Z height and thoughts on spacer boxes etc

[CaspianCmonster]

Hi I am back working on my size CNC router and am trying to ballance Z height and Z travel. This is a more or less conventional moving gantry router with 1200mm x 700mm working area for wood and perhaps some plastic. I have a real milling machine for metal.

It seems like there are two likely types of cutting I will do. Type one is cutting sheet goods or dimensional lumber (under 25mm thick) and the other would be 3D items such as patterns for iron castings which is likely to be done with a longer ball end mill or extension so I need both Z height and travel. For the sheet goods I don't really want to have the spindle at max extension from the gantry and that reduces stiffness considerably which doesn't make sense if this is the primary use of the machine.

Any suggestions for the best way to deal with this? I can build the machine to give me stiffness and long Z travel but if I don't need to I would rather not. I am thinking the most simple way to deal with this is to build a nicely braced spacer box/es that will bring the table level up to just 50mm under the gantry that can then be removed for those times I need the full Z height.

thanks

Luke

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[ger21]

You need to realistically look at how thick you can actually cut? Unless you are cutting foam, it's difficult to cut 3D parts thicker than ~50mm on a 3 axis machine, due to tool length and clearance issues with the collet.

About 125mm clearance, and 200mm of Z travel should work well for both situations?

[wizard]

Your general idea is pretty good but how it impacts Z stiffness isnot known unless we understand how the Z axis itself is designed.

Beyond that even stiffness for processing sheet goods is important. If you have two distinctly different uses for the machine designing for the condition that creates maxiumum loads only makes sense. In the overall scheme of things designing a stiff Z isnt a budget blowing exercise for a wood router.

There is also a limit to how deep you can go with a 3 axis machine when it comes to iron patterns. What that value is is debatable but in the end you need to determine realistic values for Z travel that makes sense with the tooling you have available. Oh and the actual spindle has an impact on how far you can extend a tool. In other words it isnt just a matter of the Z slide impacting stiffness.

In any event i lean towards the idea of making the axis stiff enough to do everything you need done. While the spacer block or table isnt a bad idea, i dont see it shifting costs that much over a more rigid build. The other issue here is that im kinda expecting your mold machining to be done with the blanks attached to a match plate which would raise those parts physically.

[BalloonEngineer]

On my machine I designed the gantry to be adjustable in height. The Z travel remains the same, but the entire gantry can move. The gantry attaches via a piece of angle to an 8020 extrusion vertical support, so can adjust steplessly to wherever I want it.


Of course this leads to the issue of maintaining the gantry parallel to the X axis. When I adjust the gantry (or initially positioned it) I used a matched set of spacers (1-2-3 blocks) to directly set spacing from X axis to Y axis guide rails. This ensures they are parallel





Using combinations of spacers such as these blocks and parallels I can achieve whatever gantry height I need, although 5 inches from lowest point on gantry to spoilboard has generally been where I leave it. I have 7 inches of Z travel.