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?
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.