Sunday, November 27, 2011

RapMan 3D Printer Modifications

I purchased a RapMan 3D printer from Bits From Bytes because it is, as BFB claims, a very cost effective 3D printer.  However, once I had it, I couldn't resist redesigning it. I began by replacing the plastic corner pieces with heavy duty aluminum ones, and didn't stop until I redesigned nearly everything.
The end result bears only a passing resemblance to the original RapMan.

I started by replacing the corner pieces, which consisted of 3 or 4 pieces of polycarbonate and about a dozen M2 screws, with single solid blocks of  1 1/2" x 1 1/2" aluminum.  Then I remembered I had several feet of aluminum C channel, 1/2" outside and 1/4"  inside.  This I decided would make great channels to hold side panels to replace the original X braces.  The result was this foundation for my new machine:

Wherever possible, I tightened the tolerances in my replacement parts.  The bearings now rest in bored holes which hold them in rigid alignment, while the round stock from the original RapMan is tightly pressed into holes in the corner blocks.  The C channel is also pressed into slots, and locked with screws.  The entire foundation is very stiff and square. A 1/4" aluminum plate serves as the motor mount.

Here is a side by side comparison of my aluminum corner assembly vs. the plastic RapMan corner.
The aluminum assembly is comprised of two pieces held together by a milled slot and a single 1/4-28 bolt, while the original consists of 5 pieces of polycarbonate, 12 screws with nuts, and a short piece of 8mm rod.

What must be remembered here is that my design is not economical to manufacture and sell, and that the RapMan design does work OK.  The point I'd like to make is that it is possible to take the affordable RapMan and turn it into the equivalent of a much more expensive machine.

Below: I replaced the idler pulleys made from a ball bearing-and-washer sandwich with a lathe turned pulley riding on an 8mm bearing.
The jack-screws are made from common hardware store threaded rods and nuts, with laser cut polycarbonate toothed pulleys sandwiched between washers. Everything is a sloppy fit, but I'm not complaining, for using common hardware enabled BFB to keep the cost low.  I found the holes in the toothed pulleys to be about .005" over-sized, or about the thickness of a Nine Lives steel cat food can.

This solved two problems: The sloppy fit, and the fact that friction by squeezing the pulley between two nuts was the only thing that locked the pulley to the shaft, for there is no key.  The cat food can made the pulley a tight fit on the shaft, enough to turn the shaft even if the nuts are loose.  At right is a picture of the jack-screw and pulleys, along with a strip from the can.

The original assembly is the one the left.  It has 2 small washers for spacing, then two large washers to form the sides of the pulley.  While the symmetrical appearance looks good, the resulting pulley is too wide.  I settled on the right hand version, which is asymmetrical in appearance, but the correct width for the belt.

I felt the motor pulley deserved special consideration, and made a pair of lathe turned discs to replace the washers. The assembly is held together with the original screws, and clamps to the shaft as before. 

The RapMan merely pinches the ends of the Y axis belts, which can shift over time.  I made clamps with slots that match the teeth on the belts for a more positive grip.

Then I decided that the nut assemblies which support the table were too flimsy for my requirements, so they were the next polycarbonate pieces to go.  Here is a picture of my aluminum replacements.
I retained the basic design, right down to the hex shaped hole for the nut. However, I changed from the two double guides like the original shown here, to four single guides, one at each corner. The double guides help prevent binding on the more flexible original design, but my rigid aluminum design does not require them. The slots that form the inverted "V" in the bottom of my guide are clearance slots for aluminum "C" channel which now frames the table, stiffening it considerably.

The_completed table and "Z" axis assembly is sturdy and powerful.  Here it is lifting 4 gallons of antifreeze, which is nearly 30 pounds.  Mechanically, it could handle more, but the stepper motor stalled at higher weights.
The completed frame is sturdy, and initially seemed rigid enough.  However, when I tried printing at higher speeds, the frame shook a bit.  Also, I had a misalignment of about .005 inches in the frame from one side to the other.  Adding diagonal braces solved both problems.

