Hollow shaft extruder

tombrazier

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

@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.

rqthree

@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.

oliof

@rqthree "being done is a decision" that must be made. Kudos!

tombrazier

@oliof said in Hollow shaft extruder:

@rqthree "being done is a decision" that must be made. Kudos!

Yeah, rq3's comments struck a chord with me too.

CNCModeller

Hi All,
I’ve just finished for the holiday break and wanted to look at the hollow shaft extruder, is there a good consolidated reference material somewhere on the web?

I specifically want to order some flanged bearings to go with the carriers I already have but I can’t find the specs… unfortunately I don’t have time to dig into the thread befor Xmas but I want to order parts to grind the flanges and tinker with between Xmas and new year.

Any thoughts would be greatly appreciated.

Many thanks
Barry M

CNCModeller

Update - I’ve ordered 8x3x4mm flanged deep groove bearings, hopefully I’ve made the right choice… please let me know if I should order something else…

oliof

@CNCModeller you can find Tom's stuff at https://www.printables.com/model/268255-vde100-rotary-extruder and rqthrees at the reprap forums ( no link at hand it easy to find).

CNCModeller

@oliof Thanks for the heads up, I’ve now ordered the correct bearings! Looks like I will be ready to tinker when I get the chance

CNCModeller

General question, so we grind the bearings back to 9.1 mm, how much intersection with the filament does that result in? Or to put it another way, how much do the flanges bite into the surface?

Also how significant is the drag / twist on the filament, does the contra rotating pair of blade sets make a significant difference? I’m assuming there is an amount of backlash like behaviour due to the twisting effect.

Any thoughts would be appreciated

All the best Barry M

rqthree

@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

o_lampe

@rqthree said in Hollow shaft extruder:

I never intended for this to be a high speed extruder

...which is a pity, because it's the lightest DD-extruder ever.
Regarding print-speed on my Delta it can almost compete with a flying extruder setup, but with better ooze control.
I think it deserves to up the speeds a bit. With my BLDC/simpleFOC extruder I can go 120-150mm/s before I see underextrusion.

rqthree

@o_lampe , as Tom has demonstrated, it CAN be a pretty high speed extruder, but I have found that my hot end can't keep up with the required melt rate.

But there are fixes for that, too

tombrazier

@CNCModeller Sorry I have been busy elsewhere and didn't see your questions.

I know you've bought bearings now but, for reference, here are some considerations for flanged bearings:

OD of the part touching the filament needs to be about 9mm or less. i.e. that's the diameter of the flange or just the OD for any non-flanged bearings. Larger than 9mm and the bearings start interfering with each other.
@rqthree uses extended-race bearings which helps with mounting on his machined carriers. I don't bother as I use printed carriers and it is easy to print a bezel on which the race mounts.
It is possible to use a mix of different size bearings. My best extruder has two MF95ZZ bearings which have a 10mm flange. To avoid interference I use a smaller third bearing and mount the three of them slightly non-symmetrically around the filament axis. The slightly larger bearings have more grip.
Depending on which country you're in, metric or imperial may be easier to source.

I, like @rqthree, have found a 0.1mm bite to be about perfect. More than that and it increases drag. Less than that and it both reduces thrust and also makes the tolerances much tighter, e.g. something like a 0.02mm error is a much bigger proportion of a 0.05mm bite than of 0.1mm.

There is nothing particularly special about 9.1mm. It's what I was targeting when I made my videos because it was about 0.1mm smaller than the diameter of the unground flange and I wanted to take off as little material as possible. Realistically, I could have gone for 9.15mm. My primary considerations for diameter are:

Less grinding takes less time and effort and leaves fewer chances for machining errors.
You can always take more off if necessary but you can't put it back on, so aim for larger and adjust to smaller if need be. e.g. if you want the same diameter for all the bearings and one accidentally ends up a bit smaller you can then make the others smaller too.
I have found that larger diameters grip the filament better. I am guessing this is because the bite has a different shape with a bit more overlap.
As @rqthree says, if you are printing your carrier then it's easier to make a custom carrier for whatever sizes you ended up grinding the bearings.

Drag on the filament is minimal as long as the bite is small enough. @rqthree initially designed a counter-torque mechanism into his extruder but later found it was not needed. The back-torque provided by the melted filament in the nozzle is sufficient. I benefited from this knowledge and have never even implemented a counter-torque. You do see a bit of backlash and it varies a bit by filament type. You may find retraction distances benefit from a bit of re-tuning. But I have not found it affects the print. Obviously consistent filament diameter becomes an important factor, but any decent filament these days has fine tolerances. It also helps if you minimise the path length through the hotend heatsink as this decreases the springiness of the filament section between the extruder and the nozzle.

Speed: there are a couple of guys on the CroXY Discord server who are exploring using VDE for high-speed. The simplicity and weight considerations are just too tempting. At high enough speeds VDE does under-extrude. There can also be issues with having to run the motor hotter which is problematic with the filament running through the shaft. More flanged bearings definitely help under-extrusion but they add drag and increase the likelihood of multiple grooves making a mess of each other. A mix of larger and smaller bearings mounted asymmetrically is also helpful. Firmware can help a bit as well, Marlin now has nonlinear extrusion and I think someone (@oliof maybe) said that RRF has has this feature for ages. Possibly Klipper does as well, I don't know. The guys at CroXY are, I think, experimenting with drilling a larger hole in the motor shaft and lining it with PTFE tubing. Mounting the motor on the hotend heatsink and sharing cooling air with the heatsink is also something to explore.

One idea I may go back to one day is a two bearing VDE. This required the filament path to be constrained by the walls of the hole leading up to the bearings, so adds friction, but it would allow for much larger bearings without any risk of interference.

@rqthree's observations about using PLA for a printed carrier match my experience. I have not experimented widely here but in my best round of testing I did swap from PETG to PLA when I noticed there was a difference.

o_lampe

Hi again,
we have discussed earlier to use pipe-cutter blades instead of flanged bearings.
Now I've seen a woodworking video which shows an interesting tool. The blade is very small and replaceable, so there's a good chance we can buy it separately and use it for our purpose, too.

But I don't know the name of the tool. If anyone knows, pleas let me know.

JoergS5

@o_lampe Veritas Streichmaß or DICTUM Streichmaß and "veritas streichmaß ersatzmesser"

There is also one with assembled adjusting ring on https://www.feinewerkzeuge.de/veritas-marking-gauges.html, "VERITAS Schneidräder für Zapfen" but it's a bit expensive (22 EUR), but maybe one can DIY.

o_lampe

@JoergS5 Very interesting. The idea is to bulk-buy instead of DIY.
IMHO the biggest problem with Toms handmade bearings is to keep the diameter constant.

achrn

Veritas tools are nice, but not cheap.

https://www.axminstertools.com/veritas-wheel-for-marking-gauges-477717 is a UK retailer.

The company is Canadian: https://www.veritastools.ca/en-ca/shop/tools/hand-tools/marking-and-measuring/marking-gauges/59455-veritas-standard-wheel-marking-gauge?item=05N3321 and the wheel is https://www.veritastools.ca/en-ca/shop/tools/hand-tools/marking-and-measuring/marking-accessories/102171-replacement-wheel-for-veritas-standard-wheel-marking-gauge?item=05N3511

o_lampe

@achrn Thanks for the links, but in the canadian closeup picture it looks horrible. I don't see, how it would rotate nicely.

Phaedrux

@o_lampe I believe the entire shaft rotates, not just the cutter.