Hey Ken, below are the accessories I think are important to make/have for a Sherline lathe:
First, you
need a rear-mounted parting tool. Parting from the front can be done but you will have chatter issues and resort to parting at ultra-low speeds. I normally part at about 3 times normal turning speeds - somewhere around 1200-1500 rpm for mild steel and higher for aluminum, brass. The extension of the blade in a rear-mounted post is not critical; it is from the front. A P1-N blade will cut anything from tiny out to 2" OD with no problem, as long as its sharp. If you make one, try to get the tip of the blade on center or a thou or two below center. Also be sure to make a ledge on the bottom of the post so it registers on the edge of the cross slide to prevent movement. You can find a detailed write up here:
http://www.machinistblog.com/?s=rear+mounted+sherline&submit=Search
This is 12L14 mild steel, 1-1/4 at the cut. I am making a groove. Note the extension of the blade - I was trying to see if I could get it to chatter but couldn't - check the finish in the groove. This was turned at 1200 rpm. I later parted this off about an inch from the chuck and it came right off, no problems.
Next is an exchangeable tip live center. This tool is made from 1144 Stressproof steel, with hardended/tempered O-1 steel tips. The finish is as it came off the lathe. I made this for two key reasons - accuracy and accessing smaller diameter work. It has zero run out with the standard tip and much less than 0.00025" with the extended tip. As you can see, access to small diameters is no longer a problem.
The next one is a boring bar holder. The standard holders with the set screws that mar or otherwise damage your boring bar are also the worst way to mount a bar - it will chatter more. This holder here holds a boring bar very solidly in a finely reamed, straight hole and I recommend you make one. If you do, be sure the length of the holder is 4 times the diameter of the biggest bar you plan to use in it. You can use sleeves for smaller diameter bars. This holder is made from 6061-T6 and can take a 3/8" carbide boring bar at full extension. Larger than that will be done on my bigger lathe. The third pic is a 0.010" deep roughing cut in 1144 steel using a 1/4" Circle Machine CCBI carbide bar. I have bored a hole over 4" deep with my 3/8" bar and it had a consistent diameter all the way down and a very fine finish. None of my bars chatter in this holder.
I highly recommend you make or buy a Graver Tool Rest and make up some gravers. This was built from an article by WR Smith, a fine gentleman and a legendary Horologist. He wrote this one up in Home Machinist but plans can be purchased directly from him in the form of a booklet. He has done several books and videos that I highly recommend for all Sherline owners. The videos can be rented from Smartflix. If you ever wondered how well tooled a Sherline lathe can be, watch his videos.
For those of you who are not familiar with gravers, these tools are used in much the same way that a wood turning tool is used. The tool rest adjusts for height, distance to the work and angle to the work. It allows you to cut balls, concave cuts, and almost any shape you can imagine and it will do it in almost any metal/material, including hardened steel. If you look at the pics of the live center above you will see that the edges of the tool and tips are gently eased with gravers; you wouldn't notice it if I didn't point it out but the tool feels good in your hand, with no sharp edges. Almost every piece that comes off my lathe has been kissed with a graver in some location and I would not be without this tool. You can work with gravers up near a spinning chuck but its a bit unnerving, which is why I also recommend an ER-32 chuck for this kind of stuff.
You can buy a ready-made WR Smith tool rest from Sherline for about 5-6 times the cost to make one but it will get you up and running fast. The gravers are simply 1/8" or 3/16" square and round HSS tool bits. I recommend you only buy 1/8" bits; I almost never use the larger tools.
Finally, the knurling tool. This tool is patterned after one by Chris Heapy of the UK - search. It is made from mild steel, has 1/2" thick arms and is rigid as heck. It indexes on the edge of the cross slide and will not turn in use. It will make a full-pattern knurl in most common materials in a single pass and I haven't mis-tracked with this tool in over a decade. The arms have zero side play, yet move smoothly and easily to accommodate diameters up to about 2-1/8" OD.
Hope this gives you some ideas. If you ask me, the most important tools will be your turning tools. This is a small machine that can cut like a much bigger one with the right tool geometry. I recommend 3/8" square bits for this lathe, not 1/4"; the larger bits are far more rigid and will cut better for you.