Best posts made by rqthree

rqthree here (known as Rip in real life). I'm the clown who started this whole VDE thing, and have been using mine for well over two years now.

A few items of potential interest:

1. I started with a "rolling screw" approach, based on a Rohlix linear actuator. It finally dawned on me that any tilted axis drive can have only one mathematically defined SINGLE point of intersection with the filament. Hence, the single knife blade.

2. I have found that my current NEMA 14 pancake stepper will stall and skip steps long before the drive knife starts to grind up the filament (at least PLA, PETG, and ABS). The push force on the filament at that point is about 1 kilogram (measured by pushing on a sensitive scale with the actual filament after it leaves the drive knife. For this reason I decided that multiple drive knives was a waste of material and weight. The motor quits before the filament does. This is also why testing this design by measuring the pull on the filament is not valid, as the filament tends to break at one of the knife cuts. This cannot happen in real life.

3. I see a lot of effort being made on making it easy to introduce ad remove the filament. I have never had an issue with this. To use new filament, I just cut the existing filament off at the motor, start the extruder, and push the new filament into the top of the motor. It just pushes the old filament out of the way (hot nozzle, of course). Both Tom and I independently arrived at a "motor clip" arrangement, so that the entire motor and drive assembly can be lifted off within seconds, effectively performing a "cold pull" on the filament if needed.

4. If you really want an easy feed, increase the cant angle of the knife edge and pressure rollers to 30 degrees. I use 15 degrees, and I believe Tom settled on 20. Increasing the angle to 30 degrees does several nice things:
   a) it makes the filament feed faster per motor revolution
   b) it makes the drive mechanism "back-driveable", so that the motor will spin if you push or pull on the filament. This makes loading or removing filament trivially easy.
   c) it makes the mechanism subjectively feel "smoother". I can see no difference in print quality, and this is strictly a "feel" thing.

5. For those trying multiple drive knives, good luck. I never had any success in getting the "multi-starts" impressed in the filament to co-operate during retraction. And I've never seen the need. Again, the motor skips before the filament does. The design philosophy behind the VDE was ease of build, simplicity, low weight, and extreme repeatability.
   The best multi-drive approach I've tried has been two NEMA 11 pancakes, mounted back-to-back in a push-me-pull-you arrangement. The dual drive VDE weighs about 65 grams, and both motors are driven by the same signal with the coils reversed on the second motor so that it will spin the other way.

6. The bearings I use are extended inner race Imperial units made of 440C stainless steel. This has a Rockwell hardness of about 60, which is darn hard. 52000 series steel is even harder. In any case, the drive knife on my current VDE has seen pretty hard use for over two years, and is still going strong. I have never tried carbon or glass filled filament, though.

It is pretty critical that the bearings be of high quality. I use ABEC 5 units, but see no need to spend over twice the price for ABEC 7. Note that ABEC standards just set limits on the manufacturing tolerances, and do NOT have anything to say about the radial or axial play of the bearing. That must be specified when the bearings are purchased, and increases the bearing price as each unit must be measured and selected.

One neat trick to effectively eliminated wobble in the bearings is to place 20 bearing balls of 0.026 inch diameter in the groove on the top shield of each bearing. The head of the shoulder screw effectively traps them in place, where they collectively act as a thrust bearing. 500 of these balls costs about $14, and are yet another reason I use Imperial bearings.

Have fun!

@o_lampe said in Hollow shaft extruder:

@tombrazier said in Hollow shaft extruder:

At 700mA I can drive the filament at 50mm/s and accelerate at 1500mm/s^2 without a motor stall

Is that the retraction speed or can you actually extrude at 50mm/s? What are actual PLA print speeds with 0.4mm nozzle and say 0.3mm layer?
It's always hard to compare apples with oranges, but in the end I'd like to know if it's worth going the extra mile with the dual motor setup I'm working on.

Tom, although I've been using my "original" NEMA 14 VDE for well over two years, I still tinker with ideas. I never could get a NEMA 11 pancake to drive reliably, but TWO of them, rotating in the same direction but mounted back-to-back, seems like the cat's meow. Because they are back-to-back, they eliminate torque on the filament. Of course, you need two motors, and two bearing carriers. But one carrier is "right hand thread", and the other is, of necessity, "left hand thread", so the grooves scored in the filament only cross each other once per revolution. There is no issue with getting the two grooves to track each other, because they can't, and don't need to. They are independent of each other. There is still only one knife edge per carrier, per my reasoning above.

Of course this bumps the complexity and cost by 2x, and the weight up to 65 whole grams! Prototype with 30 degree cant angles.

EDIT Nov.1, 2023: A very brief video of the Push-Me_Pull_You VDE running (change the .doc extension to .mov).
Cons: as noted, twice as heavy and twice as complicated, needs left AND right hand drive carriers.
Pros: handles extremely flexible TPU with no hot end anchoring at all, 5 times the filament force (5 kilos vs 1 kilo) at 0.2 amps (paralleled NEMA 11 motors, no heat issues at all), motors can attain 4500 RPM, and 3000 RPM while actually driving filament (150 mm/sec at 30 degree cant angle). Feed and retraction very repeatable, as the two drivers don't interact.
The Take Away: Not worth the size, weight (80 grams), and expense, unless you are after extreme extrusion speeds. I'll stick to my tried-and-true single knife NEMA 14.

IMG_1951.DOC

@o_lampe said in Hollow shaft extruder:

@tombrazier said in Hollow shaft extruder:

grinding an edge down to the exact diameter.

Both precedures aren't mass-production friendly. The skrewder wraps the screws around the filament which results in adjustable bite and fast changeable filament ability.
If we could change the VDE100 design to be wrappable, it would be best of both worlds.

I must respectfully disagree on both counts. Holding sub-micron tolerances is common practice in manufacturing these days, and the VDE does not need "adjustable bite" and "fast changeable filament ability". It already does these things by design, albeit perhaps serendipitously.

I believe that one must be very conscious of the goals of a particular idea, declare victory when achieved and then move on, rather than try to force fit what has worked before into what is needed now.

Personally, I believe that increases in FDM printing speeds will only come from smaller diameter filament. All existing extruders (the VDE included) are limited in their extrusion rate by how quickly you can melt a column of plastic 1.75mm in diameter.

@CNCModeller, less is better. As Tom said, it may be better to print a carrier that works with the bearing flange diameter that you've made.

Playing with the VDE-337 (which I won't pursue anymore), it became even more apparent that a gentle kiss of the knife edge (0.1 mm or 0.004 inch) is about ideal.

If you are printing the carriers, plain PLA is infinitely better than anything else, at least in the short term. PLA is very stiff, and acts very much like aluminum as regards machining, drilling, tapping, etc. It's just not tolerant of higher temps, and cold flows over time under pressure.

Filaments like PETG are, in my experience, way too "rubbery" to make a bearing carrier of any use in this application.

I have had all of my carrier designs CNC milled from 7075 aluminum to avoid these effects, since we are talking about micron level precision, and just today finished an exercise regarding print surface finish versus extruder knife edge bite depth.

Results: again, the less, the better.

I never intended for this to be a high speed extruder. I intended it to be a very accurate and reproducible extruder.

Regards,
Rip

On the Veritas tool, neither the cutter nor the shaft rotates during use. The cutter scribes the wood, and the shaft is rotated to a new cutter edge position when and if the cutter becomes dull.

The screw just holds the cutter firmly on the end of the shaft. The cutter is not supposed to rotate at all.