[Hack]

I am looking at building my 4th router. This one will be a 4' x 8' machine and it's primary use will be cutting cabinet parts out of MDF, Melamine, Plywood, etc as well as doing a bit of Solid Surface Material and Solid wood. Sheet Goods Primarily. Will be cutting for myself as well as others, so I condiser this a serious hobby machine. My goal is to route sheets in a single pass at good speeds - perhaps comparable to a Shop Bot.

I have drawn a machine up in CAD made with 8020 extusions. The X and Y axis to be supported by 3060 extrusion, the legs out of 3030, and the rest out of 1530 and solid aluminum. Drives will be Rack and Pinion.

My questions is, how solid is this design and could someone critique it? In the back of my mind I am thinking I may want to go with steel instead of extrusion for the vibration dampening ability and sheer mass.

My problem with this is I cannot weld, do not have the tooling to cut it, and would have to farm this out at additional cost.

With the cost of extrusion being so high, I was considering steel, but with the soutsourcing needed to cut weld, paint, etc the cost is closer to what the extrusion would be.

So I come back to design. Is ther a big benefit either way?

Thanks

Dan

[LeeWay]

I think steel might be more rigid of course, depending on the sizes and types used. However, I think that either material is well suited to do what you want.
I built my first small router using steel tube. It is rigid. I didn't weld anything but the lower shelf onto the legs. The rest was all either drilled and tapped or through bolted. You could easily cut all your parts with a hand held bandsaw. They are cheap enough.

I built a small mill using the 3060 as a base and column. It is very rigid as well. It is much easier to modify an 8020 design than a steel one, but given the steels strength, I would mark them as near equals with the edge going to steel.

You can save a lot of money on your design as well. Use only 3060 for the gantry and the sides of the table. Use steel or aluminum tube for the rest. Make your own corner braces and brackets using steel or aluminum.
You can make it incredibly strong and cheaper this way. You would be able to over brace for less money.

[jhowelb]

Check these guys.
http://www.faztek.net/

[dertsap]

nothing says steel has to be welded , why not simply concider bolting the steel frame together, the extrusion uses brackets and is bolted together
all you should need for cutting the steel is an angle grinder and a handfull of cutting discs ,or you can buy a chopsaw cheap enough
brackets can be made from chunks of angle iron and or pieces of plate , once again you'll only need 2 tools , an angle grinder and a drill

[ger21]

You should look at building a Mechmate. www.mechmate.com

[Oldmanandhistoy]

nothing says steel has to be welded , why not simply concider bolting the steel frame together, the extrusion uses brackets and is bolted together
all you should need for cutting the steel is an angle grinder and a handfull of cutting discs ,or you can buy a chopsaw cheap enough
brackets can be made from chunks of angle iron and or pieces of plate , once again you'll only need 2 tools , an angle grinder and a drill

What would you guys recommend as the best type as in cold/hot rolled steel to use in a frame construction?

John

[dertsap]

i would say structural steel channel and tubing ,which for the most part would be hot roll

i agree with Ger , the mechmate is a nice looking setup

[Oldmanandhistoy]

i would say structural steel channel and tubing ,which for the most part would be hot roll

Thanks dertsap

[Dougal]

Steel is three times stiffer than aluminium and also three times heavier. I'd make it from steel.

Regarding the sections, the geometry usually dictates that and gives you a few choices on size. Hot vs cold rolled isn't going to matter as they only affect the strength where your frame will be mostly concerned with higher stiffness and low stress.
You can't use higher strength materials without deflecting the steel a lot, so even pig iron will do the job well.

[Hack]

Now if I were to substitute the base for steel and still use the 8020 to mount the rails on the x and y axis, would this be any better or will I still be limited by the 8020 gantry? Kind of the chain is as only strong as it's weekest link principle. Would the gantry limit the strength and vibration dapening properties? I am considering this in order to save on drilling all the holes for the linear railes as the t-slot seems to be much easier.

I had a previous version drawn up in cad that I had considered building. It is based mainly on c-channel and a bolt together principle. Also, how rigid will bolting get me vs welding?

I had given up on this design as well as a shopbot or mechmate design as I do not want the X rails above the table height. I would rally like to be able to slide the material on / off from the sides without fighting the rails.

Thanks again.

[Oldmanandhistoy]

Here's a nice machine that might give you some inspiration

http://www.cnczone.com/forums/showthread.php?t=30751

john

[Dougal]

Now if I were to substitute the base for steel and still use the 8020 to mount the rails on the x and y axis, would this be any better or will I still be limited by the 8020 gantry? Kind of the chain is as only strong as it's weekest link principle. Would the gantry limit the strength and vibration dapening properties? I am considering this in order to save on drilling all the holes for the linear railes as the t-slot seems to be much easier.

Stiffness and strength are treated quite differently.
While a chain may only be as strong as it's weakest link, it's stiffness is the result of the combined flex in each link.

It's difficult for me to comment on the stiffness required because I don't know what forces are encountered at the cutting tip and I don't know how much flex is tolerable.
It looks like the load from the cutter is taken from the gantry down almost directly into the rails and table edges. In which case the design of the gantry itself is going to be your biggest concern. You'll also gain/lose far more in each rail than you will in the table itself.
Bolting will be ample for joins. If you don't think a particular joint is stiff enough then just add more bolts.

