Monday 19 July 2010

A bit of a drag

One problem I have had, on and off, with Mendel is a tendency for the infill not to meet the outline. This was particularly bad with PLA. I have combated this by having some infill overlap and also extruding the plastic slightly faster than it should be, so that the solid layers are well stuffed. I don't have to do either of these things with HydraRaptor. I couldn't figure out what the difference was until I changed a reel of plastic recently. The first print on the new reel came out like this: -

The gap is always at one side like this, it is as if the infill is not centred within the outline. The reason, I have come to realise, is that the belt has some play in it because it is not infinitely taught. When the extruder pulls filament off a reel it exerts a force on the carriage, which displaces it slightly from where it would come to rest without any external force. Because the carriage moves, the filament only gets pulled from the reel at the local extremes of movement, the rest of the time it is slack. This causes small offsets in the filament paths. In particular, when it is doing zigzag infill it is using filament relatively quickly, so at the end of the zigzag furthest from the centre of the bed it is likely to give a little tug of the reel each time, causing the zigzag to stop short of the outline.

The conclusion is that the filament feed for a belt-driven moving-head machine needs to be very low drag. The hanging basket technique that I used on HydraRaptor is no good because it has to pull plastic out from under its own weight. Making the feed point high above the machine reduces the lateral drag on the carriage, but you can easily get enough vertical drag to deflect the x-bars upwards, or even lift the z-axis slightly because the backlash in the thread is only taken up by the weight of the x-axis.

The reason it was bad with PLA was because I was pulling it from a hanging basket and being very stiff, even a small coil needs a lot of tug.

The system I now use is a vertically mounted spool big enough to take 5kg coils, which last me a couple of weeks.

The bearing is just a stainless steel axle running in PLA bushes, lubricated with some lithium grease. It is low friction, but not as frictionless as a ball bearing. It needs a little friction to stop any in-balance in the coil causing the spool to spin to its low point. Also the faces of the spool need to be quite big to stop a loose loop of filament coming over the side. It pulls tight and jams if that happens.

I can take the spool apart to insert a new coil of plastic.

In general though I have to wind it all off and on again to get it tight and balanced enough to wind off smoothly. Since 5kg is about 800m it takes a long time to wind it onto a garden hose reel and then back on again. Someday I will get round to making a machine to do it for me. In the meantime I will make a second identical spool so that I can just mount the coil on one spool and wind it onto a second one to use.


  1. This problem should also be avoidable by using a bowden extruder, although I've noticed in building my mini mendel that you have to be careful about the cable sliding along the threaded rods across the top, which would probably have similar results.

  2. Yes I fitted a smooth rod in the spare holes in the middle of the top frame brackets for the filament to slide over. I didn't like the noise of it scraping on the threaded bars.

  3. What if you don't reel it up so tight? In other words, take the spool you get from the manufacturer and make it hang slack on the spool. You wouldn't get a low point because any rotation would make that the new low point.

    Not sure how that works when you pull on it, the first pull might pull it tight against it and ruin the thought :-)

  4. If I don't re-spool it then it does end up slack and hangs off centre. To turn it requires a lot of force. As you say when it turns you get a new low point but the net effect is that it is always pulling against the off centre weight. As it weights 5kg to start with a small in balance needs a lot of force.

    The other problem is that when the coil is loose the outer loop can pull tight to the hub of the spool (as the layers below part to let it through) so you are turning all the weight with very little leverage.

    A horizontal spool might solve these problems but then the feed path would have a ninety degree bend in it.

  5. I was seeing similar problems with my Mendel, and assumed I had the skein forge settings wrong or a problem in my X axis. It's always good to know what problems are mine alone, and which might happen to all the other owners!

  6. It seems to me that a machine that manages the spooling would be easier to build, and consume less overall time and materials than a machine that re-spools everything.

    All it needs to do is maintain a given amount of slack between the spool and the extruder. It would have to be a bit stronger than the extruder so it can unspool the filament from a regular spool. Add a microswitch with a wire to detect when there's little slack between the unspooler and the extruder, and have the microswitch turn the unspooler on and off. Add a little logic if you want more than simple bang-bang control, but it should take care of everything for you, and you won't have to worry about whether the spool you're putting in the machine is too tight, too loose, has too much friction on the axle, etc.


  7. Cool stuff.

    It may be worth considering building a combined bowden/spool assembly (similar to mig welders but with more push)

    I am currently looking for brake cable outer that has an in ID just right for the Bowden PTFE tube. Having a bowden assembly that has less linear flex may help reduce ooze.

    Great work as ever.

  8. I bought 1/8" PTFE braided hose, more here, from ebay which seems to be a fairly good fit for 3mm diamater filament. The braid is a bit of a pain to trim back at the ends, but I figured would give less flex in the tube. M6 nuts also seem to screw quite well onto this, which is what I use to secure the tube at each end.

  9. I just bought an electrical spool to use temporarily ( until I can build the Mendel Feed Spool on Thingiverse (

    What a pain in the #$@!? getting it onto the spool. That filiment is very springy - I hope I never have to do that again...

  10. Yes my wife and I spool it onto something like that, and then back onto the one above, about every ten days. It is hard work!

    I don't think the plastic I get would work on the thingiverse design without being re-spooled first. The coils don't start at the outside and work inwards, they are far more random, so I have to pull filament from the depths of the coil sometimes and that takes a lot of force.

  11. I think this is a good reason to buy your plastic from someone that sells it on spools, then you can either use the spools directly or respooling could be done automatically.

  12. Yes it would be nice if it came on a spool, but then soon I would have too many spools.

    I like the ABS from reprapsource because it has a very consistent diameter.