very high accuracy

[reefy86]

what should i be looking at for achieving very high accuracy for 3d wood carving? taking the high quality ballscrews and hiwin rails i already have aside, also has anyone been able to achieve 300mm z height cutting space with high accuracy too? my bed frame is made out of 100mmx100mmx5mm mild steel box sections filled with sand and then the gantry is either going to be 2x 160mmx80mm aluminium extrusion in a L shape or thick walled steel rectangle section.

so goals are

to cut wood up to 300mm in height
very high accuracy for 3d models

any advice people?

cheers

Similar Threads:

• Recommended router for ultra high accuracy! Budget of 30k
• High accuracy, small CNC
• CNC/Etch recommendations - high accuracy
• Small Parts Need High Accuracy

[Jim Dawson]

Define ''high accuracy''. Anything is possible.

[reefy86]

the detail of a human hand or face kind of accuracy

[Jim Dawson]

You should be able to hold +/- 0.01mm or better. Depends on how stiff your machine is, but it sounds like the frame will be well built. A moving table with a fixed gantry is much more rigid design than a moving gantry, allows you to put much more mass into the gantry. I would use servo motors with linear encoders on the axes, and a good closed loop motion controller.

[ger21]

Any average machine should be able to do pretty accurate 3D carving. With a 300mm Z axis, it needs to be fairly rigid. However, the finish pass is usually only removing a small amount of material so even then, it's not that critical.

[reefy86]

how straight are aluminium extrusions? trying to decide if i should go with a full steel gantry which means alot of work and epoxy resin for mounting linear rails or if aluminium extrusions are just as straight to mount the rails on.

[ger21]

how straight are aluminium extrusions?

You'll have to measure them to find out. The manufacturers specify an allowable tolerance. It's possible that you could get perfectly straight extrusions, or extrusions that are at the maximum allowable tolerance error.

[KH0UJ]

Steel Gantry is perfect for future use on my opinion (all-in-one-machine), unless you stick to wood / plastic machining forever, Aluminum will be flimsy on machining metals but great for wood / plastic machining, cost less, easy to install and assemble, though you can work on metals on an aluminum gantry but it will sacrifice "time" and building two separate machines for specific use takes "space", so I guess it`s your call man for me steel for the WIN

[routalot]

I think you will struggle to make use of a 300mm Z axis.To satisfy yourself that it is achievable,I suggest you draw the spindle you propose to use and the longest cutter you can imagine using and you are likely to find that ramping down a 65 degree slope is the limit of what can be done without fouling.It gets worse on the Z axis side of the spindle as the hardware is bulkier and there is more of it.I have faced vertical surfaces using 5 axis machines with the A axis rotated 25 degrees but that is several orders of magnitude more complicated than building and using a 3 axis machine.The other factor to consider is that at full extension to work on thinner jobs your long z axis will suffer from all the play in the system interfering with maximum accuracy unless you mount the job on lifter blocks.

[dixdance]

Speaking as an artist and not as an engineer, I don't think that extreme accuracy is at all necessary for figurative 3d sculpting in wood. I doubt you'd see any difference between a piece done to +/- .005" or one at +/- .0005". Not trying to discourage you, I like as much precision as I can get too, whether I really need it or not.

[steve323]

Speaking as an artist and not as an engineer, I don't think that extreme accuracy is at all necessary for figurative 3d sculpting in wood. I doubt you'd see any difference between a piece done to +/- .005" or one at +/- .0005". Not trying to discourage you, I like as much precision as I can get too, whether I really need it or not.

I agree. The OP mentions a desire for super straight rails. I doubt that this would have much impact, especially for a wood product that will be subject to expansion due to humidity.

Low backlash would be much more important to repeat a second pass with no carve lines. A quick touchup with sandpaper might still be needed to smooth out a few fuzzy lines.

Steve

[reefy86]

Thanks for your input, i just don't want to under make the machine that's all and considering that 3d carving requires slow feedrates most of the time then why not make it strong to cut aluminium and maybe steel too and for a personal project i would like to cut 10mm acrylic to a high standard too.

[KH0UJ]

I guess you need to attach a rolling wire cut on your z axis for cutting 300mm on wood, it`s like a bandsaw but wire. I did not encounter yet a tool bit that had a 300 mm long I was thinking for a second, how on earth would you install a 300 mm tool bit on your spindle? hehe

[ger21]

and considering that 3d carving requires slow feedrates

3D carving does not "require" slow feedrates.
The more rigid, and powerful your machine is, the faster you can cut your 3D carvings.

If you want a really good finish on acrylic, you also want a lot of rigidity.

[A_Camera]

You should be able to hold +/- 0.01mm or better. Depends on how stiff your machine is, but it sounds like the frame will be well built. A moving table with a fixed gantry is much more rigid design than a moving gantry, allows you to put much more mass into the gantry. I would use servo motors with linear encoders on the axes, and a good closed loop motion controller.

Very high accuracy costs a lot of money... Most people use steppers and with stepper motors you will not get higher accuracy than +/- 1 step. Add to that the inaccuracies of the software, especially when the travel is not a straight line along one axis. Also, having high accuracy expectations and using cheap cutters which may or may not have the diameter they are specified for, tool wear and so on... not to mention that measuring 0.01mm REPEATEDLY accurately demands also very high quality instruments.

Anyway, you don't need that accuracy for 3D milling in wood, what is needed is repeatability, low or zero back lash, and very high rigidity as well as high speed and acceleration with high torque.