William,
I had that happen to me with my Grizzly (first lathe) also. If I had to guess I'd say it snapped right where the top of the rest meets the post, right? That's where they all break. I think it's caused by having a sharp right angle corner right there where the post meets the top part of the rest, right at the Tee. Any time you have a sharp corner like that it creates a stress point where it can snap. You can do several things: Call Grizzly and have them repair it by replacing the cast post with a steel post. They did it for me - no charge. They will drill up into the top section and insert a steel post with a pin. After that you should have no problems. They'll tell you it'll take about a week. After 3 weeks of waiting I went to Woodcraft and bought their 6-inch modular toolrest, which is very nice. Soon thereafter the Grizzly toolrest came back and it has worked fine ever since. Then I had both a 6 inch and 12 inch rest, which was nice to have. When I upgraded to my Stubby lathe which takes a 1 inch post all I had to do was buy a
1 inch post from Woodcraft to continue using the 6 inch rest (it's a modular system and you can get different bars and different posts and they screw together). The 6 inch rest is very handy for doing smaller stuff like pens or ornaments. My Stubby toolrest is cast iron also, as are most toolrests, but it's rounded at that point where the post and the top meet so it spreads the forces better. It's also a much better casting. The fact that your rest is cast iron isn't a problem in itself. It's the design and the casting--not the material--that caused it to fail.
Incidentally, the Jet toolrests are made exactly like the Grizzlys and they also break. I was at a 6-club competition picnic a few years ago and all the lathes being used were Jet mini's. The competition was for each team to make a natural edged bowl in one hour using 6-person teams, each person getting 10 minutes. All the rough blanks were mounted before starting the clock. When the clock started there were at least 4 broken toolrests in the first 10 seconds, all broken at that Tee join, all at the exact same spot. Both Jet and Grizzly obviously figure that it's cheaper to replace broken rests than to redesign them to make them right.
Take my advice and (1) Get Grizzly to fix the rest with a steel post, and (2) immediately order the Woodcraft 6 inch rest. You'll be glad you did.
-Jim Gott- San Jose, CA