Z-Axis Rollers Contribute to Tool Chatter

During our chatter-dominated CNC testing session, we used our fingertips to feel around machine structure. Most people’s fingertips are sensitive enough for identifying the presence of relative motion between mechanical parts, though only very few people can accurately quantify the distance of that motion. In this case we wanted to know which parts are moving relative to other parts, and our fingers were great for the purpose.

One of the weakest links in our machine rigidity were the four rollers aligning our Z-axis vertical extrusion beam. Two each on left and right sides of the spindle, one above the other. We could feel the vertical extrusion beam vibrating within these rollers clamping them in place.

Examination after our cutting session found the lower two rollers loose. Before this session, all four were tightened up against our vertical beam allowing no movement and enough friction they were difficult to turn by hand. By the end of the session, the lower two could be moved by hand. It appears the upper two held tightly enough to act as a fulcrum, and our cutting tool had enough leverage to move the lower two loose.

Movement of the lower two rollers were a consequence of this modular design built out of aluminum extrusion beams. These rollers are held by square nuts inside the slot of an extrusion, meaning they were held in by friction. When forces build up enough to overcome that friction, these square nuts would slide within their slot, loosening our rollers.

Until we find a better way to arrange our Z-axis, we will have a constant maintenance task of re-tightening these rollers. We also went looking in Fusion 360 CAM for settings to take shallower cuts, and together they made follow-on session a lot more successful.

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