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Indoor Free Flight Forum => General Discussion => Topic started by: dslusarc on September 13, 2017, 09:56:18 PM



Title: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 13, 2017, 09:56:18 PM
UPDATE: September 27, 2017

The torque meter design has been updated to reduce friction from the pull of the rubber. The new version build starts at reply  #29
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=22468.msg213741#msg213741

The rest of the thread is being left for reference.
Don


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I am in the process of building a digital torque meter that Jake Palmer designed and I am going to document my steps for others to follow if they desire to build one. Jake's design uses 3D printed parts and the files can be found here:
https://www.thingiverse.com/thing:2441787

The electronic parts are taken from a TBBSC 200 x .01 gram scale. I bought mine off Amazon for $8.99
https://www.amazon.com/TBBSC-Electronic-Digital-Balance-Jewelry/dp/B013L9DJRC/ref=sr_1_2?ie=UTF8&qid=1505353456&sr=8-2&keywords=tbbsc+200+scale

The first photos is the 3D printed parts. I have a Prusa i3X printer, and printed it using PLA, 40% fill density, 0.2 mm layer thickness. The buttons and collars I printed together and used an external fan to cool them as printing. I found this produces better quality small sized parts. The lettering on the face plate I just filled with some spackle and wiped the excess off, this way the contrast in colors makes it easy to read. Once you have the parts printed then start by taking the scale apart.  

Remove the battery cover. Then remove the two case screws on the right side (of the photo) and the two load cell screws near the center. The load cell screws will be tight from the factory. Use a plastic pry tool (you can get these at harbor freight or ebay or amazon, they are for removing car dashboard panels etc). Use the pry bar on the opposite end from the battery cover. Once it pops loose, carefully open it up as seen in the last photo.

 


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 13, 2017, 10:04:53 PM
Now remove the four screws holding the bracket to the load cell, and the four screws holding the green pcb/display to the case. SAVE these 4 screws!

Carefully flip it over and lift the silver weighing pan that is connected to the load cell. Using the pry bar, pry off they silver pan from the bracket. It is just held with double sided tape. Be careful not to damage the load cell (note how I am holding the pan when prying).

Remove the two screws to free the load cell. These screws will be tight.

Cut the two red wires from the battery case. You should have what you see in the last photo. The display,load cell, and the four small screws.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 13, 2017, 10:24:26 PM
Now it time to start assembling. The first photos shows some items used in this step. I chose to print eh version that uses  1/4 x 1/8 teflon tube instead of the ball bearings. When I went to order the tube, McMaster Carr showed it was discontinued. So instead I ordered low friction dry running sleeve bearings made from Rulon J. They come in the same size and are precision machined. The OD is .251/.258" and the ID is .129/.131". Mcmaster Carr Part number 6377K49, and are $2.33 each. Two are required.

Also in that photo you see the shaft material I used. Since I was ordering from Mcmaster Carr anyhow , instead of 1/8" music wire I went with .125" rotary shaft material. It is 316 stainless so will not corrode, OD is ground so the tolerance is tight +0/-.0002" and has a very nice surface finish, all this will reduce friction. This material is not as hard as music wire or hardened shaft material whic is OK for this application as iot means the set screws used will bit into the shaft better. Mcmaster Carr part number 1263K39 cost was $9.29 for 12" (enough for 3 torque meters).

I also ordered the 2 x AAA battery holder. McMaster Carr part number 7712K17 cost was $1.48.

The holes in my case were close to .250 but I could not get the sleeve bearings to go in. Fortunately Dremel has a .250 stone bit that works well to ream the hole. I uses the tool with my hand to size the hole. Once I sized them the sleeve bearing pushed in snugly.  I then ran the shaft material through to see how well they sleeves lined up and it was perfect. No friction.

I then put some silicone glue on the back of the black batter holder and glued it to the floor of the case. Make sure those red and black wires go through the opening.

 


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 13, 2017, 10:33:40 PM
Next photo is the back of the display board, The 4 wires on the left go to the load cell. The two red wires on the right are the battery connections. I decided to unsolder the four load cell wires and remove the two red battery wires. I suggest doing this only if you are good at soldering small items on circuit boards and have a soldering station with a fine tip. If not I suggest using a connector in the middle of the wires like Jake did on his (see the photo on his Thigiverse page).

Once the wires were removed, I then used those four small screws to attach the display to the back of the face place. Make sure to put those printed button in first! Carefully screw those four tiny screws in and don't over torque or you may break the plastic stud off or strip it out. 

