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> Proof That I Am Absolutely Nuts, The next CNC
crane550
Posted: April 14, 2012 05:07 am
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It is true that I am still finishing up my 3x6, but it's nearing completion and won't need much attention soon. Time to go on to the next thing.

This is my new plan. It's made to be a bit...uh....heavier. The previous machine surpassed all expectations for wood routing, and has even done some good aluminum work, but now I want to attempt a metal cutting machine.

There are a lot of proven designs out there, but this time I think I want to try something different. I am a big fan of the THK profile rails, and have seem used in many high quality metal working machines. I purchased an SHS45 rail (45MM) on accident a couple years ago, and it has sat in my garage waiting for a project. I was fortunate enough to come across two more just like it (86" ea) for a very reasonable price, and purchased them as they match what I already have.

There are different rating for these, but they all seem to be in the 100,000-150,000 NM load rating, which is blatant overkill, even for this application. As you have seen from my previous builds, however, I like overkill.

Here is a close up of the rail:

user posted image

Now for the design. The goal is to have about a 3ish x 6ish work area, be able to tale fairly liberal cuts in mild steel, and have all of accuracy and tolerances that you would expect.

Here is the current design:

user posted image

This is a very early concept, most of it will change by the end of the design.

The first thing you will notice is that the gantry is actually longer then the supported sides. Yes, I would agree that this is a weaker option then using the long rails on the supported axis. I completely get this. However, I would like to explore this design as usability is a big factor. I don't want a machine you have to crawl into. This makes everything nice and easily accessible from the front. Having a long gantry certainly will pose challenges, which will need to be met. Also, if I get thru all the math and it does not add up then the idea will be scrapped. My current math skills are getting a lot better, plus I have access to several university professors who specialize in structural design who are more then happy to lend a calculation or two. If the math says no, then I will accept it. However, someone scratching their head and saying it does not look right will gain no ground with me. I need data. Plus I know this can be done one way or another, so I have already won this one in my opinion.

Servo's will be needed for this task. Also I plan on full flood lubrication. Aka, the "crib" like design. It will probably use a 15HP or so spindle and high speed machine methods.

As you can see ballscrews will drive the Y axis, and the gantry will most likely run on the same, although I have not counted out R&P. I plan for the accordion type covers to protect the Y ballscrews and slides.

I fully admit I am completely nuts. It's also possible I will take on a smaller project before diving into this one.


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AwesomeMatt
Posted: April 14, 2012 05:21 am
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Are you also planning on buying your wife's half of the house in the divorce so you have somewhere to keep your armada of CNC machines?
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crane550
Posted: April 14, 2012 05:23 am
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Haha, the first machine is in the process of being sold at the moment, so I will only have 1 machine for a bit.


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JoOngle
Posted: April 14, 2012 09:52 am
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QUOTE
Proof That I Am Absolutely Nuts


Meh...news at 11, tell us something we don't already know, skip. wink.gif

On a more serious note, it's no doubt that you gain an awful lot of experience building and constructing all of these, thus making them better, improving the technology etc.

Are we taking bets anyone? When will Crane550 finish his 11th CBC?



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damien
Posted: April 15, 2012 01:37 am
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And if you get the mini cnc/engraver/mill sorted out, I can start using it. LOL

Damien


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Wow you look quite normal for a geek.

The statistics on sanity are that one out of every four persons is suffering from some sort of mental illness. Think of your three best friends -- if they're okay, then it's you.
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tekwiz
Posted: April 15, 2012 05:48 pm
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Do yourself a favor & study the design of a few commercial CNC gantry mills.
Steel cutting is a whole 'nother ball game, especially when metal removal rates go up. A decent cutter should be capable of removing 1in³ per minute per spindle horsepower, in mild steel. Rates go down considerably when machining the tougher grades.
It's not just the ultimate strength of the ways that counts, but how much they deflect under load. Or more importantly, under overload.
The last thing you want is a crash to warp all of your ways to the point where they must be completely removed & remounted, & this is virtually certain if only screws are used to hold the way components to a smooth surface.
Way components should be keyed in along their entire length for a metal cutting machine. This means pressed into a shallow machined keyway, then bolted, or with the use of a full length separate key. It's the only fastening method that comes close to the solid cast in ways that most commercial machines use.


