Best posts made by tombrazier

@dc42 and @chrishamm thank you for the EMF calculator. I was busy replicating this work when I found it.

I think I have found several errors in the calculations of the "Speed at which torque starts to drop" values. (Just to give some credence to what I suggest below it might be helpful to know I am a Marlin dev.)

• When calculating revsTorqueDropLowSlip and revsTorqueDropHighSlip I think just dividing speed by the number of motors is incorrect. Instead, the value of supplyVoltage should be divided by the number of motors. i.e. motors in series form a potential divider for the supply voltage. An easy way to test my assertion is to consider what happens with two motors in series when resistance * motorCurrent > supplyVoltage / 2. That should result in "n/a" because the voltage cannot even drive the requested current through the two winding resistances. But, instead, the web page just halves the speeds.
• driverVoltageDrop is double counted. It is incorporated when voltageDrop is calculated and then, later, when revsTorqueDropLowSlip and revsTorqueDropHighSlip are calculated, it is added on to voltageDrop again.
• revsTorqueDropHighSlip is missing voltageDrop in two places. This formula is the result of applying the quadratic formula to solve for revs in the equation supplyVoltage = sqrt( (revs * inductiveBackEmfPerRevSec)^2 + (revs * motionBackEmfPerRevSec + voltageDrop)^2 ). This expands to0 = revs^2 * inductiveBackEmfPerRevSec^2 + revs^2 * motionBackEmfPerRevSec^2 + 2 * revs^2 * motionBackEmfPerRevSec * voltageDrop + voltageDrop^2 - supplyVoltage^2. The b coefficient in this equation is 2 * motionBackEmfPerRevSec * voltageDrop and it is this voltageDrop which is lost when the quadratic formula is applied to calculate revsTorqueDropHighSlip.

Hi all, I made the VDE-100 video that has been referenced a few times. I have been using the VDE-100 for a long time now and I love it. Several comments in response to the thread above:

@nikscha your maths was correct, tan(15) * 1.75 * pi is the theoretical thread pitch. In reality the knife edges tend to drag a bit through the filament resulting an a somewhat lower pitch.

Loading/unloading is pretty easy but, yes, it is done by instructing the extruder stepper to turn. I have my whole VDE-100 assembly attached via a quick release clip which also helps because occasionally a blob on the end of the filament went retract through the extruder. And the quick release is super cool for dealing with nozzle clogs, etc.

The extruder seems to just work on any filament (e.g. floppy TPU at one end of the scale and hard PLA at the other). But I think definitely avoid abrasive filament. I did print recently with GF filament and specifically swapped out my extruder for another that is not as nice as the VDE but has bondtech gears.

There have been suggestions about tensioning. I played around a lot with tensioning ideas, including using a similar idea to @nikscha's printed spokes. In the end, it just is not needed. The knife edges cut into the filament and that provides the tension. But it does mean you need fairly precise diameters @o_lampe. Since I found a way to cut those even with rudimentary tools, I am satisfied with that. Of course, you also need precise filament diameters, but that's pretty much a given these days.

Steppers getting hot: with the VDE-100 the stepper stays nice and cool because there is so little torque needed and so not much current is needed. I would think Ingenuity would have the same benefit.

Filament twisting in the VDE-100: this is surprisingly far less of a problem than you'd think. The melted plastic in the nozzle provides a fair back torque. And the shorter the part through the heatsink, the better this effect is. Once again, though, precision edge diameter is key. Cut too far into the filament and that creates more torque.

VDE files to play with: I published my files with FreeCAD model on printables.com.

My latest VDE design is even better than the one I used for the 16 minute benchy. It features two pairs of flanged bearings mounted back to back plus a plain bearing.

I'd like to experiment with three pairs of flanged bearings but need to buy more bearings before I can do that.
Here are some graphs to give an idea of how different designs are affected by compression of the helical thread:

Y is thread pitch / theoretical thread pitch and X is actual linear extrusion speed in mm/s (i.e. taking compression into account).

• The baseline is what I was using when this thread started. It has two chisel edged bearings and a plain bearing and is extruding through a traditional 0.5mm nozzle.
• The red line has larger bearings with knife edges (i.e. a / shape rather than a |/ shape). Both of these reduce compression effects. These are lower quality bearings and I believe this accounts for the red line performing worse at low speeds.
• The yellow line replaces the nozzle with a 0.4mm CHT clone from aliexpress which I have modified to enlarge the three holes so that there is a sharp edge in the middle rather than a flat face. These clone CHT nozzles actually increase back pressure at low flow rates but they do not have the sudden drop off at around 6mm/s.
• The green line is the extruder I used for my 16 minute benchy. It replaces the plain bearing with a knife edge bearing so it has three knife edges rather than 2.
• The last line is my latest experiment with 2 pairs of back to back knife edged bearings. This is slightly better and is the only extruder where I have been able to extrude at an instructed 12mm/s (and getting 8.54mm/s) without the compression causing the knife edges to start falling into the wrong helical thread path, causing inconsistent extrusion.

