The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

Thing-O-Matic / MK5 Extruder: Resistor Autopsy

Cooked thermal compound

Having built cartridge heater mounting blocks, I autopsied the two aluminum-case power resistors I’d been using on the MK5 Thermal Core. They weren’t dead yet, but I have some spares in case the cartridge heaters don’t work out as expected.

First observation: the blue-tinted thermal compound I’d put under the resistors turned white! It has a 200 °C maximum rating, so it’s been cooked well beyond any reasonable limit. On the other paw, it was still soft and didn’t have any air bubbles; the resistors were pretty firmly glued in place.

Based on those thermal measurements, I had replaced the original parallel-connected 5 Ω resistors with series-connected 2 Ω resistors, thus reducing the power dissipation in each resistor from 28.8 W to 18 W. While that’s still far beyond the specification, every little bit of reduction helps.

In round numbers, the resistors ran at 50-75% duty cycle to maintain Thermal Core temperatures in the 200-230 °C range. I guesstimate I had 10-15 power-on hours on the resistors, but that may be a lowball estimate: time passes quickly when you’re having fun.

Anyhow, I slipped a brass tube around one resistor terminal, braced the other end on the drill press vise, and pressed the cores out.

Resistor elements

The top core literally fell out without any urging, which means that it had shrunk and separated from the housing. That means the resistor was well on its way to failing: a loose core gets hotter and deteriorates faster.

The bottom core was still firmly attached and disintegrated as I forced it out, which means it was in good condition. Paradoxically, the crumbled resistor core in the picture came from the resistor in the best shape.

Given that I ran these resistors at 63% of the original power level, the fact that one was well on its way to heat death after only (at most) a few tens of hours suggests that you shouldn’t expect much life from the stock MK5 resistors. If you haven’t already done so, electrically isolate the thermocouple bead from the Thermal Core to protect the Extruder Controller.

I’m unwilling to sacrifice a new resistor to see if that discoloration is normal, but I suspect it’s not. The ends should be the coolest part of the resistor, which means the middle is discolored, but that picture suggests the opposite, so I really don’t know.

I’d hoped the ID of the resistor bodies would match the OD of the cartridge heaters. That didn’t work out: 0.275 vs 0.250. They’re also a bit too short. If the match was closer, I could see slipping a shim in there, but having two air gaps around the heater just doesn’t make any sense at all.

2011-02-24
Cartridge Heaters: Mounting Blocks
Drilling SHCS head clearance

MBI sent me a selection of 1/4-inch cartridge heaters to evaluate, seeing as how I’ve been such a pest on the subject of those poor aluminum-case power resistor heaters. Thanks, Zach!

I initially thought I could punch the cores out of the resistors and slip the cartridge heaters into the holes, but it turns out the resistor bodies aren’t quite the right size: slightly too short with slightly too large holes. So it goes. Some earlier thoughts live there.

This is a first pass at building mounting blocks to attach cartridge heaters to a stock MK5 Thermal Core. Ideally, you want a solid Thermal Core with a hole or two for the heaters next to the filament extrusion nozzle, but that requires fancier machining that I’m ready for right now. The fabled nophead shows how that looks for a ceramic power resistor.

The obvious question is whether you want a single high-wattage cartridge heater or a pair of low(er)-wattage units. I think a core-with-hole can get away with a single heater, which is also the lower-cost option. My thermal measurements suggest the Core is pretty much isothermal, so there’s no problem with distributing the heat evenly from one side to the other.

However, adding two lower-wattage heaters to a stock MK5 Thermal Core makes more sense, because the interface between the blocks and the Core seems to run a bit under 1 °C/W. A single 40 W heater would thus run 30-40 °C higher than the Core: call it 260 °C. IMO, that’s much too high for something an inch away from a plywood frame and an acrylic support structure.

A pair of 25 W heaters would run at 245 °C-ish. That’s still pretty hot, but every little bit helps. I’ll start with that arrangement and see how it works.

Block top and bottom

The blocks are ordinary steel from the Scrap Box: a convenient length of 1×1-inch bar stock that somebody else had made into something else a long time ago. I bandsawed off four 1×1-inch slabs, each about 5/8″ thick. A second bandsaw cut turned the square slabs into rectangles. I finished two blocks; the other two slabs await more experience with how these work.

I squared up the blocks with a flycutter in the Sherline, then sanded down the bottom surface a bit. The thermal tests suggest the contact is Good Enough with a reasonably flat surface, so I settled for a used-car finish: high shine and deep scratches. They’re actually smoother than the pictures would have you believe.

