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Forward Kinematics/D-H parameters for perpendicular joint axes

I am trying to compute forward kinematics of the Kuka youBot using DH convention:


The arm joint 1 and arm joint 5 are revolute and rotate about the world z-axis (pointing to the sky)

But the other 3 joints are all revolute and rotate about x-axis, let's say (points horizontally)

DH convention says the "joint distance" is along the "common normal". But unless I am mistaken, the only common normal is the y-axis, and that is also horizontal, meaning there is no joint distance.

I was thinking I would use link offset for joint1 - joint2, but then I ran into a problem with joint4 - joint5. Link offset is supposed to be along the previous z-axis, and in that case it would point horizontally out to nowhere. But link distance STILL doesn't work either, because that is the common normal distance, and as established the common normal is x-axis, also horizontal. So now I feel very screwed. I am sure there is a simple solution but I can't see it.

So I guess the question is, how do I use the DH convention for the links between 1-2 and 4-5, when the joint rotational axes are perpendicular?