Ah, bother, there is an error in one of my formulae and akismet won't let me correct it. The third bullet point should have "This expands to..." with 2 * revs * motionBackEmfPerRevSec * voltageDrop in the middle of the equation (where revs is not squared).

@chrishamm @dc42 Did you see my comments about errors in the EMF calculator in this forum topic?

Posting a new topic just in case you didn't.

Success! I've finally completed a 16 minute benchy with my VDE extruder. Somewhat under-extruded in places, but pretty good considering. The rest of the machine hardware is also pretty low spec. 12V bedslinger running on an 8 bit AVR mainboard. Posted a video on youtube.

That's been a long-running project. Glad to put it to bed.

Next up: a new VDE idea to try with 3 pairs of flanged bearings. (And @o_lampe then I'll send three of the bearings to you. It was while I was grinding them that it occurred to me that three extra bearings might be an interesting thing to test!)

Hi guys, I have been on holiday for a few week, hence the radio silence.

Nice work from several of you.

@zero-K how does your extruder do?

@breed Your pressure bearings that aren't canted will probably be fine with many filaments. Quite some time back I tried a VDE-100 with only plain bearings (i.e. no edge) and it had very little grip. I think that was with PLA. I conclude that filament slides over plain bearings very easily. Something like TPU might be a different matter, though.

@o_lampe I got the carriers, thanks. Now I have my work cut out for me grinding bearings! On the plus side, one of the users on the 3D Printing discord server is working on a Dremel attachment to help with this.

@rqthree Rip and I have collaborated extensively on VDE. By which I mean I have second guessed pretty much everything he has done, tried it myself and generally found that he was right after all!

Some updates from me...

I have experimented further with the LDO motor. For everything other than ridiculously fast printing it's great. The primary limitations I have found are that continuous current much above about 600mA will eventually cause it to get too hot for PLA and at any current E jerk needs to remain quite low (e.g. max 7mm/s) for the VDE which has quite a high steps/mm ratio. On the other hand it has great acceleration and speed, I can get 1000mm/s/s even with the high step/mm count of VDE and top speed is about 60mm/s (with no load) and over 10mm/s driving filament into the hot end.

One day I might figure out how to get TMC CoolStep to help with the current. And I have dreams of implementing a jerk-less version of Linear Advance in Marlin which would pretty much remove the jerk limit as an issue.

In other news, I have uploaded a video about the bearing grinder jig.

And, further news, with a PLA printed carrier the 3 back-to-back bearing pairs version of the extruder seems to work pretty well. I have used a bite of 0.07mm for this version. @o_lampe it works a lot better than with your printed carriers which leaves me wondering whether something like material flexibility might be an issue. I am also coming to the conclusion that it's best to grind the bearings first and then measure them and print a carrier to match their edge diameter. That's a lot easier than grinding an edge down to the exact diameter.

@o_lampe said in Hollow shaft extruder:

With all due respect, but grinding an edge to a flanged bearing is not my favorite evening activity

Really? Why?

If you can work out a way of self adjusting I'd be very interested to see it. I didn't really follow the conversation above. I need a picture, I think.

@nikscha I have not tested but I think that my VDE extruder could not lift 4kg - or if it did it would experience some much widening of the grooves that the helical thread pitch on the filament would reduce to pretty much zero. However, 4kg equates to a nozzle pressure of something like 16MPa, which I think is probably a lot higher than ever would be needed for extrusion. With fluid dynamics, there is a point where flow stops being laminar and flow rate pretty much stops increasing with increased pressure. And with feeding filament through an extruder, there is a point where it is feeding too fast to fully melt, which also places a fairly abrupt upper limit on flow rate, regardless of pressure. Volcano and CHT type nozzles help but even they have their limits. I would be interested to know at what pressure the cut-off typically is, but naively I suspect it is below 16MPa. Stefan from CNC kitchen has done a lot of work exploring these limits but I can't remember whether he has put a load cell on his extruder to measure extrusion force at the limits of flow.

High flow rate is a bit of a weakness with the VDE because of the groove widening thing I mentioned above. I have vague ideas of experimenting with more flanged bearings and different edge shapes. But in reality it is only sub 20 minute speed benchies that challenge my VDE for flow rate so it's not my highest priority.

