What are the advantages of Fixed Double Column Machining Centers

[Watcher]

Someone asked me today what would be the advantage of moving the table instead of the column in a gantry machine...

 

I could not give an answer better than "Maybe for accuracy if the column structure is very heavy..."

 

 

http://www.kentcnc.com/products/cnc-vertical-machining-center/double-column-vertical-machining-center/

 

Any insights for this dumb machinist?

[Reko]

I have run some pretty big gantry mills, and I would have to say weight distribution.

 

I'm not sure how a standard C shaped vertical mill would even be possible past a certain designed size... the same would be true for weight... at some point it is better for a static foundation... and a table that is part of that foundation.

[Sticky]

Huge difference in rigidity. Much easier to square and keep square as well.

[Guest]

Moving the bed is way easier and cheaper for the most part.

 

JM2CFWIW

[Colin Gilchrist]

Having run both types, I think both have advantages and disadvantages. For the move able column, you can have an almost unlimited bed length. I programmed Cincinnati routers at Boeing that had 420 inch long beds. The entire column moved on rails.

 

One disadvantage of this construction method was a limited amount of tools. Because the column moved, the tool changer was carried on the column. So our particular machines had 12 tools. There is also a requirement of more floor space, since there was a cable track on a "flying bridge", to plumb air to the moving column. The machine gantry itself was less rigid, since it had to move.

 

One advantage of moving the columns instead of the bed is load capacity. We could handle fixtures that weighed several tons, with no issues. Since the load itself is "static", the machine does not have to accelerate or decelerate the load. We had a system where we could be loading or unloading parts and Fixtures while the machine column was Cutting on another section of the bed. Our fixtures were all vacuum based, and it was easier to plumb the vacuum lines on a fixed bed.

 

The longer your potential parts are, the more a move able gantry makes sense. Imagine having a 250 inch long part. You'd have to have 500 inch long ball screws to move the table below the spindle. At certain point, it just becomes impractical to move the bed.

 

For smaller parts, a move able bed gives you more rigidity and allows for more aggressive metal removal. It also takes, in general, less floor space, since the bed length limits the part size.

[Watcher]

Gents,

 

I appreciated  everyone´s input on this topic but colin Clearly won the prize!

 

Very enlightening and just make me love you guys for being so knowledgeable! I owe this place a bunch of stuff in my career...!

[Colin Gilchrist]

Gents,

I owe this place a bunch of stuff in my career...!

Me too Daniel. And thanks for the praise. I just like giving back to this forum as payback for all the things I've learned and gained over the years. I literally owe much of my success to this forum and the members...

[YoDoug®]

I just want to add a few comments here. Experience is a huge factor for building and installing large machines. Most builders ship 20-30 large double column machines per year. Okuma builds and ship almost 30 per month. In my time at an Okuma distributor we have sold a few double column machines and we have lost a few sales to competitors. Because of the volumes of Okumas that are shipped they have a full assembly and testing process before the machine leaves the factory. There is no custom fitting in the field. They also have dedicated install teams that can install these machines in 3-4 weeks. The few sales we lost to competitors all took months to install and get running. Just something to consider when purchasing a large volume machine. 

[mkd]

^^i've seen a small amount of drama on the field installation of a toyoda^^

pretty solid once up and running.