[dinkyguitar]

Hi All,

I'm having a hard time deciding what to use. My cnc will have a working area of 24" x 36", mainly wood project with occasional aluminum work.

I already have my linear round rails and ball screws, so my next step is making the frame.

I like going extrusion, but the cost is killing me.

Aluminum tubing is my next choice, but I have to drill holes as opposed to the extrusion already having a channel for the screws. Accuracy will be a challenge.

Steel is nice and strong, but again need to drill holes and maybe epoxy leveling is needed to mounting the rails...not sure how much leveling I need to do with the others. I'll be bolting the pieces instead of welding.

Which one should I be doing?

Any other choices for frames?

dinkyguitar

[louieatienza]

There's no rule stating you can use only one or the other. On my current build (on hold right now due to work) I used a combination of aluminum extrusion and plate, and will be using othr materials like phenolic as well. All materials will have their benefits and disadvantages. There's a build in the log section of a very nice machine made with square aluminum tube, and connectors milled on a smaller machine. T-slot extrusions cost more, but are modular, adjustable for misalignment, and very strong for the weight.

I personally believe steel tube is not as worthwhile unless you use thick wall tube. Even then there will be a lot of drilling and tapping, and possibly machining. It is cheaper, but you pay for it with the extra work and machining.

You might be able to find surplus aluminum or steel plate as well. Also, 8020 has a lot of surplus sales on eBay for a fraction of cost.

[harryn]

Hi Dinky, welcome to the club of people struggling with this decision.

As you noted, each approach has its advantages and challenges:
Extrusion
- the extrusion itself is expensive, and even worse, the fasteners end up being nearly 2 x the price of the extrusion. For my project, this is just too much.
- If you get really good quality material that is really straight, then it might be worth considering, but that is hard to determine in the used market. If it is not really straight, then you have gained nothing over cheaper methods.

Al tube
- It is cheaper for sure than extrusion, and frankly pretty easy to drill, cut and file. I find it isn't much worse than old growth hard maple.
- The material I have found really isn't all that straight though, so you should plan on using self leveling epoxy or similar
- I would use at least 1/4 in wall so you can tap it. Anything thinner will just flex way too much.
- If you buy large enough tube so that your hand can get inside of it, that can be really handy for fastener assy.
- The down side of Al tube, is that it is pretty soft. If your plan is like me - hole drilling in Al, that is pretty light use. If your plan is reall to "mill Al in light passes", that will start to strain your frame a lot more than is obvious.

Steel Tube
- In most cases, steel tube will be the cheapest to buy.
- So far, I have not found any "off the shelf" tube that was straight though. The specs on A30 / 36 tube are pretty liberal. As far as I can tell, it means "its some kind of steel like substance" and "even if it is run over by a car, it still meets the straightness spec."
- Because of this problem, I have been looking around (for over a year) for alternative methods to get a straight, flat, rectangular steel tube. I now know 2 methods to do it, but neither are particularly inexpensive. The quality is really high though.

One area often forgotten is temperature range of operation. If you have a steel frame and steel rails, they all will grow / shrink proportionally, so while the dimensions change, it will tend to stay straight. If you have an Al frame and steel rails, a 40 F change will make it warp.

Take a look at Carveone's thread on rebuilding a router a few times. He tried a number of methods before settling in on steel tube.

[harryn]

Just an idea, since you have the main parts already, consider to make a proto frame from 2x4s and plywood. It does not need to be anything fancy, but at least for me, I find it easier to make "adjustments" in wood.

I don't have any precision metal cutting tools, so my plan is to take what I learn in wood, and then get help with the metal framing. I jokingly call this approach PAD (plywood Aided Design).

My personal favorite build structure is to make a 5' x 8" x 8 ft box structure using 2 kiln dried 2 x 4s, and 8 in wide 3/4 plywood strips. I make the strips slightly wide, and then use a trim router to finish it out.

It isn't perfect, but if you are careful it can work for a decent starting point.

[wizard]

You might want to dampen that frame some.

Why? Experience at work with special machinery is the primary reason. Eventually Aluminum extrusions come apart! Mind you I'm not one to baby a wrench but most extrusion systems I've worked with do not result in rugged structures. I'm certain somebody will pipe in claiming the opposite but my personal experience is to save the aluminum for the least stressed parts of the machine.

