[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]

... 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.

[Hobbiest]

But it is going to have to be located so as not to disrupt the straightness in any direction. Here is another idea, just JB weld some allthread to the rail. Do them all at the same time. With 1/4" allthread, lay the rod on a table, then lay a piece of shimming stock 1/8" less than the radius of your rod. With the allthread laying on the shim stock, it will center and square the allthread on the rod. Clamp everything down to the table until it dries.

Doing this, you can then adjust the height of the rail with on nut above, and one nut below watever your frame is. With enlarged holes and washers, you can also adjust for side to side parrallel.

[maxxgraphix]

I think I have an easy solution. First let me describe the design. This is 8020 T-Slot 1515 aluminum frame. It's already straight. 2" x 2"x 3" precision drilled blocks mount the 20mm rod to the T-slot. The rods run parallel with the T-Slot. It's not moving. The only problem was the 20mm rods would flex downward. So I have used split linear bearing blocks. Now all I need is a few center supports that are adjustable. Duh, a few t-nuts and bolts will do that. I can V cut the head of the bolt if needed. I could even mount a solid rail support the entire lenght of the rod. If the support was grooved with a router bit then it would even provide stability side to side. This should work well.

[Nonoriginal]

Duh, a few t-nuts and bolts will do that. I can V cut the head of the bolt if needed. I could even mount a solid rail support the entire length of the rod. If the support was grooved with a router bit then it would even provide stability side to side. This should work well.

I have thought about that too, as I have SS shaft with open pillow block bearings. I don't want to be a bummer or rain on anyone, but tramming in a round piece of stock to a vee along the longitudinal axis is very, very hard. (if you want to do it with any sort of accuracy)

This is what I see as potential game killers with the bolt idea:

First, you are limited to one half revolution of travel based on the TPI of the bolt that gets vee'd. what if you need something in the middle? 1/8 turn or 1/4 turn?

The vees themselves can become the problem at any point. a small burr of .002 can translate into .002 (acceptable...close to perfection) to .250 TIR (runout), again based on how many bolts get cut, and the accuracy they are cut with.

Second, You can't just slap a craftsman level on the rails and call it good. You have to have something perfectly strait (or close to it) to tram against. Then you need run a dial indicator along the axis to dial her in. fussing with the bolts just seems like a waste of time.

you could;

Cut the head of the bolt off.

buy some durable plastic (delrin, Nylatron) that is .0005 to .010 larger than the I.D. of the bolt's O.D.(or a bit smaller to press fit) Make sure that the O.D. is smaller than the width of your gap on the open bearing.

Cut them into 1 or 2 inch increments.

take them to a machine shop and have them mill a groove onto one side using a 20mm endmill. ensuring that they are all XX.XX + or minus .002 (or whatever you are comfortable with. Should cost you around 75 for the milling based on 10 to 15. + materials.

fit to bolt and slide the rail on!

After all of that I would probably just wait to buy some supported rails on Ebay or go to a liquidator. Because over time the plactic will deform, and you will have to start again.

Hope that helps! Good Luck!

[maxxgraphix]

I see your point. So no V-Cut bolt head. I'm just looking to make this thing work for now. Later I will upgrade to better slides when a good deal comes my way. With a little center support will this machine cut to .015? That would be just fine with me! It's just for plastic sign letters!

Thanks for all your advice. I'll show some pics of it this weekend. It's building time. Whether it works or NOT.