Flip it over and check that buttons work when you press on the, they should move with no sticking.

I then installed the load cell using two 3mm x 25mm screws from underneath. Tighten these until snug. You don't want the load cell to move in operation. Notice the four wires are feed though the hole in the case to the front where the display will be.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 13, 2017, 10:42:14 PM
I then put the case on its side and the display next to it as seen in the photo and resoldered the four load cell wires and the battery box wires. Note the polarity on the battery wires (the pcb is labeled V- and V+). The display was then attached to the case using four 3mm x 6mm button head screws. You can also use the four 3mm x 6mm screws that held the load cell to the weighing pan during the scale disassembly. There was no need for me to tap these holes for the thread. The hole size was just right that the screws self tapped when threading. Since this is plastic no need to go overboard torquing these screws down. I then took off the screen protector from the leftover scale parts (it peels off) then attached it to the face plate to nice finished look.

The last picture in this post is my testing the unit to make sure it powers on OK. I found that the back light intensity is based on the strength of the battery. I had some old batteries I threw in and the scale worked but the backlight was very dim. New batteries made it bright and easy to read.

That is all for tonight. More photos as I do the rest.

Don


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 12:08:13 AM
Up next are the hooks. I printed a couple of the hook collars as I wanted a thicker wire hook (.039 wire) for heavier motors and a thinner wire (.025) for events like F1R, ministick etc that use small o-rings. I printed all three size Jake had with his file. The largest hole collar printed the hole all the way through the part. I did have to size it with a 1mm drill. The middle size collar part of a hole on the first layer so I had a reference point to drill my .025 hole, the third one printed no hole as the diameter is just too small for my printer to handle. What you do is bend your favorite hook shape, then install into the collar, then make a 90 degree bend to fit the wire in the slot that is on the inside.

The metal stop collar fits right inside and you have to replace the short 1/8" long set screw with one 3/16" or longer. I happened to have 1/4" long ones so used those.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 12:19:45 AM
Next I cut the shaft to 2 7/8" long with my dremel. I also bought a few #6-32 x 1" long screws at my local Ace hardware store and the washers and nuts needed to make the arm that touches the load cell. The first one I made was using the parts suggested by Jake, and it works out just like he says on his Thigiverse page. I then decided I wanted to make a special washer from some thin sheet metal stock. The fender washer is around .040" and I wanted it thinner so it would have a very precise point of contact. The metal I used is .010" thick and is from a small apple juice can. It is a mild steel compared to thin aluminum on pop cans. I cut the washer out using a 7/16" diameter hole punch and a 1/8" diameter hole punch (the punch set was bought from Harbor freight). The photo I have shows the thickness difference in the purchased washer compared to the one I made from the sheet metal.

I did a test fit of the parts and found that the washer comes very close to the front of the case. So I took one of the spare sleeve bearings I had and slid it on some spare shaft material then used a razor to cut a spacer about 1/16" wide. The Rulon J material cuts easy like teflon so you can get a clean cut. I then did a test fit up with the spacer to make sure it fit OK. .  


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 12:36:09 AM
This next step is my own addition due to me being paranoid. The hook the rubber attaches to is secured to the shaft by a set screw. I am paranoid the set screw will come loose and the hook will fly toward me some day. Especially as I plan to take the hook on and off depending on what event I am flying. So I used my dremel and made a slot on the shaft so they the set screw sits down in that slot. So there is both mechanical friction of the set screw plus a physical barrier for the set screw.

I also made a similar flat about .015" deep where the torque transfer arm is attached to the shaft. When the rubber is pulling this takes the pull load of the rubber and I am afraid it would come loose. This 6-32 screw does have a nice large head so you can really torque on it but the added groove give me peace of mind. Now adding that slot in this location means you have to make sure your washer is still 1" from the center of the shaft if you want accurate in-ounce readings. I used calipers to measure the washer position. If necessary you can file off the end of the 1" screw to get the correct length.

Once the flats were made the parts were reassembled. Now you will notice there is some shaft sticking out the back. In reality the shaft can be flush with the back of the case as the load is not carried at the back sleeve bearing. The front takes the load. But as an additional safety feature I added the rear collar to act as a safety net if ever the main collar should become loose and try to pull forward. In that case this rear one will stop it from going forward. The rear collar does not touch anything, there is a gap between it and the case. Yes I know it is redundant but years ago I had a solder joint break on a torque meter when winding and the parts flew right at me.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 12:40:56 AM
And here is the final product!

