CNC lathe work holding – when to use tailstock and steady rest
The part held in the chuck is a cantilever, and the radial cutting force of the tool tends to bend the part. You use a tailstock or steady rest to prevent the bending. Here’s a CNC lathe work holding thumb rule that tells you when you can hold in a chuck, when to use a tailstock, and when to use a steady, based on the L/D (Length to Diameter) ratio of the part.
Chuck only: Use if L/D is less than 3. You can go up to L/D 5 with reduced cutting parameters, which reduce the cutting force. Tailstock: Use if L/D is between 3 and 6. You can go up to L/D 10 with reduced cutting parameters. Steady rest: Use if L/D is between 6 and 12. You can go up to L/D 20 with reduced cutting parameters.
If you want to cut with the full recommended cutting parameters, every 3 L/D requires a new holding point. E.g., A chuck is a single holding point, chuck+tailstock is 2 holding points, chuck+tailstock+steady is 3 holding points, etc. Use this formula to determine the number of holding points: No. of holding points = ((L/D) / 3), rounded off to the next higher number. E.g., L/D 4.5 would require 2 holding points – (4.5/3 ) is 1.5, and this rounded off to the next higher number is 2. If you’re willing to cut with reduced cutting parameters, in this formula change the 3 to 5. Caution A rule of thumb is not the best way to do something. It is not always right. It is just a tool for quick decision making, for figuring out what you can get away with. Text and pics. source: CADEM NCyclopedia multimedia CNC training software.
