A Serline Chuck

The Problem

Back of old Sherline chuck

I picked up an old Sherline 4-jaw independent chuck on eBay, intending to use it on my Taig lathe. The Taig lathe spindle has 3/4-16 threads, and so do Sherline chucks, but there is a problem. This old Sherline chuck is threaded all the way to back face of the chuck, as you can see in this photo, while The Taig spindle nose has threads that stop a little over 1/8 inch from the reference face. Apparently, Sherline now makes chucks that are manufactured to fit the Taig spindle, but this is not one of those.

You can buy 5/32 inch thick precision flat washers to fit the Taig spindle nose. With such a washer, the Sherline chuck screws on and registers against the washer which registers against the reference face of the spindle. This works, but when I compared my Taig chuck with the Sherline chick, I saw that the total thickness of the back of the chuck is the same. The Taig chuck has fewer threads because the back 5/32 of the threaded hole is bored out to to fit the unthreaded portion of the spindle nose.

I carefully measured the length of the Taig spindle nose to verify that it would not interfere with the chuck mechanism if I screwed it in all the way, and then decided to take on the job of boring the back of the Sherline chuck to match the way Taig chucks are bored.

Mounting the chuck backward on the spindle

Truing the mandrel

I have a slitting saw mandrel that I made to fit Harbor Freight's little and very cheap slitting saws. It fits the Taig's 15° spindle taper, held in place by a drawbar. I mounted it, and then set up my dial test indicator to check the runout. There were a few mils of runout so I took a skim cut on the fat part of the mandrel and checked the runout again.

The photo of my mandrel also shows the shoulder on the Taig spindle. The reference face of the spindle is the flat rightward-facing surface of the hexagonal portion of the spindle. The threads on the spindle nose stop well before they reach the reference surface.

With the mandrel trued up, I checked that the face of the Sherline chuck jaws was no longer than the trued up surface of the mandrel. Fortunately, they were very close to the same length, so I mounted the chuck on the mandrel with its back open to the world.

Truing the chuck

It's just a matter of tightening screws to clamp the chuck to the mandrel, but the key job here is to center the chuck on the mandrel. That had to be done carefully, first centering by eyeball, and then centering using a dial test indicator.

Because the chuck jaws are clamping a fixed mandrel and you are moving the chuck body, all the screw motions are backward, but with work, I got it dialed in to under a mil of runout. The runout at the very back, by the reference face, is what matters most, since this is the point concentric with the very short bore we want to make. That's why the photo shows the indicator finger running so far from the chuck jaws.

Boring

Setup for boring

The photo to the right shows the setup for boring the recess. I set the tip of my boring bar against the reference face of the chuck, then used a 5/32 allen wrench as a spacer while I adjusted the carriage depth stop. The red arrow in the photo shows the allen wrench pinched between the depth stop and the lathe carriage.

The boring bar is adjusted very close to parallel to the the lathe axis — the cutting edge of my bar is relieved behind the tip, so setting the bar this way bores to an almost flat face.

With the allen wrench removed, I adjusted the depth of cut so the boring bar would take a shallow skim cut off of the threads in the bore of the chuck. I repeated taking off skim cuts until my caliper showed that I'd opened the bore to just over 3/4 of an inch.

Setup for chamfering

The next step was to chamfer both ends of the new bore. I resharpened the front edge of the boring bar first, because I wanted to absolutely minimise any distortion to the threads. I adjusted the boring front edge by eye to about 60 degrees, so the chamfer would be at the same angle as the threads, and then redid the depth stop adjustment, setting the outside corner of the boring bar on the edge of the mouth of the bore while I set the depth of cut with an allen wrench.

After I chamfered the end of the threads, I also chamfered the opening of the bore. My chamfer there was minimal, about half the depth of the chamfer on that point on my Taig chuck. The photo here was taken after finishing the two chamfering cuts and cleaning out the swarf.

After removing the chuck from the mandrel, I tried to screw the chuck onto the spindle and failed. After re-cleaning the bore and using a scraper to try to remove any burrs from the cut ends of the threads, it still didn't screw on. The solution was to chase the threads using a borrowed 3/4-16 tap.

The Result

Testing the runout

Once the threads were chased, the chuck screwed onto the spindle with no discernable resistance. I mounted the dial test indicator again and found that I had about 1 mil of runout on the back of the chuck, the only uninterrupted surface where runout measurement is easy.

Chucking a piece of drill rod in the jaws, however, I found that the rod was not held on axis. Zeroing the runout at the chuck end of the rod left the far end wobbling visibly when I turned on the lathe. There are three possibilities:

• The rod is bent. This is possible but seems unlikely.
• The chuck jaws are not good. This is a second-hand chuck, who knows what has happened to it over the years.
• The back of the chuck is not resting flat against the reference face of the Taig spindle.

