Solving a nagging question about print adhesion

Unlike most of you, I don't use an electrically heated print surface.  Some time ago I bought a Rapman 3.1, which used an acrylic 3 mm print table.  I soon discovered that 3 mm was far too thin and quickly warped beyond use.  Switching to 10 mm solved that problem.


After a long time of successful prints, I noticed that with winter causing colder temperatures in the print room I was having more and more trouble getting my prints to stick to the acrylic.  I tried cleaning it and sanding it with little avail.  Electrically heated print tables were just coming available but insofar as printing was concerned, I thought that things were already complicated enough without adding that sort of equipment to my Rapman.


I had an IR heat lamp in the lab, detritus of another experiment, and discovered that using it on a tripod to raise the temperature of the acrylic print table above 40 degrees Celsius measured with a non-contact IR thermometer gave me consistent adhesion.  I soon discovered that I could turn off the IR lamp after 4-5 print layers with no ill effects.  It was not needed for the rest of the print.


The rig looked a bit like this...

Note that the lamp is placed at a 45 degree angle to the acrylic print table.


I soon noticed that adhesion at the near side of the print table was much less firm than that at the back and less firm at the left side than the right.  I attributed this to various things, uneven heating being one of the possibilities.  While the left/right difference made sense the front/back difference didn't seeing as the IR lamp was aligned with the left/right axis.


Cranking the terminal heating temperature before starting a print to about 50 degrees solved most of the problem for the center of the table and I was able to print along the front/back axis with reliable success.  Unfortunately, the back side of the print area seemed to have the print pad melting into the acrylic while the front side would separate easily.


It made no sense.  I thought for a while that it had something to do with the acrylic plate and rotated it with no effect.  Swapping ends and sides always left the back side of the print table very firmly attached to the print pad.  While that wasn't a horrible situation it was annoying, because it meant that processing the printed objects after separation became more time consuming.

A few weeks ago, I purchased a FLIR E30 thermal imaging camera with the intention of learning more about what was happening with prints as they were being laid down, the ultimate goal being building in advanced heuristics into my Slice and Dice app which converts STL files into Gcode.  I also had hopes about eventually doing some research into what actually happens thermally with extruder hot ends with the notion that I might be able to design a better one.


Yesterday, the E30 arrived and I decided that a good beginning exercise might be to look at the distribution of heat on my acrylic print table when I used the IR lamp in its standard configuration to heat it.  The results were quite unexpected.

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The lamp put down a marked hot spot at the upper right rather than at the right as I expected.  The upper right was exactly where I had the most trouble with print pad melting.  Obviously, the IR lamp did not give even heating when tilted but overheated in on the upper right.


This was nasty.  I had previously thought about using several smaller IR lamps at the corners of the Sampo printer that I have been developing.  If the smaller lamps behaved like my single, large one, however, this might not be a good idea at all.


I then got to thinking about how IR lamps are actually used in food heating cabinets.  They are almost always placed point straight down.  I rearranged my tripod to place the lamp almost vertically over the acrylic print table.

That sorted out the temperature distribution problem...

A new acquisition...

Details

Sampo's touch screen begins to work...

Adriaan has been working hard learning the TFT programming protocols and has his first touch screen menu working on the Sampo controller board.

Bogdan makes a measurement suggestion...

After I got the x and y axes operating independently, Bogdan suggested that I measure the steps between the limits switches to see how much difference in measurements might be attributable to the mechanical microswitches that I am using.  I had already been recording that with the y axis, so I decided to extend the monitoring code a bit.


With the y-axis, I had been simply writing the number of pulses to an SD card whenever a limit switch was tripped.  In that I was running the axes at 65 mm/sec, I was getting a rather substantial thump whenever a switch was encountered.  When I thought about it, I began to suspect that the impact was a result of the time it was taking the write to the SD card to happen in that I was doing that immediately after the switch was tripped.  I changed the code to record a set number of triggering events for each axis and then exit the stepper loop and print the whole set of measurements at one time.  That reduced the noise of switch triggering on the x axis to almost nothing.  It also reduced the noise from triggering events on the y axis, but not as much.  Considering the y-axis is shifting the whole weight of the x axis assembly, the extra momentum generated thereby is probably causing the larger thump.


