Thursday, April 28, 2016

3D Printed Curta Part VIII

Transmission shafts

Printing

I printed the transmission shafts in groups. There were three different shafts some with with grooves at different heights along the shafts, but most had no grooves. The grooves allow for clips to keep a gear or rotation lockout at a certain height.

I printed all shafts at 80% infill. I also printed them combined with their crown gear tops which added supports, but eliminated the need for a tool to help me offset the crown gears properly. If I were to do it again I would try printing them separately, but it might be better the way I did it. The crown gear tops were kind of fragile -- I had to reprint a few transmission shafts due to breaking them off.

Fitting

The transmission shafts needed some sanding to smooth them out. The holes in the upper main casting and the bearing plate also needed their holes widened for a sliding fit to allow for easy rotation. To widen the holes I used a set of needle files I got at my local hardware store. The Amazon link is to the set I found. There are cheaper sets on Amazon, but I like this set since the files can be used without the handle which helps for tight spots.

You might notice in the top-down picture in the next section below that some of the shafts look different from the top than others. When I initially scaled the parts up, the offset of the shaft into the crown gear top did not get scaled alongside the parts so the shafts were not inserted into the crown gears as far as they should have been. That wasn't a problem except that the shafts were now too long. Since FDM printed parts are weakest along the joint between two layers, I was able to measure out the extra length, then mark and snap off the extra smoothly at the without having to reprint them.

Count gears, lock gears, and sleeves

Once the transmission shafts were printed and in place, I printed the gears needed for the transmission shafts. They are supposed to be press fit together on sleeves that slide fit onto the transmission shafts.

Press fitting might be necessary for machining, but I printed them as one piece to avoid press fitting small pieces that would likely break in the process. That did mean support material, but it turned out well on these small pieces to use Simplify3D's supports without a higher density interface layer.
The first group of gears I printed came out nicely, but I had to do a lot of work to fit them onto the transmission shafts. To reduce that, I modified the sleeves to widen them a small amount. It was easy to do them all at once in OnShape because I based them all on one sketch.
Transmission shafts added and locking ring in place

Transmission gears installed

Lubricating

Once I had the transmission shafts gears assembled into the frame, I manually positioned the gears and test ran the Curta. It worked well, but plastic on plastic was squeaking.

To resolve this, I used a spray-on dry PTFE lubricant. I disassembled the entire Curta and lubricated each part that spins or slides against another. I didn't lubricate everything because certain parts of a Curta should not be lubricated. The results were incredible. It eliminated the squeak and an amount of friction I didn't know was possible to remove.

Tens levers

Printing


The tens levers were a trick to print. They are normally formed from sheets of metal that are cut and then bent.



I tried printing these lying flat, but the bends cause weak sections where the layers have a small cross-section of adherence.

I ended up printing them upright which came with its own challenges. Upright they have a small footprint so keeping then stuck to the bed was an issue. I first tried lining them up along the x axis to reduce y axis movement and vibration. That did not work -- they still fell over. My next attempt worked. I turned them aligning them along the y axis which gave them batter adhesion along the moving axis.

I also printed the carry lever bearing blocks they require a small amount of supports. There may be a way to re-engineer them to eliminate the need, but it was easy enough to print and remove the supports. Simplify3D showed it's value here and throughout the project.

Fitting

The carry levers did require some filing to fit their bearings, but that isn't too surprising given the narrow parts. The amount was minimal and I got pretty quick at it once I knew which areas needed work to get smooth movement.

In addition to needing to fit the levers into the bearings, the bearings needed fitting into the upper main casting and custom springs needed to be made to support the proper motion.

Carry Lever Springs

The springs have their own engineering drawing depicted below. In order to get an accurate shape consistently, I decided to design and print a jig to make the springs. I used a music wire near 0.6mm in diameter I found at a local hardware store. The jig is simply a block with a few holes in it with a diameter near that of the music wire. The holes are of the lengths necessary to make bends at the desired offsets. Music wire will spring back from the bend some so I used a pair of needle nose pliers to finish off the springs.

