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|Introduction to Lights Out CNC Machining|
This article is part of our CNC Machining and Manufacturing Cookbook. It is an introduction to Lights Out Machining, a technique designed to improve Shop Productivity.
What is Lights Out Machining?
In Lights Out Machining, the CNC machines continue to operate without operators for some length of time. Or, as one article puts it, "You go home and the machines keep on working." The goal of Lights Out Machining is to increase shop productivity by reducing labor costs, but there there are more reasons to undertake Lights Out Machining than simply reducing labor costs.
4 Reasons to Adopt Lights Out Machining
Here are the list of benefits a shop might expect to accrue with a successful Lights Out Machining program:
1. Reduced Labor Costs since no operators are needed during the "Lights Out" period.
2. Increased shop capacity and spindle utilization since the machines are able to run for longer during the Lights Out period. Sometimes it is convenient to be able to "flex" shop capacity by running Lights Out jobs on an as-needed basis. This allows the shopt to quickly ramp up without having to hire or acquire new machinery. A couple of extra hours of capacity each night may be all it takes to get a shop to the next stage of growth or to allow another customer to be taken on.
3. Ability to rebalance longer running jobs for the lights out period while the short jobs are run during manned shifts. If the job needs to run for a few hours with minimal intervention, putting it on the Lights Out "shift" may free up time during the manned shifts to handle more shorter duration jobs.
4. Ability to tolerate longer running finish passes (perhaps smaller stepovers on a 3D profiling job) by utilizing the Lights Out Hours without impacting the jobs that have to be run during normal shift time. With enough improvement in finish passes, it may be possible to reduce some manual secondary operations.
The ability to reduce Labor Costs and increase Shop Capacity are certainly the two big ticket reasons for Lights Out machining, but improved ability to handle longer-running jobs and the ability to reduce secondary finishing operations are also useful benefits.
Getting Started With Lights Out Machining Checklist
It's possible to get started in Lights Out Machining without making huge investments to try to solve every problem needed to enable unlimited Lights Out work. The way to do so is to start running just one setup on each machine at the end of the last shift. The job will be setup and the cycle started so that in the morning, it should have completed and ready to be torn down again. The ideal job candidate to maximize the value of this sort of work is one that runs for a lot of hours without requiring attention.
Suppose you're running 2 10-hour shifts. You've got a 4-hour Lights Out window that can be tacked on that adds the opportunity for 4 more hours of spindle time and production.
Success with Lights Out Machining is a matter of well-executed process. Many devotees suggest a check list be prepared that the last shift will run through to maximize the chances for success. Think of all the things that might go wrong that you can check on in advance, and make them a part of your Lights Out Checklist:
- Make sure the proper part program is loaded and ready to go.
- Make sure the job selected for Lights Out has been run successfully during a fully manned shift more than once, and that the part program has not been changed since a successful run was had. If you do change the program, a conservative approach requires it to be run again under supervision before it can be allocated to lights out.
- Make sure the job you're running doesn't require any wear offset changes mid-job.
- Make sure the setup is correct and the workpiece is tightly held in the fixture. If it comes loose, there'll be nobody around to E-Stop the machine.
- Make sure the cutting tools are sharp and have minimal wear. Just before you launch a Lights Out job is an ideal time to do any routine checks for tooling wear.
- Make sure the toolchanger is operating freely and that the right tool table information is loaded.
- Turn down the spindle rpm perhaps 20% to give a greater margin for error. Lights Out jobs need to be run conservatively and not right on the edge.
- Make sure the coolant is properly aimed for best chip clearance, and that there is plenty of good coolant available in the reservoir.
- Make sure the compressor is on and there are minimal leaks to ensure a steady supply of compressed air to the machine.
- Make sure chips are cleared before you turn the machine loose and that they will be adequately cleared once you go Lights Out. Big piles of chips can bring things to a halt quickly.
- Make sure there will be no "Bird Nesting" of chips on the cutters as there will be no operator available to clear them.
- Make sure the machine has adequate lubrication and that any reservoirs for way oil or spindle oil mist are filled with fresh lubricants. Be sure the way oil pump is actually working.