I first took a set of 50 triggering events running at 65 mm/sec.

You can see that the two limits switches on serving the x axis trigger with slightly different sensitivities, one triggering about 0.3 mm greater than the other {transition is running at 0.89 mm/step for both axes}. The both y axis limits switches trigger at the same place except that occasionally one gets moody and triggers 8 steps {~0.6-0.7 mm} longer than the first.


I then took another set of measurements at 32.5 mm/sec. The NEMA 23s were near resonance frequencies at this speed raising the noise level of the printer considerably. I will have to see about damping this.

What you can see is that the variation on the x-axis stayed about the same while the moodiness of the one limit switch on the y axis disappeared. Notice also that the steps between switches are down.


From there, I took a set of measurements at my usual printing speed for Rapman at 22 mm/sec.

Decreasing the transition velocity got us further away from the resonance frequencies of the NEMA 23s. It must be said, however, that the printer was still louder than when I was running it at 65 mm/sec. You can notice here that the variation in limit switch triggering has dropped to 1-2 steps.


This has been an interesting exercise. One thing that is obvious now is that to control noise levels I should be controlling the stepper speed both by the delays between steps and by adjusting the level of microstepping that I am using.

X & Y Axes operational from the controller

I was finally able to get time to integrate the full anti-bounce board with the x and y axis limit switches.

The two axes are playing ping pong and running at a speed of 65 mm/sec with no slippage and no heating of either the steppers or the driver chips. I've run them all morning with no mishaps.


Now I am going to have to see to writing a gcode interpreter and taking a shot at the TFT 320x240 graphics touch screen for system control.

Leveraging Bogdan's anti-bounce circuit for Sampo...

In developing the Darwin-derivative, Rapman-derivative Sampo 3D printer project as a kaizen exercise I utilized the same sort of microswitches for limits checking as are specified in the Rapman design.  I soon discovered that the switches have a formidable electronic bounce.  I was able to control that using the button function in my firmware compiler for the y-axis.  The computations taken for a firmware fix, however, were going to put a terrific drag of my MCU that I didn't want to have to deal with.


Enter Bogdan Kecman with helpful suggestions on how to put together an antibounce circuit for the limits switches.

I had last built an antibounce circuit in 1981, so his help was greatly appreciated.  I built a lashup of the circuit to check to see that the component values were right and then went on to design a board to handle all six limts switches.  I wanted six instead of Rapman's three because a lot of problems that I'd had with Rapman stemmed from the fact that it has limits switches only on one end of its axes.  When things went bad one could find steppers trying to skate off of the unchecked far end of axes.  As well, Rapman limits checking only seems to be done when one is resetting the axes at the beginning of a print.  I want to do better than that.


I bought components and dug out my stripboard and had a go at the design.  Some time before I put together a stripboard design program after having had no luck with the ones I was able to access on the web.  Eventually, I evolved this board.


Frontside...

Backside...

It has been some time since I built a board, so I found putting this one together quite frustrating.  I was about to give up this evening after making a bunch of mistakes and then got angry to the point of rage.  The adrenalin let me get the @#$#@$ thing finished.

Tomorrow I will drill out the breaks in the strips, check the board for continuity and, if I have enough time, try to rig it into Sampo and extend the firmware to utilize it. I suspect that will have to wait till the weekend, however.

Mendel z-axis stepper mounts done

I managed to get some hours together to do some more work on my son's Prusa Mendel, the z-axis stepper mounts this time.

I printed the z-axis stepper mounts at a 45 degree angle to minimize parts preparation time and avoid warping.

I'm now working on the x-axis stepper and idler mounts.  My son processed the stepper mount last night and I did the idler mount this morning.  They're printing this evening.

Mendel frame takes shape...

I had a major crash of the Rapman 3.0 printer and for several hours I thought I was really knackered. One of the leads to a phase of the extruder stepper parted because of fatigue from thousands of hours of vibration. The system shut down and reset. At first I thought I had a simple static discharge event, the first in many months. I fired the system back up and discovered that the extruder stepper would dance around but wouldn't pump filament.

