﻿ Estimating robot arc welding cycle times

The essential guide

ROBOT WELDING

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# Estimating robot cycle times

It is relatively straight forward to calculate a robot welding cycle time for a particular assembly and this generally forms the foundation for any financial justification. It also provides an indication about the utilization of the system.

# Methods

There are four different methods that can be used depending how much time you have and how accurate you need to be. At best a robotic cycle time estimate will be within 10% of the actual robot cycle time. It is also possible to simulate the cycle time on bespoke simulation software. This would be 99% accurate, but can be quite time consuming and requires a different level of expertise. In critical situations an actual welding trial can verify a robot welding time.

The first three of these methods apply to medium gauge welding only:

• Quick: Divide the manual cycle time by a factor of 3 to 4 or a factor of 2 for very large welded assemblies with long straight seams.
• A little more accurate:Divide the total weld length in mm by 6 mm/sec.
• Fairly accurate: Divide the total weld length in mm by 10 mm/sec and apply an efficiency factor of 85% for robot moves and positioner movement times.
• The most accurate: Establish times for each element of the cycle (e.g. arc-on time, time for robot moves, time for external positioner moves etc). The sum of all of these is the cycle time.

Method number 4 is easy with the help of a Microsoft Excel spreadsheet. It is also a relatively quick task. Another advantage is that it is very easy to change some of the parameters, such as the welding speed or see the effect on cycle time if two robots are working on a single assembly etc.

Estimating cycle times using the estimate sheet (method number 4)

A robot cycle comprises the following elements:

• Arc on time: the time that the arc is actually flowing (weld length/welding velocity).
• Robot movement time: the time it takes for the robot to move to the next joint and establish an arc.
• Positioner time: the time it takes for the positioner to move and move back to its zero position at the end of the cycle.
• Station interchange time: the time that it takes for the robot to move to the second work station or for a twin station positioner to index workstations.
• Other times: track movement time, sensing time etc. If the system only comprises a single work station that you would also need to add the time for roller shutter doors or hatches to open and operator handling operations.
• Torch cleaning time: after a given period of arc-on time, the automatic torch cleaner will clean the weld spatter from the shroud.

To be able to fill out the details, some knowledge is required about the welding process, type of robot movement, positioner speeds and so on, but even a newcomer in the field will be able to make a reasonable assessment. The main influence on the cycle time will be the welding speed and this depends on the size of the weld. A typical fillet weld has a leg length of around 5 mm and this can be welded at about 10 mm/sec. Depending on fit up slightly faster or slower welding speeds can be used. If you are new to the MIG welding process it is a good idea to consult a welding reference guide for any other size weld or use the information supplied in the PDF file below. The welding position also has some influence over the welding speed, particularly if the robot welds in the vertical down hand position (note: where the integrity of the weld is important materials over 3 mm in thickness should not be welded in this orientation, which is the reason why a system may need a positioner).

An arc welding cycle time estimate sheet for a cell with single & twin robots (provided the welding is shared out equally between the robots). The sheet has tabs so the calculation can be based on volumes or shifts. There is also a tab to evaluate operator handling as this may sometimes be longer than the robot cycle. Included are instructions how to fill out the sheet.

The tables in this file provide guidance welding speeds for different weld sizes in various weld positions, for mild steel, stainless steel and aluminium.