Title: Module 2: The Bar Loader or Bar Feeder
Course: Introduction to Swiss Programming

Module 2: The Bar Loader or Bar Feeder

Module 2 - The Bar Loader/Feeder

Swiss style screw machines are almost always going to machine parts using long lengths of barstock. It is necessary to use a bar loader, bar feeder or some sort of system to 'contain' the bar as it rotates. When a new 12 foot length of bar is being machined, there will be 9 or 10 feet of that bar protruding out of the front spindle end of the screw machine. Sometimes companies do use shorter bar lengths, but most Swiss screw machines are going to use full 12' bars. Note: Many materials come standard in 9-13 foot random lengths. The standard is often 12 feet for materials made here in America. Material coming from other countries are typically 3m (approximately 9 feet) or 4m (approximately 13 feet) long. Some materials may come in longer lengths and would require a special extra long bar loader/bar feeder or to be cut into smaller lengths.

Bar loaders can automatically load a new bar of material into the machine as necessary. Many bars can be staged in the bar loader. A screw machine equipped with a bar loader can keep making parts, loading bar after bar, for long periods of time without an operator's direct involvement. Some companies load bars into their bar loaders at the end of the day and let their screw machines run by themselves at night or on weekends. Bar loaders usually use a collet to actively grab onto the end of a bar. Sometimes they use more of a "pusher" tip that is a hard steel conical recess that mates with a cone machined or ground into the end of the bar of material to be used by the screw machine. If a collet is used, the bar loader will remove the remnant after a bar of stock is consumed. If a pusher tip is used, usually the remnant is pushed out the front of the screw machine's guide bushing by the next bar.

A bar loader may have a channel size barely larger than the barstock diameter. In this case it is common to need to reduce the diameter of the end of length of barstock to a smaller diameter so a collet will fit. Bar loaders monitor the pusher bar position and can tell when the screw machine it is attached to has completed a part. They can be configured to keep letting the screw machine run until a specified position has been reached or to keep track of how many parts the screw machine has made before it sends a "change bar" signal to the screw machine.

A bar feeder will support and contain the bar as the screw machine is machining but will require an operator to remove the bar remnant, load a new bar in the bar feeder and prepare the screw machine and bar feeder to continue part production. Bar feeders only work with a single bar at a time. Some bar feeders will have multiple sizes of feed tubes available. One would choose the smallest tube the barstock can fit in.

Bar feeders just control the bar as it spins. They don't keep track of where the pusher bar is during operation, they just sense when the pusher is at its end of travel. The operator must load a new bar and remove the bar remnant every time a new bar is needed.

Bar loaders or bar feeders also need to gently 'push' the bar towards the screw machine. As the headstock of the screw machine is reset, its collet opens and the headstock retracts. Without the 'push' of the bar loader/feeder the retracting headstock could drag the bar with it.

As it turns out, the larger diameter a bar is the more likely it is for the screw machine headstock to drag the bar when the headstock retracts. This means bar loaders for large stock tend to 'push' harder than small loaders. Often it is difficult to run small stock (1/8" metal, 3/16"-ish plastic) on a loader that can handle ¾" stock.

Settings can be made to the bar loader to adjust the 'push' force but it is asking quite a bit for a bar loader to be powerful enough to push a 12' bar of heavy steel down its channel and still have enough finesse to not overpower and crush a small diameter bar.

Both bar loaders and bar feeders must be adjusted for the length part being machined by the screw machine. As the screw machine consumes a bar, the bar loader or feeder will send a signal to the machine tool when the end of that bar has been reached. A properly adjusted bar loader or feeder will let the screw machine consume as much of a single bar of material as possible. A bar feeder or loader adjusted for a part length that is longer than the actual part will result in an excessive bar remnant length when a new bar is loaded. If it is adjusted for a too short length then the bar loader may over-travel and send an alarm to the screw machine. This situation may not seem like much of a problem but depending on what the screw machine was doing at the exact moment the bar loader alarmed, damage to a cutting tool may occur. Most bar feeders will not send an 'alarm' to the machine; they will just send the signal to the machine that is time for a bar change.