- Make sure the machine is warmed up and ready to produce parts within tolerances. If it has been running all day on other jobs, this may be taken care of, but remember, the shop temperature may change at night. Be familiar enough with your job to know whether that sort of thing matters. Perhaps the final finish pass is not what you want to be doing Lights Out on some jobs.
- Make sure you're not running materials (such as Magnesium) or coolants (such as Oil-based) that increase the likelihood of fire hazard. Make sure your shop's fire alarm is armed and that you can respond quickly if an alarm is sounded.
This may seem like a lot of trouble, but remember, a little labor up front will be offset by savings in labor while the job is running unattended. Also, many of these things are routine items that should be checked anyway.
Lights Out Machining Challenges
As your shop moves up the learning curve from running simple jobs Lights Out, it will want to expand its Lights Out capabilities. You're probably getting a lot of ideas from the basic Lights Out checklist about what may be involved, but let's run through a more complete set of challenges and some ideas for how they can be handled. We'll do this by calling out every area a human operator might be needed for and suggesting ways of automating that role.
1. Workpiece Loading
To gain maximum benefit from Lights Out Machining you'll need for your machines to be able to load their own workpieces.
Lathes are ready made for automated workpiece loading through the use of bar feeders.
For mills you can get a little bit more part capacity sometimes with a tombstone on a fourth axis, but for more capacity, you'll want a pallet system. You can set up an automated pallet system that let's your step pallets through the machine during Lights Out. The pallets are loaded up during the day by the operators and then the completed jobs are unloaded the next morning. There are also various systems out there that act almost like bar feeders for mills.
2. Cutting Tools
There are lots of cutting tool related issues to deal with including checking for breakage and insert wear, checking the tools running properly, and ensuring the tool table is right. A lot of this can be automated by a combination of automatic toolsetting and a load fault capability.
Automatic Toolsetting uses sensitive probes to measure the tool offsets, adjust them for wear, and check for breakage. The carousel can be probed before the job starts to establish the baselines, and then wear can be checked after each toolchange. If the tool has become broken or too worn, an alternate can be selected or the job can be halted. A good automatic toolsetter even checks for unexpected runout on the tool which may be due to improper use of the toolholder or a chip becoming trapped between the holder and spindle taper.
A load fault capability stops the machine if the spindle load exceeds a programmed threshold, perhaps due to the tool hitting something unexpectedly.
Being able to manage multiple copies of the same cutter is extremely useful. It allows a job to continue if a tool breaks or becomes too worn and it allows jobs to be tackled where tool life will be such that more than one copy is needed to finish. A good HMC with a pallet system will require a lot more tools than you'd think. If you're wondering why such machines can be had with a couple of hundred toolchanger slots, this is the reason. The more tools there are in the magazine, the more copies can be there, and the longer the machine can run without operator intervention to change cutting tools.
3. Coolant and Chip Evacuation
There are many tasks associated with the management of coolant and chip evacuation. Certainly making sure the coolant reservoir is full and the coolant nozzles are properly aimed is the minimum before starting a Lights Out job. Making sure the machine is clear of chips from prior jobs is also important.
If you have a job that requires manual chip clearing, try running just finish the finish pass on such jobs lights out, and do the roughing on the manned shifts. Use pallets to do roughing and leave parts on pallets for later finishing. Overnight may be the time to run small stepover long-running finish jobs.
Coolant filtration and pressure monitoring become much more crutial when no one is there watching. A simple bag filter to remove chips before spraying onto the cutter is always a good idea, but even more so when no one is there to "hear" the cutter going south. Pressure monitoring across the filter is also a good idea. If the pressure difference between filter inlet and exit reaches tio large of a delta, stop the job.
Chip augers and conveyors need to be working properly, and the bins chips are delivered to need capacity to hold the results of a Lights Out run. Your programs may need to be set up to activate augers and conveyors at periodic intervals.
4. Machine Faults
Consider what happens and have a contingency for all possible machine faults including:
- Power Interruption
- Fire: Especially with oil-based material and flammable workpieces. Dust is also a fire or even explosion issue.
