HDPE or aluminium??

[baker]

Hi guys,
I'm new here and I want to build a robot but don't know what material to use. I've read the tutorial about the HDPE on the site and decided that HDPE is the best. However, I'm not certain what to use so I wonder if you can help me. Bellow is a picture of the robot in 2D. Note that all of Its parts are square (I mean that the height and width are the same in all parts except the arm and some parts varies in thickness)

[Gertlex]

You gotta tell us units.  The difference between cm and ft would certainly affect things...

Is the joint in the arm a fixed joint, or is there some sort of actuator in the middle? Also the scissor lift is the only thing I can identify for sure... so labels would help

[baker]

It's in cm. Below is a new image with some descriptions:
1- The base of the robot. It will be attached with 4 omni-wheels. Uses 2 DC motors to move perpendicularly .
2- I don't know the name of this mechanical "lift", but here how does work. There is a motor at the bottom of the zigzag connected with a gear that is connected to a bar like a screw connects a side of the zigzag with the other side. When the motor starts, the bar rotates and the zigzag sides becomes gradually close to each other due to the washers on each size. When they become close the length  zigzag thing will increase and that will increase the robot height.   
 3-This is a circular plate which is connected to a servo so it can rotate.
4-(shoulder) This part is a like a beam which holds the arm. It's connected with a servo so it can rotate 90 degree.
5-This is a joint. There is a servo there so the arm can rotate 90 degree.
6- finally, this is the hand of the robot. It has a motor so the pincers can get close to each other to grab things.

I'm sorry if the description were not good enough but I'm a beginner!!
thanks a lot!!

[Gertlex]

In all probability, the lift mechanism (scissor lift, generally) is going to be very wobbly.  Even if you built fixed set of links like in your drawing, the top would probably sway quite a bit.  From what I've seen of industrial scissor lifts, they're very solidly built (aka you would need to be very precise with making each piece, and have snug holes), and don't use half as many 'zigzags' as you're using.

Other than that, the other ideas are workable, but I infer the drawing isn't an actual design.  I would recommend prototyping with HDPE (1/8" maybe?) as that will be cheaper.

[baker]

so if I make it like the pic below, will it work??

[Gertlex]

If you make it very high quality with press-fitted bushings in the holes, maybe.  If you have Lego Technic pieces sitting around, see  what happens when you use those.

[Admin]

Gertlex is right, it needs to be very rigid.

If the links are flexible and joints are loose enough to allow for ~5 degrees of bending (a very small amount), that gets amplified along the distance of your 50 inch scissor lift.

tan(5 degrees) * 50" = 4.4" horizontal sway

That's a lot!

The arm will also have lateral forces, causing it to bend even more. I wouldn't recommend the scissor feature in that design, at least not one with more than two cross bars.

[baker]

so what do you recommend instead??

[Admin]

A linear actuator/motor.

Or you can try something like this:
http://www.gizmag.com/go/8071/picture/38131/

I'm assuming what you need is very high linear travel, right? By how much?

[Gertlex]

so what do you recommend instead??

It's not clear what you want to do, other than 'build a robot'

[baker]

The maximum height between the base and the arm base would be 1 meter

[Gertlex]

The Maxwell robot on this blog is probably the approach to go with.  Money is also useful.

[baker]

That's a good idea, but how am I going to make this without buying +100$ linear actuator?

[Gertlex]

Well you could use an acme threaded rod. I've no personal experience with such designs, but they're valid.

You're building a large robot, and seemingly after some level of precision, so it's going to cost money.