The Thermal Core has hard inch dimensions (minus cleanup cuts): 1 inch front-to-back and 13/16 inch tall. I generally work in metric, so the sketch at the bottom has everything in millimeters.

The mounting blocks have holes matching the resistor footprint. I drilled clearance holes for the heads of the original M2 socket head cap screws, ran an end mill down the hole to flatten the bottom, then drilled clearance holes for the threads. Those holes are perilously close to the edge, but the blocks really don’t want to be any taller. Perhaps use a less-generous clearance?

The alternative would be to mill a flange along the edge to match the resistor mounts and put the SHCS heads in free air, but that seemed like more work and it would cramp the thermal path from cartridge to block.

I also thought about chamfering the edges to make the blocks look less, well, blocky, but that’s in the nature of fine tuning.

The cartridge heaters slip-fit into a nominal 0.250 hole; the samples are 0.247 to 0.248 and (from what I read) the diameter tolerance stays on the minus side of 0.250. I don’t have a 0.250 reamer, which is how you get a precise hole ID, so I’ll go with drilled holes. Fortunately, I have a set of letter-size drills in nearly new condition:
- A drill = 0.234 to poke a hole in the block
- E drill = 0.250 to get the final diameter
The final holes worked out to be exactly 0.250 inch, to the limits of my measurement ability, which I will declare to be Good Enough. The cartridges have a loose slip fit with no side-to-side play.

The cartridges expand when heated and squeeze against the hole to make good thermal contact. While cool, however, they can slide out without much urging, so I added a 4-40 setscrew. It’s on the butt end of the cartridge heater shell, away from the leads, so if a cartridge becomes one with the block I can drive it out with a pin punch. Putting the setscrew at the end with the wire leads makes more sense (it’s cooler there), but then you’d be beating the entire length of the cartridge out past the setscrew hole.

The setscrew and the M2 SHCSs get a liberal dose of anti-seize grease before assembly.

Here’s what the holders looked like, just before bolting them in place:

Cartridge heaters in blocks

Doodles with the more-or-less as-built dimensions:

Heater block dimensions
2011-02-23
Measuring Tape Crank Handle Repair

My Shop Assistant (who now merits a Proper Name) returned a fairly new measuring tape to the Basement Laboratory, reporting that the retracting crank handle fell off in “normal use”.

Stripped handle threads

Admittedly, this was a surplus find, but you’d think the build quality would be a bit higher. I’m sure I paid a minute fraction of list price: you could have bought it for much more in a reputable store.

Maybe this is why the whole lot got scrapped out:

Handle detail

I applied a bit of JB Industro Weld to the plastic (?) threads on the spool, twisted the handle in place, squared it up, then eased more epoxy around the top of the threads and let it cure flat on the bench.

Remounted handle

I’d say the original design wasn’t particularly good and the implementation left a lot to be desired. If the interior fittings have similar flaws, I’ll eventually regret applying JB Weld in such a cavalier manner…

2011-02-22
Homebrew Shell Drills

One of my Shop Assistant’s friends asked for help with a Science Project: building a trumpet-like musical instrument from some sort of tubing. We adjourned the meeting to the local Big Box home supply store, measured various options, and returned with a stock of CPVC pipe and fittings.

Given the budget and physical size of the valves, plus the fact that she planned to make tuning stubs from vinyl tubing, I suggested making all the connectors from fishmouthed sections of the CPVC pipe, which called for a bit of Quality Shop Time before the next build session.

A shell drill is what you use when you want a really big hole all the way through something, so the middle just falls right out. They’re handy for drilling in fragile / delicate material, because the shell supports the material until the drill reaches the far side. They’re also dead simple to make, at least when you’re drilling soft materials, which is pretty much all I do.

I always start by rummaging through the collection to find an existing shell drill that’s close enough to the right size that I can cut it down or bore it out. Here’s the assortment, some of which are obviously victims of previous modifications:

Shell drill assortment

This one was slightly too chubby, with plenty of meat:

Original shell drill

That was easy to fix:

Shell drill with reduced OD

While I was at it, I cleaned out the ID to reduce the tooth thickness. That reduces the force applied to the workpiece, which I figured would be a Good Thing considering the size of the pipe.

