[wizard]

Great reply from wizard, as usual.

Here is the sticky on ballscrews: http://www.cnczone.com/forums/linear...-software.html
Even a C3 ground ballscrew is very expensive, and only good for 0.00027" per foot. Over 10', that's almost 0.003". As wizard said, they can be mapped, or glass scales in a closed loop system, both with the right software. After that we also need:
- no sag over the 10'
- no whipping due to critical speeds
- a pure guess is that a 10' long ballscrew would need to be on the order of 50mm diameter to not sag or whip at reasonable rpm, and it would still need auxiliary support -- I wonder how much a 10' C3 50mm dia ballscrew would cost?
- a controlled temperature (high end CNCs use ballscrews with internal liquid cooling to control their temp) -- maybe could skip this with glass scale feedback.
- a preloaded double ball nut ($$) and preloaded high precision double angular contact support bearings ($$).
And that's just the mechanical drive for 1 axis!

The most basic challenge with high accuracy is temperature. Aluminum's coef of expansion is ~13 * 10^-6 in/inF. So just a 5F rise in temp of the machine makes the 10' dimension bigger by 0.008". Steel and cast iron move about half as much. So right away, we need a uniformly temp-controlled room, and maybe active cooling for the ballscrews and other parts.

Just to add a bit of color, I spent over a decade working on diamond turning machines in a nice temperature controlled facility. while the room as a whole was kept at a consistent temperature the fact that there was air conditioning vents prevented some machines from working correctly. We literally had to shut off some vents and reposition others. The parts being machined however where less than 15mm in diameter and thicknesses of a few microns. However trying to keep tolerances in the 1-2 microns range required the machines to be thermally stable, the air conditioner kicking in would throw the machines out of spec. Very frustrating.

Now a couple of microns might seem to be excessively small but I don't see keeping an accuracy of 0.001" over 5 feet to be any easier if any thing it would be harder.
we had a key benefit in that the machines ran the same part in long production runs so the machine would find an equilibrium thermally. A router on the other hand is often all over the place.

So in this day and age, on a machine this size and slow price point, one would likely want to try mapping a lower cost leadscrew to the machine. That would give you a machine accurate at a given temperature point. This assumes too that the controller can actually handle the mapping requirements. Mapping can improve things but it doesn't solve thermal issues and may not handle periodic error well.

Other challenges with accuracy are:

Room issues: floor rigidity and stability (in addition to temp). A regular concrete floor may move over time. Large CMMs aren't just machines, they are installations -- same for CNCs.

Component quality, which translates as $$, good decisions, and experienced attention during installation.

Alignment: linear (easiest but not easy), 2D (harder but there are techniques), 3D (hard), and how to maintain alignment over time (hard if the machine is not massively built out of proper materials). In 1D, for example, linear rails are not straight when you buy them, so we can't rely on them for linearity. The usual technique is to machine a highly accurate registration ledge the full length (into a very stiff and stable frame), and force the linear rail against the ledge. Next we need to get the other rail extremely parallel to this master rail, mount it there, and have some way to ensure it stays parallel (what if you move the machine?). In 2D and 3D it gets more complex.

Dimensional stability over time: will residual stresses relax and cause parts to shift over time?

Rigidity, vibration, and damping due to cutting forces, inertial loading, and local forces: FEA is good for this, but accurate modelling of a CNC machine is not simple. Some things can be certainly calculated with equations and analytic methods, e.g., how bearing stiffness and slop geometrically propagates to the cutter tip. For the complex geometry, structure, and loading of a CNC router, only experienced use of FEA will do a good job of predicting stiffness at the cutter, or vibrations of the system. The consensus view of experienced machinists is that it takes massive amounts of cast iron and $ to achieve high accuracy over a large distance.

Machine design/configuration: CNC routers are complex machines with many issues. Assuming someone already has a mechanical engineering degree and many years of machining experience, I'd guess it would take about a year of CNC-specific study, *from the right people*, to be qualified to design one that could hold 0.001" 3D accuracy over a long distance, for years. I'm an experienced ME, have done design/build projects for 35 years, have been studying CNC design on and off for 5 years, and I'm not sure that I could do it (nor do I have much interest in it... seems like a bunch of headaches!) This kind of accuracy is just very difficult and complex.

So hopefully you can see why 0.001" accuracy over a long distance will cost an amazing amount of $, and more importantly, a team with lots of knowledge and experience.