Also visible in the first picture is another improvement, a case for the electronics.  As another cost-shaving idea, the electronics come with only a top panel, while the sides and back are exposed. This does not present a hazard to the operator, but it does make the electronics vulnerable.  In addition, there is no strain relief for the wires.

Here is a rear view.  The wires go through the holes in the board, and that is it. 
I fabricated a case from the metal case of an old VCR.  VCR's are a great source of a variety of parts. I wished to keep the original RapMan electronics assembly as original as possible, but I did make 2 modifications. First I countersunk the screws in the top cover for a nicer appearance, and then I milled a relief around the perimeter so the cover would fit flush with the surface of my case. 

The electronics slide in from the side, and are retained by polycarbonate pieces which lock everything tightly in place.   The cable protectors are twisted into threaded holes in the polycarbonate and will not pull loose.

The picture below shows how the side pieces support the electronics cover and circuit board.

Below, a close up of the assembled controls. The side panels are .10" steel, and the whole thing feels very substantial.  There is plenty of room for air circulation underneath.
The screw terminal "D" connector will be replaced with a conventional one, further improving the appearance.
 With all of my experimenting, I have now fried two stepper motor driver chips. I repaired the board, but some of my repairs are a little unusual. For more on that, click here

More to come!  After going this far, I've decided to redesign the carriage the print head rides on.  There will be very little polycarbonate left, and the the machine will gain a few more pounds in weight.  It will likely tip the scales at around 60 pounds when complete.
The print head development took a while, for I developed an entirely new design.

The Rap Man printer head works OK, but is excessively large in size, and is complicated, with the body made from 10 pieces of laser cut Plexiglas. The head is over-sized because the motor , feed screw, gears and bearings are mounted at an angle while the raw material being fed to the printer is vertical.  BFB did this to move the drive gear and bearings out of the way of the path of the raw material.

In  the more expensive BFB3000, they made the head smaller by mounting the drive gear vertical and bending the raw material.  This also works OK, but is a nuisance to reload, as one has to snake the material through the curved path.

My idea was to have the best of both worlds by positioning everything vertical.  This would make the head small, and reloading simple.  But, of course that meant the bearings would be in the way.  The solution here was to move the bearings out of the way.  But how to do it?  Instead of having the drive screw go through the center of the bearings, as is normally done, I put the drive screw on the outside of the bearings. Now, instead of the center of the bearing rotating while the outside is stationary, the outside rotates, and the inside is stationary.

This required two bearings at each end.  The feed screw now rides in the valley between the two bearings, leaving the top of the feed screw unobstructed, creating a straight path for the raw material.  

Below is a picture of my feed mechanism. 
Very compact, and a much more attractive design than the original RapMan head.

Now that the material feed design was complete, it was time to develop a mounting system. Primary requirements were ease of removal, and a versatile design which would permit me to experiment with a variety of print head designs, including the ability to swap the 3D print heads with a mechanical or laser engraver head, or perhaps even a pottery extruding head.
I settled on a dovetail style mount, which eliminated the need for threaded holes in the carriage, and bolts to attach the heads.  A single screw on each print head expands the dovetails, clamping them in position quickly and securely.
The carriage is machined from a solid block of 1 inch thick aluminum.   This allowed me to make a very low profile carriage whose top surface is just 2 mm above the linear bearings.  This helps to reduce the overall height of the machine. Combined with the low profile print heads, my machine is about 4 inches lower than the RapMan.  My carriage is also smaller in both the X and Y dimensions, resulting in more x and Y travel than the RapMan carriage.
One design problem I struggled with was how to get power to the extruders in a way that looked tidy and also gave me the versatility to experiment with different styles of heads.  I rummaged through my collection of old Apple hardware and found a 50 conductor ribbon cable from a hard drive.  It was an ideal length, and I was able to cut the connectors off the hard drive and motherboard.

I cut one of the connectors up into several small ones, and made individual plugs for each motor and extruder.  This way I can easily remove them separately, or swap their positions for troubleshooting.  
Unlike  the RapMan, which required removing several screws and wires, I can remove each head in under a minute by loosening only a single screw.
Since ribbon cable has light gauge wires intended for carrying signals, and not large amounts of power, I used 3 wires in parallel for each extruder and stepper motor.  This arrangement works well, giving me the flexibility of a ribbon cable, and the current carrying capacity of a thicker, and stiffer, wire. Out of the 50 conductors available in the ribbon, I used 46.