Adjusting; Eliminating bend


The first spring I made was scaled directly from the engineering drawing. It was a little bit too long to properly snap the tens carry lever into position.

I altered and reprinted my jig for the springs and had a better length, but I had problems with the legs of the spring catching on the shoulders of the tens carry bearing. That problem was solved by eliminating the 10° bend at the opposite end of the spring from the legs. I guess scale changed the requirements of the spring. 

Calibration

Once I could crank out springs that would consistently work for the carry levers, I had to actually calibrate each carry lever. The tens bell has a ramped section which resets the carry levers from their lowered carry position to their raised no carry position. The springs help keep the levers from being between the upper and lower positions, but each lever will have slightly different friction and each spring will have slightly different pressure. This means that the amount the tens bell has to press upwards on a carry lever to get it to reset may vary a little bit per carry lever.

In order account for that variation, the carry lever bearings allow a range of height positioning in the upper main casting. Each carry lever is adjusted height-wise until it can easily be triggered then reset by the tens bell and the lower position aligns the carry gear with the tooth on the tens bell.

This step took some time and patience. I had to adjust everything multiple times to get them right. Once each carry lever was working properly, I tightened down the screw next to it to prevent it from shifting.
Tens carry lever install complete

Selector shaft assemblies

Printing -- knobs, shafts, guide screws, bearing pins, and plates

The selector shafts are comprised of multiple printed pieces. There is the shaft itself, the knob, a guide screw which facilitates the shaft's rotation, a bearing pin, and a couple of plates which hold a set of four shafts on the Curta.

I printed the shafts in two parts in order to reduce the support needed and to avoid having to force fit the number roll onto the shaft. I added a key to the two parts in a way similar to how I did on the main shaft in order to make sure they mate properly. I printed both at 100% infill. I also added a raft to prevent them from falling over during the print.

I considered printing the selector knobs and guide screws together as one piece, but decided to try printing as separate pieces and cutting thread into them. I printed the selector knobs at 30% infill and the guide screws at 100% infill. I also printed the ones, thousands, and millions digit selector shafts in red. The rest were printed in black.

I printed the bearing pins at 100% infill for strength (the pin tips are narrow and must keep the selector shafts in place) and the bearing plates at 30%. There wasn't anything special with these pieces -- they printed well with no special settings needed aside from infill settings.

Threading

Each selector knob needed thread to accept the guide screws. Each plate also receives two M4 screws to hold it in place. The bearing plate needed threading there to accept the M4 screws. The holes were easily threaded. The guide screws were more difficult. I sheared two off inside the die before I figured out I was using too much pressure and moving too quickly. It also helped to turn the die backwards more frequently to clear out the shavings which helped eliminate unnecessary force.
Tapping the selector knob
Threading the guide screw
The guide screw with thread

Engineering drawing error

After completing my first selector shaft print and adding it to my Curta, I realized that it did not align the transmission gear to the step drum properly. I looked around the internet at pictures of Curtas and at vcalc.net's disassembly images and came to the conclusion that at the zero position, my selector knobs were aligning too low by about 1.2mm.

I wasn't sure at first why the knobs were too low. It could be cummulative error in the sizes of the parts of the curta causing misalignment or a mistake in one of the models I made or even a problem with the engineering drawings (I had already seen one in Part VII with the main shaft alignment).

In order to figure that out, I got measurements made directly off of a real Curta by Cody Brocious who is working on a project to fabricate a 1:1 scale Curta by machining the parts. His measurements showed me that the distance from the top of the selector shaft to the first divot where the selector knob sits was different on an actual Curta from the engineering drawings by ~0.4mm which corresponds to my misalignment of 1.2mm when scaled up to 3:1.

In OnShape, this was an easy fix -- I altered a couple of parameters to the model and was able to print copies of the selector shaft with the correct dimensions.

Assembly

I assembled the selector shafts using springs from McMaster Carr and #TT sized steel balls from ballisticproducts.com using the method I described in an earlier post. One notable difference is that I added the use of the PTFE lubricant.