For a point of reference, here is Multicam's top router, I'm guessing about a $70k machine: 7000 Series CNC Router - CNC Cutting Machines for Your Application & Budget | MultiCam
They only spec a repeatability of +/- 0.001". No accuracy is stated because it's considerably worse than that. But few people need 0.001" accuracy over a long distance.

If they do need that sort of accuracy they most likely would end up with a Bridge mill with a moving table design.

Some suggestions:
There is a lot of good info in the archives -- search and learn, follow current threads -- for a few months.
Also search on practical machinist dot com -- lots of experienced machinists there sharing their knowledge, especially the user 'milacron' -- I have learned a lot from his posts.
Spend as much time as you can in the shop at your university, and learn from the smartest machinists there. Ask to be involved in their special projects.
Don't order parts until you've done at least a few months of external study, refined the design through many iterations and analysis, and it's been vetted by others. Exception: ordering some sample parts is often helpful, especially when less experienced.
Design and build a smaller machine first. You'll learn a lot by just completing one, and it's much easier to build a small one than a big one. Then you can make the beginner mistakes on the little one, and have much better skills to build a big one.

[dmalicky]

Just to add a bit of color, I spent over a decade working on diamond turning machines in a nice temperature controlled facility. while the room as a whole was kept at a consistent temperature the fact that there was air conditioning vents prevented some machines from working correctly. We literally had to shut off some vents and reposition others. The parts being machined however where less than 15mm in diameter and thicknesses of a few microns. However trying to keep tolerances in the 1-2 microns range required the machines to be thermally stable, the air conditioner kicking in would throw the machines out of spec. Very frustrating.

Now a couple of microns might seem to be excessively small but I don't see keeping an accuracy of 0.001" over 5 feet to be any easier if any thing it would be harder.
we had a key benefit in that the machines ran the same part in long production runs so the machine would find an equilibrium thermally. A router on the other hand is often all over the place.

Good story, Wizard, thanks for that!

I agree steel and cast iron are ultimately better for a 'real' machine tool, but there are some things I like about aluminum for DIY machines...
- To get steel without internal stresses, we get hot rolled, but it has scale, inclusions, loose tolerances, and isn't straight or flat. In aluminum, we get MIC 6 tooling plate which is ~stress-free, ground flat, and can be cut on a table saw -- great for Y and Z car backplates. Or 6061-T561 bar stock, which is mostly stress-relieved and stays pretty flat after machining.
- Hand drilling thick steel is fairly challenging; less so through aluminum.
- Unless one has high $ motors and drives, an aluminum gantry does help acceleration a lot.

[OneWound]

Good story, Wizard, thanks for that!

I agree steel and cast iron are ultimately better for a 'real' machine tool, but there are some things I like about aluminum for DIY machines...
- To get steel without internal stresses, we get hot rolled, but it has scale, inclusions, loose tolerances, and isn't straight or flat. In aluminum, we get MIC 6 tooling plate which is ~stress-free, ground flat, and can be cut on a table saw -- great for Y and Z car backplates. Or 6061-T561 bar stock, which is mostly stress-relieved and stays pretty flat after machining.
- Hand drilling thick steel is fairly challenging; less so through aluminum.
- Unless one has high $ motors and drives, an aluminum gantry does help acceleration a lot.

I concur, for a 3x8 thick piece of aluminum running across 5' weighs just south of 200 pounds. To help with tapped holes, helicoils will be used.
All, I'll try to reply to everything the best I can. If I miss something, let me know.

@wizard, this gantry will machine high-density foam, ranging from 15lbs to 40lbs. We can't use a moving table design due to the sheets being in 4x8 form. The reason I want .001 so much is because I'm a perfectionist. While I understand that .001 is unobtainable, I'll do everything I can to get as close to .001 tolerance as possible. I'll be happy with .01-.005, honestly.
FYI - This machine will be used to make carbon fiber molds for 2-3 student organizations, and while the molds may be sanded, if I can ensure accuracy and use small enough stepdowns I can greatly reduce sanding time.

@dmalicky, I understand what you are saying. An active A/C system for this maybe may be viable, but for now I want to get this machine design done at least. I will be posting a design review on here, and I do hope that you are willing to look at it. There will also be an assembly review, as I understand this thing will be a MAJOR pain to install correctly