Hi Steve
All the slitting saw arbours I have are home made. They are all RH threaded, and none are keyed. I allways have mine set up to tighten up if it jams. It all depends on what way you have the teeth on the blade going. I used to worry about jaming up and the blade breaking, and I used to flip the blade over, and run the spindle in reverse. But, I have found with just a light snag, it will un screw the fastener on the arbour. So, I have stopped doing it that way. Over the time, I have found that it does take a fair bit to bust a blade. I have done it in the past, but its rare these days.
You wil probably find slow spindle speed is much safer than trying to run the spindle fast, the slower the better really. The feed rate should allso be very slow, and if your mill is on power feeds, this makes it easy for you. If not, then its easy to want to rush through the job and feed to fast, and that will usually be the root of trouble. The other main thing is keep the cutting oil handy, and dont let the blade run dry, it can heat up and jam on you.
You can run the slit saw either direction, but I perfer to have the arbour bolt tighten, rather than loosen. They loosen to easy on a light snag. Being I dont run them with keys, thats my prefferance. I have used keyed arbours, way back when, but in my opinion, the key help to break the saw blade when it does snag.