Assembled selector shafts

Reversing lever

Threading

I threaded the top of the reversing lever shaft in much the same way as I did the selector knob guide screws. I had more trouble keeping the shaft normal to the die than I did the guide screws. I was concerned that this might cause the threads to be torn off as the shaft turned deeper into the die, but that fate was avoided. The M4 nut that holds the reversing lever on doesn't sit perfectly flat, but it seems to do the job.

Assembly

Assembling the reversing lever and shaft went smoothly after having done the same for all of the selector shafts. I used a narrow hex wrench to press the ball bearing and spring into the reversing lever knob and then just slid the shaft on to the point where the edge of it covered enough of the bearing to hold it in place. I then removed the hex wrench and slid the shaft the rest of the way through. On the reversing lever there is no guide slot or guide screw so a little bit of toying with it was necessary to find the right spot for the ball bearing to engage one of the two divots on the reversing lever shaft.

Breaking the Curta's main shaft

The reversing lever's knob engages all of the turns counter gears. The gears need to spin freely in the portion of the reversing lever that holds them in place. I did not check that thoroughly enough and combined with other gears getting caught up in the device, I managed to break the main shaft. The shaft broke right at the hole for the pin that connect the crank handle. It broke there because that is where I was applying pressure to turn the main shaft (I was using a small hex wrench to turn it).
Boo!


The main shaft was printed as one piece combined with the step drum and all its toothed segments in a twelve hour print. I have already reprinted the entire thing, but disassembly and reassembly of everything I have done so far is still in my future. While I am in there, I will be adding the parts of the Curta back one at a time and checking friction at every step of the way, filing and re-lubricating any part that shows trouble.

The device should run smoothly and it wasn't doing that when it broke. In fact, there were a several times the Curta got hung up and I couldn't immediately see why it was seizing. It ran pretty smoothly before I added the digit selectors and reversing lever, so I am pretty sure the problem is that the gears don't spin freely when mated with their respective knobs. In some cases I was lazy and pushed harder instead of doing what I should have which is to find what was catching and fix it. This caused stress where the hex wrench pressed against the edge of the hole it went through that weakened and eventually broke the main axle at that point.

I decided to improve the strength of the main axle while I was reprinting it. Instead of the 30% infill I printed it at originally, I was able to tell Simplify3D to print the main shaft portion of the model containing both the step drum and the main shaft at 100% infill. I shouldn't need the added strength once I fix the issues as I reassemble it, but it will be nice to have that piece of mind.

Monday, February 29, 2016

3D Printed Curta Part VII

Much more printing

I have done a lot more printing since I posted last.

Step Drum and upper portion of the main axle

The main axle is just a little bit taller than I can print. I might have been able to modify my printer a little bit to print that tall, but the print bed on my printer is the y-axis so the parts move back and forth as I print. That could cause a tall print to dislodge from the print bed. Instead I divided the main axle into two keyed parts. I divided the axle such that the parts made inside the middle of the step drum. The upper part I combined with the step drum and printed as one piece. The lower piece will slide into the bottom of the step drum with a tight fit -- no glue necessary.
I also could have tried printing the main axle horizontally, but the z-axis resolution is limited by the layer height you specify. I might have been able to get a pretty round axle, but the first layer is often slightly squished when printing (it takes a lot of time to tune a printer to be 100% perfect on the first layer and the tuning must be repeated often) and the footprint area that the first layer adheres to the print bed would be very small which also could lead to the print dislodging from the print bed.
After printing the step drum and main axle I found that I had a mistake in my models. The step drum teeth were misaligned by 20 degrees. I had to fix the model and reprint. That was a painful mistake as it took 13 hours for each print.

Main axle bottom portion

Since I split the main axle, I needed to print the bottom part separately. The keying of the two parts is important. The bottom portion of the axle must be oriented properly and the key ensures that it is. After having printed it and carefully fitting it into the step drum, I had a completed main axle. Only I discovered later that the engineering drawings are either incorrect or from different versions of the Curta Type 1. The orientation of the lower portion of the main axle was 90 degrees off. That was easily fixed by altering the direction of the key I added after dividing the main axle into two parts.