[Don Kellogg]

Hack for what it's worth 80/20 sent a few people to my house today. Man they droped off tons of books and cds. Anyway after reading the manuals well so far the strength of the AL vs Steel is the same if not better pounds for pound. Of course this could be marketing but I would think in a case like this they would have to tell the truth.

If you go to the 8020.net site and look at the online version of the catalog it should have a few things in there that compare the extrusions to Steel, Iron, Copper, and Brass. There was only one extrusion I've seen so far that steel beats.

Also the 8020 rep told me they can cut any part I need if they don't sell it and they will beat anyones price. Plus they will help with the design all I had to do was have a rough drawing and size etc..

[Geof]

......My problem with this is I cannot weld, do not have the tooling to cut it, and would have to farm this out at additional cost....

How do I say this politely and without causing offense? Nonsense of course you can weld; the only thing missing is maybe 4 or 5 hours of practise.

Maybe I should clarify; with a small mig system and a bit of dedication you will be able to gain more than enough welding skill to weld together 1/8" wall tubing for the frame of your machine.

Regarding the tooling for cutting I am sure for much less than the difference in material cost between steel and aluminum you would be able to buy a very nice swivel head horizontal band saw with a capacity up to at least 3" square at 45 degrees.

Never say "I cannot"; instead say "I am going to learn.....".

[YZF]

Steel is about 3 times stiffer than aluminum. It has a Young's modulus of ~200GPa vs. Aluminum at ~70GPa. A piece of material having the same cross-section will deflect 3 times more under the same force if it is made out of aluminum. Steel is also heavier than aluminum which can be an advantage or a disadvantage. Aluminum is very strong for its weight but for a CNC machine that's probably not a driving factor unlike something like an airplane. Aluminum extrusions are easy to handle and work with which is why a lot of people end up using them for all sorts of stuff...

Just my 2 cents,
Guy

[Dougal]

Hack for what it's worth 80/20 sent a few people to my house today. Man they droped off tons of books and cds. Anyway after reading the manuals well so far the strength of the AL vs Steel is the same if not better pounds for pound. Of course this could be marketing but I would think in a case like this they would have to tell the truth.

Strength is not the problem. To use a materials strength you need to deflect it a long way. This is not a lot of use on a machine which requires a stiff bed.

The "Second Moment Of Area" combined with the materials stiffness and bending moment applied will tell you which extrusion and material are best for the application. But this is beyond your average hobbiest.

[harryn]

I would go to 1/4 in thick wall (regardless of material) so you can tap a thread when and where needed. 3/16ths is not enough IMHO.

Since this is going to get serious use, perhaps consider designing it with an enclosure from the beginning, not as an add-on item.

You could of course make the long axis the Y. That would allow easier side loading, but would require a pretty solid gantry. Depending on what you are cutting (lots of detail, or longer cuts), this might not slow you down at all.

It might make sense to raise the side rails even more so there is virtually no "gantry side" extending down. Then a long Y will work better.

[irving2008]

Strength is not the problem. To use a materials strength you need to deflect it a long way. This is not a lot of use on a machine which requires a stiff bed.

The "Second Moment Of Area" combined with the materials stiffness and bending moment applied will tell you which extrusion and material are best for the application. But this is beyond your average hobbiest.

Not so, all you need is some basic understanding of structural framework analysis (and there are many tutorials on the web) and a suitable piece of software. I've been using Framework which can be downloaded from http://home.wanadoo.nl/wolsink/ (the file is mechanic.zip). Its not perfect, but you can do basic structural analysis and if you remember that displacements are additive you can model quite complex structures in simple parts.

[Dougal]

Not so, all you need is some basic understanding of structural framework analysis

I suspect if someone had a basic understanding of structural framework analysis, they wouldn't be asking for advice on steel vs aluminium.

Analysis software is dangerous in the wrong hands. A mistake as simple as a decimal point in the wrong place can cause wildly inaccurate answers. If you can't do the hand calcs to verify your answer, then you should seek someone who can.
If you can do the hand calcs to verify an answer, you probably only need the software to pinpoint individual hotspots.

I use COSMOSworks, which is a full finite element analysis package. But it is next to useless to an amateur.

[towertek]

Having built an 8020 5x10 table, and then subsequently adding more aluminum stiffening members to it, I have come to the following conclusions,

• Stiffness is good but gravitas is better. Inertia fighting mass/weight trumps stiffness.
• 8020 is stiff and true and awesome to work with. It also looks great but its lightness may be its weak point.
• When routing aluminum or even heavy wood, there is a lot to be said for weight of the gantry. The heavier the gantry, the more inertia required to move it, and hence the harder to make it vibrate or bounce around.
• When peck drilling or plunging, seeing the gantry bow up just makes me sick. I hate that.

If I had to do it all over, I would design at least the gantry portion of my table out of bolt-together steel sections and drive it with bigger heavier motors. Why bolt-together? Because you will always want to tweak out any torsion and stresses thereby promoting flatness and trueness. Bolted structural connections will allow you to do this. Welding twists the pieces irreparably and there are no second chances.