[GME]

Say if you nested 20 of the same part, such as a 6" x 6" square with 1/2" rounded corners and a hole drilled at the corner radius center point. Will the features in all 20 parts measure to within +/- 0.002"?, +/- 0.005" of the CAD specs? Assume generally accepted speed and feeds for wood and aluminum.

In my experience, assembling precision machined/cut parts, especially the size and length of a CNC machine, requires precision setup instruments. A new machine generally requires commissioning from the OEM to "tweak" and adjust for each sub assembly to be square, flat, and true to the required tolerance. Especially after shipping. I see no mention of this when anyone is assembling parts from a kit or built by themselves from scratch. Everyone says to use high precision rails, bearings, etc. But assembly is more critical. When scratch building a base for example, what techniques is the builder using to make sure each corner is square, flat, and in the same horizontal plane. What are the generally accepted tolerances for those?

I am building my own extrusion-based 4 x 4 CNC. I understand your concerns about assembly and took great pains to assemble mine as precisely as I could

I'm the guy wmgeorge mentioned who had issues with the extrusions. Virtually all extrusion-based machines in the U.S. use extrusions from 80/20. IMO, 80/20's cut tolerances are less than optimal. For cut at the ends, the tolerance is .002" per inch. So, for a 1.5" x 3" extrusion, you are looking as .003" across the short 1.5" dimension, and .006" across the long 3" dimension. Extend that over a 5'+ extrusions length, and you are going to be way out of square on assembly. 80/20 also has a cut length tolerance of .015". Quite at bit, IMO and makes a precision assembly pretty much impossible. I found that 80/20 doesn't always stay within their published tolerances. Some cuts were off more than .002" per inch. I didn't try to determine whether 80/20 stays within the length tolerance. Since I was milling anyway, it wasn't important to me. My mission was to get everything right.

I solved the 80/20 tolerance issues with my benchtop mill and an auxiliary support structure. I milled the ends of the extrusions square and milled them for length. Rather than repeat the process, here is a link to my build thread, which is still open and ongoing: https://www.cnczone.com/forums/diy-c...machinist.html

The CNC Router Parts Pro machines are pretty popular, but unless CNCRP has some special deal with 80/20, or mills the extrusions themselves, (which I doubt), their machines are bound to suffer from the tolerance issues. Most folks apparently don't have a problem with tolerance issues and probably don't even know they exist. I decided to build my own, because I wanted a machine that was as accurate and repeatable as I could make it, and substantially beefier than the usual kit machine.

Most first time CNC buyers wouldn't know where to start with designing a CNC. I was one of those guys, when I bought my first kit. However, once I saw how it went together, designing my own turned out to be fairly easy. I've had to make modifications on the fly, and have run across some unanticipated issues, like designing hard stops and limit/homing switch/sensor flag placement. I'm not an engineer or a machine designer, so for me, those kinds of issues were inevitable. So far, I haven't encountered any unsolvable problems. Long story short, I wouldn't discount the idea of building your own.

Keep in mind that extrusion-based machines are never going to be as rigid as a welded steel. However, using larger extrusions, you can make as machine that significantly stiffer than the kits.

I tried the Fineline Saturn 2, but the one I bought was so badly put together that it wasn't worth the effort to even try to get up and running. Others have reported similar problems, albeit in less detail than I documented. From the best I can tell, the earlier models were fine, but any semblance of QC went away over a relatively short period of time. Needless to say, I cannot recommend the Saturn 2. Here's a link to my experience: https://www.cnczone.com/forums/finel...cnc-posts.html,

My assembly was accomplished with a precision straight edge, dial indicators and a large precision machinist's level. The whole process proved to be very fussy and time consuming.

Gary