Tramming With Taut Wires

[Kitwn]

I like the way you use the spindle to rotate the sensor contact using the spindle itself. and the way you use reversing it on the same wire to establish that the spindle is at 90 degrees from that wire.

I am a little confused about relying on the method to align both Y (Along the table) axis bearing alignment wires,

In order to do this the carriage must be geometrically perfect, any twist making the carriage rails deviate from co-planar would mean that the long axis rail alignment wires would be adjusted to the error introduced in the carriage.

Taking a measurement from another measurement system is always difficult.

Ideally the long axis wire need to be aligned first with the lased method then using your method the carriage twist could be minimized using the y axis wires.

I hope I have understood your photos correctly.

John,
Thanks for having a look at this, I have seen your PM.
I haven't used wires along the long, fixed axis of the machine (this is the X axis on my machine which might be some of the confusion) but have assumed these rails are straight enough. Checking they were coplanar was done using a precision spirit level and adjusting the feet of the frame which is not especially rigid. Sometime in the future I will make a suitable camera mount and use Joe's method to get these rails as perfectly straight as possible, but for now they will have to do as they are. My aim for this work was to get the gantry better aligned than it has been since I've always known this was the least accurate part of the machine.

Based on YouTube research, tramming the spindle with dial gauges usually involves some way of creating another plane using a piece of glass and shims that is coplanar with the fixed rails. This seemed overly complex and prone to error, hence going for the taut wires across the rails themselves. I think the video shows this best, though the opening still actually shows the gantry positioned for the final measurement I made for checking the spindle vertical as you mention.

In practice it has proved an effective method. Averaging front and back measurements allowed me to shim the gantry height at one end to remove any end-end tilt and taking the difference of the same measurements indicated a front-back lean. A difference in the differences indicates a twist in the plane of the rails on the gantry (Y axis). By taking measurements with the Z axis assembly positioned over each set of Y rail fixing screws it has been possible to shim the upper or lower rail at each point quickly and easily.

Some iteration was required but a day's work has got pairs of measurements all along the gantry to match within 0.3mm over a 600m measurement base. Hardly the highest engineering standards but more that an order of magnitude better than I started with. Using the Z axis DRO as the measuring device has limited accuracy but made measurements quick and simple at no extra cost!

I've since been able to square the gantry so that the diagonals of a 500mm square are exactly equal as far as I can tell using a set of made-for-purpose dividers. This is a noticeable improvement from the past when it has never been possible to get them within 0.5mm, presumably because of the distortions in the gantry set-up.

Kit