All of the photos here that show the back of the chuck show significant rust. I cleaned the back of the chuck very carefully, wiping it down with a solution of white vinegar and salt to remove the rust, then wiping it with clean water to remove the acid and salt before using an oil wax mixture to seal it against further rust. All of this was done very carefully to keep water, vinegar and salt from getting anywhere but on the reference surface at the back of the chuck.

I also took the opportunity to remove, clean and replace all of the jaws and jaw adjusting screws. I did this one jaw at a time so as not to interchange the jaws from their original slots. There was significant gunk to clean out of the slots. I found that two jaws, even after re-cleaning, refused to slide smoothly in their slots, but each slid smoothly in the other slot. I opted to exchange those two jaws, and then I oiled the screws and jaws. It was at this point that I first noticed some of the damage to the jaws and chuck; see the next section for more on this.

With the jaws cleaned and back in place, I mounted the chuck and studied it with my dial test indicator. What I found was reassuring. The face of the chuck indicates flat to within a mil all around its perimeter. The work-facing flats on the jaws are not quite as good, but they are within a 2-mil range. These measurements leave me content that my boring on the back of the chuck was successful.

However, when I clamped my drill rod in the chuck and dialed it in to under a mil of runout at the chuck, it still wobbled considerably at the far end. I repeated this experiment with three different drill rods, all with the same result.

Conclusion: My chuck jaws are not ground to hold work parallel to the lathe axis. I need to skim off the inside faces of the jaws with a grinder to fix this.

Jaw numbering and assessment

Jaw and slot numbering

After the jaws are ground, they will all be a little different and will only run true when in their own slots. (And they will only run really true when set to the same radius as that at which they were ground, but that's another matter.) Because of that, I took the time to mark each jaw with its jaw number and to stamp matching numbers by the adjusting screw for each slot.

The chuck body is not hardened, so it was a simple matter of using number stamps and a small hammer to number the slots. The jaws, on the other hand, are hardened, so I used a diamond point in a dremel tool to mark the sides of the jaws.

Jaw and slot damage

With the jaws out of the chuck, it became obvious that the chuck had been involved in a serious crash. The lathe must have been spinning very fast when something (probably a tool in the toolpost) got tangled with the jaws. It seems to have taken a divot out of the middle step of Jaw 4, bounced away by the force of that impact before taking a chip out of Jaw 2 and finally digging in hard on Jaw 1. The final impact took a fairly large chip from the leading edge of that jaw.

That is not all, there's a big flake taken out of the shoulder of slot 1 in the chuck. It's impossible to say if this flake came from the same crash or a different crash, but something hit the jaw in slot 1 hard enough that the far side of the jaw cracked the steel chuck body and sent a steel flake flying.

Looking back at the photos distributed with the original e-bay listing, both the damage to the jaws and the flake taken out of the chuck body are visible, if you know what you are looking at, but not at all obvious. I certainly didn't notice when I put in my bid. With the jaws in the chuck as they were in the listing photo, they just looked a bit worn, an the flake could have been a shadow.

If the jaw and chuck body damage were caused by the same accident, given the point of impact on the jaw, the chuck must have been holding something very small. That, in turn, would justify spinning it at the kind of high RPM that could lead to such severe impact damage.

The big question, on finding this damage, is simple. Is the chuck still usable? I mounted it back on the lathe spindle, minus jaws, mounted my dial test indicator on the toolpost, and carefully adjusted things so that the chuck jaw slot was exactly horizontal, with the finger of the DTI resting on the jaw guide within the slot.

Driving the cross slide in and out, I measured each of the 8 jaw guides and found that none of them were distorted. That suggests that the chuck is still usable. I should probably try to do the same on the 8 inside surfaces of the jaw guides, but my DTI finger is not long enough for that.

Jaw faces before grinding

Looking closely at the working faces of the jaws, the jaws I'd numbered 1, 3 and 4 looked factory fresh, but jaw number 2 looks like someone ran a grinder in on a slight diagonal. That grinding would certainly make the lathe tend not to hold work very straight, but why would someone do that to a lathe jaw, and why jaw 2, when it was Jaw 1 that seems to have had the most serious crash.

Recall that when I started out, I found that two jaws didn't slide well in their slots and switched them. I'm pretty sure that I switched the jaws now numbered 1 and 2. That means that the jaw now numbered 2 was in the slot now numbered 1 when I got the chuck. Both those jaws are chipped, but but the damage to jaw 1 seems greater, making me guess that I restored it to the slot it was in at the time of the crash.

Did someone, after the accident, switch the jaws (perhaps unintentionally) and then try to repair some damage by sticking a grinder down the throat of the chuck? It's hard to say, but it's also apparent that whoever did that didn't do a very good job. I'll see if I can do better.