Now doing steel right isn't without it's challenges. It may be advisable to find a machine shop willing to square the weldments and cut pads for the rails. Drilling the mounting holes might also be advisable. You could always hand scrape the frame instead of machining it.

The above is after weldment, before welding it really pays off to machine the parts to be welded as accurately as possible. This applies to all frames not just welded steel. Adjusting a frame after welding can be a challenge. We are talking tolerance wood workers may cringe at. One big advantage with extrusions is the ability to order a parts kit of sorts pre cut accurately. Steel requires an effort on your part or working with a good fabricator / machine shop. Plus many parts can only be machined successfully after welding. Someplace in there you may find the need to stress relieve.

This sounds like a lot of work and in a way it is. Just remember with extrusions you are paying for somebody else to do much of that work.

[dinkyguitar]

Thanks guys,

I know it is a loaded question, but I keep changing my mind everytime I think of using each type of material...I just can't get myself to commit to one of them...

dinkyguitar

[harryn]

Dinky - It is going to come down to tolerances you want to hold, and how much use your router will get.

A lot of hobby people build routers for fun with use being secondary. It sounds like "use" will be closer to your plan. That will push things toward a tougher frame.

Do you have access to a mill to do the machining part ?

[dinkyguitar]

Thanks harryn,

I don't have access to a mill....everything will be done by hand tools.

Although I keep thinking I should make it out of MDF first, then use the machine to make the parts for itself..

dinky

[harryn]

Thanks harryn,

I don't have access to a mill....everything will be done by hand tools.

Although I keep thinking I should make it out of MDF first, then use the machine to make the parts for itself..

dinky

That can be a great approach, similar to my PAD approach.

I dislike building with MDF and instead use plywood, but either one can work to build up a usable system. Using this proto to build the final version is a great idea, relatively low cost, and commonly done. It also will give you time to think about the Al vs Steel frame question.

The rails, motion gear, and electronics are the big expense anyway, and nearly all can be reused in the final version.

[Benonymous]

Unless I'm barking up the wrong tree Dinky, I'd hazard a guess that you'll be cutting out guitar bodies or components for guitars. If I'm right, then I'd say a steel machine will cover everything you need. I haven't updated in a while but my no-weld machine is made of common ERW rectangular steel tube (Electro Resistance Welded) This gives you a tube with an internal seam. The wall thickness is 4mm. I have constructed mine with a multitude of drilled and tapped holes/fasteners primarily for the ability to tweak the structure true and square. So far this has worked out well.
http://www.cnczone.com/forums/diy-cn...1_x_0_a-5.html
When I purchased my steel, I just bought a random length, put it on the bench and used a taut length of piano wire to test the straightness. On the 4mm side wall material it was surprisingly good and I couldn't slip a 005" feeler gauge under the wire anywhere. It's not perfect however, still, for the purposes of a home made machine it is more than adequate. It would have been much easier to weld the frame and there are numerous, excellent steel frame machines on here made that way.
Aluminium extrusion in my experience, isn't made to very close tolerances and with 80/20 they do the old "razors and razorblades" trick on you the extrusion is fairly cheap but all the other stuff is dear as poison, as you have already discovered.
MDF, ply and timber can be made to work but I don't think you'll get a machine as durable as steel out of it and may have to check it regularly to correct things going out of whack or fasteners pulling out/loosening (waits for howls from the MDF guys!)
Anyway, those are my thoughts. BTW, I hope you haven't bought unsupported linear rails!!

[louieatienza]

Keep in mind there are thousands of aluminum t-slot machines here and there's good reason for it. There are even some commercially-avavilable.

While many would say steel tube would be the best of the three, the commercial machinebuilders woud say otherwise. CR Onsrud has a moving gantry router, rack and pinion with a steel tube bridge that they only recommend for occasional hardwood use (Panel Pro). And this is a 15HP spindle, and probably blow away most steel tube builds here. Their heavy duty is a solid cast iron bridge and moving table design with ballscrews. The base is steel tube, but HUGE steel tube.

Heck, you may like your MDF build so much that you may never upgrade it. I've tested some extrusions from Misumi and Bosch that were as straight as my precision straightedge could read. Misumi even offers extrusions with ground flat sides if you really need higher precision. \

The point I'm trying to make is that it's easy to get caught up in "what is best" and then not have anything cut from your machine to show for it, or worse, not even have a running machine to show for it. Make your machine as best you can with the resources at hand,and start making stuff, instead of pondering... and pondering... and...