Don


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Tapio Linkosalo on September 19, 2017, 01:05:15 AM
Next I cut the shaft to 2 7/8" long with my dremel. I also bought a few #6-32 x 1" long screws at my local Ace hardware store and the washers and nuts needed to make the arm that touches the load cell. The first one I made was using the parts suggested by Jake, and it works out just like he says on his Thigiverse page. I then decided I wanted to make a special washer from some thin sheet metal stock. The fender washer is around .040" and I wanted it thinner so it would have a very precise point of contact.

If, instead of the screw you attach the washer between two nuts (like I did in my original design (http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=3650.msg36872#msg36872) a few years ago) you can adjust the arm length in high precision! Also note that the desired arm length (one inch in your casse I suppose) is the distance from the meter shaft to the washer edge touching the scale element, not the distance of the washer attachment point on the screw.




Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 01:29:39 AM
I am not sure what you are correcting in my post. I stated:  "... you have to make sure your washer is still 1" from the center of the shaft if you want accurate in-ounce readings."

How is 1" from the center of the shaft to the washer that is touching the load cell incorrect information? There is a spacer washer between the bolt head to offset the transfer washer so it ends up 1" from the center line.

The 6-32 x 1" bolt measured .990" from the end to under the head, the spacer washer is .030 thick, my transfer washer is .010" so the center of the washer is .955" from the bolt end. Half the 1/8" shaft diameter is .0625 minus the -.015 flat I made  = .0475" from shaft centerline to where the end of the screw will start. So .0475 +.955 =  1.0025" which is close enough to 1.000" for me.

Using Jakes original design. Assuming the same .990" length screw, same .030 spacer washer, and .040 thick transfer washer (fender washer) then .990-.03-.02(center of washer) +.0625 (half of 1/8")= 1.0025"

Don


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Tapio Linkosalo on September 19, 2017, 04:26:50 AM

Sorry I cannot quite follow your description with my pidgin English. But you do not mention the washer diameter in the calculation, so I made the drawing below to make my case clear. In the picture, the circle on the left is the collar around the meter shaft, the rectangle to the right of it is the bolt/arm, and the narrow vertical rectangle is the washer. The two nuts holding the washer in place are omitted. With red arrow I drew the distance from the center of the shaft to the edge of the washer. That diagonal is the dimension which should be one inch.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 06:54:05 AM
This design has the screw/arm parallel to the load cell. So the load cell gets only vertical force transmitted from the thin washer as it is perpendicular to the load cell. So that is why I have the washer set to 1.002" from the shaft center. If the arm was not parallel to the load cell then the diagonal distance like you show has to be used to know the effective arm length.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 12:42:58 PM
Hope this explains better how this specific meter is set up.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 19, 2017, 12:49:09 PM
As Don mentioned, the screw is parallel to the load cell, so I believe the setup he's using is correct.  The load is being applied perpendicular to the load cell at 1" from the center of the shaft.  If I were to use the dimension in the drawing, that would imply that the load is being applied at the angle you have drawn.  I don't believe this is correct.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 19, 2017, 01:05:59 PM
Also, thanks for doing this write up Don.  I put quite a few hours into the 3D work and wasted a lot of plastic on prototypes, so in the end I was too lazy to do a write up like this with photos.  

I like the use of Rulon J as it's supposed to be good for eliminating stick-slip.  I made a new version that completely eliminates the bearings, but I might try revising it to incorporate the Rulon J before releasing it.  It also has a sliding top so the batteries are easy access when they need to be changed.

Eventually I'm planning to make a handful of these torque meters to sell as a fundraiser for the junior team.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 19, 2017, 10:03:30 PM
Jake,

Hopefully this thread will help people out. I am glad you did the 3D stuff. I am not strong in 3D CAD as we design in 2D 99.9% of the time due to the nature of what we make. I have no issue visualizing, its knowing the ACAD command to do it :-)

I like the sliding top cover idea. I think the only thing I would want different based on my build would be for the c-bore that the forward sleeve bearing goes into to be about 1mm shallower so I would not have needed to add that additional spacer between the stop collar and the Rulon J sleeve to make sure the washer cleared the front of the case. 

Don 


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Tapio Linkosalo on September 20, 2017, 12:48:10 AM
Hope this explains better how this specific meter is set up.

