Feb 13, 2017

[2.70] Seek and Geek #1: Austin's supercool leadscrew preloader

Austin has been making a super cool granite-base CNC mill at MITERS, and he also has a blog with his documentation.

Both man and machine are exceptionally photogenic
This particular seek&geek is going to focus on a tiny mechanism in his project - a tiny sliding wedge that allows adjustment of the preload in his leadscrew. Threaded brass nut 1 (yellow, left) is rigidly attached to the carriage, but threaded brass nut 2 (yellow, right) can move in the axial direction outwards. The adjustment screws (blue) shoving against the wedge creates a preload, forcing brass nut 2 to contact the shaft threads on the opposite side as brass nut 1 and removing overall backlash in the carriage.

With this, even $13 ebay leadscrews can become precision items!
Backing up a bit to talk about leadscrew backlash. Leadscrews are super common, simple transmission elements that convert rotary to linear motion using sliding contact between a threaded rod and nut. Their most common incarnation has them attached to a motor and simply-supported on bearing blocks.

As the screw (grey) rotates, a nut (blue) moves linearly along the shaft.
Usually this nut is constrained to prevent it from spinning in addition to axial motion
gif from W.Rebel, Wikipedia
Since nothing is perfect, there's some clearance between the threads when the nut is threaded onto the shaft. This is fine when you only move in a single direction - the nut will contact on one side and the clearance does nothing.

However, when you want to switch directions, there will be some dead space where the shaft turns and the nut doesn't move until the other side of the thread contacts the shaft. This backlash positioning error induces sadness.

Austin's preload mechanism allows him to tune the position of Brass Nut 2 such that backlash in the carriage is eliminated in either direction, without causing binding.

Current state of Austin's mill

No comments:

Post a Comment