Other printing

  • bearing plate 
  • anti-reversal plate, zero positioning lever, and anti-reversal lever 
  • body support columns 
  • some of the transmission axles and gears

Making springs

To make the zero positioning and anti-reversal mechanisms work, I needed to make a couple of springs. All other things kept the same, a torsion spring scales up the force it exerts with the diameter of the wire used to make the spring. That made things easy for me -- I simply multiplied the wire diameter from the original engineering drawings by three for my 3:1 scale Curta.

In order to make springs, you need to have the right kind of metal tempered the right way to make it springier than it is malleable. To this end I got some music wire from a local hobby shop. To actually form the spring, I printed out a couple of mandrels -- basically cylinders with a hexagonal end that I could fit into the chuck of my drill. I made the mandrels a little bit smaller than I needed the springs to be because the wire will want to relax after winding.

This can be very dangerous, so take all of the necessary precautions if you try this (eye protection, gloves, thick clothing, etc)!

I placed the wire and mandrel into the drill so that the chuck grabbed both and very slowly turned the drill while using pliers to hold the wire taught at a tangent to the mandrel. My drill chuck would not turn without the drill turning. Ideally I would have turned it by hand. As the mandrel turns, it will begin wrapping the wire. Keep the loops tight together (it may take some practice to get this right). Once I had the number of loops required, I put the drill in reverse and slowly let the springy wire unwind as much as it wanted to.

It's very important to do let the wire unwind slowly. If the wire is released too early, it will violently unwind while whipping around and possibly causing injury. I accidentally had this happen to myself while winding the thicker wire. When the spring unwound it whipped into my hand. The force broke the skin and left a nasty bruise. I thought for a moment I might have broken it. I've broken a finger before and it actually hurt less than that wire.

Once I had wound my springs, I put them into a toaster oven for as hot as it'd go (450-500 degrees Fahrenheit) for 30 minutes and let them cool slowly inside the oven without opening it. That helped temper the wire so it would keep its shape.
The springs I wound in the toaster oven getting tempered.

Making thread

Many of the parts on the Curta have either external or internal thread so that they can receive screws or nuts. There are some shoulder bolts I couldn't find as well so I set out to cut thread into 3D printed parts using a metric tap and die set I have.

After several tries, I gave up on the 3D printed shoulder bolts. They were M5 bolts and I printed them head down on the print bed so I wouldn't need supports, but that meant that the layers aligned normal to the length of the bolt. The weakest part of a 3D print is between the layers, so as I tried to thread the bolts, they sheered off leaving plastic bits in the die I was using. I eventually decided I could produce those bolts by printing a sheath for a normal M5 bolt to create a shoulder at the size I needed.

After solving that issue I still needed some external thread. The support columns are threaded at the ends so they can be connected to the body and the bearing plate of the Curta using hex nuts. I tried a lot of different methods, but what worked in the end was printing the support columns horizontally to change the layer alignment and prevent the part from shearing as I threaded it. I used a combination of raft and brim to prevent them from coming loose off of the print bed. They aren't perfectly round, but it doesn't matter much for those.

Cutting internal thread was not nearly as difficult. I was able to just turn the die into the plastic a quarter turn at a time. After each quarter turn I would reverse direction to help clear out some of the excess plastic. Depending on how good the first cut was, sometimes I would put the tap through a second time (being very careful not to cross thread it)

The top of the main shaft with the washer and printed circlip connecting the Curta body and tens bell together.

The bottom of the Curta showing the bearing plate and zero positioning / anti-reversal plate. The nuts at the bottoms of the support columns are also visible.

The Curta looking top-down.

A side view of the Curta with the step drum visible.

The anti-turn plate bolted into the bearing plate -- an observant eye will notice that this picture was taken before I fixed and reprinted the lower part of the main axle.

