[Doug4d3s]

I am pondering building from scratch and have what is (apparently?) a rather odd idea.

I am thinking that if a gantry-style machine was built to only use tiny bits, then the stress on it would be much lower, and it could be built of steel 3" x 1.5" x 1/8"-thick rectangular tubing (hand-scraped flat) instead of solid iron. Much cheaper + easier to move around as well....

Also since the stress of cutting is intentionally kept so low, would it be possible to use split nuts instead of ballscrews? The ballscrews I'd need would probably cost ~$500+ alone. If precision acme screws were used instead, the price drops to ~$100 or so.

It would cut a lot slower than larger+heavier machines, but for hobbyist use that's not a huge drawback. -Unless there's something else I don't know about it. The longest 1/8" end mills I've found have a 1" depth of cut, which would be one limitation. It would also be possible to build the spindle to accept bits up to 1/4" or so, and just limit the toolpaths to light cuts. That way you could get bits that had a depth of cut up to 2" or so.



I have a Grizzly G0619 mill (SX-3) that can be converted, but the work envelope is too limited. I'd want the CNC machine to have a X-Y-Z of maybe 30" x 12" x 6", and to hold maybe .001" over most of that.



I haven't found a lot of people who have built 3-axis metal-cutting machines from scratch, so maybe I haven't been using the right search terms. All I find is people making routers (with a limited Z-axis) or plastic extruders (much less stress and precision needed), or people converting existing manual milling machines. Does anyone do this much?

[awerby]

I am pondering building from scratch and have what is (apparently?) a rather odd idea.

I am thinking that if a gantry-style machine was built to only use tiny bits, then the stress on it would be much lower, and it could be built of steel 3" x 1.5" x 1/8"-thick rectangular tubing (hand-scraped flat) instead of solid iron. Much cheaper + easier to move around as well....

[Cutting forces from large bits are just one factor that stresses a machine. And if you build a machine that can only deal with tiny tools, every part run will take a whole lot longer. The main issue you need to deal with is rigidity, or the lack thereof. Gantry-type machines don't have much of that, which is why you rarely see them used for cutting metal, especially the harder metals. And hand-scraping is not easy to do - if you're not good at it, you can make things worse, not better.]

Also since the stress of cutting is intentionally kept so low, would it be possible to use split nuts instead of ballscrews? The ballscrews I'd need would probably cost ~$500+ alone. If precision acme screws were used instead, the price drops to ~$100 or so.

[That really doesn't have much to do with it. The reason people use ballscrews is to reduce the power needed to drive linear axes, not to combat the stress of cutting. Split nuts are used because you can adjust them for wear.]

It would cut a lot slower than larger+heavier machines, but for hobbyist use that's not a huge drawback. -Unless there's something else I don't know about it.

[How are your neighbors going to feel about hearing your machine going all night? The combination of a large work area and a small bit adds up to very long part runs.]

The longest 1/8" end mills I've found have a 1" depth of cut, which would be one limitation. It would also be possible to build the spindle to accept bits up to 1/4" or so, and just limit the toolpaths to light cuts. That way you could get bits that had a depth of cut up to 2" or so.

[That would be twice as good. But remember, the smaller the bits you use, the faster the spindle has to run. The bigger they are, the more torque it has to have.]



I have a Grizzly G0619 mill (SX-3) that can be converted, but the work envelope is too limited. I'd want the CNC machine to have a X-Y-Z of maybe 30" x 12" x 6", and to hold maybe .001" over most of that.

[You're dreaming. Shoot for +/- .005 and you might hit it with your homemade scratch-built machine.]



I haven't found a lot of people who have built 3-axis metal-cutting machines from scratch, so maybe I haven't been using the right search terms. All I find is people making routers (with a limited Z-axis) or plastic extruders (much less stress and precision needed), or people converting existing manual milling machines. Does anyone do this much?

[Nope. People try occasionally, but tend to be disappointed with the results. Your best bet, if you want a machine like that, is to find an old CNC machine with good iron but a broken or obsolete controller, and retrofit it with something more modern. Metal appreciates a machine with mass, and it's hard to beat a big hunk of cast iron for that.]

Andrew Werby
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