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...

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.

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.

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.