Finally, I wanted a convenient means to hold several spools of material without taking up valuable table space.  I mounted the spools on top of the machine, where they are readily accessible.  A picture of the completed machine is below. The whole thing fits nicely on a small 2 foot x 2 foot  table.

  I ran the "Duck" test program provided by BFB, and made some very nice ducks in a variety of colors.  The surface finish is quite good, and I'm impressed with what can be done without using support structures.  These ducks are ABS, but I've also made them in PLA.
 I modified the duck file to make two color ducks as a test of the second extruder head:
After making a whole flock of ducks and other things, I realized that there is still room for improvement.  One thing I discovered is that PLA stays soft for a longer time than ABS.  This made it difficult to make small parts because the material was too rubbery and unstable when the extruder returned for a second pass.   I solved this by adding a second fan on the rear of my carriage.  Below are pictures of the brackets, and how they mount on the carriage.

 This solved one problem , but created another.  The RapMan extruders always struggled to reach maximum temperature, and now with the increased airflow, and the addition of a heat sink on the inlet side, they really struggled. It didn't help that BFB did not insulate them well, or used 3 stainless steel standoffs to mount them.  I turned the center portion of the standoffs down in my lathe to about 2.6mm to reduce the heat transfer.  As the cross sectional area of the standoff is now less than half the original, the heat transfer should also be reduced.
 I added additional high temperature RTV to the exposed back of the extruder, and covered the slimmed down standoffs. It did not make a huge improvement, but it was enough to allow the heads to maintain 260 degrees.

I ran these extruders for many hours, and they generally worked well, but would occasionally lose their grip on the filament.  Increasing the pressure on the filament against the screw helped, but then the motor would occasionally stall.  I realized that the blunt machine screw style threads took considerable pressure to bite into the filament, so I designed a much sharper tooth profile.  Below is a drawing representing the original profile and my design.

I made these screws from water hardening drill rod, which I hardened after machining.    The new screws are working well so far, and require much less pressure to maintain a grip on the filament.  It remains to be seen whether the hardened drill rod maintains a sharp edge.

Another improvement was made to the material feed arrangement.  Feeding from overhead quickly proved to be less than ideal, so I added a pair of turning pulleys to bring the material in from behind.  The top pulley removes quickly without tools.  With room for 6 materials overhead, material changing is far easier than most other machines I've seen.


With the machine now running well, I decided to tackle the issue of ABS shrinkage and build a heated bed.  Since my existing bed was made from 1/8 inch aluminum, I was off to a good start.  I experimented with a variety of heaters and decided that 300 watts was about right.  The heating elements were removed from food warming trays which had a nice length of Nichrome wire insulated with fiberglass.  I would not recommend going beyond 300 watts, for if your temperature controller sticks on, the bed would get extremely hot.
I had a old British made CAL 5000 temperature controller in my junk box, so I built an enclosure and used that.  It uses a type J thermocouple.  The heated bed was a huge improvement when printing in ABS.  It also helps when using PLA as a raft material, for it keeps the PLA slightly soft and sticky.  
     Next problem.  I was not satisfied with Kapton as a bed surface, and I heard of someone using a stone surface, so I decided to give it a try. I tried a marble tile and that has worked well so far.  I'm using the rough back side of the tile as my surface. As long as the tile is warm, the PLA raft sticks well. When it cools, the PLA pops off by itself.

Finally, here is my copy of Emmett's Heart Gears, a popular download from the Thingiverse site:

Sunday, July 10, 2011

John Deere 300 / 316 Mower Deck Rebuild

While my John Deere 316 and overhauled mule hitch are now in fine shape, the 46" mower deck is not.  Some of the mower bearings began to fail, and I decided to replace them all.  When I removed the deck and began disassembly, I discovered that the deck was badly rusted.  Now the bearing job turned into a major overhaul.  I removed the loose rust and and sprayed the badly pitted surface with a rust converter, then painted it.  While the paint adhered well, I did not like the pitted surface, which looked like it would be difficult to keep clean.  So, I filled the pits with auto body putty and re-sanded the whole thing.  These two pictures were taken before I filled the pits.

