Dealing with detaching rafts

In which your narrator seems to have come up with a way to prevent raft peel for ABS on an acrylic print table.

About two weeks ago, the rafts for my ABS prints started detaching from the right hand side of my print table.  I was losing one out of two to one out of three prints that way.  At first I decided that my acrylic print table had simply got too warped and I had too much variation in level on the acrylic.  I removed the acrylic print table and checked its flatness with a milled straight edge.

Indeed, it was a little warped so I used a belt sander on it till if was as flat as the milled straightedge.  That seemed to work for about an hour but I was soon back to where I began.  I then decided that the table was now adjusted properly and went to a great deal of trouble getting it so on my Rapman.  Same result.

In desperation I began to print on the left hand side of the print table and the problem went away.  It was still troubling, though.

On Tuesday, I finally caught on.  In the last weeks the weather had cooled to where the outside temperature was in the teens more often than not and dropped into the single digits {Celsius} in the early mornings when I began to work.  I looked at the printer table and noticed that the window I used to ventilate the work area was on the right hand side of the printer.  The next time that I had a raft detach I measured the acrylic work surface temperature with my IR thermometer and discovered that whereas it was about 25-26 C on the left hand side of the table the draft from the window dropped that to 22-23 C on the right hand side.  I was rather shocked that I got that wide a variation in surface temperature over a few centimeters distance, but I certainly did.

I closed the window and the peeling instantly stopped.

That remedy wasn't workable because of the ABS fumes, so I rigged a portable heat lamp onto my camera tripod to shine on the print table.

The radiant energy keeps the acrylic print table at 34-40 C with the window open.

I haven't had a raft detach since then.  I've done a few dozen prints, mind.

Second go at the carpal ensemble

Acid test

I decided to give the new non-loop road finder routine a tough workout to see how robust it is. For that I designed a 20 mm diameter herringbone pinion gear.

I did a solid print for strength.  As you can see, there were no problems.  The stl's processed without drama and the gcode is good.

Changes to Slice and Dice

Slice and Dice was fine as long as your parts were more or less continuous along the z-axis and not so complicated that you got a lot of clashing with print loops. When I got to working on the thumb joint on my telepresence hand, however, I started having a lot of trouble with both of those limitations.

A major strength of Slice and Dice is that if you have a dodgy STL file you can clean up little imperfections on the slice images. That's wonderful until you have a part that you want to print that has little in the way of commonality between slices. I found myself fixing faults on 30-40 slices in Paint. That was seriously not fun. The main problem, as it developed, lay in the implicit dependence of Slice and Dice on looped road descriptions. Once you get into complicated parts it becomes very difficult to meet the app's expectation of clean looped print roads.

This is the part that started causing me trouble.

Virtually every slice is unique.

I rewrote the road making routine to deal with non-looped roads this evening. This is the resulting test print of the part shown above.

A problem that the old Rapman firmware was that it expected minimum print road line segments to be about 0.6 mm long. When I finished the rewrite, I was not looking forward to fitting line segments to the non-loop print roads. I had the output, but it was all 0.1 mm print roads.

Andrew at BitsfromBytes has said that the updated firmware would handle 0.1 mm roads with no problem. I frankly didn't believe him because the older firmware would slow the print down to 6 mm/sec with 0.1 mm roads. I decided to give it a try with the 4.0.2 code, however, and it turned out that Andrew is absolutely correct. The Rapman firmware has no trouble print sequential 0.1 mm line segments at my chosen print speed of 16 mm/sec.

That made the print file for the two halves of the thumb joint some 8 meg long. Since I have a 1 gig SD card, however, that's no trouble at all.

Wondering what the fuss is about

In which your narrator wonders what all the fuss is all about?

I guess I just don't get it. When you work with a plastic, after a while you get a feel for what it can do and what it can't. I started getting along along quite happily with no heated bed and ABS.

I print with a 0.3 orifice at an axis speed of 16 mm/sec. I suspect that I could kick it up to 22 mm/sec without a lot of drama, but I don't want to take the time out to play "who can print fastest" games at this point.

Once I started doing thin walled pieces and no infill my warping problems virtually disappeared. Most of the pieces I design have a largest dimension <= 90 mm though I've printed a herringbone rack that was 250 mm long. My acrylic print table temperature stays at about 25-30 degrees. You also don't find my prints warping after a few days from the internal stresses that Bogdan has talked about. I've seen that with HDPE. I was also printing with cross-hatched infill in those days, too.

