. At least, among the manufacturers of threaded-mount spindle accessories. As I mentioned above, South Bend made their 1-1/2"-8 spindle noses with a 1.509" register diameter. A quick survey of the register diameters of threaded-back spindle accessories that I own provides the following result:

Cushman 3-jaw chuck 1.528"
Skinner 4-jaw chuck 1.530"
Faceplate 1.528"
Jacobs headstock chuck 1.523"
Jacobs headstock chuck 1.535"

Now, it may be true that the spindle thread "can never provide a real accurate radial location," but as long as the thread form is reasonably accurate, and the thread is relatively short (length < diameter), I believe that when it is tightened up against a right angle face, it will come back to the same position with reasonable repeatablility. This has to be what's going on, otherwise, how to explain the universal existance of "oversize" register bores?

Paula

I realize what you are trying to do to keep overhang to a minimum. But there is a potiential problem being caused, that is, you allowed the spindle thread to be exposed to the "Dirty" end of the chuck adapter. In other words, you get chips or swarps in the spindle thread, just takes one small little bity one to get in there and you will have bloody heck getting that chuck off the spindle.

Click to expand...

I have to admit, Ken, that never occurred to me. But in all the years that I used that 10k pictured above, with the low-overhang 3-jaw chuck, I never once experienced the problem you mention. Maybe I was just lucky. Now, I have experienced the situation where an unnoticed chip was present in the threads before screwing the chuck onto the spindle, and that can cause instant wedging and lock-up. It tended to make me more careful about cleaning out the threads beforehand.

Paula

I think oversized register bore diameters
on toolin are simply a fact of life, based on
the idea that the tooling may well be used
on different machines, or it may be sold for
a machine that has an unspecified outer diameter
for the spindle.

If the backplate bore that a manufacturer sells
is three thou under, that's a piece that's going
to be returned. If it's three thou over, it
won't come back.

When fitting backplates I strive for a pretty
snug fit-up. Maybe it improves precision, maybe
not. One things for sure, if that chuck gets
switched to somebody else's machine, it may not
fit, even for teh same nominal spindle size.

But I don't fit tooling like that for universal
interchange. I fit it for maximum accuracy, on
one machine.

JIm