My Longworth Version 2

[joe greiner]

Here's Version 2 of my Longworth chuck variation.

I mounted a disk of cabinet grade plywood in the Version 1 chuck - seemed the easiest way to secure it. I turned an annular slot to receive a hex nut matching my headstock spindle. I left the core at full thickness to receive a pivot screw to anchor the front disk. I glued the hex nut into the recess with J-B Weld, and machined the overspill later. This was my second attempt. The out-of-date (learned later) epoxy I used on the first try may never cure. I suppose I should post a warning thread about that.

Then I mounted the disk onto the headstock spindle. I'd earlier made a spacer of pvc pipe to transfer the mounting load to the shoulder on the spindle instead of having the end of the spindle bear on the plywood core of the disk. I marked the rings of the button excursions to establish the ends of the routed arc slots. In lieu of laying out the arc centers on the disk, I transferred those points from a print of the CAD layout. Satisfactory, but not perfect; "perfect enough" as we say. I countersunk the sandwich screws because there isn't enough room to remove only two of them for router clearance.

I considered several arc patterns, and made some cardboard mockups from my CAD layouts. The one in the lower left of the pic (with the yellow pushpin) is the conventional Longworth arrangement: center of the arc midway between the inner and outer rings. The one at the middle of the pic (with the red pushpin) also has the center at the middle of a line joining the rings, but the line spans to the ring at the far side of the hub. This pattern minimizes the crossing angle of the two arcs (zero at both excursions). Minimizing the crossing angle reduces a tendency for self-rotation; somewhat like the flatter helix on threaded fasteners. ** This pattern unfortunately leaves precious little material to maintain the integrity of the disk; also doesn't allow both holes and half-holes for hand adjustment. The third pattern (with the green pushpin) uses a radius of half the router's base plate. I had thought to just swing the router's base plate around a pivot nail (not a great idea, really). But I found that my world-famous Router Compass could be modified to place the pivot at just the right location. This was pure luck; there's no way I could have planned the compass with this extra feature in mind. Testing the cardboard mockups enlightened the choice of adjustment holes and half holes, and I think I still might have more than I need.

** Comment: The tool produced by a chap in Portland OR USA seems to deviate from this principle, and reinforces his assertion that it was developed without knowledge of Longworth. His crossing angles are quite steep; he overcomes the tendency for self-rotation by adding a locking hub and providing pliers for rim rotation. The crossing angle can be made constant by employing logarithmic spirals for the arcs. But I know of no way to generate such arcs mechanically; a template and a router bushing would likely be needed. And a constant angle over the entire range of radii may not be best anyway.

For the Version 1 chuck, see http://www.woodworkforums.ubeaut.com...ad.php?t=50923

For the Router Compass, see http://www.woodworkforums.ubeaut.com...ad.php?t=50372

For the Portland tool, see the link in rsser's post of 9th June 2007 at http://www.woodworkforums.ubeaut.com...t=29038&page=3

More follows in Part 2.

Joe

[joe greiner]

Version 2 of my Longworth chuck variation, part 2.

This chuck accommodates bowls too small for Version 1. The disks are just small enough to clear the lathe motor. It can mount rims from 45mm to 137mm in compression mode, and 94mm to 185mm in expansion mode (for undercut rims).

The buttons are attached substantially the same as in Version 1. I used talcum powder for lubrication on this one also. Again, the truncation on the washers avoids overhanging the edge of the disk. Oh, and this time I scraped both faces of both disks before routing the slots.

Upon first mounting of the back disk, I had a wee bit of runout. I massaged the pvc bushing with sandpaper, face up on a flat surface, and achieved a satisfactory minimum runout. I found, though, that the two parts had a preferred assembly; so I drilled match-mark divots in the bushing and the hex nut.

The holes and half-holes allow one-handed adjustment. The small bowl shown in the pic was made several months ago. I haven't actually used this chuck yet, but it looks good to go.

Joe

[dai sensei]

Thanks Joe, this post also links to your others, so I've booked marked it for later.

[Skew ChiDAMN!!]

Good idea epoxying the hex-nut for mounting in lieu of a faceplate.

Question time: how did you ensure that the hex nut was square and centered? Did you position the nut in the annular ring and then "pack" with the weldbond, or did you apply glue first then insert the nut? (The latter's what I've done and I've had headaches with it not quite sitting square for some reason. )

[joe greiner]

G'day, Skew. I got the recess reasonably square by turning it with a DIY Oland-type cutter in a shortened shank - usually use it just for sharpening the cutter, but I obviously didn't need the full-depth tool with the handle banging into the tailstock. The recess might have been slightly conical at the bottom, but doesn't affect squareness. Centering wasn't a great concern because I didn't scribe the work circles until I had it mounted on the headstock (with the pvc spacer and with runout corrected). Even so, I turned the recess to an almost snug fit for the nut.

I'd considered using a coupling nut instead (to eliminate the spacer), but often they're slightly smaller OD and would have had less purchase on the disk. Also, sometimes hard to find that size at retail without a bunch of chasing my tail, and when found likely in the neighbourhood of US$8 or so; plus the fuel cost of searching, as well as time. Furthermore, the only way to correct runout would have been to face the disk. Initial runout was about 1.5mm at the disk rim (about 190mm OD), and took very little sanding on the spacer to correct.

I applied the glue in stages. I first masked the exposed part of the threads with filament tape (to have any hope of removal), and also the surface of the disk beyond the recess. I smeared a thin coat on the inner face of the nut and put it in the recess with a wringing motion; I think that gave the glue a reasonably uniform thickness. Then I used a toothpick (actually, many toothpicks!) to pack the inner and outer gaps. I also clamped the assembly overnight in a book press (DIY from an old romance with bookbinding) with substantially equal gaps at all four corners.