Most modern bar loaders and bar feeders use the principal of "hydrodynamic support." Hydrodynamic support means the length of barstock in the bar loader or bar feeder will be in a tube or a clam-shell channel with oil pumped through the tube or channel. As the bar rotates, the bar will naturally climb to the center of the bar feeder tube or bar loader channel. The oil in the feeder tube or loader channel will lubricate the tube/channel as the bar touches the walls of the tube/channel while the spindle starts, slows down or is stopped. The oil also dampens vibrations while the barstock rotates. Often slightly warped small bars can run fine in bar loaders due to the dampening of the oil. The oil helps to dampen severely warped bars or larger bars with small bends but those bars will still not run well. If the warp is more than 'slight,' the bar should not be used or the bar needs to be cut where the bend is and both pieces of that bar can then be run through the machine.

Bar loaders and feeders work best with heavy weight or very viscous oil. The heavier or more viscous the oil is, the better it is at dampening vibrations. All bar loaders/feeders have a method to remove oil from the barstock before a section of bar reaches the screw machine. Sometimes there are brushes or air knives to remove the bulk of the bar loader/feeder oil from the bar as the bar exits the loader/feeder. Sometimes there is a tube the bar passes through between the loader and screw machine that will catch the oil slung off the rotating bar and drain it back into the bar loader. Often both methods are used.

None of these systems are 100 percent effective at removing barloader oil from the barstock, so some oil will mix with the cutting oil or cutting fluid in the machine tool. This contamination can affect the performance of the cutting fluid/oil.

Some shops run cutting oil in their bar loaders to avoid the contamination issue. Screw machine cutting oil is not as viscous as bar loader oil so the bar loader dampening performance is not as good. The tradeoff becomes cleaner cutting oil versus better performance for warped bars; each shop or company will need to make their own decision. Some shops even run cutting oil in their lubricators to avoid cutting oil contamination.

Some older, less sophisticated bar feeders are still in use and are known as 'gravity' bar feeders. These feeders do not use any oil and rely on a series of pulleys and ropes to push the bar into the screw machine. Since there isn't any oil to lube the tube the bar is in, these feeders rub the outside of the material, sometimes causing damage to the bar's exterior. Also since these don't use oil, the barstock is free to rattle around quite a bit and these feeders are usually annoyingly noisy.

Major components of a bar loader:

Channel set: Available in many sizes. Often the channel set is sized for the maximum diameter a particular screw machine can accept.

Pusher bar: This bar has a collet on the end of it, that collet is sized for the barstock the machine will run. These are usually around 4 feet long and this is what lets the bar loader extend up into the back of the screw machine's spindle. A collet sized for the diameter barstock the machine will use is fitted on the end of the pusher bar. Some brands use collets that screw onto a threaded end of the pusher bar, some brands use a pin to connect the collet to the pusher bar. RRCC's C320 loader uses a pin.

Feed carriage: The feed carriage handles positioning a new bar in the loader in preparation to be fed into the machine, it also attaches itself to the pusher bar and moves the barstock and pusher bar forward. The pre feed carriage is what allows a bar loader to only be a couple feet longer than the barstock. Without the pre feed carriage, bar loaders would need to be 4 or 5 feet longer than the longest bar they are made for.

Bar gripper: The bar gripper grips the bar remnant allowing the pusher bar and its collet to be removed from the remnant. After the remnant is removed the bar gripper opens, waits a couple seconds and closes again. If the bar gripper jaws encounter a bar the loader will 'alarm out" and stop operation. The bar gripper also grips the back end of a new bar of material and holds the bar stationary while the collet at the front of the pusher bar is pressed onto the end of the bar. Some bar loaders require the gripper mechanism be adjusted for the barstock size during the bar loader setup. The C320 loader on RRCC's A-20 does not need to be adjusted.