After serious prayers that the stepper driver chip for the extruder hadn't fried I rewired the connector and got the extruder pumping ABS again only to discover that the intense vibration from the stepper before the reset had actually shaken apart the Arcol extruder hot end that I had bought from László Krekács in Hungary.

This sounds worse than it was. I simply screwed it all back together and cleaned the extruder end and it worked perfectly again. Unfortunately, I will have to recalibrate Rapman now. That should take a few hours that I didn't have available today.

In any case, I was able to cobble the Prusa Mendel frame together this morning.

It's a dinky little thing, but interesting all the same.

Printing a Mendel derivative

After talking with my son recently, I concluded that he needs to start printing a lot faster than I'm going to have Sampo debugged and duplicated. I have a lot of 8 mm linear shafting and linear bearings in stock, so I decided to build him a Mendel derivative using Sampo firmware and controllers. That should get him printing a lot faster than would otherwise be the case.

I started printing parts for a Prusa Mendel yesterday. So far, so good. Got the gantries finished and am printing the rest of the parts now.

5/16th inch threaded rod is pretty much a one on one replacement for M8, #8 machine screws replace M4s and #4 machine screws replace M3s.  I couldn't see much point in printing the SAE Mendel.  I'm going to have to redesign the extruder carriage to seat linear bearings instead of those strange PLA things it ordinarily uses.

Picking up speed

The MIPS core that PIC32 uses is a very high performance CPU that was used in high end Windows workstations in the early to mid-1990s. It doesn't behave much like the 8 and 16 bit PIC chips, so it's taken me a while to get down the learning curve. The button function in the Mikroelektronika compiler library works, but requires about 10 msec to filter out the bounce when a limits switch is encountered.


Processing a button function for each step when I was running at half step slowed the y-axis down to 12-15 mm/sec. To get around that problem in firmware I wrote a smart limits switch routine that runs slow until it finds the first limits switch and then kicks the stepper motor up to full speed until it nears the other limits switch.


Using this approach lets me increase the maximum transition speed for the y-axis from 15 mm/sec to about 65 mm/sec for my firmware testing as you can see in the video clip.

It's worth noting that the Allegro driver chip has a maximum rating of 0.75 amps and the NEMA 23 is a six wire model drawing 0.5 amps per phase. I've wired it in series which brings that amperage down considerably. In spite of this I'm getting 65 mm/sec and both the stepper and the NEMA 23 are running quite cool. The driver chip requires no heat sink or fan.

Y-axis test firmware operational

After a delightful chat with Bogdan this morning about the PIC32's MIPS core processor, I was able to get the y-axis test firmware working.  Bogdan has done a lot of work with the PIC32 and is very generous with his knowledge.  A few hours later, I had the y-axis responding to the limits switches.

Right now with sampling from the limits switches in the same loop that runs the stepper I'm getting 15 mm/sec.  On its own the stepper can do about 52 mm/sec.  I suspect that the speed of the axis will be getting a lot closer to that upper limit once I get the limits switches into an interrupt loop.  :-)

Printing flexible cable guides...

There is not much to say about this.  Once I got two of the links printed and assembled so that I knew everything fit together, I bought a few hundred #4-40 3/4 inch machine screws and nuts to hold the parts together.  I'd designed the parts to perfectly seat a 3/4 inch machine screw and nut.


When I got home with my trove of fasteners from my stockist I discovered that his Chinese supplier had been making a little extra money by trimming his 3/4 inch screws (0.75 inch) down to 0.714.  What that meant was that the screws went all the way through the guide assembly but didn't emerge on the other side to allow the nut to be seated on the end of the machine screw.


My stockist is getting me some 7/8 inch machine screws as replacements and writing a hot note to the warehouse.  Quality assurance at the Chinese plant needs a bit of a rework, I think.