Thanks! Yeah, a picture is worth of a thousand words, indeed. I was thinking about the math in the way to wrote it down, but did not have the time to do it yet. Now I do not need to.

Yes, I stand corrected, I think your math is correct. And luckily this also explains why my meter readings are so low, I have always wondered how people han wind their motors to such huge torque readings, while my motors always break at much lower TQ. My meter is adjusted the wrong way, and as I am using gram*centimeter, my arm is much shorter, and hence the ratio L2/L1 is much larger, giving a considerably higher error.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 20, 2017, 10:18:19 AM
Glad it helped :)

Jake has a new version he has built to eliminate the thrust force friction. As soon as the files are available I will do a new build thread.

Don


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Skymon on September 20, 2017, 11:37:12 AM
This is a really nice device.
Well done on the design and development.
The only issue I can see is that the particular electronic scale chosen doesn't seem to be available in Europe...
I'll keep looking for a source.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 20, 2017, 03:13:44 PM
Skymon,

I'm not sure how bad shipping would be, but I could send you one.  They cost me $9 delivered.  I would only ask for that plus the cost of shipping.  Or I could sell you a complete torque meter with profit from the sale being donated to the US junior F1D team.

Jake


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: spr on September 20, 2017, 04:19:21 PM
The only issue I can see is that the particular electronic scale chosen doesn't seem to be available in Europe...
I'll keep looking for a source.


Skymon, the Amazon link Don posted above claims that the scale will be shipped to UK or any European country as  well. I'm sure you can also find the same scales in any Chinese web store like Aliexpress.com

Thank you Don for an informative build description. Good to have an answer for the torque arm measurement issue that I have discussed with Tapio a few times:). My meter is a Bill Gowen-style with a plywood frame. It is a good alternative to 3d printing if you don't have a 3D printer (and skills to use it)  easily available.

BTW, can you change the batteries in Jake's design without removing the axle?

Simo


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Hepcat on September 20, 2017, 04:43:56 PM
Thanks Don,an excellent tutorial, almost as good as your description a few months ago of how to make a No-Cal fly for minutes on end!  There was one little point, brought to mind by your discussion withTapio about the precise length of the load arm.  I have a feeling that on these cheap scales they give slightly different values depending on where the load is applied to the load cell and that the precise centre is not necessarily,consistently the best.  I am not trying to make an issue out of this, I don't think that checking torque on a rubber motor warrents thousandths of an inch but I just wondered if you had come across this aspect.
John


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Andrew Darby on September 20, 2017, 05:36:29 PM
John,

Generally these "binocular" load cells (binocular refers to the dog bone shape in the middle) are single point load cells.  What this means is that the measured force is insensitive (to an extent of course) to the position of the applied load making them ideal for weighing scales with platforms.  So as long as the torque arm length from the shaft centre is accurate (as has already been discussed) the actual point of contact on the Load cell need not be accurate and you will still get good readings.

Andrew


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 20, 2017, 06:08:11 PM
BTW, can you change the batteries in Jake's design without removing the axle?

Simo

It's a bit tight, but you can change the batteries without removing the axle.  The updated design I'll be publishing soon is also tight, but it too allows for changing the batteries without disassembling anything.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 20, 2017, 11:38:56 PM
I have a feeling that on these cheap scales they give slightly different values depending on where the load is applied to the load cell and that the precise centre is not necessarily,consistently the best.

If you look at the photo before I take off the 2 screws holding the load cell to the bottom of the pan, the stamping has a smaller rectangular section that touches the load cell. I found that if a weight was added to the load cell anywhere in that width (about 3-4 mm on either side of the screw centerlines) the weight was the same, as I got closer to the center of the cell, about 6mm from the centerline it changed .01gr. So there is a pretty wide range for the load to be applied. On this design, the transfer washer at the 1" arm length is right at the centerline of those tapped holes.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Tapio Linkosalo on September 21, 2017, 12:30:49 AM

Maybe use rechargeable NiMh cells and charge them in-situ?


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Hepcat on September 21, 2017, 08:01:27 AM
Response to #23.

Thank you Andrew,
Of course I immediately opened my scale to prove you wrong. I had a convenient bottle that weighs about 20g.  I put it on each corner (to be honest overlapping a little) of the platform. I was amazed that the only difference was an occasional flicker in the second decimal place (20.12 or 20.13). Education can be embarassingly quick with the right teacher!
John

 


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 21, 2017, 11:50:26 AM
These are pictures of a newer prototype, but as you can see, it's not difficult to remove the batteries.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 24, 2017, 11:09:19 PM
I printed out the new version that uses a fiber supper for the thrust load and it is not completed. The new design required a new enclosure to be printed as well an  new cover (slide on and off cover) , and the new collar to hold the thread fiber.