I got into thin walled, no infill after I realised that if I went that way I got pretty fast prints that way at lower print head velocities.

Watching you guys reminds me of the first and second year architectural studio students that I used to lecture to back in my university professor days. They'd make models of buildings out of either "shipboard" {2 mm solid cardboard} or carve them out of expanded polystyrene foam treating it like it was so much cool butter or cheese. We used to say "form follows chipboard". If you followed the careers of those students they usually spent the first five years of their careers designing actual building that got built that looked as if they'd been carved out of cool butter. That's an extremely expensive way to design and build buildings. Those guys either got out of the habit fast or wound up doing architectural detail drawings for other designers who'd developed a feel and respect of the potential and limitations of the materials that went into their buildings.

Around here I see parts designed like you were more used to using an expensive CNC milling machine to carve parts out of a block of steel or aluminum.

That looks really cool but begs the fact that you're not taking advantage of the strengths and avoiding the weaknesses of the material you're working with, viz, plastic and extruded plastic at that. Somewhere along the line with Reprap we got the idea that we ought to be able to design parts in any shape we wished and whatever the material wanted to be be damned. You can see the trouble it's caused us in the chase after things like heated beds and support materials. We're spending a lot of time on that chase when we could be designing killer apps that make having a much simpler reprap machine very desirable.

That's just an old architectural technologist talking, I suppose.


I've included a link of the telepresence hand that I'm currently designing. That is human sized, btw. The largest part dimension is a touch over 80 mm. No warping whatsoever on any part.

Thin walled, no infill, snapped or glued together. There'll be a few screws in it to secure some elastic bands that return fingers to their rest positions. I haven't figured out how to secure elastics bands with snap on parts or glue yet. I'm thinking about how that might be possible all the time, though. :-)

Making a lid for a potentiometer mounting box

In which your narrator shows you a few tricks of the trade with thin-walled prints.


In my telepresence robot hand project I'm not using off-the-shelf servomotors but rather making them from scratch. A servomotor basically consists of a gearmotor, a potentiometer and some electronics, in my case an MCU.

You saw in my previous posting how I'd developed the hand segment and sorted out the gearmotor positioning therein. That done, I had to turn to see to mounting a potentiometer. I decided to simply add an extension to the back of the hand segment to contain the potentiometer.

I'll not go into detail about the design of the box itself but rather cover the trick one uses to design the lid for the box. Here is the box.

The mounting box is nothing special.  It's a simple square box with mounting rack groves in the sides, seating for the potentiometer and screw posts to secure the lid.

The box is simple to design.  It took me about seven design iterations to make everything fit properly and to get the proportions worked out.  Using the Reprap 3D printer at every step meant that it was a very low risk exercise.

The box itself in Art of Illusion 2.8 was a simple box in which I'd removed the mounting slots on the sides with two boolean ops.

Once that was done I simply removed the voids where the potentiometer would be seated and where the mounting screws would be fitted.  I've switched Art of Illusion over to wireframe mode so that you can see the voids since they don't penetrate the surface of the mounting box.

Designing thin walled parts is a bit like designing an old photographic negative.  You design the  skin of the object and the holes inside of it rather than trying to design the object itself.  When you print the object you simply don't use any infill.  The top of the box is just another kind of infill, so it is omitted, too.

Here you can see how the printed box seats the potentiomenter.

Now, designing the lid you simply take the Art of Illusion file for the box and leave out the void that seats the potentiometer.

You want the lid to have holes that match the screw posts in the box so you leave those voids in.  

Next you slice off the top of the box leaving the screw hole voids exposed.  You do this in Art of Illusion by simply creating a block and merging it with the top of the box down to where the screw voids begin.

Then you remove the top of the box using Art of Illusion's boolean ops function.

While you don't want to have a hole big enough to slip the whole potentiometer through, you do need to accommodate its shaft.  A simple way to do that is to simply take a copy of the seating void cylinder and put it back into the object as a solid rather than a void.

Since we've sliced off the top of the box you can see the top of the cylinder now.  Now it's just a simple matter of reducing the radius of the cylinder to a bit more than the radius of the protruding shaft.