Interestingly, I had designed the holes for the #4 machine screws so that the threads engaged the sides of the holes, so actually nuts weren't required.  With that in mind I went ahead and assembled the flexible cable guide for the x-axis.  It seems to work perfectly.

I get a tight turn like I'd hoped with no clashing.  Right now I am up to 16 inches of a 24 inch assembly for the x-axis.  When I get the full 24 inches printed and assembled I will design and print the end mounts.


It will be interestingly how many hours of operation this kind of flexible cable guide will handle before something wears out.  

Flex cable carrier

I've never been happy with the way that Rapman handles axis and extruder cabling, so I decided to print my own flex cable carrier system.  I saw several possibilities in Thingiverse.  Most of them were knockoffs of existing injection molded parts, however, and printed very poorly.


A few looked as if they were designed specifically for a 3D printer like this one...

I didn't much like this one largely because of the large turning radius for the flex.  I wanted something more like this...

Since my wiring was considerably more modest, however, I wanted something with a bit sharper turning radius still.  After several hours of trying out alternatives in Art of Illusion, I came up with this as a first try.

It is shown here with #4 bolts 1.5 inches long.  It can work interchangeably with #4 UTS/SAE - 3/4 inch or M3 - 20 mm bolts.  With fasteners it costs about $1.50/ft.  Commercially available alternatives average about $12.50/ft.

The system can make a 180 degree flex within 50 mm.  The next move is to get a couple of packets of #4 bolts and nuts and print a few feet of this to try with the x-axis cabling.

Another battle in the string wars: cabled z-axis working

It took me 10 hours and fifty minutes to print the first print table bracket and I got a few measurements wrong, but it appears that the cabled z-axis concept is going to work.

Since the speed of the z-axis is not particularly critical, I intend to use a fairly high gear ratio between the NEMA 23 stepper and the lead screw that drives the cabling to insure that I have adequate force to overcome friction in the system.

Getting into threading...

When I bought my Sherline lathe, I went for the loaded option.  As I am finding out, it was a very good decision.  I don't think I'd have been able to use at all it without the digital readout box.  I understand how people in times pass could have used a micrometer to do measurements between cuts.  I doubt that I could have had the patience, though.


After my last attempt at doing a deep drilling and cutting down a piece of aluminum bar stock, the next thing I wanted to do was use the threading attachment.  Between Joe Martin's Sherline book, Tabletop machining and the Sherline Accessories Shop Guide I was able to puzzle out how to re-rig the lathe for threading.  My motive in getting a threading attachment was to have the possibility to cut my own lead screws and thrust collars.  The studding used in the Rapman and Mendel 3D printers, not to put too fine a point on it, suck the middle out of canned Vienna sausages.  When you buy studding it is rarely, if ever, straight.  When you try using it as a z-axis lead screw, it does not very pleasant things to your print quality.


As well, I am quite fond of the polymer pump design on the Rapman.  My primary reason for wanting to do my own threads was to be able to make the threaded drive shafts for those, too.


The biggest shock for me in this adventure happened when the instructions book told me to demount the lathe motor.  It turns out that you cut threads on the Sherline lathe with a hand-powered handle.  Using it reminds me of my grandmother's old foot pedal powered Singer sewing machine that I learned to use when I was about four.  It has a very similar, 19th century feel to it.

By and large the threading system design is quite good.  The real pain in the tail, however, is that little black lever that you see below the spindle in the above picture.  Getting that to work right had me taking the whole lathe apart about three times.  Sherline needs to rethink that part of the design.  It really sucks.


Other than that, the threading attachment was quite easy to use.  It is set up to do standard and metric 60 degree threads.  The carbide cutting tool did that quite easily.  


For a first try I did a really coarse metric thread, 2 mm.  

I figured that that deep a thread would require the most muscle.  It was a bit tiring but not at all impossible.


For lead screws, the Sherline will handle about 15 inches of threading.  If the diameter of the stock is less that about 9 mm, a longer unthreaded piece of stock can be snaked through the spindle.  Given that manufactured lead screws about cost $1/cm and thrust collars typically cost $30-35, this capability will save me some money over the mid-term.  

