Tramming With Taut Wires

**Kitwn** · 01-05-2020

If you can't build it precise, build it adjustable. Regular readers of this forum and anyone who has tried to build a machine using only a drill press and hand tools understands the significance of this. If you've welded up any of your machine out of steel box section scavenged off the local tip as I have then this is Design Parameter Number One.

The next trick is what to measure and how to measure it in order to make useful adjustments that will turn your whirring monstrosity into a precision instrument?

The most basic alignment is to get the fixed rails (X axis on my machine) straight and parallel so that they present a level, flat plane as the foundation for everything else. Whether you've done this by testing the base on a monster slab of known flat granite, used the epoxy leveling method which relies on the consistency of the Earth's gravity within the floor area of your shed, built a Devmonkey type optical laser alignment device or used a precision spirit level and adjustable feet (as I do for now) is up to you. Once this is done the next thing is to get the gantry aligned. That's what the rest of this blather is about.

For want of knowing any better at the time, I built my machine using 20mm continuously supported rails for the X axis which sit a little above the height of the machine bed. This allows me to use a pair of taut wires stretched across the rails as a reference for measurements. This has to be more accurate than messing about with pieces of glass and playing card shims and was certainly quicker to set up. My thanks go to John McNamara for inspiring this method. I appreciate that the sensible people don't have their rails and linear bearings sitting right where all the chips fly and might not be able to use the method described here, but your comments and suggestions will be very welcome. Raucous laughter will also be tolerated within limits.

As with all tramming methods an arm carrying some measuring device is needed to fit in the spindle. Mine is just a piece of wood with a 12mm rod sticking up at one end to fit in the spindle and a piece of brass bar at the
other facing down to act as a probe which makes contact with the wire. A 5V power supply and an LED indicate when contact is made. Precision is not required in the probe design but it must be sufficiently rigid to give accurate results.

The length of the arm is set to allow the wires to sit outside the footprint of the gantry feet linear bearings and the wires positioned so contact is made when the arm is aligned parallel to the X axis (90 degrees to the gantry). The Z axis drive is used to lower the probe until it makes contact and then raise it again until contact is only just lost. The Z axis position as displayed on the DRO is then the required measurement. The machine is used as it's own micrometer. The probe is rocked from side to side a little to ensure the lowest point of the probes rounded end always makes contact if it can. Measurements are made on both front and back wires at each end of the Z axis' travel along the gantry and at intermediate positions if required.

Ideally all measurements will be the same

More on processing the measured values later.