Welcome to the RobotShop Community!
Wow! Amazing project! That large scale spider is surely a challenge!
What made you choose the Nema Stepper?
If you are curious to discover other projects, here is a recent Octopod project: https://community.robotshop.com/robots/show/spooder-octopod-robot
Looking forward to seeing your progress! 
I tried the elbow joint with the 5V steppers but they are to weak to get the joint moving let alone hold some weight. The Nema steppers are much stronger.
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@tacotijsma it seems that you are back on this project after 1 year
In general, I am concerned that you are dramatically underestimating the forces needed. Also, you would not normally need two actuator motors for each joint. Though, you would need to add something to take up slack on one side of the cable.
I recommend that you cheat.
Depending on the situation, you could put the weight of the spider’s body on wheels, since spiders often walk fairly close to the ground, the wheels would be hidden. Perhaps put the main body on a bit of a spring-loaded suspension, so the body will wiggle a little as it walks. You can also add a spring to the main joints where the legs attach to the body. Then the motors don’t have to work as hard to lift the leg.
If the robot is inside, another approach is to support it from above with a cable or bungee cord. There are cable reels that have springs in them to keep a relatively even amount of force if it is changing elevation a lot. If it is going up a wall, that approach is easy. But, if it is walking along the floor, consider putting a bungee cord on a curtain track, which is motorized, so that it will keep above your spider.
-joe
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Wow! what a build! Earlier I only saw DIY spider robot kit and some more like this: Quadruped Spider Robot (3D Printed Parts, SG90 Servo Motor, Arduino Nano) - Share Project - PCBWay
But your spider is different. It really has the thin body of a spider.
Hello Joe,
The steppers only drive the hips. The ankles and knees are going to be powered with 25Kg servo’s. Most of the holding force will be absorbed by the knees and all eight of them combined can hold 200Kg. I believe that is going to be enough power.
Grtz,
Taco
tacoijsma, earlier in the discussion (which was quite a while ago), you commented about using servos that didn’t have enough power. So, I assumed, incorrectly, that this was still the case. Though I have seen people take on projects like this, without being able to do the calculations to determine the forces involved. Though, some people seem to succeed suing trial-and-error, and just have a good feel for where to start.
But, good to see that you are making progress.
-Joe
So cool!what a wonderful building 
