Methods for parting off larger diameters

Try going part way in with your tool, then back out, slide the parting tool, over a bit to make the groove wider. You make be getting a lot of side friction. May be basic info, but on deep cuts, it is hyper-critical to make sure that the blade is perfect in alignment.
As mentioned above, good steady feed.
I have made 3 to 4 in. cuts on my old Atlas (lantern style) at 500 rpm. On my SB Heavy 10, it is a breeze
 
I part 3" aluminum 6061 T6 on my little 7x16 HF with 3/32" t type blades regularly. Batten down the hatches first ah I mean tighten the gibs, lock down the saddle and set the parting tool for just enough stick out to get about 1/4" deep into the victim. Don your protective head gear and apply oil and set the spindle speed very slow. I start out at 30 rpm. Then advance the tool to get a nice curl going and as you get further in you can set the stick out deeper and slightly speed up the spindle. Keep dripping oil on it. Also if it starts to chatter get more aggressive with the feed. Works great for me. The chips come off in these tiny little curls. It has actually become a fun thing to do. I don't know what lathe you have but the procedure should be adaptable to it. Then again I might be crazy.

Roy
 
I don't have any issues parting normally. I have a 10x28 lathe. It is reasonably well set up, decent rigity. I don't usually have any issues parting 0ff. I've been doing a lot of it lately since I've been making this belt conversion or the mini mill. But I hadn't parted of such a big item before. I assume my previous attempt to part off a large diameter with lack luster results was due to the blade not being straight. I have thought about that but forgotten.

I usually just part off at speed. Works fine. I do like the little curly bits. I had picked a few out one day and wanted to do something with them as they looked neat... as soon as I opened the gear head lid to change the pulley, I lost them. Lol.
I'll give this a shot tonight. See how it goes. I hope I don't screw it up. It's my only piece of stock for this pulley. Pressures on.
 
I have parted off 5 inch aluminum with a 3/32 tool... I started with the blade fairly short, cut as far as I could, then extended the blade and cut more. I parted the 5 inch diameter in 3 steps like this.
I have done this also with great success.
The few times I haven't done this I have bored the hole so I don't have to cut as deep. If done right a bottomed hole can be bored deep enough to be in the middle of the cut off.
 
Two more things. When cutting, keep it cutting, don't let it rub without cutting. Also, don't even think about parting off with a tailstock center pushing on the work...
 
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