Open-source CNCing

Posted Jul 29, 2020 23:44 UTC (Wed) by vadim (subscriber, #35271)
Parent article: Open-source CNCing

For people thinking of getting into this area: do your research on what you want to accomplish. There are many machines that move in X, Y and Z directions that you can attach a spindle to, but construction matters a lot. Machines like the ones in this article mostly cut wood. Most don't have the rigidity necessary to work with metal.

If you want something for working with metal, probably want something more like a Sieg SX3. They're relatively easy to adapt to CNC, and instructions and kits for doing so are quite common.

Unfortunately this is an area where physics matter a lot, and where you actually need large amounts of heavy metal in your machine for it to have the necessary rigidity to perform well. If the machine is a light one, then there will be very significant limits to what you can do with it.

Cutting metal

Posted Jul 30, 2020 2:22 UTC (Thu) by michaelkjohnson (subscriber, #41438) [Link] (3 responses)

It's worth noting that it's not just mass: cast iron absorbs vibration, but aluminum rings like a bell, which ends up giving surface artifacts. There are two good materials like this in common use; cast iron and granite-filled epoxy. Both are expensive. This is why experienced machinists were skeptical of SwissMak's aluminum-frame construction. But it's worth considering routing and milling separately.

For routing aluminum plate, an aluminum frame driven by a leadscrew can produce fine results. The R7-CNC router uses OpenBuilds 4080 C-Beam extrusion for its frame and is lead-screw driven. It works well for cutting aluminum plate (I don't have one but I've seen pictures of very nice clean edges it has cut). With a brushless spindle capable of high torque at low speed with good speed control, it could work for cutting mild steel sheet and some thin mild steel plate as well. There's more in the R7 category at MakerForums if that sounds interesting.

But for milling, the cheapest way to get started if you are going to mill metal is indeed a Sieg-family mill; there are CNC conversion kits for most if not all sizes and many variants, not just the SX3. Also, some companies occasionally sell skeletons for CNC conversion so you don't end up throwing away an otherwise perfectly good trapezoidal lead screw when you substitute ball screws as is more appropriate for CNC.

Another forum friendly to hobbyists and with a lot of DIY/hobbyist mill conversion information is The Hobby-Machinist. It's not about open source, though it's friendly to open source when I've seen the topic come up, but it's the only machinist forum I've found that combines active members with deep machining experience (including lifetime work experience) with being friendly to those new to the craft. They understand limitations not only in material but in financial resources and can celebrate both "doing it right" and "making do with what you have" which is a nice balance.

Cutting metal

Posted Jul 31, 2020 3:23 UTC (Fri) by dgc (subscriber, #6611) [Link] (2 responses)

So.... milling metal with Sieg mills.....

Years ago I did a CNC fusion kit conversion of a Sieg X2 with an eye to milling aluminium parts for my race car. I now have a Sieg KX3 - a beefed up, factory CNC version of the SX3 that I got near new at a bargain basement price....

The bottom line is that the sieg mills just aren't designed for _regular_ milling of metals. The spindles are not powerful enough, and while the KX3 is far more rigid than then SX3, unless you are really careful and take small cuts the head still vibrates enough to cause chatter even on soft aluminium.

So while you can make decent parts from aluminium and various steels with sieg mills, it takes lots of time and care and many prototypes to dial in the feeds and speeds necessary to minimise vibration and chatter to get the desired accuracy and tolerances in the milled part.

Hence I think the most important question people looking at CNC mill conversions have to answer is this: do you just want to "learn CNC milling" or do you need a CNC mill to make parts for other projects?

If you just want to "learn CNC", then great - buy a second hand sieg mill off ebay and a conversion kit and off you go. That's exactly what I did with the X2. But be aware that you'll quickly outgrow it's capabilities if you want to do any serious metal milling with it - I found the X2 really couldn't mill aluminium with better than ~0.5mm accuracy or repeatability even after substantial modding to improve rigidity, minising play in the table, etc. Hence it was only as I graduated from "learning CNC" into "building other stuff" that I realised I'd started with the wrong mill for what I really wanted to do with a CNC mill.

Which brings me to the other side of the coin: Sieg mills really aren't that cheap. Here in .au, you can get a decent low end Hafco industrial mill/drill with a 1.5-2kW spindle with the weight and rigidity of a KX3 for not much more than a new SX3. These will do a much better job of milling metal than an SX3. And for a bit more $$$ you can get a really solid, good quality industrial mill that will perform far better on metal and run more reliably for longer than any heavily modded, high end hobby mill will ever do...

Hence before you start on your CNC mill adventure, decide/know what you are actually going to do with the CNC mill once it's built. Then go and buy a mill appropriate for that job rather than something that is cheap or "simple to convert". Finding out you can't do what you want to do with the mill after the conversion is complete is not much fun....

-Dave.

Cutting metal

Posted Jul 31, 2020 15:16 UTC (Fri) by joib (subscriber, #8541) [Link]

Sometimes one can find good deals from some metal workshop closing due to retirement/bankruptcy/etc.

Of course that's a 2m side cube that weights as much as a car and needs a sturdy concrete floor to stand on. Not really a solution for apartment dwellers. But hey, much less vibration..

Cutting metal

Posted Jul 31, 2020 21:32 UTC (Fri) by michaelkjohnson (subscriber, #41438) [Link]

Completely agree with your conclusion to start with the destination in mind!

There is an astounding variety of quality available in Sieg and Sieg-derived equipment due to different levels of quality control by different sellers... There are a lot of people getting way better than an astoundingly bad 0.5mm tolerance out of even small sieg-style mills in CNC conversions. Sorry for your bad experience; there have definitely been others with better experiences. Heck, 0.5mm is bad tolerance for a 3D printer, let alone a mill.

Availability of "old iron" for conversion is a highly localized phenomenon. In the US, it's often been relatively easy to come by in the "rust belt" — but you're pretty lucky to find something reasonable in other places.

But we're going way beyond LWN here, so these details probably make more sense in one of the machining forums populated by machinists. ☺

Open-source CNCing

Posted Jul 30, 2020 15:39 UTC (Thu) by coogle (guest, #138507) [Link]

I avoided discussing the LongMill in too much depth as a tool in my article in order to focus on the open-source side of things, but to address this point it is worth noting that LongMill absolutely is rigid and well-designed enough to mill some metals (and specifically aluminum):

https://www.youtube.com/watch?v=BYkk0dFohZs&feature=y...

Open-source CNCing

Posted Jul 30, 2020 17:58 UTC (Thu) by chrismakesstuff (guest, #140526) [Link]

vadim, yeah it's certainly really important to ask yourself what you're looking to get out of your machine. Our LongMill CNC is a great and reasonably solid build for the price and accessibility that we offer but certainly can't compete with a converted milling machine or even a bulkier built tabletop CNC router or mill. If anyone calls up and tells me they're looking to cut metal all day long with our machine, I tell them that they're in the wrong place and try to point them in a better direction.

What I can say though is that if you're looking for a machine with a mid-sized cutting area that you can have fun with - cutting woods and plastics without breaking the bank or having to source individual parts for yourself - that's what we offer with the LongMill. It's not for everyone, but there are certainly garage woodworkers who don't want to tackle their own build and like the idea of a reasonably priced, mid-sized machine. Designing our CNC around more readily available components also gives people the option to tackle the build themselves if they so choose 👍