First photo is the load cell and battery holder transferred to the new enclosure.

Second photo, I also installed two rulon J bearings into the new case then used a single edge razor to slice off the excess.

The new shaft is only 1.75" long for this revised design. A short piece of .047 wire about .35" long is needed to hold the thread, in this case spider wire brand fishing line. I cut about a 12" long piece of spider wire and I feed the two ends through the new collar. I drilled a .047 hole in the collar for the thread to come out. You put the short piece of music wire into the new collar so that the loop of the spider wire goes around it. Then press the music wire into the groove on the collar to hold it into place.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 24, 2017, 11:18:16 PM
Then the rubber hook was installed as well as the torque arm assembly and the new collar to hold the spider wire. Make sure the torque arm is positioned so that the transfer washer does not touch the front of the case.

Cur a piece of .047 wire about .65" long, this will hold the thread to the back of the case. Press the wire into the hole until flush with the top of the case.

Thread the the two ends of the spider wire through the hole on the back of the meter. One thread end needs to be on one side of the music wire stop and the other thread on the other side of the music wire stop. Pull the thread ends and then tie a knot making sure the knot goes down into the hole. Then I tied about 6 or 7 more knots to ensure the knots don't slip when winding when the rubber is pulling. I then used a scale and pulled on the hook on the front to about 8 pounds to make sure nothing slipped.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: dslusarc on September 24, 2017, 11:23:25 PM
Here is how it looks all tied. Again make sure when pulling on the hook that the transfer washer does not hit the case anywhere, or the collar holding the thread. When I tied mine I pulled until the hook collar was rubbing on the face of the meter, after it was tied there was a little slop so the hook collar cleared. You may need to do this step a few times to get it tied just right.


The last photo is the finished product with the new case cover slid in position. This meter when I pull on it now with a rubber motor there is no longer the friction from the pull on the front sleeve that was in the earlier version I did at the beginning of the thread.

Don


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: spr on September 25, 2017, 03:09:57 AM
How do you attach this winder to your tripod  winding stooge?


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 25, 2017, 01:48:51 PM
I wind on top of my model box, and I use 3M dual lock to attach it.  It's basically Velcro on steroids and I've never had my torque meter come loose, although I'm only winding F1D motors.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 25, 2017, 03:09:33 PM
Then I tied about 6 or 7 more knots to ensure the knots don't slip when winding when the rubber is pulling. I then used a scale and pulled on the hook on the front to about 8 pounds to make sure nothing slipped.

I used a simple overhand knot, and adjusted the length until it was just right.  Then I added a square knot behind the overhand knot, and put a drop of thin CA on it.  I've pulled as hard as I'm comfortable with, and the knot hasn't slipped at all.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: Skymon on September 25, 2017, 03:36:17 PM
Some really great development work happening here. It's turning in to a very nice product.
Had you considered putting it on shapeways or does Thingverse off the same type of printing service??
Could you look to reduce the wall thickness to reduce the cost and time of printing??
Best regards


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 25, 2017, 04:12:07 PM
Some really great development work happening here. It's turning in to a very nice product.
Had you considered putting it on shapeways or does Thingverse off the same type of printing service??
Could you look to reduce the wall thickness to reduce the cost and time of printing??
Best regards

Thingiverse does offer a printing service, but I'm also about to announce availability of complete torque meters.  I'm selling them as a fundraiser for the junior F1D team.  I haven't figured out how much to ask, but they cost me about $15-$20 each to build.  Anything beyond that will go as a donation to the junior team.

The file versions I plan to post tonight will be the final version.  I can reduce the wall thicknesses a bit, but it honestly doesn't make a major difference in regards to print time.  With my printer and slicer settings it takes about 6 hours to print a complete torque meter, and reducing the wall thickness might drop that by 20-30 minutes.


Title: Re: 3D Printed Digital Torque Meter Build Thread
Post by: jakepF1D on September 26, 2017, 11:16:09 AM
I posted what I think will be the final version of the files last night on Thingiverse.  Most of the changes are cosmetic, but I did thin out the walls a bit to reduce print time slightly.