Does Z travel make sense as a number?

I was having a ponder and it struck me that the amount of Z travel quoted on a machine makes little sense. Or I've lost the plot and would like to be enlightened :whistle:

Say you have a Z travel of 200mm and wanted to machine pockets 150mm deep in a 200mm piece. Your functional end depth will be dictated by your end-mill length surely! As soon as you run out of end-mill, the bottom of your spindle will hit the top of the workpiece.

So you have gantry height (-Z axis mount etc.) dictating the height of the workpiece.
Then max. end-mill length and Z axis movement dictates maximum cutting depth.

Of course you can also flip the piece for double the effective cutting height if the design allows it e.g. a through pocket could be machined from both sides.

Does this make sense? In what scenario would a Z travel longer than a useable end-mill length be helpful?

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Does this make sense? In what scenario would a Z travel longer than a useable end-mill length be helpful?

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You might have a part 150mm thick and only want a pocket 5mm deep. :cower:

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Originally Posted by Clive S

You might have a part 150mm thick and only want a pocket 5mm deep. :cower:

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Rotary machining,for a start,for clearance.

Ohh, I see. It would also allow you to use the shortest feasible end-mill, which would allow faster cutting rates.:thumsup:

Wait... could the same thing not be achieved by introducing a thicker spoil plate to move the work closer to the spindle?

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Originally Posted by Maurizio

Wait... could the same thing not be achieved by introducing a thicker spoil plate to move the work closer to the spindle?

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Yes of course. But you might want a small vice etc. What are you trying to achieve?

Nothing! Just exploring the interplay between variables that determine the 'true' z cutting depth (which for me means depth of pocket cut from top surface) as a prelude to building my own cnc :)

Edit: just for clarity my first post wasn't proposing no z travel, but more musing over the need for it to be combined with cutter length and table to gantry clearance to assess whether it meets your requirements. e.g. a cnc with a z travel of 100mm and a table to gantry height of 100mm could not cut 100mm narrow pockets, it would need a table to gantry height of 200mm and a cutter of 100mm. The flipside to this, as pointed out by Clive and molliemilo1 is that other operations would benefit from a z travel that goes all the way to the table.

Ohh, I guess it could with multiple cutter changes (a 50mm, then a 100mm), but you'd have to start and finish the cut within the pocket...

There is sometimes an advantage to being able to move the spindle up and down on the Z carriage a bit to accommodate different cutter lengths provides you have clearance.

Similar to Clive’s response you might want to drill holes in the end or side of say a rectangular piece of material so need to have space under the tool.
As mentioned being able to move the spindle up and down in the holder is also really helpful and extends the range of the machine.

If you are making a router then 150mm from the tip of the tool to the bed is a good start point. This is because it allows a modest vice and work piece to fit under the tool whilst not requiring the Z axis to have to hang down too far from the gantry when machining parts down on the bed. Long travel on Z will reduce stiffness.

If you are making a CNC mill then you can make the column taller to give more options.

Of course this all depends on what you need to make but a win in one area will comprise another area.

Also being able to retract above the work and get your hands in for manual tool changes is also helpful.

Many reasons for a high Z-axis but no right or wrong for either, it's all down to usage.
For instance, I recently built a machine with a lifting gantry that had 400mm of Z-axis travel but was never going to cut deeper than 2 or 3mm, the height was needed because of the part size.

I've built similar machines with high gantry's and Z-axis because they wanted to drill holes thru 10" beams and machine scarf joints, pockets, etc for log cabins, yet still, they wanted to engrave slate which was only 5mm thick.!
In cases like this, you just have to accept that there is a price to pay for a large Z-axis. Raising the bed is an option for thin material but it soon gets tiresome having to surface the bed every time you want to machine something thin.

Also, regards pocketing 150mm deep then you better have deep pockets, have you seen the price of tools over 100mm in length.?

Drills are probably the longest thing you'll stick in a spindle and a good reason for a decent amount of Z-axis travel/clearance.