I might be a little late to the game here. I also have a SB9 and had problems parting.
Turn your compound to 0 and the tool holder should have clearance to drop lower and get the tip at centre.
Firstly, regrind the tool bit or an area near the tip to almost 90 degrees. The problem you likely are having is that it has way too much angle and it digs in and stalls the spindle, chatters like mad or generally pisses you off! The base of your tool holder has the needed angle of around 7 degrees. You'll find that this is about perfect for a tool with the 90 degree angle.
Second: I found that on a wimpy flexible SB9 a much thinner parting tool helps tremendously to not load the lathe as much and eliminates chatter. I can crank away with the chips coming off at a very fast rate. *You have to almost flood it with oil as you cut. Left hand oiling while your right hand is feeding the tool. Or use the powered crossfeed. Another help is to tighten the gib screws on the crossfeed to eliminate any unwanted motion.
I use a T shaped 1/16 wide tool from this company.
ICS Cutting Tools
On larger diameters initially keep the tool stick out at a minimum and only enough to cut the part diameter say 1/4 inch deep.. This wards off flex and potential chatter. Cut until the tool holder is about to touch the stock. Then pull the tool out another 3/8 inch and resume cutting, Repeat as needed. As the tool holder base is angled, when you cutoff the now different diameters you'll have to reset your tool point height for that diameter to keep it on centre. This applies to the part in process. Remember that as you pull the tool out to go incrementally deeper in a large diameter the tool also goes uphill and above centre.
Try these suggestions and you should be parting like it's nothing. **With the 1/16 wide tool don't try grinding a side angle on it so the cutoff nub stays with the stock in the lathe. The 1/16th tool is too weak to do this and it will flex towards the cutoff part. Grind it straight and live with the nub.
edit:
Last week I parted off a 3 inch bar of 4140 with no problem. It took a while, but it worked just fine.