Another thing that I have read over the internet (gotta be true) is that we are using carbide bits wrong...Carbide cutters/drills are apparently "suppose" to be run @ 3 to 4 times the speed of HSS..Most of our home based machines are not capable of reaching the speeds to allow the carbide tool to achieve optimal performance...
I have mostly carbide stuff myself...When I'm having "trouble" with something I will usually speed up rather than slow down...
Just about any modern lathe can hit 1800 RPM. Most of the work I do is 3/8" diameter and greater and I've never had issues with not enough speed. Here's my favorite SFM chart; copies of which are plastered near the lathe and mill.
A good formula for RPM using HSS is [4 x SFM / Diameter] where the diameter is measured in inches. For carbide multiply the HSS value by 2 or 3. For positive rake inserts, the 2x value should be your starting point. For neutral rake or negative rake inserts, start with 3x. For soft materials, increasing speeds is usually the right thing to do. If you go beyond 4x, double-check the material type and condition. For VERY hard materials that resist scratching with a file (or if you know its above RC 40), use the 2x speed as the starting point and drop down 10% steps to find the final speed.
As far as feed rates are concerned, for typical steel like 1045 or 4140 at 32 Rc, machines in decent condition with 1 HP motors are comfortable up to about 0.008 IPR. 2 HP an go up to 0.012 IPR. 3HP is around 0.016 IPR. Depending on insert style, best finishes are obtained at/around 0.005 IPR. Any slower and the edge drags/rubs too much. Lines start to show-up when going faster. All of this is really "ballpark" information that assumes diameters of at least 1/2" and the workpiece is properly supported with the tailstock and/or center support.
Some folks are looking for that clean bright shimmer. On some metals, it's not going to happen (like 1117 or anything with L (lead)). For shiny finish, your best bet is medium carbon like 1045 or 4140 prehard (about 28-32 Rc). Harder gives a much nicer finish. As you turn a piece of metal, hardness decreases even if the material is claimed to be "thru-hardened". Only certain tool steels can be fully thru-hardened -and you can't turn HSS that's been hardened. A 1.5" diameter piece of 1045 that is Rc 42 at the surface will be about (ballpark) 28 down at the 1/2" diameter range -and that depends on how the foundry did the hardening. The only way to control this, is to harden it yourself -which is what I do when making high-horsepower shafts etc...
Over the years, countless people have asked for advice on speeds/feeds and countless times people have responded with "hey, that worked pretty good". This is the secret sauce that I go by.
Have at it.
Ray
PS: Here's the start of a miter gear made about 20 minutes ago. 4143 Rc 38 (self heat treated). The blank was formed about 50 thou oversize in all dimensions then heat treated. The shoulder and shaft are now taken down to final size but, the 45 degree angle is still in rough dimension. The shoulder is 1.3" diameter, shaft is 1" diameter. I chose an SFM of 80, used negative rake CCMT (431) insert. DoC' were roughly 35 thou. Feed was about 0.007 IPR. Machine is a PM1236.
It will be a 30 tooth Mod-1 gear.
