Bar Straightness and Other Things to Consider for High RPMs
To run a CNC lathe at high RPMs, your bar stock must meet industry standards. Specifically, the bar should be straight within 0.007” per foot (0.58mm per meter), measured at three equal points along its length. The tolerance does not increase with longer bars — for example, a 12-foot bar still has the same 0.007” tolerance per foot, not 12 times that.
To check if the bar is straight:
- Place it on V-blocks so it won’t move around.
- Rotate it fully and take dial indicator readings.
- Calculate the tolerance for the entire length.
- Compare these numbers to ensure the bar is within specifications.
In addition to straightness, your bar stock needs to be clean and free of burrs, chips, and dirt. This cleanliness helps protect important components like the guides, pusher bearings, and the oil pump impeller.
Another key consideration is: The outer diameter (OD) of the bar collet needs to be at least 0.02” (0.5mm) smaller than the pusher’s OD. If you’re working with larger bar stock, you can reduce the ends to fit the collet, or you can use an ejection collet if you chamfer the bar end to match its angle.
Chamfers aren’t usually necessary unless the bar OD is very close to the pusher OD. However, if you’re using hex or square stock, chamfering the feeder end can help align the bar with the pusher when the stock shifts in the channel.
Saw-cut ends are generally preferred over sheared (pinched) bar ends, as sheared ends can deform the material. When a cutting blade is new, it typically produces a clean cut, but as the blade dulls with use, it can distort the bar ends, leading possibly to vibration during operation.
In all, several factors may require you to reduce spindle speed:
- Bars that don’t meet straightness standards
- Bars with an irregular profile or shaped material
- Guide channels that don’t match the bar size
- Lack of support for the pusher in the lathe spindle
- Characteristics of the bar stock (copper, brass, plastic etc.)
- Incorrect lubricant viscosity in the oil tank
- Unbalanced bar stock
- Dull blades deforming the bar ends
- Non-concentric chamfers when using an ejection collet
- Insufficient workholding force in the lathe chuck
By addressing these issues, you can ensure smoother operation at higher speeds with minimal vibration.