@o_lampe I don't think microstepping is relevant in the speed/torque calculations, but it is a factor when configuring the firmware. I have a 20° cant angle on my bearings and this results in a lower steps/mm than rq3 gets with his 15° cant angle. I am presently using use microstepping = 2 with my VDE and getting about 275 steps/mm (1.8° motor).

@nikscha said in Hollow shaft extruder:

About the groove widening: why not stack a second ring with 3 more bearings on top? If you get the spacing right, the second ring won't even have to cut it's own groove, and the load is shared between the two.

Mainly because I have not needed to. I don't usually print fast enough for it to be a problem. But it is one idea I have thought about. Another that strikes me is that the edge I grind on the bearing has an included angle of 45° and rq3 uses 60°, but I suspect the ridges on a bondtech gear have more like 90°. I would be interested to know whether a wider included angle results in better grip. And another is to use symmetric edges as rq3 does, which I now have the means to grind whereas I did not previously.

@nikscha said in Hollow shaft extruder:

I'll do some testing with lower currents to see how little torque is actually required.

The trick will be to figure out at what speed your extruder's flow rate hits the wall. Then tune current so that the E stepper does not start losing steps before that point.

@tecno It's not a problem if the motor does not get hot and that's ultimately a function of gearing ratio. I have been using a hollow shaft for over a year with no problems at all.

@nikscha Here are some filament photos for you. This piece of filament was fed at a constant 6mm/s. Initially there was no load as it moved through the heatsink. When it hit the nozzle, the load increased rapidly and the thread pitch became compressed.

Here is a close-up showing the shape of the groove in profile. As you can just see when the pitch compresses a slight ridge develops parallel to the groove and the groove widens.

It's pretty difficult to photograph this, so apologies for the imperfect images.

@o_lampe Happy to grind and send some bearings. E-mail me at tom at firstsolo dot net.

IS with delta is possible in principle. I understand resonances might vary across the print bed, though. They should not vary with speed. Marlin only has IS on two axes at present (I'd love to have the time to fix this one day!) so deltas are not really supported for IS. I have no idea whether RRF or Klipper have IS for deltas. As I understand it, though, RRF's IS implementation may have issues for any printer. If I have understood correctly then RRF attempts just to do IS on corners which makes a mess with short segments.

@o_lampe said in Hollow shaft extruder:

He might also be able to tell us more about the grinding process.

My video shows how to do it with any old sanding disk or belt sander on just one side of the flange. The same method is easily adaptable for use with a Dremel cutting disk to grind both sides of the flange.

@o_lampe In a way, the diameter itself is not all that important because you can just print a carrier to match whatever diameters your bearings came out at. e.g. one of the carrier models I sent you has bearing diameters of 9.88mm, 9.6mm and 9.1mm because, after a lot of experimenting, those were the bearings I had to spare. But if you can get them all to the same diameter it does simplify matters.

What does matter is that the edge is symmetrical. Bite depth is quite small. I use 0.1mm. So if the edge on the bearing is slightly ovoid or has flat spots you'll get bits where the extruder doesn't grip. My method of grinding produces a symmetrical edge quite reliably - as long as the bearing is free to rotate whilst it is being ground. With a bit of practice it's fairly easy. The time consuming part is when you're trying to get all the bearings to the same diameter and have to keep measuring.

@JoergS5 said in Hollow shaft extruder:

One topic would be to solve the twisting force on the filament. If you're using two screws, finding a solution to rotate both in opposite direction would be preferable in my understanding.

The same applies the the VDE100 but the torque is so minimal that in practice this is a problem that doesn't need solving. Just the back-torque from the melted end of the filament and the torque coming from the filament winding off the reel are enough to counter the feed torque. Even with TPU! Having a short path through the heatsink also helps because that reduces the amount by which the filament can wind itself up before it gets to the melt zone.

Trying to drive the filament in two different directions with counter-rotating threads / bearing edges will result in (at best) two crossing helical threads on the filament. I have not tried this but my guess is they'd interact in undesirable ways.

@mrehorstdmd I have seen a number of posts and videos of the snakebite extruder in recent days. It looks like it was a great contribution to the rapidly growing pool of FDM designs and ideas in the last decade. Nice work IMHO.

The key difference, I think, between VDE-like designs and the screw designs of 10 years ago is that the screws freely roll over the filament in these designs, thereby almost eliminating torque on the filament. In principle, one might even be able to drive the screw at the right speed that torque on the filament is entirely removed but I suspect that would be overengineering.