Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Every now and again, an upshift to the large chainring on my Tour Easy would go awry and drop the chain off the outside, where it would sometimes jam between the pedal crank and the spider. In the worst case the flailing chain would also jam in the TerraCycle idler, but I fixed that a while ago.
Contemporary chainrings (i.e., anything made since the trailing decades of the last millennium) generally have a chain drop pin positioned against the crank specifically to prevent such chain jamming.
Making a chain drop pin is no big deal if you’ve got a lathe and an M4 tap:
Tour Easy – DIY Chain Drop pin
A closer look:
Tour Easy – DIY Chain Drop pin – detail
That’s a 10 mm length of 5/16 inch brass rod drilled with a recess to fit the head of a 10 mm M4 socket-head cap screw.
The pin should be a micro-smidgen shorter, as it just touches the crank, but, if anything, moving the chainring inward by one micro-smidgen improved the upshifts and I’m inclined to go with the flow.
The kludge required to trim the coaster rims disturbed the silt enough to reveal a long-lost 5 inch 4 jaw chuck that fit neither the old South Bend lathe nor the new mini-lathe. In any event, the chuck does have an adapter plate on its backside, it’s just not the correct adapter plate for the spindle on my mini-lathe.
Making it fit required enlarging an existing recess to fit the spindle plate, a straightforward lathe job with the plate grabbed in the 3 jaw chuck’s outer jaws:
5 inch 4 jaw chuck – boring spindle recess
Carbide inserts don’t handle interrupted cuts very well, but sissy cuts saved the day. The plate is kinda-sorta cast iron, so the “chips” are dust and a vacuum snout reduces the mess; you can see some chips inside the bore.
A faceplate for the mini-lathe lathe located three holes matching the spindle plate, after I noticed the amazing coincidence of both parts having 26 mm bores. Making an alignment tool from a scrap of 3/4 inch (!) Schedule 40 PVC pipe was an easy lathe job:
5 inch 4 jaw chuck – adapter plate alignment
Transfer-punching those holes produced pips on the chuck side of the adapter plate:
5 inch 4 jaw chuck – spindle bolt spotting
I thought about freehanding the holes, but came to my senses:
5 inch 4 jaw chuck – adapter plate drilling
Of course, the Sherline lacks enough throat for the plate, so each hole required clamping / locating / center-drilling / drilling / finish drilling. With all three drilled, hand-tapping the threads was no big deal:
5 inch 4 jaw chuck – rebuIlt adapter plate
Those are M8×1.25 studs from LMS (although the ones I got look like the 30 mm version), with the long end sunk in the adapter plate to put the other end flush with the nut on the far side of the spindle plate:
5 inch 4 jaw chuck – installed – spindle nuts
And then it fits just like it grew there, although the jaws don’t have much clearance inside the interlock cover:
5 inch 4 jaw chuck – installed – front view
Now I’m ready for the next set of coasters and, if the jaws stick out too far, I can gimmick the interlock switch for the occasion.
The description of the 4 inch chuck seems inconsistent with its listed dimensions, which may be why I ended up with the larger chuck in the first place. You can never have enough chucks: all’s well that ends well.
Lacking a 4-jaw chuck for the lathe, this should suffice:
Coaster Epoxy Rim – chuck-in-chuck setup
Which is just the Sherline 4-jaw chuck chucked in the lathe’s 3-jaw chuck, with both chuck Jaw 1 positions lined up and marked on the acrylic disk fixture. The picture is a recreation set up after the fact, because I lack a good picture of the overall scene.
Now it’s easy enough to center the fixture, stick the coaster in place with reasonable accuracy, then tweak the Sherline chuck to center the coaster:
Coaster Epoxy Rim – turning setup
Because the bottom layer is a laser-cut disk, eyeballometrically aligning its edge to a simple pointer worked surprisingly well:
Coaster Epoxy Rim – locating mirror edge
Turning the OD down to match the bottom disk meant I could finally get decent results with zero drama:
Coaster Epoxy Rim – turned samples
From the bottom, this one has a 3 mm mirror, the 3 mm fluorescent green frame + petals, and a 1.6 mm top sheet:
If I never tell anybody, they’ll think the slightly granular look of the tape was deliberate; it looks OK to me.
And, for completeness, the crash test dummy from the start of this adventure:
Coaster Epoxy Rim – turned 6 petal black
I don’t know how to avoid the bubbles, as the usual torch-the-top and pull-a-vacuum techniques pop bubbles at the epoxy-air interface. These bubbles are trapped under the top acrylic sheet, even though I was rather painstaking about easing the layer down from one side to the other while chasing bubbles along.
Maybe I can define bubbles as Part of the Art?
