My printer has the z gantry supported only on one side.

My printer has the z gantry supported only on one side. I am planning to add a second leadscrew at the other end, with a timing belt connected to the current z stepper.

There doesn’t seem much chance I am going to get this to be exactly at a distance that would make a standard timing belt the right length. And I have read that cutting and joining them is difficult - no surprise.

What would be the right way to tension the belt while still having a continuous loop? Would you have an idler in the loop that is adjustable to “push out” the belt to tighten it?

Yes, you want at least one idler to change the shape of the loop. It’s technically possible to do it with just 3 pulleys, but ensuring that they all have proper contact with the belt requires a total of at least 4 (at least one has to be on the smooth side of the belt).

@Whosa_whatsis I am unclear where the fourth one would go? I get that the flat side one is to push it out, but another teeth side one to tension it back in?

I assume what you’re thinking is that you run a loop of belt between two pulleys, then press an idler against the loop on one side to tension it? The problem with that is that it gives very little room for adjustment before the teeth on the two sides start catching on one another, and you need to be able to adjust it further when the belt stretches. By creating an N or Z shape in one side of the belt, the amount of slack you can eat up is limited only by the way you mount the pulleys.

Oh yeah, I see, I was visualising it wrong, yes I would need to push it out to take up most of the slack then push it in to adjust without teeth hitting teeth

Take a look at how some people are doing it to the 3drag printer for example, you may be able to adapt it to yours

@Steve_M Ah yes, thanks that is great