Inverse Time Feed Rate Calculator

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Inverse Time Feed Rate Calculator (G93)

Total vector distance of the tool move (inches or mm).
The target speed (G94 equivalent) you want to maintain.
Calculated Inverse Time Feed:

function calculateInverseTime() { var distance = parseFloat(document.getElementById('moveDistance').value); var feed = parseFloat(document.getElementById('desiredFeed').value); var resultArea = document.getElementById('resultArea'); var fValueDisplay = document.getElementById('fValueDisplay'); var moveTimeDisplay = document.getElementById('moveTimeDisplay'); if (isNaN(distance) || isNaN(feed) || distance <= 0 || feed <= 0) { alert("Please enter valid positive numbers for both fields."); resultArea.style.display = "none"; return; } // G93 Formula: Feed Rate (F) = Desired Speed / Distance // This represents the reciprocal of the time in minutes. var fValue = feed / distance; // Time in minutes = 1 / F var timeMinutes = 1 / fValue; var timeSeconds = timeMinutes * 60; fValueDisplay.innerHTML = "F" + fValue.toFixed(4); moveTimeDisplay.innerHTML = "This move will take approximately " + timeSeconds.toFixed(2) + " seconds."; resultArea.style.backgroundColor = "#e1f5fe"; resultArea.style.border = "1px solid #01579b"; resultArea.style.display = "block"; }

Understanding Inverse Time Feed Rate (G93)

In the world of CNC machining, specifically when dealing with 4-axis and 5-axis simultaneous milling, standard feed rate modes like G94 (Inches/Minutes or Millimeters/Minutes) often fail. This is because rotary axes move in degrees, while linear axes move in distance. When both move at the same time, the controller needs a specific way to ensure the tool moves across the part surface at a constant speed. This is where Inverse Time Feed (G93) becomes essential.

How G93 Works

Unlike standard feed rates that tell the machine how fast to go, G93 tells the machine how long the move should take. The "F" word in a G93 block represents the reciprocal of the time required to complete the motion.

Formula: F = Feed Rate (Units/Min) / Distance

Alternatively, it can be viewed as:
F = 1 / Time (in minutes)

Practical Example

Suppose you have a 3D toolpath move that is 0.500 inches long, and you want the tool to travel at 25 inches per minute (IPM).

  • Distance: 0.500″
  • Desired Feed: 25 IPM
  • Calculation: 25 / 0.500 = 50
  • CNC Code: G93 G1 X… Y… F50.0

In this example, the F50.0 means the move will take 1/50th of a minute (1.2 seconds).

Why Use Inverse Time?

When a rotary axis is involved, the "distance" traveled by the tool tip depends on the radius from the center of rotation. If you programmed a move in G94 (Inches/Min), the controller might only look at the linear components or calculate rotary velocity incorrectly. By using G93, the CAM software calculates the exact vector distance the tool tip travels and converts it into a time-based F-value, ensuring the chip load remains constant regardless of the combination of linear and rotary motion.

Common Tips

  • Always switch back: Most machines remain in G93 until a G94 or G95 command is issued. Forgetting to switch back to G94 can lead to unexpected machine behavior in simple linear moves.
  • F-Value Precision: Because F-values in G93 can be very large or very small, ensure your post-processor outputs at least 3 or 4 decimal places for accuracy.
  • High-Speed Machining: Modern controllers often have "Tool Center Point Control" (G43.1 / G43.4) which can handle these calculations internally, but G93 remains the industry standard for legacy and compatibility reasons.

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