Taking a different direction

A few months ago, I had planned on acquiring another BfB Rapman with two print heads this time so that I could explore the use of PLA as a support material for ABS prints.

With the sale of BfB to 3D Systems, continuing troubles with BfB firmware and the catastrophic failure of my expensive Rapman hot end and the discovery that BfB isn't warranting such failures, I've decided to build my own second printer.

To that end, I've just purchased a Sherline 4410 metric lathe.

It will be arriving Thursday.

Repairing a catastrophic failure of the Rapman 3.0

Little did I know when I was upgrading Slice and Dice to do prints of laser-cut Rapman parts that I would soon be confronting the problem of replacing such parts without having a 3D printer to print the parts. That is an ENTIRELY different problem.

I had upgraded Slice and Dice to do a better job of printing my sample laser-cut part from last time and was doing trial prints when I noticed that I was getting an enormous number of resets. This was odd because ordinarily I only get one or two resets per month since I uploaded firmware version 4.0.2. I checked the humidity and it was a bit dry, so I fired up the hot mist humidifier and drove the relative humidity up to 52%. No joy.

The next morning I undertook a new print and noticed a mushroom of ABS had formed under the extruder.

When I attempted to remove the extruder, the mushroom of ABS proved to be too big to easily get out of the mounting hole.  I then began to disassemble the x-axis carriage that holds the extruder only to discover that the top plate was being held together by the grace of God and nothing else.  It crumbled into two major pieces and half a hundred small fragments when I began to remove the bolts.

As well, one of the tiny little bars that hold the x-axis belt had also broken in half as is readily visible in the previous picture.

As you might imagine, this all made me very cranky.  A quick calculation revealed that I had historically been spending considerably more on preassembled BfB hot ends that I was on the filament that goes through them.  I run a lot of filament, too.

I spent several hours chatting with both Iain and Andy at BfB.  They were attempting to be as helpful as they could, but somehow I came away with the same feeling that I get when I take my car into the garage and they say recursively, "It MIGHT be X.  We could work on that and see what happens."  I began to steam up a bit when they started talking about my not using "BfB gcode".  They'd latched on to that fact that I'd written my own STL processing software that produces gcode right out of their manual and tagging this as a possible problem.  They then started talking about the fact that I was using very short line segments {0.1-0.1414 mm} and that was possibly touching off heretofore unsuspected bugs in the firmware.  I pointed out that the Rapman is supposed to have 0.1 mm resolution and you can't print objects at that level of resolution unless the firmware can handle that short a line segment.

I finally decided that I had to stop talking before I said things I'd regret later {I do that a lot}, and have a good think.

Here's what it came down to...

• I needed a new top plate for the x-carriage and a new hot end.
• I was going to have to pay for these and it was going to take a week to get them
• It was entirely possible that the hot end had been destroyed by a firmware bug
• There was nothing to say that the new top plate would last any longer than the last one, viz, ten months.

No matter how I looked at that equation it just didn't seem to balance.  A big bone in my throat was the fact that BfB wanted me to pay to replace a hot end that it was very possible that their firmware had broken.  A bigger bone was that there was no guarantee that if I put the new hot end that I bought into the system that the firmware wouldn't ruin it, too, in short order.

It seemed to me that the most reasonable course was to see if the problem was with the firmware.  I typically print with a 0.3 mm hot end.  I like what that kind of resolution does for my prints.  Before I settled on 0.3 mm, however, I bought two, preassembled 0.5 mm hot ends, so I had those in stock.

I also had Bogdan's experience that replacing the top plate on the x-carriage and grounding the hot end to it would stop the resets.  This from the observation that resets were most often caused by a static charge building up on the plastic of the x-axis carriage and extruder and then discharging, causing a reset.  BfB which is apparently located in a damp environment had never encountered this issue.  I noted that resets tended to get quite common when the relative humidity in the room holding the printer dropped  below about 42%.  I'd sorted out resets, except for the ones I encountered most recently which have led to the hot end failure, I think, by using a hot vapour humidifier.  After seeing how the top plate had crumbled, however, the notion of replacing the acrylic one with an aluminum one began to sound very attractive.