Ready for some transmission axles next



Tuesday, February 2, 2016

3D Printed Curta Part VI

Time for change

I changed jobs and moved to the home of the Carolina Panthers last year and have been busy getting settled in the new house. There are still a lot of things to do and boxes to unpack, but I got my office (mostly) set up and managed to run a few prints. On a side note, Go Panthers! Play well in the Super Bowl this weekend.

Long prints and support material

The first few prints I did after getting the printer up and running again were little things to help recalibrate the printer after being lugged a few hundred miles. After I was confident in its ability to print consistently I challenged it with the main upper body of the Curta.

The main body of the Curta has a lot of overhangs which cannot be printed without support material. I slice my models with Simplify 3D which has really great support material generation that is easy to break away. It does it by producing narrow zig-zagged walls and a one layer gap (configurable) interfacing with the model. The supports are flexible due to the single pass width and are not as securely bound due to the one layer gap. The zig-zag pattern combined with the flexibility allows the supports to be compressed which helps them to easily break cleanly away from the model.

If you are familiar with FDM / FFF 3D printing, you are familiar with parts curling. As the plastic cools, it shrinks some and pulls on previous layers. If it pulls enough, it can cause the edges of your print to curl / peel off of the print bed. The plastic I use (PLA) is much better at not doing this than other plastics, but it can still happen. Particularly if the printer doesn't have a heat bed such as mine. The first time I attempted to print the main body I ran into troubles with this. The support material being only one extrusion-width wide has a small footprint on the print bed and in the case of the main body, many layers to pull against the first layer. That caused some warping on my first print which I had to cancel.

On my second attempt, I told Simplify 3D to alternate support direction by 45 degrees once every ten layers. That helped a lot. I no longer had any curling / peeling issues. In fact, the print was so strongly attached to the print bed I had to use a heat gun to warm the bed up from underneath before I could pry it up. However those alternating direction layers in the support also worked to strengthen the support material making it more difficult to remove from the main body.

On my next print, I reprinted the tens bell I had misaligned in my previous post (the tens bell handles carry operations in the Curta). I decided to go back to Simplify 3D's default support settings. I chose to do that partially because I had printed that part before with no issues and partially because of the difficulty I had removing the support material from the main body. Removing the supports this time was surprisingly just as difficult (if not harder due to the small spaces). I believe this to be due to the material I printed with this time around. I used a clear PLA from Monoprice. It prints very well and it did come out pretty clean, but I think it binds to itself much more readily making the supports more difficult to remove. Due to the nature of the Curta parts, I will be using a different material from here on out.

After printing the tens bell and main body out and removing the supports from each, I was able to assemble them together using a printed c-clip also designed from the engineering drawings. There is a washer that is supposed to be under the c-clip to provide proper spacing, but I have not yet printed that. As far as I can tell everything seems to line up correctly, but I will know more when I can assemble more. The tens bell rotates well in the main body, but I may smooth the print layers a little bit and add some lubrication before final assembly. I also printed the spring that attaches to the bottom of the tens bell. It does have some spring to it, but I don't think it is enough. I may need to fabricate that out of metal.

I recorded time lapse vide of the prints. Unfortunately my printer does move the print bed back and forth which makes the part appear to jump in the video. I will look into mounting the camera directly on the print bed, but that may not be possible without compromising the print on this printer. I am planning to build a Triple C-bot which only moves the print bed vertically. If I finish that before I finish the Curta, I will begin recording my prints on that.



The main body right off of the print bed.

The underside of the main body showing the support layers. Also note the brim.
Here I have removed the majority of the support material, but bits of it remain making the surface rough.


The roughness from the support material seen from a different angle.
The fully cleaned main body.
The main body from the top.
Another view of the main body.

The corrected and reprinted tens bell

The tens bell, spring, c-clip, and main body

The tens bell connected to the main body with the c-clip

Friday, July 10, 2015

3D Printed Curta Part V

Tolerances in OnShape

I waited through two updates to OnShape and decided that I needed to stop waiting on them for features. I used their version control to set up a branch for my tolerance work and then used expressions to add in the tolerances I calculated based on my previous tests on my printer and the tolerances specified in the Curta engineering drawings. This allowed me to begin printing and I can go back and update these expressions to used shared dimensions later when they are added.

