Articulating Arm Question

[Anonymous]

I made this 3D Printed Articulating system for mounting cameras, lamps, etc. I used an old design that those who have one of those Soldering Helping Hands can identify with.  The problem is I cannot get enough of a tight grip on the joints. The ball and joint is too smooth. No matter how tight you get it, even if you use a metal bolt, it bows the two halves and is still too loose. So why I am here is, I am hoping someone knows how to knurl the ball and joint areas, and moreover, does anyone have further suggestions on how to make this better besides that?

This has been sitting on my desk in hopes of me learning enough about this program to succeed on my own, but I'm afraid that is very slow going! Thank you!

 

[docara]

HI,

First thought - As the joints are essentially a pivoting hinge, make the black ends hollow or more specifically put a hole through them and put individual threaded parts through each joint then, when you tighten them down, they impart much more force on the actual joint. The way you have it at the moment the yellow plastic will just bend at the threaded fastener.

Also, is the contact area between the black and yellow joints sufficient.

Matt

[Anonymous]

Thanks.. The problem with the first thought is, they articulate through 360 degrees on that plane. With loosening the one bolt, the entire assembly can move any which way. I COULD do that, but then it would require the end user to use the turnbuckles for that. I THINK I have sufficient area for clamping since both the ball and the halves are concave to match each other. The only idea I have is to add knurling to the concaves and the ball. But I don't know how to do that. Keep them ideas coming! 🙂

[docara]

Hi Devek,

Sorry I had a brain dead moment and well yeah lets just start again 

So I would guess the ends of the yellow sections are lifting up you're current design when you clamp them down. Try shortening the yellow sections and / or add longitudinal ribs on the yellow to help increase the clamp force without them bending. (Think isosceles triangle in section where the peak is where the screw is located and the legs point towards the ball joints).

Try and make the contact area greater by making deeper concave section in the sockets.

Your 3D printing material might also not work in your application some 'plastics' are self lubricating.

Matt

[Anonymous]

I agree those are all good points. The real problem here is the concave and balls need to be knurled and I do not know how to do that. I can shorten the halves, but not by enough, and adding girder support like you suggest may help some, but in the end, if the parts are not knurled, they will still slip. I am thinking about 1mm knurling would solve most of the problem using the same knurling pattern on each side will allow them to connect very closely. If anyone feels like giving a short knurling tutorial that would help, I wouldn't complain! Thanks again!

[etfrench]

I expect knurling would just decrease the mating surface area and would require more force to equal what you have now.  Try roughing the surface with sandpaper (I'd start with 80 grit).

[docara]

Why do they NEED to be knurled? If your design is good and robust you don't need 'finger grips' on the inside of joints to be rigid and self supporting.

Either the mechanical design is wrong or the material chosen is wrong!. If these are to be made commercially, I would assume injection moulding, then other materials are more suitable - much stronger and have more friction than PLA / Nylon.

You asked if we had any suggestions on how to improve your design then you keep saying you want to knurl the joints and not consider your design might be weak which is part of the problem.

Good luck with your project

[Anonymous]

I am incorporating all of your suggestions in the next design. I know from how this works from experience that it will require more thought. Until I can try Knurling and toss it out as a bad idea, I have not examined every angle of this project yet that has been proposed. Sandpaper is unacceptable since the end user wouldn't have it on hand. Right now its for the grandkids for a light I have designed, but who knows where this will lead? It needs to be made out of PLA because the hope is that others can print it out easily. 

I do not think including Sandpaper because you would HAVE to since it would smooth itself out over a short period is a solution. But knurling would require much less force to do the same work if the knurls were deep (1mm  or so)  and uniform between the pieces, they would cause a locking action that does not currently exist.

[laughingcreek]

Are you wanting the knurling surfaces to lock into each other, or just for the friction?

because of the degrees of movement, your not going o be able to get the teeth of the knurling to fit together.  If your looking for friction.  I might try just adding some ridges around the mating surfaces like in the pic. easy enough if you just want something to try.

A true knurling on a sphere (as opposed to a cylinder or flat surface) is going to be tricky.  There are several posts about putting dimples on golf balls you can search for, only instead of dimples, you would be adding pyramids or rectangles.

Already suggested is adding a rib (ribs) parallel to the center of the wholes.  I second that notion.

Last idea-try making the diameter of the "cup" slightly smaller than the diameter of the sphere.  As you tighten the screw down, and force the cup over he sphere, the cup spreads out and will give you a more uniform pressure/contact.  This happens also when the diameters are the same, only the result is you are only really making solid contact right under the nut, the rest of the cup might as well not be there because it's not being pushed against the sphere.

