Inverse Time feedrate for 4th Axis

[slideleft]

So I am trying a 4th axis for the 1st time. Exciting. My MasterCam post is outputting what I assume is inverse time feedrates. Is there a setting in the Haas control to switch to this setting or is Inches per Minute my only option?
Thanks again.

[Donkey Hotey]

Straight from Haas:
http://www.cncmagazine.com/archive01/v3i08/v3i08k.htm

Dear Applications,

I have a ’97 model VF-3 vertical machining center with the fourth-axis option and an HRT 310 rotary table. Up until now I have only used the rotary table for indexing operations on simple parts. I recently bid on a job that will require full four-axis motion. My CAD/CAM system will support both feed-per-minute and inverse-time-feed modes but I am not familiar with inverse-time feed. Should I use inverse-time feed on full fourth-axis parts? What exactly is inverse-time-feed mode?

Sincerely,
Mark Curry

Dear Mark,

Inverse-time feed is not as complicated as it sounds. Inverse-time-feed rates simply dictate the amount of time a particular stroke will take to complete. To calculate the time for a stroke, divide the inverse-time-feed rate into 60. For example, an inverse-time-feed rate of F1000 dictates that the commanded motion of that line will take 0.06 seconds. This method of feed-rate command allows for more precise control of the feed rate when combining rotary and linear axes. With the Haas control you have the option of running in either inverse-time-feed mode or feed-per-minute. The Haas control can convert linear-feed-per-minute rates to approximated angular-feed-rates based on the user-definable part diameter stored in the 4th- and/or 5th-axis diameter setting(s). This feature allows the user to program a combination of linear and rotary axis motions in feed-per-minute mode, but the rotary feed rate will only be exactly correct at the diameter set by the user. Therefore, inverse-time feed is preferred when mixing linear and rotary axes because it is not a linear-feed-rate command, but rather a time-based feed command.

Sincerely,
Haas Applications

I was a bit confused about it until I read that description. It finally makes some sense to me. I hope it helps you.

Oh yeah, these two are tied to it as well:
G93 INVERSE TIME FEED MODE ON
G94* INVERSE TIME FEED MODE OFF / FEED PER MINUTE ON (SETTING 56)

[NCDataServices]

Larry Jones - NCData Services

Here is how to convert tool tip IPM feed rate G94 to inverse time Feed Rate Number G93
on almost all (all fanuc type including HAAS & FADAL) CNC. The previous answer from HAAS assumes you know the time in minutes.


Terms:
A = 3D Start position of Tool Tip in X,Y,Z
B = 3D End position of Tool Tip in X,Y,Z
RATE = Programmed FEDRAT in IPM
DIST = 3D distance from A to B (straight line)
TIME = The time in minutes to move from A to B
FRN = Feed rate number in inverse time

Given the following (pre-defined):
1) FRN = 1 / TIME or TIME = 1 / FRN (defined by the CNC)
and
2) DIST = RATE * TIME (defined by physical laws)

therefore by dividing (2) through by RATE:
DIST / RATE = TIME
or
TIME = DIST / RATE

Therefore by substitution of 1 / FRN for TIME
1 / FRN = DIST / RATE or
3) FRN = RATE / DIST

We now have the required FRN in terms of RATE (IPM) and DISTANCE
Now to calculate the DIST (Distance) "the hard part"

Each linear axis distance is the difference A - B
Dx = Ax - Bx
Dy = Ay - By (the order of subtraction is not important)
Dz = Az - Bz

The resultant 3D distance is the hypotenuse of all linear axes moves therefore
DIST = SQRT ( (Dx * Dx) + (Dy * Dy) + (Dz * Dz) )
(note: it's easier to multiply that square the distances)

Substituting for DIST in (3) above
FRN = RATE / ( SQRT ( (Dx * Dx) + (Dy * Dy) + (Dz * Dz) ) )

These calculations are all in time units of MINUTES for FRN (because we used IPM)

If the CNC needs SECONDS vs MINUTES the following calulation is used
FRN = (RATE * 60) / ( SQRT ( (Dx * Dx) + (Dy * Dy) + (Dz * Dz) ) )

This is the basic calculation for FRN (inverse time feed rate) other limitations
may be required to reduce the FRN including maximum feed rates in X, Y Z
and the rotary maximums in degrees per minute .

I have a FREE utility that computes the FRN (in minutes) by inputing the X,Y,Z
of the two tool positions and the IPM feed rate and computing the FRN. You can
also enter the FRN and back-compute the programmed IPM feed rate.

I can send you the utility if you browse www.NCDataServices.com and send me an
e-mail requesting the Inverse Time feedrate utility.

Larry