I decided to acquire the means to cut both acrylic and aluminum.  Harbour Freight in Salinas had a very nice scroll saw on sale for $69 which would reputedly do the trick.  

I bought that, a sheet of 0.22 inch (5 mm) acrylic and a billet of 3.35 mm aluminum plate.  I decided to cut an aluminum top plate first.  I began the process by simply tracing the bottom plate, which was identical to the top plate onto the aluminum with a fine tip marker.

I did the rough cut with the scroll saw and dressed it with a grinding wheel and a half round ring file after having set the plate in a small vise.

I then remarked two holes at diagonal corners of the plate, drilled 1/16th inch guide holes and widened them to 13/64th inch {as close as I could get to the 5.2 mm holes as I measured them on the original piece.   I did this with a hand drill after securing the plate in a vise. 

This done I then bolted the acrylic bottom plate to the developing aluminum one and drilled the guide holes for the rest of the holes using my Dremel drill press.

I then removed the acrylic bottom plate, secured the aluminum plate in the vise again and drilled out the rest of the holes.  

Afterwards I cleaned up the finished plate with a wire wheel and checked it for fit on the x-axis.

At this point my dyslexia set in.  The plate is not symmetrical and I'd got it flipped and completely reassembled and tested the carriage this way.  I didn't notice the problem till I tried to fit the extruder into the top plate assembly and discovered the symmetry problem.  The next pic is from the original, incorrect assembly.

The new top plate works smoothly.  Now I've got to swap out my ruined, 0.3 mm extruder with one of my spare 0.5 mm ones.  I will be able to see if I have a serious firmware problem or whether I just have a design fault with the hot end.  It could be either, or both.

I'm entering a big of a crisis with respect to 3D printing.  I bought into BfB's Rapman because I wanted to do some printing instead of screwing around with printer design and problems all the time.  At the time, a year ago, it was a good move.  Rapman was a bit pricy, but it was solid and the components of it were affordable. The 32bit MCU board was a delight after all of the Linux/Arduino/Sanguino/Bullshitino nonsense.  Some months ago there was talk of extending the Rapman MCU to where you could parameterise the firmware setpoints to deal with different machines and extruders, like the Mendel, for example, or even machines you'd designed yourself.  As it stands, it's not clear that BfB can design reliable firmware for their own machine, much less a parametric firmware app that would make it applicable to a wider range of machines.  

On top of that, they've recently jumped the price rather dramatically.

As it stands, BfB's Rapman has two Achille's heels; their firmware and their hot end.  Neither are reliable and the hot end is very difficult to repair.  I'm told that BfB is working on successors to the hot end, but that does me no good at all.  I'd like to shift over to something like Nophead's power resistor driven hot end.  The problem with that, however, is that I'll have to design a MCU to drive it and the printer both.  By the time I've done that,  BfB is out of the picture, since the those two components are what is defensible as corporate worth in the BfB.

I don't know quite what to do.

Chasing bugs in Slice and Dice

I had completed my narrow profile telepresence finger and then set about designing a mount for the gearmotor and potentiometer to drive it. The mount came out looking a bit weird.

Trust me, though, there were good reasons why it took the shape that it did ...  in my twisted mind, at least.

The first thing I discovered was that my print roads routine is not extremely happy with sharp ended print roads like you see here.

Fortunately, I was able to pull the few unique slice images into Windows Paint and clean them up rather handily in just a few moment.  Slice and Dice is very good about giving you ways around problems you encounter with difficult parts.  The messy print roads were only the beginning of my troubles, however.   When I tried to turn the image into an XML description of the roads all hell broke out.  My pathfinder routine, which worked well enough for most slices, really hated this gearmotor mount.

I finally gritted my teeth and spent the time chasing up the many bugs in the pathfinder routine.  Once that process, which took about two man-days, was complete, I was able to get a pretty good XML conversion.

Red overlays indicates well formed print roads.  The black residue shows where the routine failed.

