[blau_schuh]

I just about have the needed parts to start assembling my machine.
Here's what I've got:

2 ground screws(they seem to be at least) and 1 rolled screw. No backlash, accuracy between ~0.0003"/12" and 0.0005"/12".

2 pairs of THK linear rails, 26"-SR25s(overkill) and 14"-SR15s, and one SR35 48" pair of NSKs (super overkill)

quite a bit of 8020 3030 stock (3"x3")

I have reason to believe that the heavy duty 8020 aluminum extrusion will allow for repeatible cuts for wood and aluminum at slow speeds, but here are a couple of my concerns about accuracy.

I am assuming that the rail pairs are straight and dimensionally identical. However, even the best stuff coming off of manufacturing line at 8020 is not going to be exactly square or identical (i.e. 3.0001x3.0004 at one end, 3.0000x3.0007 at the other), and these are the pieces onto which the rails are fastened to. Sort of like tossing a ferrari engine into a pinto. So, this automatically tosses the 0.0005"/12" out the window. Due to using this type of framing for the whole structure, I now have error all over the place. welcome to the real world, I guess. AND, This is heaped on top of my own human error when mounting the rails!

What can I do to minimize this?
1) Have the framing mounted together on a milling machine and have the tops surfaced together?
This seems to be the best solution, although this could get expensive.

2) Assemble the machine and use trial and error, using depth guages, micrometers, etc to adjust the machine. making depth cuts at the extremities of the machine's cutting envelope in order to get feel for how the rails are skewed, and the them use thin shims to gradually bring the rails into the same plane. Do this for each axis.

thanks for any suggestions.

[dfstrasser]

Extruded aluminum is definately not perfect. I started assembling my Y axis and ran into alignment problems with the rails. I attempted to mount two SR15 rails on the face of a piece of rectangular extrusion and the alignment was terrible. I was able to get them parallel, but they were not in the same plane. When I attached my Z-axis to the blocks, everything was in a bind. I worked on it for quite some time and was never able to get it right. I came to the conclusion that the mounting surfaces for the rails will have to be machined in order to do the job right.

For the time being, the router is on hold and I'm looking for a milling machine.

[lgalla]

blau Schuh&dfstrasser.
I decided almost one year ago to build a large router.My first post was how to mount linear rails accurately.This resulted in a dead end.I don't think many hobbyists have done it sucessfully.From the huge amount of research I have done it appears mounting THK's to 80/20 is a binding situation and the alu ext will resonate resulting in poor cutter performance ie chatter.I am not a machinist or engineer but figured out you must have a solid parallel surface to begin with for reference mesurments to align rails,gantry etc.
Picture this a granite surface plate .0004flat as the base.EG from an E-Baystore 36"X48"X4" thick wt 800lbs cost $259US store is named Discount Machine Shop.Hope you are getting the"DRIFT".Mount the thk's to the surface plate and now you have a perfect surface to align the gantry.I ain't not no machinist or engineer but tell me if this is a reasionable solution.
Iam posting in ..... General mechanical/engineering design/polymer concrete on the zone. Please visit, lots of good stuff from members and may you guys may have something to add.
Larry

[blau_schuh]

blau Schuh&dfstrasser.
I decided almost one year ago to build a large router.My first post was how to mount linear rails accurately.This resulted in a dead end.I don't think many hobbyists have done it sucessfully.From the huge amount of research I have done it appears mounting THK's to 80/20 is a binding situation and the alu ext will resonate resulting in poor cutter performance ie chatter.I am not a machinist or engineer but figured out you must have a solid parallel surface to begin with for reference mesurments to align rails,gantry etc.
Picture this a granite surface plate .0004flat as the base.EG from an E-Baystore 36"X48"X4" thick wt 800lbs cost $259US store is named Discount Machine Shop.Hope you are getting the"DRIFT".Mount the thk's to the surface plate and now you have a perfect surface to align the gantry.I ain't not no machinist or engineer but tell me if this is a reasionable solution.
Iam posting in ..... General mechanical/engineering design/polymer concrete on the zone. Please visit, lots of good stuff from members and may you guys may have something to add.
Larry

so, did this ultimately work for you?

I'm not exactly sure how I would go about fastening to a granite block, but that is another discussion.

[blau_schuh]

Extruded aluminum is definately not perfect. I started assembling my Y axis and ran into alignment problems with the rails. I attempted to mount two SR15 rails on the face of a piece of rectangular extrusion and the alignment was terrible. I was able to get them parallel, but they were not in the same plane. When I attached my Z-axis to the blocks, everything was in a bind. I worked on it for quite some time and was never able to get it right. I came to the conclusion that the mounting surfaces for the rails will have to be machined in order to do the job right.

For the time being, the router is on hold and I'm looking for a milling machine.

this is what i was afraid of. i plan on going ahead with my build and doing the best I can given the tools available.

what I learn from this machine, Ill apply to the next one.

[lgalla]

BlauSchuh.
No this was not an ultiminate solution for me as tons of research always ended in a dead end.I was just hoping for some help on my ideas.Fastining to a granite plate?yYou are right with the question.Granite City has diamond drills at reasionable pricing.I have also contacted members who have not posted in years.It is scary most have not posted as their projects has resulted in "paper weights"and theyare em bare assed to ask for help.Hopefully we can gather our ideas on the Mechanical/engineering fourm and give a "solid base" to start on.
Larry

[blau_schuh]

BlauSchuh.
No this was not an ultiminate solution for me as tons of research always ended in a dead end.I was just hoping for some help on my ideas.Fastining to a granite plate?yYou are right with the question.Granite City has diamond drills at reasionable pricing.I have also contacted members who have not posted in years.It is scary most have not posted as their projects has resulted in "paper weights"and theyare em bare assed to ask for help.Hopefully we can gather our ideas on the Mechanical/engineering fourm and give a "solid base" to start on.
Larry

I don't think most of the machines are paper weights, the members probably got what they needed from this site and are now using the machine, or maybe just got bored with what they built. Judging from the posts I've seen, at least 10 people have had recent success building these, albeit maybe they haven't had success in seeing accuracies in the ten-thousandths.

I've been doing some basic research on laser levelling, perhaps this is an avenue to look at. With a few lucky scores on ebay I can come up with a poor man's levelling system that will at least get me into the thousandsths. I can easily see the development of the levelling system becoming a more tangled web than the actual cnc build/design.

[elogicca]

I had once thought of using the 2 part bar coating epoxy, wonder how level a surface that would make for rails?

[dshowalt]

Try this stuff instead of common 2 part epoxy. Common epoxy has a setup time too short for accurate self leveling in many instances. It's designed to set up based on ambient temp and i believe it has a viscosity too high for what you want to do.
http://www.precisionepoxy.com/

[Geof]

This thread is a very complete discussion of aligning rails. Your are correct it is not a simple process and the 8020 is not really a stiff enoug structure.

http://www.cnczone.com/forums/showthread.php?t=19418