Initial Printing

I started my first prints at 3:1 scale. I printed a section of transmission axle, a sleeve for the ratchet gears, a selector shaft, and selector knob. The initial versions of the transmission axle and sleeve did not fit well due to a mistake in the models, but that was easily fixed. Support material was needed for the selector knob. I think it turned out well. I also printed a tens bell and a c-clip for it and they turned out mostly well, but another modeling mistake meant that the rings on the tens bell were misaligned. The bottom ring also warped during printing so it will need to be reprinted. I have altered the model to align the rings properly and increase the footprint in contact with the print bed so that warping issues will be reduced.

I searched around and found some steel balls at the right size for the selector knob and shaft. It turns out that the right size matches #TT steel shot. I couldn't find it in any smaller quantity than a 10lb bag, though. When I got those delivered I improvised a ball point pen spring to assemble the selector shaft and knob. I am very happy with how that turned out. See the pictures and video below.

I look forward to further printing and getting more parts of the Curta assembled.




Misaligned tens bell. I will be reprinting it soon.
Transmission sleeve and shaft section

Close-up of the profile of the transmission shaft

Transmission sleeve on the transmission shaft. It slides smoothly as it should.
This is the #TT steel shot I used for a ball bearing in the selector shaft / knob assembly.

The selector shaft, knob, steel ball, and the ball point pen disassembled for its spring.

The selector knob with the spring and steel ball. The adjustable wrench is not tight and is there only to keep the knob upright.

In order to press the ball and spring down without the ball shooting out and taking an eye out, I placed my thumb on the short side while sliding the shaft in the opposite side (the wrong way) to limit the ball and spring's movement.

Here is the steel ball and spring pressed into place. The tube I am using to press the ball bearing and spring down with is the ink tube from the ball point pen.

Now that the steel ball and spring are pressed into place, the shaft can be slid into the knob in the correct orientation. Once the shaft is covering the ball bearing as far as it can, pressure on the ball bearing and spring no longer needs to be maintained.

As the selector knob is slid onto the shaft, the guide screw will need to mate to the slot in the selector shaft.

Thursday, May 28, 2015

3D Printed Curta Part IV

Curta Part Strength at Scale

I ordered parts from Shapeways to check the clearances I determined for moving parts and to see and feel the strength of their plastic. I ordered the digit selector axle and the selector knob. The two parts mate and does some mechanical work since the selector axle turns as the knob is moved up and down it.

The parts came packaged nicely and were in good shape. There was some powder left in the selector knob I had to clear out and the bottom of the selector axle needed a little bit of cleaning to fit on the axle, but the two pieces fit together and functioned well. I did not have a small spring and ball to add to complete the assembly. This was mostly a test of what I could expect from Shapeways for size, fit and strength.
Selector knob and axle assembled.

The held in my hand for scale.


Movement of the shaft and knob.
The parts did mate, but the fit was a little bit looser than I would have liked. The fingers on the knob where it would hold the transmission gear and the small point at the top of the axle were too small and fragile to be useful so even with SLS printing, I would need to scale up to produce useful parts. Not terribly surprising.

I may go back to Shapeways at some point in the future to build a really clean version of the Curta. However since I have to scale up anyway, I am going to produce future tests and parts with my FDM printer for now so that I can get faster iterations. It took eleven days from initial order to received parts. While that is pretty good turnaround for a process that used to take much longer, I can produce multiple revisions in hours on my printer versus one in weeks.

Tolerances in OnShape 

I could go ahead and set up all of the scaling and tolerances I need to print the parts I need, but if I ever got a printer with better accuracy I would need to repeat all of that work. Instead I want to rely on dimensions shared across the parts to define the tolerances. This way I can update the values in one place to have them applied to all of the parts at once.

