Thursday, November 7, 2013
as seen on CNC Cookbook

Gang Tooling Advantage #1:  Speed
Let’s start with the Gang Tool Gang’s view on why their machines are better:
*If the part can be made on a Gang Lathe, it can be made faster and more cheaply than on a Turret.
Remember, some parts just can’t be made on a gang lathe because you can’t use a tailstock–the gang tooling would run into the workpiece if it can’t get completely away from the workpiece, which it can’t because the tailstock is in the way.  There are some gang lathes that have retractable tailstocks, but retracting a tailstock each time a tool change is needed defeats the main advantage of a gang lathe–super fast tool changes.
The gang tool change is super fast because it uses slide motion to do toolchanges and requires no turret indexing.  If you think about it, the typical turret has to move to its tool change position before it can start to spin the right tool into place.  Once the tool is in place, it can then move back to cutting position.  This is almost exactly the same amount of motion needed for the worst-case gang tool change, but the gang tool has no indexing to do.  It moves to the tool change position (different for each gang tool) and moves the next tool into place, then moves straight back.  The most commonly used tools are placed in the center of the gang plate and they only move a small fraction of the distance to the furthest toolchange position the turret needs.
Hence the gang tool is nearly always faster.  You hear quotes from gang tool machinists like, “”The gang tool machine can finish the part, part it off, bar feeder advance the material and it’s well into the second part before the turret lathe can finish the second op.”
Gang Tooling Advantage #2:  Simplicity and Low Cost
Turrets are expensive precision devices with lots of moving parts.  Gang tooling is minimalist.  It involves a view blocks mounted on the cross-slide and that’s it. The lathe’s normal axis servos and leadscrews do all the work for a toolchange. This means gang lathes can often be a lot cheaper than turret lathes.  In some cases, shops buy 2 gang lathes for what a single fancy dual spindle turret lathe would’ve cost and feel they’re coming out way ahead.
If your part needs live tooling, perhaps to machine wrench flats onto a part or to drill a bolt circle on a flange, it is much cheaper to do with gang tooling. Pneumatic spindles, cable driven spindles, and smaller electrical spindles can all be pressed into service if the lathe has an indexable C-Axis spindle.  Motorizing a turret drives up the cost in many ways because of the difficulties of transmitting rotary power out to the tool positions on a rotary turret.  It also drives up the size of the turret, making live turret tooling impractical on smaller lathes.
Gang Tooling tends to be cheaper while turret tooling tends to be more costly, but can be larger scale and more robust.  Some of the turret tooling standards require quite expensive tool holders in order to accommodate live tooling, among other things.
Gang Tooling Advantage #3:  Fast Setup
This is one that swings back and forth, but if you look at the ability to swap out an entire block of tools easily with gang tooling, it’s hard to see how setup can’t be done more quickly since it can be done offline or saved as an assembled block of tools for various jobs.  Here is a block with 7 tools installed:
GangBar
Swapping a block of gang tools is fast and puts all the tools needed for a particular part in place…
Alternately, a lot of gang work can be done with a basic set of tools and just changing one block that holds a twist drill of a particular size, for example.
Gang Tooling Advantage #4:  Accuracy
Simplicity and a reduction in moving parts eliminates tolerance stack up and various kinds of slop (backlash) in the moving parts of a turret.  The accuracy issue is all about achieving the correct centerline (Y-axis) position with the tool. Once shimmed into place, or a custom bushing drilled for a twist drill, it’s hard to get more accurate and most turrets will be less accurate.
Another factor that gives the gang lathe an accuracy advantage is they’re used to moving shorter distances while machining a part.
Gang Tooling Advantage #5:  Robustness
This one is closely related to simplicity.  There’s not a lot to go wrong on a gang lathe.  Certainly a turret has all the wear points and parts to break of a gang lathe plus a whole lot more.  If you crash a turret, you may have an expensive repair bill, or you may simply need to realign it.

For full article from CNC Cookbook, http://blog.cnccookbook.com/2013/09/05/turret-gang-tooling-cnc-lathe/

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