Bar storage rails: The bar storage rails are where bars are stored while waiting to be loaded in the machine. As a bar is loaded into the machine the remaining bars all advance.

Bar lifter/separator: The bar lifter / separator system makes sure only a single bar at a time is lifted into the bar loader channel and also keeps the bars from climbing over one another as they advance down the rails. It is necessary to adjust this system for the bar diameter that will be loaded.

Bar support rollers: The C320 bar loader at RRCC has rollers in the bar loader at the 'front' of the bar loader and at the back of the screw machine spindle. These rollers are adjusted for the barstock diameter being run and depending on where the pusher bar is in the loader will automatically open and close during automatic operation. Not all loader have rollers, some use a polyurethane clam shell support.

Oil pump: Bar loaders always have an oil pump. The oil is fed to multiple locations in the channel.

Headstock synchronization system: The headstock synchronization system enables the bar loader to follow the movements of the screw machine's headstock. Some loaders like RRCC's C320 have a physical connection to the screw machine's headstock. When the synchronization system is engaged the screw machine headstock and bar loader pusher bar move together. The synchronization mechanism is engaged when the bar loader is in "automatic" mode and the front spindle collet is closed . It is not engaged when the collet is open. When the front spindle collet is open and the bar loader is in "automatic" mode the pusher bar will try to push the bar forward. Some bar loaders use electronic synchronization where the movement of the bar loader pusher bar is controlled by a servo motor slaved to the screw machine's headstock movements. All bar loaders for Swiss screw machines have a synchronization system.

Major components of a bar feeder:

Guide tube: This is the tube, usually steel, the bar is contained in. Some simple bar feeders only have one tube. Some have interchangeable tubes for different bar diameters and others have a housing with multiple tubes for different bar diameters. The whole housing would be rotated to position the correct size tube.

Swivel support arm: This arm is positioned to support the front of the guide tube during bar changes. The whole guide tube swivels on a pivot towards the back of the bar feeder frame to expose the front of the feed tube. Sometimes these arms fold down and sometimes they slide out.

Guide tube clamp: This clamp secures the bar feeder guide tube during operation. The guide tube must be lined up exactly behind the screw machine's spindle clamped in place. During a bar change the clamp is opened allowing the guide tube to be swiveled to its bar load position.

Nose guide: This is a tube that telescopes from the front of the guide tube to take up the space between the back of the screw machine's headstock and the front of the bar feeder. This tube has a ferule in the front of it to restrict bar whip. The front of the nose guide should be positioned just behind the headstock when the headstock is at the back of it's stroke during operation. As a point on the bar exits the front of the guide tube it is in the nose guide for some distance, this allows for excess oil to be slung off the bar. This oil is drained back to the bar feeder.

Pusher bar: This bar is inside the guide tube and pushes on the back of the barstock. It is long enough to reach inside the screw machines headstock. A hardened steel conical recess is on the front end of the pusher bar and this surface mates with a cone machined or ground into the back end of a length of barstock. The back end of the pusher bar is connected to a piston. This piston is sized for the guide tube diameter and has one or more holes through it. The size of these holes are adjustable allowing the amount of force the flowing oil exerts on the piston to be adjusted. The smaller the holes are, the more force the oil will push on the piston. The larger the holes are the more oil will flow through the piston and less force the oil will push on the piston.

Oil pump: Oil is pumped through the guide tube flowing from the back of the tube towards the front. This flow provides the pressure or push the guide tube keeps on back of the length of barstock. Just like in a bar loader, this oil both lubricates the guide tube and dampens vibration of the bar.

Part length adjustment carriage: The position of the whole guide tube assembly is adjusted for the part length by moving the guide tube assembly along the carriage.

End of bar sensor: The end of bar sensor is a mechanism inside the guide tube that is activated when the piston at the back of the pusher bar reaches the front of the guide tube. The piston can't go through the sensor so the piston and pusher bar just stop. When the piston touches the sensor, a signal is sent to the screw machine telling the screw machine it is time for a new bar.