[laughingcreek]

I didn't mean "right under the nut", but rather at a single contact point near the top of the cup.

Also, moving the cups as close together as possible, (making the linkage shorter) will probably help

[Anonymous]

Good advice. Yes, you see with true knurling that is concentric, there would be much more contact area because the knurls would fit into each other. But I can see that is going to be too hard to do. I have remade the halves following all of the advice in this thread, but I fear without the knurls it will still be too easy to move. Ridges as shown would lessen the contact area, and might work for awhile, but I think they would be short lived. I am not sure yet because it takes about a day or more to print these pieces at 100% infill. I added 1/4" (6mm) to the diameter of the ball and concave areas, brought them as close together as possible and added support and thickness. Knurls properly done would make it work nicely, but that is probably beyond asking someone to do and I wouldn't know where to start. It took me a half day just to produce the attached! I am sure that is hard doing it over a ball like this.

[etfrench]

Knurling a sphere can be quite easy depending on the look you want:

Start with two circles. One for the outer diameter of the sphere. One for the depth of the knurl.  Add a circular pattern of lines for laying out the knurls:

Revolve the knurl cutouts areas:

Create another at 90 degrees.  Use Combine|Cut to remove the knurls from the sphere:

[Anonymous]

That's nice work!  The issue with this particular application is a little different in that the valleys also have to have the square knobby areas. Think of interlocking parts. The sphere needs that pattern and also the concave areas of the halves need the same pattern. Its pretty work alright. I wont be getting this thing off my desk anytime soon because its above my paygrade at the moment, but its a start. I will play with this and see how far I get. Thanks for the effort!

[sderyckTJ4ZR]

I know this is a little late.... I was looking for a way to knurl a ball in fusion so thanks for that. It kind of works but with oddities on the sides. For an answer to this topic, what if you put some o-ring grooves in the model and popped o-rings under compression into the dished areas for the ball to grip into. Thinking the ball has at least some texture from 3d printing and you leave the o-rings 40% proud of the groove.

[Drewpan]

Hi,

I think you are missing the point here with what you are trying to do.

The problem is you do not have sufficient Clamping Force and/or sufficient Friction to maintain position

of the Joint. One reason you don't have enough Clamping Force is because the Material you are using

does not have enough Stiffness so it deforms and reduces the Force. Another is that the Material you

are using does not have the correct co-efficient of Friction for the Force applied to prevent movement.

Getting back to basics.

If Force = Pressure / Area, have you calculated these things? What is the maximum Force you can generate?

It is unlikely you can increase the Area available in this design. Knurling will increase the Area but will rely

more on everything interlocking in the Knurl. How much accuracy do you need if interlocking restricts where

you can position the arm? The only way to increase Pressure would be to stiffen the material so that it doesn't

deform and all of the Force is transferred to the Joint. The only other thing to consider is the co-efficient of

friction of the materials. Knurling will also quickly wear in this situation with PLA.

We know that this type of Joint can be clamped so there is nothing actually wrong with the design. One of the

reasons we study properties of materials in engineering is so we can solve these problems in a cost effective

manner. PLA is cheap and perfect for rapid prototyping in many situations. You have learned that your design

is feasable by using it and testing it here. Now you need to solve the problem of locking the design in place

for use.

I have seen similar Joints as you have with Stainless Steel balls and Plastic clamps. This is a matter of working

out a cost effective way create your design. You probably just need to change materials and it will just work.

Do you actually know the co-efficient of friction for PLA? Finding this out would be a good start. Then you can

look for another material with a higher co-efficient. PLA is also not stiff enough here, would ABS work? Still

cheap and available for 3D Printing.

Hope this helps.

Cheers

Andrew

[TrippyLighting]

---

@Drewpan wrote:

Hi,

I think you are missing the point here with what you are trying to do.

The problem is you do not have sufficient Clamping Force and/or sufficient Friction to maintain position

of the Joint. One reason you don't have enough Clamping Force is because the Material you are using

does not have enough Stiffness so it deforms and reduces the Force. Another is that the Material you

are using does not have the correct co-efficient of Friction for the Force applied to prevent movement.

---

Spot on assessment!

[a_antonov]

You can also apply some liquid rosin flux into the joints to increase the friction.

[wersy]

No matter what print material, I would at least reinforce it with bars.

[wersy]

With a dual filament printer, it would be possible to print the sphere surfaces with rubbery filament.
As in this case with the clamp

https://www.youtube.com/watch?v=-GwHS8BQSlo

[etfrench]

It's only been 5 years since the topic was opened😁 

Simply printing the arms at a 90 degree angle from their current position would have made them much stronger.