DIY, my plans how accurate do I need to be

[maxxgraphix]

I decided to build my own 48 x48 x 3 cnc router. I didn't like the mdf so I'm using T-Slot and .5 aluminum plate. I did make a mistake I know of already in that I used 20mm 60" linear rods that are unsuported in the center. There is flex that equates into slop. They were cheap. I'm trying to design my plans based upon measurements from the materials I purchased. I hope to be within .002 in my design. Now I'm using a plain old drill press and chop saw. There is no way I can drill / cut to .002. Assuming some adjustment might be needed just how sloppy can the parts be made for this thing to work? I'm going to plot out my parts 1:1 and overlay the plot onto the aluminum stock and center punch the holes.

[chuckknigh]

Can you design your bearings/bushings such that you can support the rail? The rails are sagging under their own weight...they definitely need support.

-- Chuck Knight

[2muchstuff]

Try using a split linear bearing instead of a conventional one. By doing so you will be able to support your guide rails.

[maxxgraphix]

I think I can use split bearings. They are available. Should the y axis have support also? The rails are doubled so it should be stronger. How should the supports be made? Drill and tap the rod then block and bolt to the frame? I can set the gap with a mic.

As in my first post, if I drill a hole that is .006 off center will it matter much? I hope this works, I have $2k invested in a pile of scrap right now. This isn't for a hobby, I guess I should have coughed up the extra $3k and bought one. I haven't drilled one hole or cut 1 piece of metal and already have problems. Even the aluminum plate I purchased is not exactly .5, some is .5, another is .493 another is .517, so now I have to go back and redesign the plans. I can't imagine building one of these with MDF and plastic.

[pen25]

as to your plate for the table top when you get it complete mill it level. use the thinest plate as the guide. do you have cross support on your table top? what you have is a pile of stuff that has a ton of potential if you take your time and think things through. before cutting anything else start measuring

[WoodSnarfer]

Originally Posted by maxxgraphix ... Even the aluminum plate I purchased is not exactly .5, some is .5, another is .493 another is .517, so now I have to go back and redesign the plans.

This has got to be true about every DIY (or commercial) builder who uses aluminum plate as part of their design. Now I'm curious, since you aren't the first person here who has mentioned the standard tolerance of plating to be an issue with machine design -- so what is it about your design that causes this to be a problem?

I'm asking because the design I'm working on is predicated on using 0.5" 6051 plating -- and I can't see where +/- 15 thou is going to cause the machine to be inaccurate. I mean, if I align the rails and screws correctly (including shimming, if required), won't it be okay?

Or, do folks that use aluminum plating first run every plate through some sort of surfacing tool that gets them to exact dimension?

I was way more worried about end cutting the plating accurately...if I also have to worry about thickness, then...yikes...

Thanks,
Chris

[maxxgraphix]

The thickness only matters when I'm stacking the plates or plates on blocks. If you have 2 that are .015 off then you could run into some alignment issuses. For the most part it doesn't cause a problem. I'm only looking to cut wood and plastic and thin aluminum. I make signs. Cross support is done with T-Slot and MDF for the deck. I figure if I miss program I only slice up a $21 sheet of MDF. I will post my plans / photos when done in the next 2 weeks along with a material costs for all who like to try this design. The nice thing about T-Slot is that your machine can be taken apart, redesiged many times. My design can be scalled up / down and is so simple that it should feel like building from a prefab kit. I hope.

[ViperTX]

You have to design having some adjustability in mind. So as long as you can adjust out the "slop" you'll be okay. If you use center supported rails then you won't have to worry about drilling the end of the rails (rods). Since your rails are not drilled to support attachment to a center support, I would consider using some clamping mechanism that clamps the end of the rails to the center support. I would place the rails on a bed of epoxy.

[maxxgraphix]

Can I tack weld a center post to the rails? I can't drill em.

[ViperTX]

maxxgraphix...I certainly would not weld them, you'll end up with more problems. Clamp the ends to the rail support.

[Hobbiest]

How about two pieces of angle iron, or aluminum, opposed from eachother, with an 1/8" or 1/4" gap between them? Set them up with a standard measuring piece (i.e. 1/4" aluminum), and use the same piece everywhere you bolt the angle down. Block a small space every here and there with some of the spacer stock, cut down to be a bit below the angles height wise, JB weld your rails to the supports. The little blocks will allow you to use less weld, and if through bolted to the angles, will keep them from spreading apart under heavy loads.

[maxxgraphix]

Let's try JB Weld. The ends are clamped. It's only the middle flex that's a problem. Just need a center support, might not really need it attached to the rail, just there to hold it up.