Why do makers make X & Y in table, and Z in spindle ?

[bobJandal]

Greetings.

I am looking for ideas as to why machines are so commonly made with the worktable moving in x,y, instead of it being stationary and the spindle moving in x,y,z.

Downsides I see to this approach are:
- a much greater load to be moved on the table axes, which will vary as the workpiece varies.
- much larger area required for the machine, since each axes length = travel + saddle, and in the case of a moving table, saddle can be quite big, relative to travel.
- inability to mount a workpiece that is bigger than the worktable (well, conveniently, anyway).

I am sure that those who make these machines (such as Haas, Cincinatti) know far more about these things than I - can someone help me understand, please.

Thanks.

Tom.

[Geof]

I think some of your downsides are backwards, you have:

- a much greater load to be moved on the table axes, which will vary as the workpiece varies.

If the table was stationary and the spindle moved the lowest set of guide rails would be carrying all the weight of everything else which would add up to about half the weight of the machine. This would be much heavier than a moving table with the heaviset workpiece.

- much larger area required for the machine, since each axes length = travel + saddle, and in the case of a moving table, saddle can be quite big, relative to travel.

I think if you draw a sketch or mock up a model you will see the overall size would not be much different. Either way your spindle centerline has to reach each corner of the table.

- inability to mount a workpiece that is bigger than the worktable (well, conveniently, anyway).

This may be an advantage but the maximum workpiece size is most often dictated by the enclosure on the machine.

Stationary table machines do exist; gantry routers and bridge mills for example but these are generally big and open.

[Al_The_Man]

Cinncinnatti and others make large travelling Gantry, multi-head milling machines, the advantage here is the required length for table takeup is halved, and part weight is not an issue.
The installation is slightly trickier due to the fixed table being a separate entity from the bridge.
A 40ft ft table would take up 80ft bed length if the heads were stationary.
On smaller machines there is less of an advantage when things like weight, inertia do not outway the space issue.
Al.

[Geof]

Looking at Al's post I realised there must be a crossover point between the two design styles. Probably something like; if the longest axis is greater than 100 inches or somewhere around there it is better to go to a moving head.

[DareBee]

The moving head machines are bridge gantry (as mentioned) because this is the only good way to move the massive weight of the Y and Z axis.
This method is very inconvenient for smaller jobs as the bridge limits operater axcess to the workpiece and makes loading/unloading parts very difficult.
Gantry machines usually have a very short Z axis travel as well due to rigidity issues with the design.

[JPMach]

Take a look at Hermle 5-Axis machines, their travels are reletively small but the head moves in XYZ. The are wicked fast too, and accurate. Their is a shop several miles away from me that all they have are Hermle 5 axis machines and they consistently work to a few tenths.

JP

[Al_The_Man]

This is like one I had in mind that I service for a customer.http://www.premac.com/images/5heads.jpg
Al.

[NC Cams]

When engineers are initially taught algebra - a beginner course that sorts out the folks who can't handle the math - they start out teaching you number lines according to a universal convention.

X is left and right

Y is up and down

These are all expressed as being ON THE PAGE with the +/- sign convention as already universally understood.

When the third dimension is added, the Z axis is specified as being PERPENDICULAR to the page.

It would stand to reason and make sense that this same convention would be carried over to machinery.

A mill is essentailly an extension of a drill press and a VMC is a beefed up version of a mill.

Due to the convenience and forces involved in moving the associated masses, it is simply easier and more accurate to move the part under the fixed RIGID milling spindle than to move the spindle.

Yes, a router moves the spindle up down and around but routers don't necessarily remove the amount and type of materials that a mill/VMC does/do.

Rigidity ultimately becomes QUITE essential/mandatory when you're removing material and a non-moving spindle is much less compliant that a moving one....

[JPMach]

On HMC,s though doesn't the spindle do all the movements. I keep reading how they are much more rigid and fast at making parts than a VMC even though they cost twice as much.

JP

[Geof]

Originally Posted by JPMach On HMC,s though doesn't the spindle do all the movements. I keep reading how they are much more rigid and fast at making parts than a VMC even though they cost twice as much. JP

Good point but I think the overall machine footprint relative to the travel of the axes is larger than a VMC. I think the "fast at making parts" perception arises out of them nearly always being configured as pallet machines or with rotary tombstones or both for high volume production. Not always because sometimes low volume big parts are done on HMCs but then part loading with an overhead crane is a factor.

[Caprirs]

There are many variations on the VMC and HMC with manufacturers trying to gain an edge in cost vs. utility. For example, here's a Mazak that has a moving column for the Y-axis. Mighty Viper and Mori Seiki have also made machines where the spindle moves in two axes.

The HMC's principal advantages are that the chips fall out of the cut zone more easily than a VMC which allows greater material removal rates and that all Z axis moves are not fighting gravity so the spindle can move at very high rates compared to a VMC with a counterweighted head. In addition, HMCs have the spindle/motor as an integral unit instead of having a separate motor driving the spindle via a belt/gearbox which means less mass to accel/decel for tool changes and rigid tapping.

On VMCs, the designer also has to consider how far out the spindle is hanging from the column. The more Y-axis travel, the further from the column the spindle must be to allow the table to move back far enough. HMCs don't have this problem to cope with.

Bridge gantry type machines are great for high speed and positional accuracy. But as noted, they lack the rigidity needed for heavier cutting. Thus, they work well as routers where there is little tool pressure and as circuit board drilling machines. They do poorly at precision side cutting because they flex too much.

[Geof]

QUOTE: ".....The HMC's principal advantages are that the chips fall out of the cut zone more easily...."

Let's take this to the (ridiculous?) extreme; the IVMC, an "Inverted Vertical Maching Center". Chips would certainly fall out of the cut zone.