Unfortunately, some of the worst rust damage was in the vicinity of the front roller and mule hitch attachment points.  Not surprising, since this area quickly fills with wet grass.  I cannot afford to have this area rust anymore, and decided to make a reinforcing patch which eliminates the corner and protects the critical areas from grass accumulation and hopefully, more rust.  The blade has about an inch clearance, which should be enough while preventing grass buildup.

The patch is 0.10" steel, about the same thickness as the mower deck.  After cutting and bending the rough shape, it was easy to pound it into a form fitting patch.  It is held by 10 5/16" bolts and is easily removable for inspection.   Satisfied that I had a good solution to the rust problem, I turned my attention to the bearings.  They were, not surprisingly, difficult to disassemble.  The nuts on top of the pulleys can be very stubborn.  I removed one with great difficulty, only to discover that I damaged the threads on the shaft.  See right end of top shaft.  Fortunately, it is still use-able, for replacement shafts are around $45!
Next nut I did the smart thing and cut the nut in my milling machine.  Now it spun off easily, leaving undamaged threads. Replacement nuts are only $3, much cheaper than a replacement shaft! See cutout in my old nut, and notice that the new one is a vibration proof crushed nut.  The old one probably was, too.  That is part of the reason they are difficult to remove.
Note:  All nuts are standard thread.

I begin disassembly by removing the nuts at both ends, an then the snap ring that is exposed when you remove the round nut under the mower blade.  I use a pipe wrench to turn the round nut, but be careful, for I believe this nut is cast iron.  Whatever you do, don't hit this or the shaft ends with a hammer.
 See snap ring in this picture.  Only the bottom bearing is held by snap rings.  I leave the inner snap ring alone.  No reason to remove it.
Now press on the shaft at the pulley end.  If all goes well, the shaft will push through the pulley, and this bearing will pop out.  Remove the pulley and press the shaft's blade end, while using a block to prevent the lower pulley from going back into it's hole.   The upper bearing and shaft will now pop out the top, and the lower bearing is now free.
   I found every key locked solidly in place, so I didn't bother them.  Instead I pressed the upper bearings down and off the bottom of the shaft.
 not having a hydraulic press, I pressed them off in my industrial sized Athol vise, made by the Athol Machine Co., which apparently was bought out in 1920.

Not sure how much longer Athol brand vises were sold after that, but I'm guessing mine is anywhere from 1900 to 1930.  It is a very rugged vise that opens up to 8" and has an unusual handle with a dog clutch in it.  Pull towards you and you can reposition the handle anywhere you wish. Unfortunately, my clutch is worn and slips, so I have it locked most of the time.  For arbors to press against, I use my Harbor Freight deep impact socket set.  They press against the bearing in the right places, without damage to them of the bearing

   I replaced all of the hardware with new 3/8" bolts.  The originals were 5/16", and the more rusted ones turned inside the square holes.  That makes them very difficult to remove.  Once I had them out, I realized that the holes were nearly a perfect fit for 3/8" bolts. Only a small amount of filing the square holes was necessary, along with filing a slight flat on the heads to clear the step in the casting. These bolts will never turn in the square holes, no matter how rusted they might become.

More bad news.  One pulley was badly rusted and
needed to be replaced.  New ones from John Deere are about $30, and used ones are, in my opinion, overpriced at over $20.  As much as I like this old John Deere, I try to avoid putting more money in it than I have to.  Hard to justify a $30 pulley on a 30+ year old, and rusted mower deck.   Looking at the selection of pulleys at Tractor Supply, I came up with an $8 solution.   I could turn down the rusted John Deere pulley in my lathe, and then attach a Weasler pulley to the John Deere hub.  Weaslers are inexpensive because the hub is bought separately.  But I didn't
want the hub anyway.
Above right:  The rusted 4 1/2" inch John Deere and the 5" Weasler.  While they look very different, where the belt rides is nearly identical, and that is what matters.
At left, the old John Deere with the rusted half removed in the lathe, and the new Weasler pulley. Now, simply attach them together.  They fit almost perfectly, with the John Deere nestled under the rolled edge of the Weasler, which I then pounded tight over the edge of the Deere.

