Posts Tagged Mini-lathe
After dismantling the tailstock to apply the tweaks, it was grossly out of alignment, as seen from the top:
Seen from the side, the tailstock center is way too high:
No surprises there.
The object of the game is to make the tailstock bore collinear with the spindle bore in all four degrees of freedom:
- Yaw angle
- Pitch angle
The first step is to match those two points, then measure the angular error.
Loosen the (new!) screws holding the tailstock top & bottom castings together:
I set them snug enough to prevent casual motion and loose enough to allow adjustment with gentle taps from a plastic hammer. Tapping the top casting forward lined up the dead centers horizontally, leaving only the vertical alignment.
Then I clamped the tailstock’s bottom casting to the lathe bed:
Loosening the screws a bit more let me tilt the top casting to the left and slide a brass shim between the two castings, adding just a little more height to the left side to move the tailstock center downward.
This could do any or all of:
- Correct a pre-existing pitch angle so everything is fine again
- Pitch the tailstock ram axis out of line with respect to the spindle axis
- Confuse the issue
I started with a 6 mil = 0.15 mm shim that didn’t quite do enough and a 16 mil = 0.4 mm shim was a bit too much. Pinching a brass shimstock snippet between the centers show how they match front-back and don’t match up-down, with the tailstock center now too low:
Some back-and-forth fiddling showed a 10 mil = 0.25 mm sheet came out about right:
With the two linear degrees of freedom accounted for, measure the yaw angle by comparing the position of the tailstock ram’s far end:
With its near end:
Note: measure the offset by sliding the tailstock along the ways, not by retracting the ram. Reassuringly, the ram slides out parallel to its axis.
Measure the pitch angle, similarly:
As it turns out, the far end of the ram is 5 mils down and front from its base near the tailstock. Over 1.5 inches of travel, 5 mils works out to 0.19°.
Although it’s a small angle, the huge Jacob chuck supplied with the lathe puts a typical drill 125 mm from where you see the tailstock dead center’s tip. In round numbers, the drill point will be 16 mils low-and-front, about 25 mils radially off-center, which agrees reasonably well with what I actually see:
Because I don’t do much turning between centers, I retinkered the alignment to put a point held in the drill chuck on center. Deep hole drilling won’t work quite right, because the ram extends along those 0.19° angles, but it’s Good Enough for now. It’ll be much easier to correct the yaw misalignment than the height mismatch.
Those of you who read image metadata surely noticed the pix aren’t in ascending temporal order. Verily, this was an iterative process, with pix happening all along the way.
Filling the mini-lathe’s tailstock ways with epoxy made it slide easily and lock firmly. Some upcoming projects urged me to perform The Canonical Mini-Lathe Tailstock Upgrades, as shown nearly everywhere on the Intertubes and detailed in various HSM articles.
For unknown reasons, the screw clamping the tailstock’s top and bottom castings together threads into the top casting from below:
Although it’s faintly possible you could adjust it by reaching up from below the bed, it’s easier to just drill out the threads for a clearance fit around a 5 mm SHCS:
The drill went through the tailstock so easily I think the hole had been capped with body filler, which would eliminate the need for a bottoming tap.
Then build a square nut from a slice of 7/16 inch square stock:
… and be done with it:
If the tailstock ever needs more adjustment range, I can knock those corners down a bit.
The screw clamps the castings vertically, with a second screw under the tailstock ram handwheel forcing the top and bottom not-quite dovetails together horizontally. I replaced both with 5 mm socket head cap screws:
Tightening a cup-end SHCS against the square not-a-dovetail tends to shift the upper casting; the original screw had a narrow pin end to reduce the torque. Having brass rod close at hand, this seemed easier than machining the screw:
The little tip comes from using a square-ended cutoff tool. Purists will dress the tool at a slight angle to cut off one side first. Of course, which way one dresses it depends on whether you want the remaining stub on the stock or the cut-off part. Sooo, I still have a square tool.
The tailstock has a cam lock handle clamping a square-ish plate against the bottom of the bed ways. Unfortunately, the manufacturer cut the plate with a dull shear, producing two beveled edges:
Flipping it over dramatically improved the clamping action, although I must eventually scrape the paint and grunge off the bottom of the ways. While I had it off, I turned a small aluminum bushing to replace the pair of washers:
The plate hangs lower toward the rear because the clamping bolt isn’t in the middle. The tailstock originally had a mighty spring holding it level, but the spring tended to snag against the front side of the bed, so I removed it.