OnShape does not yet support shared dimensions, but it is a feature that they told me they are working on. I look forward to that update so I can continue my work.

In the meantime, I am back to the idea of using my FDM printer which means supports for overhangs. I would prefer to keep that limited so I am going to revisit the models to consider further alterations that would 3D print more easily.

Tuesday, May 12, 2015

3D Printed Curta Part III

Modeling Refinements

In part 2 I finished modeling the parts for the Curta. Now I have completed an assembly of all of the parts in OnShape. For the most part I added mates that allow the proper movement of the pieces, but in some circumstances I had to limit the movement so the assembly didn't suffer. While the mates allow movement, there is no collision detection or other relationships between the parts, so sliding a digit selector does not turn the selector shaft or move the transmission gear.

Despite that, it was a good exercise. I found a number of problems that have now been solved. The digit selector knobs were not the right size -- the engineering drawings show all of the knobs being machined from one piece. I did not account for the material that would be cut away when the knobs were cut apart. The step drum parts did not want to mate correctly and I had to manually adjust them because the edges of the teeth were intersecting the transmission gear sleeves. There were a few other similar issues as well.

The assembled step drum

The assembled Curta (no lettering and the clearing ring is on upside down)

3D Printer Accuracy vs Resolution

I first set out to figure out what clearance between a hole and a shaft would lead to different mechanical fit types. I printed a series of holes and shafts where the shafts varied in diameter while the holes were all the same. The range was fairly large with a pretty course step (0.05mm). I only got about half-way through fitting the holes into the shafts before I ended up with a forced fit. I believe I printed these at a 0.23 mm layer height at a tame speed for my printer of 3000 mm/s. Here are the (very basic) results:
  • 0.35 mm -- Loose sliding fit
  • 0.3 mm  -- Sliding fit
  • 0.25 mm -- Tightish transition fit. (pressed into place by hand; removable by hand; twists with some effort)
  • 0.2 mm  -- Forced fit. (pressed into place by hand; cannot remove by hand; cannot twist; removal with pliers damaged the parts)


While doing some research on hole and shaft tolerances, I came across an article discussing accuracy vs resolution. The difference is something akin to precision vs accuracy. A printer can print at a high resolution, but if the layers it deposits vary in width or position a lot, it has a low accuracy.

That article led me to the thought that instead of adjusting all my parts by a larger tolerance, my tolerances should be the same as the engineering drawings, but I should also account for accuracy by subtracting it from shafts or adding it to holes (or half and half).

To test this, I devised multiple test prints. The first was to find my printer's accuracy. Again I printed a series of holes and shafts where the shafts varied in diameter. This time I started at 0.2 mm clearance and increased the gap by 0.01 at each step while also adding in a 0.02 mm for the maximum H7/h6 mechanical fit tolerance. I ran these prints at a smaller layer height of 0.1mm. The one that fits best should indicate my printer's accuracy.

After printing the test, the smallest accuracy clearance (0.2 mm + 0.02mm fit tolerance) turned out to provide the closest sliding fit after the pieces were worked together a little bit. That tells me that at 0.2mm I am clearing most of the ridges that the 3D printing process creates, but there are a few outliers (mainly where there are joints) which get worn down as the parts are moved.

While it is probably not 100% correct, for my purposes I am going to call my printer's accuracy +/- 0.1 mm (or +/- 0.2 mm counting two walls -- one on either side of the shaft). 

Next I printed some smaller fit tolerances with the same clearance for accuracy. I used the following fits:

  • H7/h6 for an LC2 fit (this was the same as earlier to have a comparison)
  • H7/js6 for an LT1 fit
  • H7/k6 for an LT2 fit
I was hoping for a noticeable difference in how these fit, but they all have similar resistance after working the parts together which is probably the problem -- any difference in width is worn away.

These tests give me an idea for how to proceed and give me a starting point to run some tests with Shapeways.

I also printed the same test on the printer at my office, but it is not as finely tuned. The results showed that difference with pegs that were visibly not as straight vertically. They did not fit as well as a result and got stuck when I tried to push the peg through.