You can see the failures circled in blue.  Looking a bit closer at a few of the failures you can see that the routine breaks down when the distance between two parts of a print road drops below 0.2 mm.  That's a ridiculous case, but slice and dice routines regularly encounter ridiculous cases.  Here is a typical one.

Here you can see that the pathfinder routine jumped the 0.1 mm gap between two sections of the path.  Another thing that became obvious is that the routine can't handle paths which are less than 1 mm in their greatest dimension.

None of these faults are serious enough to cause me to continue working on the code at the moment.  I'm back to printing.

Finishing the reel

I discovered that while the mismatch of inner and outer loops can be sorted out on a single slice fairly easily, things get tougher when you have successive slices getting smaller as I do with the 45 degree angled slopes on the servo reel.

What I did was to make two halves of the reel with boolean operations like this...

Slice and Dice lets me control the print roads on each slice, so I simply arranged it so that inner and outer loops did not meet on one side...

...and left off loops on the other side to make the reel into two halves that fitted together.

So, it was effectively back to glued together plastic model airplane dodges again.

Improving print roads calculations

In which your narrator rewrites the Slice and Dice routine that calculates print roads on a slice.


Having got a working finger design for my telepresence hand project, last weekend I set about to rig it to a servo motor to see what issues would come up. One of the items needed was a reel which screws onto the servo which takes up the tendons on the finger and moves them in concert.

The 40 mm reel looks like this...

I wanted to make the reel solid since it will be under considerable torque.  To achieve that I used my concentric print roads option rather than cross hatching.  I am not fond of cross-hatching mostly because the join between the perimeter of a print and the cross-hatching is so often mechanically poor.  Using concentric print roads yields a much stronger print.

I calculated concentric roads very simply by painting a strip of pixels on the boundary of the slice to create an interior print road, deleting the strip to define the print road and then painting another strip inside of the new, smaller boundary.  I repeated that simple process until I ran out of slice.

Heretofore, I'd used concentric prints for relatively shallow prints like this phalange...

This looks pretty good.  I had been quite happy with the concentric roads routine till I applied it to the spool.  The spool was a deep object, 40 mm in diameter with an interior and exterior boundary.  You can see what happened...

The roads track the original boundary quite nicely till you get 6-8 roads away from the boundary.  Then, geometric features of the original boundary are sufficiently magnified that the roads bear little resemblance to the shape of original boundary.  The reel provides a really nasty example of this kind of distortion.

The reel that resulted was actually very strong, but looked nasty.  Last night, I undertook to see if there was another way to do the job.

After several false starts, I discovered that if I began with the original slice boundary in one picture box and then mapped a filled circle of the proper radius onto each pixel in the boundary I got extremely smooth print roads.  The reel slice you were first shown looked like this with the new routine...

You will notice that the interior and exterior boundary roads match each other very well but show a gap between the interior and exterior roads.  The hole in this slice of the reel is a screwdriver access port for securing the reel to the servo drive shaft.  The dimension in this slice is just big enough to pass the screwdriver tip.  I widened that access port by two millimeters and the problem with the roads mismatch disappears as you can see here...

The reel consists of two parts; the reel itself and an insert which is glued into a recess in the reel which seats over the servo drive shaft and allows for a screw to secure it.  You can see how this works in this exploded view of the ensemble...

As designed the print road pattern for the reel slice defining the recess for the insert looks like this.

If you look closely you will notice that the meet between inner and outer roads is a bit tight.  The outer dimension of the insert is not critical and can be adjusted a bit to even that out.  Care must be taken, however, to not compromise the roads in the insert in the adjustment.

The more experience I get printing dimension critical parts the more I understand what a balancing act getting the proportions is.  When you are printing a critical part you can't just slap it together in CAD, pump out an STL and print it.  The print road cross section is an integral part of your design and you have to exercise considerable cunning in making everything work together.  This experience is much like what I learned after I graduated from architectural design studios eons ago.  It's easy to design something that looks nice if you have any sense of aesthetics at all.  Designing something that looks {or works} nice that can actually be built is a far harder task.

In any case, I've got the new print road mapping routine running rather well and am going back to work on my telepresence hand project.