Now do a boolean op to remove that cylinder so that you there will be a hole in your lid.

When I looked more closely at the potentiometer I noticed that there was a little metal tab on one side to act as a stop to keep it from rotating around its shaft.  I had to design a little slot into the lid to accommodate that tab.  I did that by simply locating a little block where I wanted the slot to be.

Then I did a boolean op to remove the space occupied by the block.

At that point I brought back the cube I used to slice off the top of the box and moved it down so that exactly the thickness of the lid was exposed.

I then did a boolean op that chopped off the bottom of the box so that only the lid remained.

At this point I discovered that I'd made a mistake.  I'd designed the hole in the lid so that it fit the shaft.  Actually, the shaft fitted into a seating hub in the potentiometer that stuck out about a millimeter.  I brought back in the shaft cylinder that I'd used a moment ago and widened the radius to a bit over the radius of the hub.

I then removed that cylinder via a boolean op which widened the hole to accommodate the seating hub.

Now comes the tricky part.  The lid as designed will sit on top of the box.  I want it to recess into the box.  I could try to do this using Art of Illusion, but there is a much simpler way much less likely to fail.

Remember that the box will be printed without infill.  In this particular case we will print it with two 0.8 mm print roads describing the perimeter with a thickness of 1.6 mm.  Print roads tend to be rounded on the inside of the box, a fact that makes working them with Art of Illusion a bit messy.  It is easier to just process the  lid in Slice and Dice.

Once you've done that you simply go into the Filled folder and pull out the image of the print roads for the lid.

In the present state of development of Slice and Dice with complex prints roads like this it is necessary to go in with Windows Paint and clean up the print road image a bit.  Here you can see a few flaws in the roads image.

I've circled two of the most obvious.  In the upper right hand circle we have a feature that is less than 1 mm in diameter.  The lower circle encloses a loose print segment that is too small to print.  It's best just to remove those in Paint.  You will also, when inspecting the print, note that several of the print road loops are broken by a missing pixel or have tags of pixels hanging on.  Those should also be fixed or removed.

You usually only encounter this kind of problem on slices with very complicated print roads.  That's not a big deal and it is a quick thing to fix in Paint.  Since Slice and Dice processes images rather than arrays of numbers at each step you can go in and make changes with Paint if there is some flaw that you want to fix.  You can even alter the print roads in Paint if that suits you.

Altering print roads is exactly the trick we will be using to make the lid recessed.  The box has a perimeter of two print roads.  We simply go into Paint with our lid and erase the outer three print roads.

That lets the print box lid fit into the box and sit atop the screw posts inside.  We could have simply removed two.  In practice, however, that requires a bit of touching up with fine grit sandpaper on the edges to get the lid to fit.  Removing three roads leaves a 0.8 mm gap between the box and the edges of the lid.  That's good enough for this job.

Here you can see the potentiometer box with the newly printed lid lying beside it.

Now you can see that the lid fits nicely over the potentiometer.

After that it's a simple matter of securing the lid with metal screws and putting on the potentiometer's washer and nut to complete the job.

Now you check the fit between the potentiometer box and the hand segment.

You are now ready to connect your potentiometer box onto the back of the hand segment with boolean union ops in Art of Illusion and then print out the resulting large part.

This is how you create a large complicated part by designing it as a set of smaller, simpler parts.

Designing with a Reprap machine in the loop

In which your narrator celebrates a return to craftsmanship made possible with the addition of a Reprap printer into the design process.


In the five years I've been on Reprap team I think that possibly the most terrifying and depressing transition of the many I've made during that time was actually having a 3D printer on my worktable and having to face up to turning some of my pipe dreams into physical realities.

Let me tell you, it's a lot easier to dream up something than to actually cobble it together and get it working.

To begin with I had discovered that thin-walled prints tend to be quite strong, properly designed, and don't have nearly the trouble with warping that we've had with large, filled objects.

For a project I wanted to create an animatronic hand for a telepresence robot project I've wanted to undertake for several years now. If you go out to and buy one you're looking at anywhere from $5-50K for one hand. I haven't got that kind of money so I dug around on the internet till I found a guy in Indonesia who'd made one for $300 in parts.

