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Discussion Starter · #1 ·
I could use ideas for the mechanical parts of the sliding mechanism and a hand that comes out to grab the lid and goes back when the lid closes.

I thought of using a servo, stepper or small DC motor. I know a lot of people use deer motors, wiper motors and linkages, etc. but I'm an electronics and software guy. I want to use a a microcontroller like an Arduino, ESP8266, etc. I have a ton of them. I also have small servos and a couple of steppers and a box of DC motors similar but better than those in toy cars. I could use the expertise of you mechanical guys on here!

Initially, I was working out the angle and arc of the lid and thinking of 3D printing a linear gear (like a curved rack of a rack and pinion) and then a round gear for the servo, but then it occurred to me that the easiest method may just be an electric motor. I also have some motor controllers laying around (SN754410 chips and L298 boards) to control them. But I'm open for you guys to steer me in any direction.

If I go servo or stepper, I still would need some kind of gearing or linkage or at least to know how to mount the idea I was thinking is simplest, a string. I suppose all 3 devices could use the string method. The idea there is to have a spool on the end of the the spinning thingie and pegs on the lid where the ends attach. Spin said thingie one way, lid opens, another way, lid closes. I need ideas for where the device should go and where the leverage points should go, etc.

As for the hand, there are those old boxes where you open the lid and a spider jumps out. I think it is just a curved piece of somewhat rigid metal, maybe a gear or spring? And it pops up in the arc of that metal piece. Maybe a string can work there too, or something that pushes against the side of the coffin to push the hand down?

It will have sound of stone sliding and will open jerkily as if someone is pushing it and it catches in places. The programming for that is easy, I need the mechanics. I'm also not sure about my proportions. The base pieces are 1" foam and the lid is made from a 1" piece glued to a 5/8" piece. I am wondering if I need to use 2" pieces for the 3 parts of the base or take some off the edges. Each layer has 2" spacing all around except for the top which has 1"
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Interesting project. If you have the software side down, then that is half the battle. On the mechanical side, you might start by looking up "useless box" type mechanisms - that does something very similar where a lid pops open, hand/finger pops out. So might be some inspiration there.

As for motors, I see dimensions in multiple feet. So while you do get positional command reliability with servos and steppers, I'd be a bit worried that unless this is the lightest and smoothest moving / bearing pivot pieces of styrofoam, (or the servos/steppers are bigger/more powerful than I imagine) it might be getting a bit heavy for them. If this is outside, it might also encounter rain, leaves, wind, cold temps, etc. which could also make it harder to move. ...and nothing is worse than a prop which needs constant fidgeting and fussing with to make it work (don't ask me how I know!)

If I had to do it, given the scale, I'd probably look at driving it with a wiper motor and a couple limit switches. That would have a built-in gearbox and provide plenty of torque. With the arduino you could do PWM speed control if needed, so you could maintain torque across a wide range of speeds.

Of course, with that said, small servos and steppers can put out quite a bit of torque these days, so that could be an option. Given how fast my 3D printer jerks around with the small nema17 stepper motors, you could likely get some pretty good movement out of thsoe. Believe I'd still stick with metal push-rods, small cogged belt drive/pulleys over string, though. ...again for the reliability perspective.

Seems like you'd want some sort of pivot point in the lower, left hand side of your lid, then some sort of drive up toward the top. It sort of becomes a balancing issue at that point... the further toward the 'head' you go, the more leverage you have, so the easier things move, though you also need more displacement from your motor/mechanism. If you have the motor a bit closer to the pivot, the lid would be harder to move, but a small movement on your motor could be a larger movement of the lid.

Hope this helps with some ideas.
 

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I just tackled one of these a couple months ago. I did use a deer motor to power it as I already had it onhand. Here are a couple photos that I had taken while working on it that might give you some ideas on the linkage and whatnot. I used two links of what is basically Erector set pieces that I got from Lowes, I believe it was a 3 inch piece connected to the motor and at the last hole I connected a 6 inch piece which is then connected to the lid. I also used fixed casters to bear the weight of the lid to help the motor out. For the mounting location I measured half way across each side of the crypt, installed a diagonal board to connect the two points and then mounted the motor so the shaft was at the midpoint of the diagonal board. I also built the lid, framework & mechansim as an insert so it simply just sits inside the base of the crypt and I can lift it out if I need to work on it for some reason.

Crypt by Riff_JunkieSFR, on Flickr

Crypt 2 by Riff_JunkieSFR, on Flickr
 

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Discussion Starter · #4 ·
Thank you! @Riff_JunkieSFR How thick is your pink foam? and are you putting 2 different height together on each level or did you just route the edges? And what is the inset spacing from one level to the next? What is the overall dimesion? I am using forced perspective since this will be the 2nd or 3rd row back of tombstones, so at the moment my total length and width is 5' x 2" and height is only 4 - 5/8".