From left to right, attempts at LC2, LT1, LT2

The assembled parts from above arranged in the same order.
They all fit closely, but allow easy sliding and rotational movement.



Wednesday, May 6, 2015

3D Printed Curta Part II

Initial Modeling Complete

I completed modeling all parts for the Curta Type I from the engineering drawings. All parts are modeled to the nominal sizes (no tolerances are applied yet). It took me about a month to get through them all (working on it during evenings and on weekends). I used OnShape to model the parts and the excellent disassembly at vcalc.net as an additional reference to the engineering drawings.

There were a few parts in the disassembly at vcalc that differ from the engineering drawings (namely the results and turns counter carriage casting). It may be that those parts were easier to manufacture and assemble in a slightly different way that they were drawn up. As long as it was functionally the same, I opted for the design that would be easiest to print.

Print Process Change

After modeling the parts, looking at the tolerance specifications, and watching a video on assembling the Curta; I decided that even at 4:1 spo FDM printing may not be up to printing a Curta. It may produce something that could be assembled, but function may be inhibited by parts that have too much friction or too much wiggle room depending on how the layers match up between parts.

Instead of FDM, I have decided that I will use SLS. I toyed with the idea of using SLA, but decided I might as well go with the process that requires no support material to make things easier. I have not decided yet whether I will use Shapeways or build an SLS printer via the You-SLS project (I contributed to the project). While I have a ways to go before I have anything printable, You-SLS is very experimental. We'll have to see where it is and what quality it can produce when I get to that stage. I have not fully decided what scale to print at. I will need to print some tests before I know for sure, but I want to approach a 1:1 scale.

Design Considerations

When producing the 3D models for the Curta parts, I took into consideration 3D printing constraints (initially for FDM printing) as well as affordances gained by using 3D printing. Some parts manufactured separately could be combined and manufactured together as one piece. For instance, the gears and sleeves that the step drum turns in order to add or subtract each digit are combined. Unfortunately they cannot also be combined with the shafts or I would have no way to assemble it. The tens bell and step drum are normally made up of segments that are stacked together and fastened. Those will be printed as one piece. From 145 engineering drawings of parts, I have 116 models. This is partially due to the combination of parts and partially to somth the parts being fasteners and springs which will not be printed.

Threading

There are parts that must receive machine screws and parts that are designed to thread onto one another. I will need to use a micro tap and die set for this. However, two of the parts that screw together require M50 thread (larger if I scale up the Curta). M50 or larger taps and dies are expensive. Instead, I will either design these two parts to snap together in a keyed way or drill and pin them together in the correct orientation.

Screws

The Curta uses many very small metric screws. Thank goodness for the Internet because I don't know how else I would have found them without it. A friend, Ray Kholodovsky, maker of the Cohesion 3D printer suggested a site where I found all of the screw sizes I need. There are some screws which have a nub tip that are intended as a pin on the digit selector knobs and a guide for the rotation of the crank handle which are custom that I won't be able to find. Instead I will integrate those into the design of those parts or print the body of the screws and thread them myself. Some of the screws are shoulder bolts at very specific sizes. I will likely use standard screws combined with printed sleeves / spacers to add the shoulders.

Springs

There are also a few very small springs that are needed. I found equivalents to many of them online, but there were a couple of torsion springs (for the zero positioning lever and anti-reversal cam) that I could not find equivalents for. In addition, there are non-standard springs in all of the tens carry levers that are needed. Due to all of that, I looked into making springs.

I found a few articles on the subject (Making Springs, Homemade Torsion Springs, Make Your Own Springs in Seconds). At the small size I need, I think I should be able to fabricate the springs I need. I will need to create a couple of jigs for it, but making them and the springs should not be terribly difficult.

Tolerances

Tolerances between parts will be the defining factor on whether I can pull this all off. The engineering drawings specify tolerances which are smaller than even an SLS printer's accuracy. I will need to experiment and find values which create parts that have the proper fits.