Definitely fancier than chipboard, although not nearly as absorbent.
The ID is bigger and the OD is smaller than the fixture, so it won’t get in the way of further proceedings:
Coaster Epoxy Rim – 3-jaw lathe setup
The pad on the live center came from the cookie cut from the fixture, with a just slightly off-center 3 mm hole poked into it to hold the point away from the coaster.
A ring of carpet tape on the fixture provides traction holding the coaster in place:
Coaster Epoxy Rim – carpet tape
That turned out to be more trouble than it was worth; scissoring a pair of strips to fit the OD works just fine.
In any event, the live center applies enough pressure to keep the adhesive happy.
The fixture disk is sacrificial, so it now has a notch around its front face where the cutter cleared the coaster.
Although I intended to shim the fixture against the chuck jaws to center the coaster, my first attempt at manually centering the thing on the fixture was Close Enough™ that I just turned the OD to see how well the whole process worked:
Coaster Epoxy Rim – turned 6 petal black
The edge finish is arguably not Good Enough™, but it looks much better in person. In particular, the difference between the transparent acrylic top layer and the black acrylic frame around the petals is much more prominent in the photo, perhaps due to scatter from the overhead desk light.
This was the original crash test dummy acrylic coaster, so more care will be in order for the next set. In particular, shimming the fixture requires removing and replacing it for each adjustment, which can easily become a non-converging process.
Because the Sherline mill can’t cut all the way around a 4 inch OD coaster clamped to its table, I set up the 4-jaw chuck on the rotary table and centered the nicely round fixture:
Coaster Epoxy Rim – centering fixture plate
Admittedly, the centering need not be so precise, but practice makes perfect.
A few sissy cuts demonstrated the tape lacked sufficient stickiness to hold the coaster in place against the milling cutter’s uplift. I managed to mill most of the perimeter with those clamps in place, moving each one from just ahead of the cutter to just behind the cutter.
That way lies both madness and organic damage.
There are better tapes and better adhesives, all trading off a really sticky fixture against difficulty extracting an undamaged part.
A more complex circular fixture with built-in mechanical edge clamps extending around a major part of the perimeter seems like entirely too much of a diversion for a couple of obscene-gerund coasters.
The gotcha: epoxy oozes from between the layers to form a slobbery edge.
I tried introducing a similar coaster to Mr Disk Sander with reasonable results:
Coaster Epoxy Rim – disk sanded rim
The coaster on the bottom has its original generous epoxy slobber around the acrylic disks.
Assembling the layers inside a mold seems fraught with messiness, particularly if I eventually want to get it out of the mold.
Using a finer abrasive disk would certainly help, but the whole process requires intense concentration and is utterly unforgiving of mistakes.
I figured I could attach the coaster to a lathe fixture and turn the rim, so I made a fixture from scrap acrylic:
Coaster Epoxy Rim – cutting fixture plate
The lathe chuck inside jaws fit inside the hole and I set up to turn the OD to a nice even diameter:
Coaster Epoxy Rim – turning fixture rim
The fixture sat flush against the middle step of the jaws with plenty of clearance from the outer step, so I could turn the OD without whacking the carbide insert.
I planned to grab the OD and turn the ID to a (reasonably) concentric finish, but the outer jaws have an absolute diameter limit a few millimeters less than the 4 inch = 101.4 mm coaster OD.
After some increasingly desperate attempts, I concluded that, lacking a 4-jaw lathe chuck, there was no way to mount the coaster on the fixture and have it sit it even approximately centered on the spindle axis.
When you (well, I) get fussy about angular alignment on the laser cutter’s honeycomb platform, an adjustable stop or two may come in handy:
Laser Honeycomb – Adjustable Pins
That’s a serving suggestion based on a true story, because I really wasn’t all that fussy about precise engraving alignment on those signs.
A more typical situation on a smaller scale:
Laser Honeycomb – Adjustable Pins – engraving
The scrap of MDF with three holes provides angular alignment for the little two-color acrylic test coupon, so you can tuck successive squares into the corner, hammer them with slightly different patterns, then compare the results.
The stops are an off-center hole (the ±3 text gives the offset) in an MDF disk with an acetal post:
Laser Honeycomb – Adjustable Pins – detail
The 3 mm SHCS provides a convenient way to turn the post and disk, so the threading isn’t critical. Sufficiently snug threading will let you turn the screw counterclockwise without loosening it, but that surely depends on how tightly the 8 mm section fits into the honeycomb. The larger top section is 9mm, cleaned up from the rod’s nominal 3/8 inch OD, for a jam fit into the 8.8 mm + 0.1 mm kerf hole.
The Smell of Molten Projects in the Morning 
