CNC Router

I've started assembling the bits and pieces to make a CNC router table. My mini mill is nice and does a good job, but I want to rough out some larger stuff from time to time. My problem is figuring out how to measure and mark my holes with what I have for the larger pieces to make the frame of the new machine. My mill (with some custom mods) has a working range of about 6 x 11 x 6. I'm planning a CNC router with a range of about 20 x 20 x 4-6. I figured machined and braced aluminum plate would make up most of the pieces, but I'm having some difficulty figuring out how to get my precision distances between screw holes on the longer dimensions. I'll probably go with a fence and stop configuration and make my primaries equal distance from each corner. Is there is a better way?
Precision length and square is also a trick I am trying to figure out for the cross pieces. Leave them semi loose & just cut small(er) squaring braces maybe?
I could probably cut upto 5 x 8 braces accurately enough with no problems.
The table frame is planned to be an open configuration with threaded holes for semi quick changes of table surfaces. Surplus t-slot table pieces most of the time, with a couple precut MDF surfaces when I need to get savage with a work piece (which is really what this machine is planned for).

Bob La Londe wrote:
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Look at the various plans and designs on cnczone.com
You will note that nearly all of these designs do not involve precision construction, they provide adjustment points to allow for fine tuning during assembly and alignment.
One other suggestion if you haven't thought of it yet, is to use one control computer and stepper driver for more than one machine since for hobby work there isn't much need to have multiple machines running at the same time. I share a control PC and drive between CNC plasma and CNC mini mill currently, and if/when I CNC a mini lathe it will run that as well. My big mill and lathe are manual.

"Pete C." wrote:
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Oh yea, on table surfaces, plan for vacuum chucks for a lot of work, i.e. account for an inch or so for the thickness of a vacuum chuck. People report good results using MDF to make vacuum chucks if they paint the MDF with a sealer first, and then route the channels and whatnot in the CNC router, finishing the last ports with a hand drill and long bit.

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I'll probably get another controller and motors setup because I want to go with some much heavier NEMA 34 motors for faster travels and faster cutting. I'm using 280oz motors on my mill and they really are not enough for a heavy router table unless I go with gear reduction. That means a more fabrication and slower working speeds. Also plan to use a 30,000 rpm trim router for the spindle and just add an external router speed controller for slower speeds. Maybe at some point add an optical tach if I feel a need to know exactly how fast its turning.
Computers are cheap. Might even go Linux / EMC on this machine, but I'm not sure I want to get into that. I tried to set it up once before and got all turned around. I know a little more now though.

Bob La Londe wrote:
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Computers are cheap, stepper drives are not. I don't know what you have for stepper drives currently, but they may support larger motors. I have larger motors on my mini mill 600oz-in than on the plasma table 200oz-in since the plasma table has no cutting forces to fight, just acceleration of the gantry. The same drivers handle either set of motors just fine.
Your routing speeds won't be super fast anyway since rigidity of the machine is more of a factor than stepper strength. Been there, done that, and the machine could move far faster than you could route while maintaining any rigidity.
EMC2 is supposed to be a lot better than EMC, but I'm not going to bother trying it since Mach3 works well and covers everything I need. Last I looked, the Mach3 license covered unlimited machines for non commercial use.

Bob La Londe writes:
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Dye and scribe. Large import calipers are cheap. It helps to engineer the required tolerances. If you need better accuracy than that, then you're in a different game altogether. Consider the ultimate source of accurate lengths, squareness, etc. Don't assume structural stock is specifically square.

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Oh, I know how far off mill stock can be. I was figuring to order plate aluminum close to size and setup a fence on my mill to machine the ends and edges square after rough cutting to size with the carbide wood saws. Table, miter, radial, etc. I figure if I can get the fence square to the X axis I can machine about twice the length of my table to square by sliding the piece along the fence. I picked up an optical edge finder which makes that sort of squaring go pretty fast, but I might just use a large piece of sacrificial material instead.
I figure if I can get the table frame square and true the top edges together for reference then get the Z axis perpendicular to the table base every thing else can be machined in place to the machine... More or less... LOL