[wizard]

Seems I replied a little to quickly to your first comment Gerry and missed your subsequent post and Wizards. Thanks for the advice, and I will do just that .
I can live with 5" of travel.

There is a great thread in the stickies that goes into a bit of beam engineering for a gantry. Well worth the read before settling on the Z working envelope. You can design a machine for any sort of Z working space you want but it quickly becomes a massive machine.

Don't confuse Z clearance with Z axis travel either. Sometimes additional Z travel can be justified. This especially if you intend to use long drill bits, or other cutters. Before you jump on a Z axis value it is best to understand how you intend to use the machine.

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To put this all in perspective, it will be used to machine closely interlocking armature pieces.

Armature as in a DC motor or as in a artists sculpture?

Eventually looking at accuracies within a few thousandths of an inch. I'm planning for this first machine to be the first stepping stone towards something that can do this. Doesn't have to get all the way, but a significant percentage of that capability will allow a proof of concept.

A few thou is certainly possible but it won't be easy.

Wizard, your reply was awesome, thanks! As for the fabrication from steel, this is doable. A friend has a small machine shop I'm sure he will not mind me stealing for a little bit.

A small machine shop with a big mill I hope!

Thanks to your comments I am now seriously looking at a complete redesign to transition to a moving table design. Though the footprint may be prohibitive, I'll spec it out and see.

The general problem with a moving table design is the space it takes up. However I kinda think it will be required for this project to give you the best hope of hitting the tolerances you want.

With regards to this, what are peoples thoughts about picking up an older mill that fits the specs and adding control mechanisms and steppers to that?

It depends upon the machine. Frankly a dead CNC machine that needs a controller upgrade is often a better deal.

I'm unsure how worn the screws can get with the older machines, but have the possibility in mind that there might be a few savings to be had. Is this something done often?

Yes it is done often. Sometimes by people that want a machine for their home shop and sometimes by companies wanting to improve productivity on a large piece of machinery. Companies are less likely to upgrade smaller machines if you can buy new off the shelf.

From the standpoint of wear it spends upon what you buy. Buy a machine that was in an R&D lab and you will see little wear. Buy a machine off a production floor and it might be worth scrap metal prices and not much more.

To be clear there is little to no overhead for me to weld a frame from scratch. Just thinking out loud.

You will need to decide if a router makes sense or if a mill would be better! Small mills can be cheap but a mill to cover the same space as your router specs might not be so cheap.

If I was to go the welded steel frame route, what would you consider "not a light weight steel frame"? Should I be breaking out the 1/2 inch steel plate?

A rough guess would be to go for square tubing of six inches in size, with 3/8" walls for the gantry.

As for plate you will need plate in more than one thickness. You will need internal gusseeting in the gantry beam where 1/4" thick plate may do. The Y Axis saddle might benefit from thicker material. External gussets and brackets may be in the 3/8" to 1/2" range.

Steppers and Acme leadscrews it is.

You will likely want to upgrade those in the future but this is a good place to keep an eye on the budget and at the same time get a working machine.

For all the good reasons you mention, I am going to ditch the idea of using aluminum and just go with steel. As you say, we will be running coolant, and the very thought of the extrusion in that context has me realizing what a very dim idea it really is.

Well do realize we are talking T-Slotted aluminum here. You can get aluminum in square and rectangular beams and sheets just like you can steel. It is however expensive to buy and weld. I just don't think it is worth the expense. Also aluminum doesn't hold threads well so you end up spending a lot of time dealing with that.