If you don't know what size tool post you need, that's the first thing to figure out. Chances are it's an AXA, but to be sure you need to
measure the distance from the top of your compound to the spindle centerline and compare that figure with the manufacturer's specs.
If for some reason an AXA is too big, there is a smaller style available. (Or, if someone with a Jet chimes in you won't have to go through
Basically you tool post choices are either Expensive (Dorian or Aloris) or Chinese (Shars, Phase II etc.). People around here seem
pretty happy with their Chinese tool posts.
Be sure and ask more questions as you think of them.
I bought a QCTP for my 6" Atlas a few years ago from littlemachineshop. It was a slightly different tool but it worked
fine and their service was good. I think I bought some tooling from them as well with no problem.
I recently bought a BXA from Aloris for my lathe. A larger size than yours, probably AXA but you'll need to check as suggested above.
For what it is worth I'd sugest getting the better quality post such as Aloris I think the wedge type clamp is better than the piston type Then you can always buy the much cheaper tool holders of the internet they are mostly interchangable. Aloris will supply the post nut machined to your dimensions if you supply them when ordering
I don't own a 9x20 so ignore me if you like but I wanted to add some perspective on tool post material. Aluminum posts are fine for little lathes; here, I'm talking about the Sherline and Taig class of lathe. I own a Sherline lathe and have several aluminum QCTP for it and it works fine. However, above that class I would go for a steel tool post.
A tool post has to be rigid when subjected to cutting loads. We tend to think that a solid chunk of anything will remain solid but this is not so. Each material has a Modulus of Elasticity that describes the tendency of a material to move under load and aluminum has a rating 1/3 that of steel. That means it will tend to move much more easily than steel. This translates into a much less rigid set up so chatter/inaccuracy can result. Bottom line is to go for a steel tool post.
I use AXA tool posts on my 11" lathe. Mine are Aloris and Dorian posts and both are wedge type posts. They work well but from what I have read, so do the Asian versions of these posts. Although most of us feel the wedge type posts are more repeatable under heavy loads, that has not been proven to my knowledge. What seems to separate the Asian versions from the higher end posts like Aloris and Dorian is fit and finish. In the past, Dorian was better than Aloris in this regard but it seems that the quality of Dorian posts has fallen off of late. In any case, a steel Asian post appears to be more than adequate for a smaller hobby lathe and the wedge type is preferred.
From what I have seen here on HM, Phase II posts seem to be a bit better quality among the Asian posts. Shars also has a decent following. Can't say about the other also-rans.
+1 on what mikey said. The tool post I bought for my 6" lathe was aluminum and it seemed to work fine, but honestly
if I were buying again I would look for a steel tool post. Other than cost there's no benefit to aluminum and small lathes need all the help they can get with rigidity. For a 9" machine it would be steel for sure.
My 10" machine has a Phase II and I'm happy with.
If budget is an issue, consider a used tool post and holders from eBay. Good tool posts turn up there, and I've bought toolholders with no problems. They don't really wear out and they're hard to break. . Having said that, Shars toolholders are so cheap the price is hard to beat. Depends on what you want.
Grizzly, Shars, and Bangood all sell mini QCTP sets. An AXA requires about 15/16" from the top of the compound to the center line of the spindle in order to utilize 1/2" tooling. If you want to use 3/8" or 1/4" tooling, you can subtract 1/8" and 14" respectively from that dimension.
I am not sure about dimensions for the mini, sometimes referred to as OXA. Shars says 5/8"less 1/64" for a 1/2" tool for their OXA set.