Calculating Cycle Time Using CSS......How?

I have come across something I have no idea how to figure (Ox if you're reading this.....I mean "finger" ). I normally work on all small parts 1.5" diameter and under in my cnc lathes so CSS has not really made a big difference in calculating my cycle times, but now after having gotten my feet wet doing some larger parts (over 6" dia.) a couple weeks ago, I am looking at some 4.125" dia. parts of 4140, and some 6.375" Dia. pieces of 6061. I need to face these and drill a hole through......no rocket science here.......but my question is, how do I figure a cycle time for facing the pieces? I will be using constant surface speed when facing, but it starts out at around 556 RPM's for the 4140 material (600 SFM) at that diameter according to MEPRO and will end up around 2500 RPM (max RPM) when it gets to the center. I realize the time difference is not much at all, but I am curious none the less how it's done. I'm looking at 325 pieces and with both ends needing to be faced, I just want to be as competitive as possible so shaving off a few seconds here and there could be just the ticket. What do you guys do? Base your time on the start point of the cut at that speed, or is there some funky equation to use?

Later,
Russ

If you can drill the hole first (?) so the calculated RPM never exceeds your max RPM, I think you can just use the average diameter.

Example - 6" OD, 2" hole = 4" average diameter

You can input the expression in the Turn Diameter box in MEPro
(6+2)/2



Software For Metalworking
http://closetolerancesoftware.com

Like MRainey said, and you said...not much difference. Drill then face. In these cases, I try to make up cycle time seconds elsewhere. Index position, tool location, drill feed rates (diff of .0005 per rev can make a diff sometimes), etc. Calculating times with CSS can be easily done...you know the surface feet area and how far you are moving per minute...shop math anyone?

How about a volume calculation. You know the SFM, which is easy to turn into IPM that the material is moving past your cutter. You know your width of cut, you know your depth of cut.

Say 600sfm, or 7200ipm(cutter speed through material) * .030 width of cut * .010 DOC = 2.16 cubic inches per minute.

At 6" diameter facing taking off .090" = 2.54 cubic inches, so a little more than a minute, not taking into account maximum RPM or rapid time.