When it came time to machine away the excess glue, I had to experiment a little to get it re-centered well enough, because I had neglected to mark its position on the larger chuck. And I merely sanded the outside of the disk because it doesn't need to be exactly round. In fact, IIRC I might not have even sanded it at that stage.

Before making these chucks, I'd been mounting bowls on a piece of plywood attached to a faceplate, with tape wrapped around the assemblage. One would think the plywood would be square; one would be wrong. I still had to face the plywood before cutting the mating groove.

Joe

[Skew ChiDAMN!!]

Thanks, Joe.

I've tried a similar process for a jam-chuck, a size I use frequently, except I pressed the nut with the tailstock while the blank was still in the chuck. It was less than successful, but I suspect that may've been partially due to the horizontal orientation.

It also made a mess of the ways.

[joe greiner]

It also made a mess of the ways.

Food wrap and old newspapers are your friends.

Joe

[wheelinround]

I have been reading with much interest in your Longworth Chuck design, having just joined the ranks of TTIT. This appealed to me as a project to use my skills with tools so I can better perform new skills..........waffle here....

So anyway I did a search and came up with the following pages
http://www.fholder.com/Woodturning/chuck.htm
which lead to
http://www.fholder.com/Woodturning/lwc-wtm.htm
and
http://www.fholder.com/Woodturning/longwrth.htm
with its link http://www.fholder.com/Woodturning/longw-2.gif and http://www.fholder.com/Woodturning/longw-3.gif
The photos are pretty good also
and a pdf file
http://www.fholder.com/Woodturning/L...th%20Chuck.pdf

Question as to supply of rubber stoppers and size you used

[hughie]

Fine looking chuck Joe. It sure is sturdy, bullet proof even .............

[joe greiner]

G'day, wheelin, and welcome aboard. Yep, tons (tonnes?) of info on the net; I never searched deep enough to find original citations; would have been on a WT club newsletter, I think.

I shopped high and low for appropriate buttons. Some sources sell replacement buttons for Cole jaws, but mighty dear. I think Skew (3 posts back) used some sort of crutch tips or chair leg tips on his IIRC; the ones I found upover here weren't suitable, though. I wound up with "screw bumpers" from local hardware stores (Ace & Home Despot for Murricans), 7/8" (22mm) diameter by 3/8" (9.5mm) high, all from the People's Republic of course. Higher buttons would be more versatile. Occasionaly I've come across laboratory supplies including cone-shaped rubber "corks" with central holes at one of my surplus sources, but didn't need them then and not available there now. Only downside to them is they're black and might stain the work. Or might not.

Joe

[joe greiner]

Thanks, Hughie. Quite sturdy indeed. Unfortunately, the tiny bowl I used for a test wasn't. Paper thin walls are no match for only four buttons. And my shortcut of transferring the arc centers from a CAD printout was perhaps a bit too clever. I just made Version 3 with 6 buttons; new thread with text and pics soon.

That said, even with slight eccentricity, this 4-button chuck could be useful for turning rosettes in a square blank, for door and window interior architraves for example.

Joe

[hughie]

. I just made Version 3 with 6 buttons; new thread with text and pics soon.

Hi Joe,

I am in the process of marking up one with 8 jaws out of 3/8" poly-carbonate/ Plexiglass around 24" dia. Version 2 for me, the only thing that concerns me is the likely-hood over time of cracking around the screw holes onto the face plate.

Like wise pics to follow when completed.

[scooter]

the only thing that concerns me is the likely-hood over time of cracking around the screw holes

I can see the benefits of using plastic for the discs, and they look speccy too, but can't help worrying at the thought of a stress crack in one of them travelling, & letting go in a big way.

This is where ply would be better in theory I reckon. Some sort of reinforced plastic would be good.

Having said that, I'm way short on experience so who knows.


Cheers..................Sean

[joe greiner]

Hi Joe,

I am in the process of marking up one with 8 jaws out of 3/8" poly-carbonate/ Plexiglass around 24" dia. Version 2 for me, the only thing that concerns me is the likely-hood over time of cracking around the screw holes onto the face plate.

Like wise pics to follow when completed.

On my Version 3, as you'll see in a day or so, the arcs are very crowded at the minimum diameter. I found it advisable to locate the arc centres only 1/8" from the rim to provide sufficient material for integrity of the disks. This is a deviation from Longworth's guidance. With 8 buttons, you may find the minimum bowl diameter is governed by such crowding. This could also permit use of a large dedicated face plate, with additional screws, in added new holes, to enhance purchase; new hole locations wouldn't need to be overly regular. You also might consider using machine screws in threaded holes in the back disk for stronger gripping. Ordinary tap drills provide about 65% thread engagement - satisfactory for similar bolt and hole materials; but for softer hole materials (plastic, aluminium, etc.), I get better engagement (100%) by using a tap drill equal to the bolt's root diameter. Needs a gentler touch for tapping.

Joe

[joe greiner]

Version 3 of my Longworth chuck variation is at

http://www.woodworkforums.ubeaut.com...ad.php?t=52167

In a previous post, I mentioned using the 4-button chuck for cutting rosettes. Here's a Q&D example; no particular pattern, just made up as I turned it. For production, a template would be advisable for duplication. At about 3 1/4 inches diameter, this exceeds Grizzly's largest of 3 inches (YMMV). And it doesn't utilize the largest blank my chuck can accommodate, so even larger rosettes could be made. An 8-button chuck would also work for this task. For a 6-button chuck, a rectangular blank would be needed, to be trimmed to square after completion.

Joe