Dual lead speeds for acme thread

[BobMachines]

Hello I am wondering what spindle speed you use when cutting a dual lead acme thread in carbon steel. I'm trying to cut a dual 5tpi acme thread internal left hand thread. I'm using a g76 code, but im getting a lot of chatter. Since the feed is doubled I can assume a lot of tool pressure is generated. How much would you take per pass? Ive seen youtube videos on conventional lathes and it seems they are going slow. I do have the correct seat shim as it is a steep helical angle. Thanks

 

[sinha]

Try 29 degree in G76. This may reduce chatter because only one edge will be cutting material.