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Ball Screws and Ball Nuts Axial Measurement using PGI Novus

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An example of a pitch circle diameter measurement on a ball screw. Before we can measure the pitch circle diameter accurately, we must make sure that we've aligned the ball screw to the traverse unit to the direction of the measurement, and you can see in the picture here if we measure accurately across the top of the part, we will get good results.

If we measure with the parts slightly tilted or slightly angled, we will get incorrect results particularly on these gothic arch shapes.

There are two examples here, one shows where we’re aligning on the plane shaft area. This is example one.

Aignment on shafts

In example two where there is no plane shaft area. We're measuring on the crest of the of the thread and finding those crest positions and rotating the part such that it is aligned as shown here.

Alignment on thread section

So, one of the things that is critical before we can measure diameter accurately, is a highly stable low noise instrument such as Taylor Hobson’s PGI Novus system. This instrument allows us to establish the precise relationship between opposing thread profiles.

PGI Novus Profilometer

To achieve all of this we need appropriate calibration routines so that we really know the relationship between the lower stylus tip and the upper stylus tip.

Ball screw and ball nut measurement

So, in the video we’ll see the alignment taking place. We'll see measurements of both sides of the ball screw and then we'll see the analysis and all the results.
So, in this example the first thing we’re doing is auto aligning the thread and we're taking a measurement on this top thread here and here as well and in each case, we're cresting to find the crest position. Once the crest positions are known the ball screw will rotate it sitting there on a rotary stage and it will rotate in a line in line with the travis movement.

We’ve now taken the first trace across the top. You can see that we've identified from the drawing the points to which to move. The stylus is moving using something called smart move to those points very precisely and we're taking the lower measurement now.

So once that's finished, we now combine these measurements. We know the relationship between those two traces and we’re going to combine that data together and here it is so what we can see here is a very clear diagram of all of the results and in particular you can see that we've fitted the balls of known size to the gothic arch shapes.

We know the center positions of these balls and we've done the same thing underneath here and we're looking at the distance the pitch circle diameter the PCD between these center. As well as looking at other dimensions such as pitch and other characteristics such as roughness primary profile and gothic arch dimensions. So, we've looked at surface finish here form gothic arch pitch circle diameter and we've tolerance the results as well if we want to.

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