Fishmouthing CVPC tubing

If you must start from scratch, all you need is a rod that fits inside a tube of some sort: the rod must be chuckable in your drill press and the tube must be about the right diameter for the hole-to-be-drilled. Turn them to suit on the lathe, then press / bash / braze / epoxy / pin them together, paying some attention to concentricity and alignment.

Cross-drill two holes near the business end of the tube, with diameters 1/4 to 1/2 of the tube diameter. Cut off the end to remove about 1/4 of the drilled holes. File some relief on the web between each pair of holes and you’re done.

The holes provide all the rake you could possibly want (take off more of the hole if you need less rake) and filing gives plenty of relief (what you see is grossly too much). None of this is critical for drilling soft stuff; you’ll need more attention to detail in a steel-cuttin’ shell drill.

Then clamp the pipe in the drill press and have at it! The teeth have enough rake that it’ll be grabby, so control the downward force and be sure the vise has a good grip on the pipe.

The trick is to support the pipe by ramming a dowel into its snout from one end or the other, thus preventing the sideways forces from deforming the ever-thinning end. This will take some practice, so buy a spare length of pipe.

After some of that and a bit of cleanup, we had a handful of connectors like these:

Fishmouthed tube connectors

Which eventually became the trumpet’s valve assembly:

CPVC trumpet valve assembly

My Shop Assistant turned wood dowels to a slip fit in the pipe, we drilled suitable holes and Dremeled passageways to convert the dowels into pistons, and it actually worked pretty well. Not nearly as resonant as a brass trumpet, but that wasn’t the design objective.

Haven’t heard how they fared in the competition, but it was a fun project!

2011-02-21
Thing-O-Matic: MK5 Plastruder Feet

The MBI assembly instructions blithely direct you to:

Using superglue, or ideally acrylic cement, you’ll want to attach the spacer feet to the bottom of the supports.

As it turns out, though, the tabs on the Support sides stand just a bit proud of the Bottom plates, so that any attempt to glue the Feet in place will simply attach them to the side tabs and nothing else. Not what you want…

So I rubbed the Bottom plates on a sheet of coarse sandpaper until everything was nice and flat:

Flattened Plastruder Support bottom plates

Then the spacer feet glued neatly in place:

Gluing Plastruder feet

I tried to keep the acrylic cement off the tabs, so it’s theoretically possible to dismantle the whole thing, but I suspect that’ll never happen.

2011-02-20
Miniature Ball Bearing Sizes
Having had to look up ball bearing sizes far too often, here’s the table…

Bearing ID OD Thick

603 3 9 5

623 3 10 4

633 3 13 5

683 3 7 3

693 3 8 4

605 5 14 5

625 5 16 5

635 5 19 6

606 6 17 6

626 6 19 6

608 8 22 7

629 9 26 8

~~The first digit is something like the bearing type; I think 6xx = miniature bearings.~~

~~The second digit has something to do with the overall size, but is a code rather than an actual dimension.~~

~~The last digit is, hal-lay-loo-ya, the actual bore diameter.~~

[Update: Shows what I know; an excellent explanation of the numbers lives there. The short summary:
- First digit: bearing type, 6 = single row deep groove
- Second digit: series, 0 = extra light, 2 = light, 3 = medium duty, 8 & 9 = thinner
- If three digits, third digit = ID in mm
- If four digits, last two = ID/5, except 00-03 = 10/12/15/17
Moral: always verify everything you read on the InterTubes!]

Of course, a randomly chosen eBay listing will list the bearing size as:
- ID x thickness x OD
- OD x thickness x ID
- ID x OD x thickness
- and be wrong in at least one dimension
Of most interest to Thing-O-Matic hackers: a 635 bearing ought to fit a NEMA 17 stepper shaft (pay attention if you’re buying surplus: not all are 5 mm) and slip into the same hole as a 626 bearing.

Alas, there seems to be no 5 mm ID bearing equivalent to the 606 bearing in the MK5 extruder head, but a 0.5 mm = 20 mil shim around the outside would adapt a 625 to that hole. Might take some careful forming, though.

Buy ’em in bulk and save…
2011-02-18
Thing-O-Matic: Ouch!

This should be obvious, but don’t reach across the build platform of your Thing-O-Matic with the extruder at 215 °C: you might bump the nozzle with the back of your hand.

Scorch mark from TOM nozzle

It never really hurt, but the nozzle tip made a nasty punch mark in the middle of a disk of scorched skin.

Ah, you’re not that stupid, are you…

Memo to Self: Gloves?

2011-02-17