The lever handle actuating the tailstock cam lock had no stops and rotated freely counterclockwise when loosened. The tailstock casting had enough meat for a 5 mm threaded brass insert in a useful location, so I drilled a suitable hole:
The vacuum hose slurped up the cast iron dust, which is a Very Good Idea.
Butter up the insert with JB Kwik epoxy, slide into the hole, wipe off most of the excess, then pause to admire the result:
The lever stopper won’t win any design awards, even though it’s a dramatic usability improvement:
The finger-tight nut serves to lock the SHCS in place against the lever’s impact. I conjured a small bumper around the head from a rubber foot intended to fit under a random box of electronics.
Pushing the lever leftward to the stop lets the tailstock slide freely and pushing it to the right clamps the tailstock to the bed. The cam’s limited rotation keeps the plate close enough to the underside of the bed to prevent it from tipping left-to-right as the tailstock slides, so it no longer snags.
While I had the tailstock up on jackstands, this is what the ram thrust bearing looks like:
The flange over on the left bears against the steel disk on the right, with no real thrust bearing to be seen. A dab of grease improved its disposition.
Now, to realign the thing …
The OEM spring now sits slightly compressed with the screw tip flush at the far end of the block:
That OEM screw head knurling leaves a bit to be desired, doesn’t it?
Actually boring the hole would be a remarkably tedious process for little gain. Instead, I lined up the block in the drill press using a ¼ inch drill (the OEM hole isn’t hard metric!) in the unthreaded section, enlarged it with progressively larger drills up to an O (0.316 inch = 8 mm), then finished with a P (0.323 in = 8.2 mm).
As it turned out, my guesstimated relaxed spring length was a bit off, so I turned a brass bushing to shorten the hole by 2 mm:
If I don’t mention it, nobody will ever know!
The original doodle, with close-enough sizes:
After the faceplant caused by the crappy compound way finishing, I decided to try repairing the tailstock ways as a means of gaining experience before tackling the real problem. The general idea is to see whether filling the gouges with epoxy will suffice.
I’m using good ol’ JB Weld steel-filled epoxy, rather than graphite / molybdenum disulfide loaded epoxy, mostly because:
- I have it on the shelf
- This is a non-sliding joint
- My technique needs polishing, too
The key point: the tailstock is (astonishingly) well aligned and, if I can manage to not change how it sits on the lathe bed, this should be a zero-impact operation. Scraping / filing / fiddling with the high spots will change the alignment; I expect I must eventually do such things; this represents a first pass at the problem.
Applying a fat blue Sharpie to the tailstock ways:
After sliding the tailstock back and forth a few times, the remaining blue shows where the ways did not make contact. Those shiny and silvery spots rubbed against the lathe bed ways.
The flat way looked like this:
The patch along the upper-left edge and the small dot near the upper-right corner are the only contact points across the entire flat.
The outside of the V groove:
As nearly as I can tell, that’s actually a reasonably flat and well-aligned surface, with small contact points scattered all over. Granted, there’s a larger contact patch to the left and less to the right.
The inside of the V groove:
There’s a single point near the top left, another over on the right, and that’s about it.
I cleaned the tailstock ways with acetone to get rid of the Sharpie / grease / oil / whatever. Under normal circumstances you’d roughen the surface to give the epoxy something to grip, which definitely seemed akin to perfuming a lily.
To prevent permanently affixing the tailstock to the lathe, some folks put a generous layer of oil / graphite / soot / release agent on the lathe bed ways. I used some 3 mil = 0.08 mm Kapton tape, figuring an impervious layer would pretty much guarantee I could get the tailstock off again, no matter what.
So, we begin.
Butter up the tailstock ways with epoxy and smoosh into place atop the Kapton:
Make sure the tailstock remains well-seated where it should be:
Do other things for 24 hours while the epoxy cures, pry the tailstock loose by hammering The Giant Prying Screwdriver between the lathe bed and the underside of the tailstock (just right of the V-groove, where nothing slides on the bed, but I did use a bit of plastic as a shield), chip off excess epoxy, clean things up, etc, etc.