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crane550
Posted: April 15, 2012 07:05 pm
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Tekwiz, keep an eye on this one for me. In the end it needs to be reasonable and in budget- so even if it's an entry level machine in the end I will be happy. I'm not made of money. Still, my goal is to create the best machine possible with the resources available. Your ideas on a spindle for this machine would be most certainly welcome.

On a side note....got asking price (which was the lowest I would go) for the old Solsylva. She has now found a good home to a hobby electronics guy who builds enclosures.

Bitter sweet.


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AwesomeMatt
Posted: April 15, 2012 07:33 pm
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Man, I remember when you first started building her. I was thinking "Wood? Really? I dunno about that.."

Ahh well, that's what pictures are for. You can't keep all your ex-s to play with, only 2 or 3 at the most. tongue.gif
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tekwiz
Posted: April 17, 2012 04:51 pm
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The main consideration for such a machine is rigidity. Choose your designs to maximize rigidity...make it as solid as you can. If you use square tubing, make sure it's heavy wall & weld caps on both ends. For keying purposes, it will be necessary to machine slots in the tubing. 1/8" deep should be adequate & will preserve enough wall thickness to take threads if you use 1/2" wall or thicker.
If you use premade ways, like you did on the others, they must be as low & as heavy as possible.
The other main difference from what you have been building is you will need a heavier table with clamping provisions. I doubt that you want to go through the work to make a T slotted table, but a grid of tapped holes in a steel plate will be enough. You will want to farm the plate out & have it Blanchard ground on at least one side, to provide a flat accurate surface onto which to clamp the work. Both sides would be better & the whole grinding job should cost no more than $50. If you plan a 3'x6' table, you will need 1.5" plate, minimum. This may sem like a lot, but it's not. You may want to consider going a bit smaller, perhaps 3'x4'. Even that's pretty big for a small gantry mill. Most commercial designs have a small table sliding along longer slides for the X axis. One bonus is that if your slides are adequate, dual ballscrews won't be necessary.
There are also a lot of solid slide components available. These are widely used in the diemaking industry, for cam slides & whatnot.
Anchor Lamina is one major manufacturer. It would be a good idea to peruse their website & see what is available. Solid slides, similar in design to those in your milling machine, are the most rigid design available. They will take forces that would tie the rod & linear bearing setups in knots, without damage.
Having the table move for the X axis will also help increase total rigidity, for obvious reasons.
Note also that a steel cutting machine is going to end up much heavier than your previous builds. You are going to need a cherry picker, at least.

On your current design. You would get much better rigidity if you set things up so that the gantry is in between it's slides, rather than on top of them. On top leaves too long a moment arm for oscillation(chatter) to occur. That whole assembly would oscillate, swinging on the points where the lowest risers are fastened to the main frame.

More as I think of it.

As for the spindle, I don't see why those Chinese spindles shouldn't work. The main consideration here is low speed torque, because most of your machining will be at less than 1000RPM, much of it less than 500RPM. 7.5kW would be a nice starting point. A 5,000RPM top speed is more than enough. Look for something with an R-8 spindle taper, as then you can use standard milling machine cutters & cutter holders.


BTW: How much making of parts do you want to do? There really isn't anything you couldn't make with your milling machine if you wanted, depending on how much work you want to go through.


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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Jawno
Posted: April 18, 2012 03:44 pm
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I really like the keyway idea. I might try to incorporate something like that in my machine. thumbsup.gif
I prefer CAT30 to R8 but it might be hard to find CAT30 for a diy machine.
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