So, yesterday, I was thinking about trying out the Support options in Skeinforge. Something on the (ever shortening!) list of things MakerBot’s aren’t really good at are overhangs and when people talk about “support“, they’re talking about having the printer print extra material below areas that would otherwise be over open air..
(edit: skip down to the picture below if you already know why support structures would be nice to have)
Take, for example, the letter T. If you were to try printing an object that looks like a T standing up, that is with the narrow part at the bottom and the wide part at the top, you’d run into trouble when you got to the top. The printer would do a nice column all the way up but at the top, the plastic for the wide parts (or at least the parts that aren’t over the column) would just drop down – there’s no plastic layer below to support them.
It’s not all bad.. Take, different example, the letter H. Two columns going straight up with a horizontal bit between them. It might seem like a MakerBot couldn’t do this but it’s really suprisingly good at it. The trick is to use a profile that has a high enough feed rate (feed rate = speed of the build platform that moves on X & Y) that the plastic gets stretched out and doesn’t have time to drop down to the bottom. The first layer or two that it does this on might be a little saggy but the ones above that will be just fine – a little exacto work & sanding and it’s great!
That doesn’t work for everything, though. I fired up Google Sketchup, something I’m trying to learn but still stink at, and threw together this object:
Nothing nice, just something I know ain’t gonna print.
Instead of making you read all the babble below before seeing the result, here’s how it eventually came out:
Still needs a bit of cleanup but I was suprised how well it came out!
Anyway… The support options are in the Raft page in Skeiforge. I’m using Skeinforge 27, dated 2010-06-28, so if you have an older version, this stuff may be different. There aren’t a lot of options related to support:
Support Cross Hatch checkbox: I used this option. This controls how the alternate layers of the support structure are laid down. With this option, each support layer is rotated 90 degrees from the one below it. So, first layer is all X, next layer is all Y, next is all X, etc. Without this option, I think it’s all X .
In other words, if you’re looking down at the top of the completed object with Cross Hatch, you’d see the build platform as a bunch of tiny boxes through the support grid. Without Cross Hatch, it’d be a bunch of lines.
I think I won’t use this again soon. The crossing support structure makes it pretty strong, which also makes it pretty hard to remove. Then again, I haven’t tried printing without it so.. we’ll see.
Support Flow Rate over Operating Flow Rate ratio: This is what you want your extruder speed to be, in relation to what it is on the real object. I’m printing the object itself with Speed\Flow Rate set to 255 – full power! – and used 0.85 for this support option, which is (255 * 0.85) a PWM of about 217.
There isn’t an option for changing support feed rate (platform speed) so turning the flow rate down here is really the way to control the density of the plastic. I think the short answer for this option is to turn it down so you get the lowest PWM that you ran reliably extrude at. I may try 0.75 next time.
Support Gap over Perimeter Extrusion Width ratio: I left this at 1.0 for the print but messing around with various settings and looking at Skeinview, this looks like it controls how much distance to put between the object and the support structure. At 1.0, the distance is the width of one perimeter. At 2.0, it’s twice the width of one perimeter.
For example, if you’re nicely dialed in with a layer height 0f 0.3mm and a width/thickness of 1.5, setting this value to 1.0 would mean 0.45mm (0.3mm * 1.5 * 1.0) between the object and the support. With a value of 5.0, you’d have 2.25mm between the object and support.
I think the optimal value here really depends on your feed rate – set this value so you get a distance that you can easily cross (easily cross = not have the filament drop much, if at all) at the speed the platform is moving. I think I’m never going to use 1.0 again; 2.0 is probably my lower limit now. I tend to print at fairly fast feeds and predict I’ll keep this around 5.0 or so.
Support Material Choice is a choice of none, empty layers only, everywhere, exterior only: Didn’t really play with this but “none” means not to print support structures; “exterior only” means, I think, to only do support on the outside of the object – no support on internal areas, inside the perimeter; “everywhere” is both exterior only and internal areas; not sure what “empty layers only” means – maybe that can be used to print multiple objects on top of each other?
Support Minimum Angle as degrees: this is how much of an overhang Skeinforge has to detect before it will start adding support structures. I’m not sure if this is from horizontal or vertical though, so I left it at 60. 🙂
Some pictures of the build, as it was happening..
Only a ~dozen layers in and the raft’s already lifting.. Notice that Cross Hatch is enabled. Also the Gap, which looks a bit more than a perimeter width here but I’m convinced the text above is at least close to true..
Few layers into the middle block. Notice that the support structure is also now above the lower block.
Finally on the last block!! 10 minutes left to print!
Arggg… Extruder jammed with only 5 minutes left. I thought the ABS coil was looking a bit weird – probably a kink or something. Close enough, though – it’s been 45 minutes and I’m tired of waiting.
A view of the object from the bottom with most of the raft removed. Note the scars of passage from where the print head plowed into the platform, back when I was new.. :). Note again the Cross Hatch. I’m also looking at that Gap and wondering if I really do understand what that setting does – I still think I’m mostly right. Mostly.
Cutting off the support structure.. This is a HUGE pain in the neck. I think my settings, cross hatch and 1.0 gap in particular, made the support stuff a lot more solid than necessary. This takes me close to 30 minutes to get it all removed (mostly around the middle block, where it’s too tight to get nice cuts with a razor)
A chunk of the support structure.. This is maybe 5-6 layers thick, going from the bit facing the camera towards my bench.
Finally, again, the (mostly) finished piece. Finished as in not totally cleaned and missing the top because of the extruder jam..
One is that I need to mess with this stuff some more. Top on my list are disabling cross hatch and cranking up the gap to 5.0 or maybe 10.0.
If I had a choice, and I don’t think I really do, I’d rather have a feed rate tweak instead of a flow rate tweak. That way, I could move faster (shorter build time!) and keep the extruder turned all the way up. Or, better, options for both feed and flow rate tweaks.
I think an option to control the density of the support structure itself would also be really nice. By density, I don’t mean the size/shape of each line but how far apart the support lines are from each other – how many lines it prints. For what I was printing tonight, having the support lines be 5x farther apart (waaaay shorter build time!) would probably have resulted in very similar quality..
Next up: Part 2!