And I watched a video last night that is pretty great. There are several if you just google "3d printed rack and pinion servo". Or substitute "linear actuator" for "servo" to see other ideas. This is the first one I watched:

I'm really tempted to do this just for cool factor, reliability, and lack of having to tweak it. I would need to make a different rack that has a round open hole, like the end of a wrench, for the end so that it can go over a post on the lid. The post could then spin so the fact that the lid prescribes an arc won't be a problem. So at least i am down to 2 ideas. The simplest is a string connected to 2 posts and a winding grear/spool on the servo/motor and printing one of these somewhere. I'm not sure if the servo mount should also be able to spin so there isn't and torsion with the lid not moving straight. Guess I should think of parts count and assembly simplicity too.

This is the sliding hidden compartment picture frame.

 

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I used 1" foam board to build it, but I layed the pieces on the sides to make them 2" thick which looked better to me and then routed the edges as I built it up. The base of it is 6'x2' and it is roughly 8" tall at the lid. I stepped it in 1" from the edge of the route on each level. I actually almost did a complete design change once I got the base assembled that I beileve would have been closer to the movement you are going for. I had a skeleton sitting close to the work bench, and I thought wouldn't it be cool if I made the lid slide out of the way to reveal a skeleton inside the crypt. I was going to use a drawer slide at the top & bottom of the opening to guide the lid and attach the motor linkage to the midpoint of the long side so instead of pivoting it would simply slide to the side. The problem I ran into was the deminsions I used did not produce the interior space I needed to conceal the skeleton so I decided to continue on with the original design. Anyway I believe the drawer slides would work to both guide the lid & bear it's weight. I really like how compact the 3D printed rack & pinion design is, and you are right it would be easier to setup & more reliable. One more reason I need to quit stalling on getting a 3D printer.
 

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The dc motors from Surplus Center are mostly from the back windows of SUV's and work in an arc not continuous rotation. It wold be extremely simple to put an armature on the shank to slide the lid back and forth. I like the idea of the hand working like the spider in a box gag. should be very simple and cheap. It probably wont need any printed parts or Arduino to run add a ultrasonic mist maker and a green led's.


It does 30cycles per minute at 12v you could drop that down to make it much slower with a rheostat. It will have plenty of power to move a secondary gag like the reaching hand.
 

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Discussion Starter · #8 ·
Thanks for the ideas. I think I tend to overengineer things. I'll look at that mist maker. I may rebuild a fogging cauldron with a mist maker instead of the fogger. There is a part I can't identify so have no idea what temp it is supposed to open or close on.

As for the coffin lid, I had 2 more thoughts. The first is that it would be cool if the middle knuckles on the hand were hinged. Probably they could be so that when open, the backs of the fingers have a stop so when under the lid and rising in an arc, the fingers are curved but "locked". As the hand curves more when rising above the lid, gravity would let the fingers move and they would fall onto the lid of the coffin. This would give the illusion of the hand rising and fingers gripping the lid to move it. So I have to figure that one out. That may be an upgrade for next year ;)

The other idea was back to the sliding mechanism. I have 3 and am drawing them all out, though as I type this I may eliminate the latest option. That option is a curved slot in the lid running along the long axis with my push/pull assembly running diagonally towards the long end corner. That way the lid would open in 2 directions. The whole lid would follow an arc so that the lid opened right to left at the top corner while at the same time sliding down a bit. It is a more complex motion, and make look kinda cool, but might not be worth the extra time to work out how that would work. It would be a simple mechanism though of one or 2 channels that pegs in the lid follow as it is pushed and pulled.

The other 2 ideas are just string on a spool gear on the servo with spools to guide the string and 2 pegs in the lid for the open and close, or the rack servo I showed above with a custom made rack with a loop at the end that fits over a peg in the lid. In both these cases, I would have a pivot point close to the front left corner with the lid opening at the back right corner.

But as I write this, I have a little cardboard model and experimented by putting the pivot point on the same side of where it opens. So the movement is different if the pivot point is front left and it opens back right, vs. the pivot being in the front right with it opening from the back right. Hmmmm! What do you think?

https://flic.kr/p/2mnMDSk
 

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The good thing about using a programmed microprocessor is that you can introduce a bit of randomization. The eye picks up pretty quickly when a prop has a short cycle. One interesting variation on the "coin grabber" it to use a arduino to provide multiple hesitant trials for the hand to grab the coin before it actually does grab it.
 
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I think the complex movement would be awesome to sort out although it most certainly would be tougher than a simple pivot. I also like option two of the model; option one is more like the traditional setup of this prop, but option two seems like it is closer to the actual pivot if the lid was being pried open.
 
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