- Compressed Air: Police your leaks so compressor runs less often. Consider a pressure switch that shuts down the machines and allows remote monitoring if the air pressure in the lines falls too low.
- Way Oil: Certainly check its full before “Lights Out” operation begins and that the pump is delivering way oil. In some cases it may pay to increase the reservoir capacity.
Depending on the sophistication of your machine, it may already have sensors and logic to automate theese functions or you may need to add them.
5. Workpiece Unloading
You'll need to think about the automation of workpiece unloading at some point. Pallet systems and lathe parts hoppers work well. Make sure the Lights Out prior shift checklist makes sure they are emptied, working, and ready to go.
6. Monitoring and Data Collection
Some decent monitoring and data collection will be important early upgrades to your Lights Out system. Being paged when there is a machine fault is the minimum first step. From there you can add cameras and Machine Data Collection. These days being able to monitor the machine table and control panel with a camera is fairly cheap to do via web cameras. You can even get them with remote tilt/pan/zoom and low light capability.
If you're running a lot of parts, even more sophisticated monitoring may be needed. You'll want to be able to identify which parts had excessive tool wear or which ones were out of tolerance so they can be checked and dealt with the next day.
7. Part Program Management
Part program version control, knowing you always have the most recent and correct version of the program for the part that is supposed to be made is very important. This can require a fair amount of specialized software if you make a lot of different parts and want full flexibility over the mix being produced for the Lights Out runs.
You'll need to make sure you're running proven programs that have been well-tested during manned shifts and that haven't changed since they were proven on a shift.
In some cases, you will want program behavior to change depending on whether it is run on a regular shift or Lights Out. One shop wired up a switch that could be monitored by the parts programs that allowed them to tell what position the switch was in and change behavior based on that. One position was for shift running and the other was for Lights Out running. Similar approaches can be used to let the part program monitor sensors and change behavior accordingly. In the case of the shop with the Lights Out switch, it was primarily used to scale back the spindle rpms during Lights Out running. Cutting back to 80% of normal rpms adds a lot of tool life and increases the margin for error.
Parts must be solidly held. Pallet systems and hydraulic or pneumatically operated workholding is common place. If the part comes loose during Lights Out operation, there's nobody there to hit the E-Stop.
9. In-process probing for QC checks and wear/temp updates
In-process probing can be invaluable for Lights Out operation. Probes can do Quality Control checks on tolerances, and update to compensate for thermal expansion, insert wear, and other variables. See our article on RAMTIC manufacturing for some ideas on how this works.
10. Other Manual Operator Processes
There are a myriad of other things operators do that you may have to improvise in order to deal with them during Lights Out machining.
- Chip fans can help clear chips and coolant out of the way and off the table. Mounting a wire brush somewhere on the table allows the program to be set up to swipe the wire brush over a tool in order to pull chips off of it. Running the spindle in reverse while the tool is working against the wire brush will clear the chips handily.
- You may need to clear chips from holes between operations, for example before trying to tap a blind hole. You can fabricate a nozzle affixed to a tool holder in such a way that the through-spindle coolant can blast the hole clean, or remove chips from deep pockets and the like.
- Another shop mentions brazing corkscrews to an extension and using that rig to pull chips out of holes.
- Tapping often works better with a good tapping fluid, like Moly-D. You can program your g-code so it dips the tap into a reservoir mounted on the table before tapping each hole.
The list of miscellaneous tasks operators perform goes on ad infinitum. Use your ingenuity to figure out how to automate those tasks so they can still be performed during Lights Out machining.
11. Lights Out Process Maturity and Testing
The biggest key to Lights Out machining success is not the gadgets and technology, it is process maturity and security. You need to have sufficient experience with your machining process so it is predictable and you understand how to recover from potential problems. Test during shifts. Don't try new things for the first time on a Lights Out run. With some effort and investment, you'll be able to run more and more jobs Lights Out, and you'll likely wind up increasing automation and reliability for your day time jobs too.
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