Locator Design: A Custom Fit Between Part and Chuck

Complex-Locator-editAlong with the chuck and jaws, the third component required for high quality workholding is the locator, also known as the part stop or Z stop. The locator is the device that controls how far off the face of the chuck the workpiece is situated. In a recent post, we talked about selecting the right jaw design. This article will provide detail on the locator and how it works together with the chuck and jaws for precision machining.

Unlike our chucks and jaws, you won’t find a lot of examples of locators in our catalog. That’s because every part is unique and requires a custom fit for maximum effectiveness. We have a dedicated engineering team that specializes in top tooling (the part touching details of workholding). It’s all they do.

So, how do our experts begin a design project? Each locator is unique but there are some basic underlying factors that they will consider.

Simple-Locator-editPoints of Contact
The first point of concern is how the workpiece will contact the chuck. Typically you must have at least three contact points because you want to create a plane. So, the minimum you can have for a locator is three points standing up off the face of the chuck. The maximum would be a full ring for the part to sit on.

The only time we opt for a full ring is when the workpiece is very thin and there is a risk of deflection. This is especially true if you have pull down action in your chuck. Without full ring support, the workpiece may bow or flex between contact points which can create an undesired waviness in the final part. Even a few microns of bow or flex is enough to affect high precision machining.

Part Detection
A feature commonly utilized via locators is air sensing. This is used to detect whether the part is in place and/or if it is completely seated. This is done by plumbing air through the back of the cylinder, through the drawbar, up through the chuck and into the locator assembly where it flows out of a tiny 1mm hole. When you seat the part against the locator and clamp it, the air is sealed off. This activates a backflow sensor (digital manometer), which measures the pressure or air flow. Establishing the pressure setting is easy and the setting is good for all subsequent parts in the run.

Chip Flow
Where will the chips go? This is always a question to be asked, especially when using a full ring locator. If you're boring the part or cutting into the middle, chips are going to go down the center. The chips may collect there instead of flowing out and being cleared by the spinning action of the chuck. If there is no path for chips to escape, they could build up and affect machining.

If you do need a full ring, then it must be designed so that the chips have somewhere to flow. One way to do this is to make stand offs that support the part, but lift the ring off the face of the chuck so the chips will flow underneath it.

Another popular option to help control chip flow is to run coolant through the spindle. It is designed the same as the air pressure system, though there are ways of using both options together. This is especially useful for chip flow in parts that are bored or I.D. work because it is hard to get the machines spray nozzles in by the tool. It's easier to control chip flow if you have high-pressure coolant spray coming up through the locator assembly or through the center of the chuck. It can help to break up the chips so they don’t create a bird's nest. An added benefit is you may be able to eliminate a wash cycle after you finish each part. Although this can only take 10-15 seconds, it adds up fast when working parts in 30-40 second runs.

Automation
If the locator is being used with automation, whether it be a robot or a gantry, it is important to take the size and shape of the locator into account. The engineer must be aware of the range of motion of the robot and its end-of-arm-tooling so it will not conflict with the locator when the part is being loaded. As automation becomes more common, workholding and automation companies are getting used to working in partnership during design. Air sensing is also useful for robotic systems so it knows that the part has been loaded properly. Feedback for robotics is especially important if you are setting up an unmanned cell.

Why SMW Autoblok?
A crude locator made in the back of a machine shop might work okay for rough jobs. But the fact is, the higher the tolerance level of your parts, the more likely you're going to need a custom-designed locator to fit your particular needs.

We have a dedicated engineering staff in the US – along with resources around the world – who have designed locators for almost every part imaginable. New ideas are constantly being shared and put into a global database of what works and what doesn’t. All this knowledge is put to work for you when you place your order.

If you would like to discuss customized solutions for your specific workholding applications, please contact us.