  At right, an original John Deere, and my Weasler Deere pulley. 
When something fails prematurely, like the lower bearings did - they were less than 10 years old, I analyze it and look for a fix.  In this case, the round nuts, which are supposed to protect the bearings, had too much space between them and the spindle. There was about .015" - .020" of space, which let things like bits of bailing twine get wrapped around the shaft and eventually destroy the bearing seals, causing the bearings to fail.  I machined the back of the nut to allow them to be closer, about .005".  See the tight gap in the photo at left.  Since each spindle is a little different, the nuts are no longer interchangeable, and must be returned to the spindle it was on.
The idler pulley arm has a brass bushing that is easy to miss.  I didn't notice it at first under all the dirt and corrosion.  It is worth the effort to remove it, clean it up and lubricate it.

Meanwhile, I continued to work on the deck.  Filling some of the larger rust pits made a dramatic improvement, so I filled some more.  It was really starting to look good! I repeated the process once more, re-sanding each time.
   I used a can of John Deere Top Sail white I had on hand as the finish coat.  It is some of the nicest spray paint I've ever used, much better than the quick drying hardware store brands. The high gloss finish almost has a wet look appearance to it. 
There is actually very little putty on the finished deck.  I sanded until I had a surface of shiny metal and filled pits.  The putty adhered well to the pits, which were treated with the rust converter.
I'll give the paint a few days to cure before assembling the mower.  Unfortunately, there is no need to rush, as the heat wave has turned my fields brown.
 Finally, the time has come for reassembly. First question.  Will the Weasler work?  It fits nicely on the left side, but the spring has to be installed carefully, for the clearance is very close, about .010".  There is plenty of room to get the belt on.
 It was a tight fit on the right side, and rubs a little. Also, the spindle has to be loosened to replace the belt.  Fortunately, I needed only one replacement pulley.
 I made over-sized washers out of  0.10" steel, for the region surrounding each bolt hole was distorted into a concave shape.  These should prevent that from happening again.  I also epoxied them on place to prevent rust from creeping under them.

Below:  A close up of the edges of the patch.  A very close fit with no sharp edges to cut through the paint on the deck.
No more deep, grass accumulating corner, plenty of clearance, and easy inspection of the critical areas where the brackets are welded on.
 A view of the installed deck.  I still have to restore and install the covers over the pulleys. White, instead of John Deere yellow, is the finish color, because I had white paint on hand. The previous owner painted the tractor frame and mower red.  Apparently, he had plenty of red paint on hand!
 Visitors have come to this site for instructions on removing the deck.  It is simple, no tools needed. First, put a 2x4 under the front roller to take the weight off the mule hitch.  Loosen the belt adjustment knob and remove the belt.  Pull the spring-loaded J hooks by the front roller towards you and rotate them clear of the mule hitch.  The mule hitch is free now and may even fall off. Pull it forward and remove it from the tractor.  Next pull the retaining pins out of the links which raise the deck. In this picture, the link and pin are directly over the belt where it stretches between the two outside pulleys.  Finally, pull the round knob visible in the center of the picture and rotate it counterclockwise.  It should rotate into a slot which keeps it in the open position.  The links will likely drop free at this point. 
     Here is a view under the deck after mowing 2 acres.  Only a small amount of grass has accumulated, a big improvement. No, the blades are NOT spinning!  I'm not sure why it appears like that other than it happens to fall  on the dividing line between the bright sunshine in the background and the dim lighting under the mower.
The deck, with newly sharpened and balanced blades, is much quieter and smoother than before.  I am pleased with the way it turned out, and believe the old Deere has many more years left in it.

Here is a link to my John Deere mule hitch repair
and my Kohler K341 engine overhaul
I have now added a Harbor Freight winch to the rear of the tractor. You can read about that here.