Want to improve your cell cycle time by 20% or more? Here's how to calculate what a dual gripper could do for you!

Machine tending of multiple CNC machines is a great way to improve the productivity of a manufacturing cell, but it has its limitations. Even though a collaborative robot is able to load and unload several machines at once – which greatly reduces cycle time – two factors are always at play:

1. CNC processing time—This is hard to reduce further once you've optimized your machining process.
2. Travel time between different machines—This can definitely be improved!

So how do you reduce the travel time? By adding a dual gripper to your cobot!

We've discussed the advantages of a dual gripper in a previous post. But how can you be sure that a dual gripper is right for your cell?

This article gives you the tools you need to calculate the benefits a dual gripper could bring to your cell. You'll find out whether a dual gripper is the right solution for you, and how much cycle time you could shave off your process.

Can your robot tend multiple machines?

You might already know that you can tend a single CNC machine using a collaborative robot. (If that's news to you, check out our post "What the Heck is Robotic Machine Tending Anyway?".)

But have you ever considered tending multiple machines at the same time?

The advantage of tending multiple machines is pretty clear: you can shave a lot of time off the process! Glidewell Laboratories in Canada reduced their cycle time from 27 hours to 18 hours when they added a UR5 robot to tend four CNC machines at once. That's a 33% improvement over their previous non-robotic process.

As you can see from Glidewell's case study, it's certainly possible to tend multiple machines with a cobot! You can tend several machines just by moving the machines closer together, while keeping the robot fixed to the spot. This approach is much less complicated than the traditional method of using industrial robots attached to moving rails. Although it's possible to do the same with cobots, it's often not worth the hassle.

Could you rearrange your CNC machines so a machine tending robot could sit between them? If yes, keep reading to find out how much time this will save you. 

How to calculate the cycle time for tending multiple machines

The first step is to calculate how much time you would save using a single gripper robot with multiple machines.

Draw a rough sketch of your cell on paper, like this:

Then, calculate the values listed below. If you've already set up the robot cell, you can measure the real times using a stopwatch: start the robot, take several readings of each value throughout the process, and then average the results.

If you haven't yet implemented your multi-machine cell, you will have to approximate the values. You can either perform a small test application with the robot (this is ideal) or use theoretical values and pen-and-paper.

The values you will need are:

• Nm—Number of machines.
• Tl—Time to load or unload one machine. This includes dropping off the previous part at the output buffer, picking a new part from the input buffer, and loading this new part into a machine.

• Tm—Travel time between two different machines.
• Tp—Processing time (use the longest time from among the four machines).

You can then enter these values into the following formulas, which come from researchers at universities in Texas and New York. These two formulas determine the effect of the travel time vs. waiting time:

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To calculate the "cycle time per unit" for a robot with a single gripper, simply take the maximum of these two values:

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A worked example

Let's take an example cell, which contains four CNC machines. The values are:

• Number of machines (Nm)—Four
• Time to load/unload a machine (Tl)—60 seconds
• Travel time between machines (Tm)—7 seconds
• Longest processing time (Tp)—324 seconds

These will give us the following values for the per-unit cycle time:

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How to calculate improvement from a dual gripper

You can use your results to determine how much a dual gripper could improve your cycle time. However, first you'll need to calculate another value:

• Ts—How long it takes (in time) to switch between grippers with a dual gripper setup.

Now you can calculate the improvement of your cycle time, which we'll call Ti. Remember that Ti depends on which value was bigger in your previous calculations – Tt1 or Tt2 – as follows:

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This gives a new per-unit cycle time of:

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You can then use the following equation to work out the percentage improvement using dual grippers:

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Some examples for two, three, and four machines

Finally, let's apply the equations to a few examples. The table below shows the results for two-, three- and four-machine cells (all time values are in seconds). 

As you can see, how much of an improvement you'll get with a dual gripper varies hugely depending on the times in your processing cell. That's why it's important to run these calculations before you invest in a dual gripper, to make sure it's sufficient for your productivity needs.

Have you thought of any cool ways to use a dual gripper in your setup? Got any questions about machine tending? Tell us in the comments below or join the discussion on LinkedIn, Twitter, Facebook or the DoF professional robotics community.