This time, I applied Sharpie to the lathe bed, then slid the tailstock back & forth a few times. As a result, the blue areas now show the contact patches and the gray areas just slid by without touching.
The flat way looks pretty good:
That round dot over on the right seems to be a steel protrusion; I think it’s part of the same lump appearing in the “before” picture above. That rather sharp point seems to have indented the tape and produced a low area in the epoxy around it, which may not matter much: it was the only contact point before I did this.
The V groove isn’t anywhere near perfect:
On the upside, the ways have much, much larger contact patches spread across nearly their entire lengths, which isn’t to be sniffed at.
While reassembling the tailstock, I added a pair of M6 washers above the clamp plate so it cleared the bed with the screw tightened into the cam-lock post:
Which definitely calls for a small bushing, of course. If you put a lockwasher under the screw head, it won’t clear the end of the bed casting. So it goes.
Another washer under the ram lock screw changed the phase enough to keep the knob out of the way in both the fully locked and unlocked positions:
I slobbered some Mobil Vactra #2 Sticky Way Oil (thanks, Eks!) on the bed ways, snuggled the tailstock in place, and wow does that thing move! Verily, it slides smoothly and clamps solidly in place: a tremendous improvement over the status quo ante.
- The tape (perhaps the adhesive layer) produces a slightly textured epoxy surface
- The tailstock way’s small contact points indented the tape, even though it’s only 3 mil thick
- Filling the low areas in the way works well
- The high areas may not have enough epoxy for good durability
- I expect the epoxy will wear faster than steel, so contact should improve with time
- This is not a permanent fix
What I’ll do differently next time…
- Apply more epoxy to avoid those small gaps along the edges
- Use a real release agent: smoothed in place, it might provide a better finish. Might not matter
- Verify a good prying spot before epoxying, say, the compound
All in all, though, this worked much better than I expected!
The plug had a rather large cable entry that cried out for a touch of brass:
Fancy plugs have a helical spring strain relief insert about the size & shape of that brass snout; might have to buy me some fancy plugs.
This time, I got the alignment right by clamping everything in the lathe while the epoxy cured:
I flipped the drill end-for-end, which was surely unnecessary.
It’s now sitting on the kitchen table, providing a bit of light during supper while I wait for a WS2812 controller failure. Again.
A length of aluminum hex bar became a nice 10-32 screw trimmer:
The hex neatly fits a 5/8 inch wrench, so I can tighten the jam nuts enough to run the lathe forward, part off the screw, and clean up the end just fine.
Unfortunately, the second test cut didn’t work nearly so well:
With the cross-slide gib adjusted to the snug side of easy, the cut put enough pressure on the parting tool to lift the way on the tailstock side about 4 mil = 0.1 mm. The parting tool submarined under the cut, dislodged the fixture, and didn’t quite stall the motor while the chuck jaws ate into the aluminum.
Well, that was a learning experience.
After tightening the cross-slide gib to the far side of hard-to-turn:
- Put a longer screw in the fixture
- Grab it in the tailstock drill chuck
- Crunch the hex end of the fixture in the spindle chuck
- Remove the screw through the spindle (*)
- Put a slight taper on the end of the fixture threads with a center drill
- Deploy the live center to support the fixture
Turns out that angling the bit by 10° dramatically reduces chatter. If I had BR and BL turning tools, I’d be using them with the QCTP set to 0°, but they weren’t included in the set that came with the lathe.
It’s a good thing I’m not fussy about the diameter of that cylindrical section:
I knew the craptastic lathe ways needed, mmmm, improvement and it’s about time to do something.
(*) By concatenating all my ¼ inch socket extension bars into an absurd noodle capped with square-to-hex adapter holding a Philips bit.
By and large, when you follow the recipe, you get the expected result:
That’s another length of the same aluminum rod, this time with a full-length M3x0.5 thread down the middle, and a screw with a neatly trimmed end.
Running the lathe spindle in reverse prevents the screw from loosening the jam nuts on the left:
Running the spindle forward does move the screw enough to loosen the nuts. Perhaps I should put wrench flats on the big end of the fixture so I can really torque the nuts.
That front nut was mostly decorative, rather than tight, because I didn’t expect the first attempt to work nearly as well as it did. A bit of filing to taper the end of the thread and it was all good.
That was easy…