Toggle Nav Shop All
Loading...

Setting up work datum for repeatability and consistent part quality

Machining centres, horizontal or vertical, are required to achieve consistent accuracy in their machining capabilities.

Whether you’re machining for mass production requirements or more intricate small batch production, part rejections are a devastating loss.

To reduce rejection, regular machine calibration is a must. 

Here, we’ll explore the role of datum, pre-part probing and ball bar testing in helping you to achieve this…

Datum - explained

If you imagine planning a journey, you’ll more than likely use your house as a starting point. This is the same when machining a part. A starting point (or reference point) needs to be set, so that the machine knows where to move to.

So, everything within the part programme is linked to the datum, ensuring the machine understands where the part is.

Especially when machining large volume batch work, having consistency of your work datum is a must and is key to producing high quality parts with repeatability.

Your part datum will usually be determined by the engineering part drawing from which you will manufacture the part. The features that come from this datum will have set tolerances to machine to.

So what is pre-part probing?

Pre-part probing involves finding the position of the part with reference to the machine and the machine control.

It ensures the machine spindle knows where the part is. So, when the tool comes in, it knows where to go to machine the part without any collisions.

The probing will inform the machine control what position on the machine bed the part is.

Part probing can be done manually or automated.

 

The manual way of doing this involves something called a 3D Taster. This will be used in the X Y and Z axis manually by hand and entering the Work Coordinate System (WCS) into the work offsets by hand too.

This is one of the cheapest ways of setting up a datum. However, that said, human error can play a part in this, as all it takes is to input a wrong figure and your WCS is out.

The automated option would be to use a Renishaw Machine Probe.

This way of setting your WCS is the safest and most accurate. The process can be added to your part program and fully automated. 

This eliminates human error, as the system updates the WCS automatically measuring to sub-micron measurements.

What is ball probing?

Ball probing relates to positional tolerances that ensure the machine knows what the datums are.

Some engineers use a ball bar, others use the edge of the component - or the edge of the fixture - to create a datum.

Either is fine, it depends on which method you prefer.

If the machine has a recognised datum from its’ X, Y and Z position (on 3 axis machining), it doesn’t matter where the datum is set from. You’ll programme the job from that datum regardless of where the datum is.

Ball bar testing

In practice, the ball bar is a calibration tool that analyses the motion on a machine tool. It’s a telescoping linear sensor equipped with precision balls at both ends – a sturdy metal component that is bolted to the machine bed with a known size to a very high tolerance.

The QC20 ballbar from Renishaw serves as a precision measurement device to assess a machine tool's performance. It does this by detecting minute variations in radius as it follows a programmed circular path around a fixed mount. 

During operation, the sensor's balls are precisely located within magnetic cups, facilitating the measurement of radius. 

When you probe it, you know that it’s accurate and that distances travelled are accurate from that position.

Ball bar testing and ball bar calibration are an effective method that will help you to achieve consistent accuracy in production and reduce part rejection.

The use of a QC20 ball bar kit can diagnose any problems you may have on your machine tool, checking X Y and Z axis movements.

To check the movements in the 4th and 5th axis, the use of a Renishaw Axi set can diagnose any issues.

Kinematics

Machine kinematics is the study of how machine parts move relative to each other, including their position, velocity, acceleration, and displacement.

Accurate kinematics can help prevent collisions and detect errors. The kinematics of a machine also dictates how quickly the machine will work and how accurate it is.

Temperature, in process management, and incorrectly set tooling are just some factors that affect kinematics - so it’s important that any issues spotted early on are dealt with to prevent any unsightly and costly accidents on the machine.

A great way to stay on top of this is with regular and consistent calibration of the machine, and the QC20 ballbar is just one piece of kit to do this.