Andreas Maryanto's  approach is both brilliant and cheap.  I used his project as a starting point and was able to use his blueprints for getting proportions and sizes right.

 I was also impressed with the Meka Robotics hand design.

Meka's hand was expensive, but had several interesting points.  It was self contained and used elastic bands to return the hand to an opened position.  I liked that.

I started small, with a fingertip.  Actually, half of a fingertip.  I did the design with Art of Illusion 2.8.  AoI has always been about the easiest to learn, free 3D CAD app around.  With the introduction of version 2.8 the problems with the boolean operators have been largely solved.

While AoI 2.8 will, occasionally, create a flawed STL file, these can be easily repaired in the free Netfabb Basic app.  The combination of Art of Illusion and the free Netfabb app, now available on the cloud as well, makes for an extremely powerful combination.

I wanted the joint between phalanges in the finger to be integral with the print, that is, not requiring any additional hardware. I printed and worked with the finger tip for quite some time.

Eventually, after a few dozen prints and redesigns, I got it to do what I wanted.

After that, the rest of the finger came together rather easily.

So, I had the joints right, but how to integrate the actuating tendons was a puzzle.  Within a week or so I had a tentative solution.  I could incorporate slots in the phalanges of the finger to guide and protect the tendons.  This meant that I had to import the STL files for the parts back into AoI and carve out the slots to seat the tendons.  Here you can see the STL for the second phalange slotted.

After slotting the second, third and fourth phalange in AoI and reprinting them I had a tentative solution for flexing the finger.

At that time I used a single tendon running through guides along the top of the finger.  That was later to change.

I quickly explored using servos very much like the ones Andreas had used.

...and just as quickly rejected them as overspecialised and too bulky for use in my hand.  From there I went on to creating my own servos from scratch using gearmotors and potentiomenters driven by a micro controller.

Once I'd settled on a gearmotor that I had a supply of...

...it became a matter of designing a section of the actual hand and accommodating the gearmotor inside.

Notice that the design of this part is only aiming at holding the outline of the gearmotor and providing a recess for the finger.  The goal is to work out the seating of critical parts and getting the general shape of the part right before trying to accomplish more.

At that point, I needed two finger assemblies to get a feel for the clearances and volumetrics between finger ensembles.

Attention to the reel that would wind up the tendon that contracted the finger was the next step.  I'd decided by this time to use a Meka-style elastic band on the top of the finger to return it to the open position.

The reel was a tiny, finicky part, given the clearances in the hand section.  I printed two of them at a time to avoid having to do a lot of pausing for the reels to cool between printed layers.

That done, I needed to do a bit more work with the hand section.  First, I needed to seat the gear motor.  The hand segment was a big, rather complicated part, so rather than trying to redesign that I simply designed a plate that could be glued onto it that would achieve the effect.

By this time I had slotted the reel to make attaching it to the nylon tendon easier.

While I glued that gear motor support plate to the hand section, I could just as easily do a boolean union in Art of Illusion between the two parts and print them as a single part, or not, as convenience dictates.

From there, I designed a housing for the potentiometer that butted onto the base of the hand section.  You can see it on the left-hand side of this picture.

Notice how I haven't bothered gluing this part to the bottom of the hand segment but rather just clamped it.  I've not settled on all the dimensions and placement of this part yet, so gluing isn't justified.

Similarly, you can see here that I just took the hand section into the free Netfabb app and flipped it into a mirror image as a start towards completing the hand segment.

The point of all this is that you can take on some really challenging design exercises if you will just take them a little bit at a time.  Having a Reprap 3D printer {in my case a Rapman 3.0} makes it possible to print out a partially designed part and try to match it up with the bits that go in it and the bits with which it must integrate.

While you can do all that in a 3D CAD system you have to have a very good sense of volume and space to do so.  With the Reprap printer in the mix, you don't have to be that good.

Don't be afraid to use filament during the design process.  You're not wasting it.

Cloud CAD

Ordinarily, I publish only my own work on this blog. In this particular case, however, I'd like to pass along a nascent project to make OpenSCAD available in a computing cloud environment.

Tony Buser is the man on this undertaking. It's a very worthy one, imo.

Introducing Snakl!

He used to live on Tommelise 2.0 but recently took up residence on my Rapman 3.0...

Snakl does all the hissing!  :-D