Kb58's granite CNC router build

[kb58]

I've been away from this forum for a few years. In that time, I built a CNC router but completely forgot that there's a subforum for such things here. As a result, the thread exists elsewhere, so over the coming days, I'll transfer it over here, where it'll be more at home. The original posts will be move here in order and without change, so what I say early-on may have changed later. The project started in mid-2022, with this first post:

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As a "blue sky" project I've been thinking about this for a few years. Until recently I'd all but decided to go with a laser because it's so much cleaner and quieter compared to a router. One issue with a laser is that the length of the tube is fairly proportional to how much power you get. Knowing me, I'm not going to build something small and want something that can cut 12mm plywood. That necessitates either multiple passes, or higher power. Getting a 150-200W tube... they're roughly 1.5-2 METERS long! That's simply out of the question in a one-half a garage setup. There are fiber lasers (that have the nice feature of not needing a series of mirrors), but they're even more expensive, and emit a different frequency of light. Good for cutting metal, not so good for cutting wood. I'm not kidding myself into thinking I'm going to start cutting sheet metal, so it sort of rules those out. With both laser types somewhat off the table, I reluctantly starting thinking about a router again.

They'll do what I want, cutting wood, and if rigid enough, cutting aluminum. As mentioned, they're loud and messy, which kind of sucks. In addition to the noise is all the fine sawdust, so another noisy vacuum is needed, and I doubt that'll get it all. Being in a garage connected with the house brings with it certain consequences. Then there's zooming out and asking the hard questions: What am I going to do with this that I actually "need" one? I don't like that question because I don't have a ready-made answer all queued up. Yes, there's wood gear clocks I keep mentioning, but another hard question is "well how many of those do you plan to make?" And: "If you're only going to make one or two, just send out the files and let a pro shop do it." So, yeah, there's that.

 

[kb58]

Okay, for fun, here are some arbitrary requirements:

• 91mm x 61mm (36" x 24") workspace
• Ability to cut aluminum without chatter and vibration
• Use a spindle cutter, not a router, for versatility

Building one would mean doing so in reverse, based on what's found to use as the side rails, gantry, and gantry pillars, as they need to be fairly robust to prevent vibration. Due to the mass, these bits need to be fairly large, which typically means expensive, so surplus material could be adapted. I'm not necessarily sold on using aluminum beams, as its stiffness/weight is no different than steel, but it is easier to machine, but harder to weld (granted, there would probably be no welding to avoid heat distortion). As of now, I don't know how stiff is stiff enough, and it's easy to "go big", and if large and cheap can be found, that's a win. Still just thinking about it for now.

 

[kb58]

I like this fixed gantry design, though the question becomes: once hold-down plates and/or a waste layer is added, which weighs more? In other words, for the same size work area, is a fixed-gantry better than a traditional moving head design (where "better" means stiffer, more accurate, and repeatable).

https://www.wadeodesign.com/design-details.html

 

[kb58]

Watching many videos, I get the impression that there is a long learning curve getting a CNC router to produce clean cuts. For production quantities, it's normal (I imagine) to spend a lot of time testing settings on a sample piece before telling the machine to make 100 identical units. BUT, if you need only one of something, I can see how getting a nice piece on a CNC router can waste more time and material as the proper settings are figured out (feed rate, spindle speed, depth of cut, etc). It may even take more time than making the same part on a manual mill. That said, I realize that this comparison only works with one-offs, also assumes that all the settings aren't available from previous runs, and probably most importantly, only applies when no curved cuts are needed. It's not keeping me from building one, but does make me realize the reality that from hand-drawn sketch to a piece-in-hand is not guaranteed faster - depending what it is. Repeatable, yes, faster... meh.

 

[kb58]

I seem to have won a 36x24 granite surface plate (to serve as the baseplate). I made a low-ball offer to offset fuel cost for the 200-mile round trip to pick it up (being 400 lbs, it's not realistic to have it shipped), and since such things cost a fortune to ship, no one else bid on it.

Getting it out of my truck into the garage at my end, yeah... that'll be a problem for Future Me.

I looked into doing a cement/rock/sand sort of thing for a router base but while cheaper, it ends up needing to be leveled and flattened. That can be done via aluminum inserts and surfacing it on a large mill, but then there's "how do I get this 600-lb thing to the mill?" Buying a used surface plate avoids that, as it's already flat and can be drilled as it sits.

 

[kb58]

Sunday morning I drove up to get the surface plate. Traffic was wonderfully light, the 107 mile trip made in under two hours.

The surface plate turned out to be better than presented. The ad had only one distant photo with no description, so I was taking a chance on it. It turned out to be a "Grade A" surface plate, the calibration sticker showing a flatness as 0.0002", way better than I'll achieve with any other part of the assembly. It was from a machine shop that's being sold off, the owner having recently passed away. The son helped load the stand and plate into my truck, and I talked with him about the Burbank airport area. It's a fabricator's heaven, endless shops supplying just about any mechanical parts or services that you'd want, in addition to many surplus metal yards. There are many businesses in the area associated with aviation, race cars, and the movie industry, so it's not surprising that all the eBay ads for mechanical/machinery all seem to center around this area.

Literally right down the block is Jay Leno's shop, so I couldn't help but snap a photo of his rather unassuming storefront. The son said, "Oh yeah, I see him about once a month at local restaurants. He's got some really unusual-sounding cars and we can always hear him coming down the street." In nearly all of Leno's videos, it shows him driving right past this machine shop, the irony of that only sinking in when I was there. I admit to still being disappointed that Leno's organization never responded to my offer to drive Midlana. Even a "no thank you" would have been sufficient, but that's a rant addressed elsewhere. Traffic on the way home was initially light enough that I caught myself thinking, "I could have driven Midlana to Leno's shop", then traffic ground to a halt for a very long time, a reminder of one of the reasons I sold it...

Getting the surface plate out of the truck... ahaha, yeah, it wouldn't budge. I thought that maybe the truck bed and stand would be close enough in height that I could slide out the 400+ pound lump straight onto the stand. Nope, too heavy, and the stand was about 8" higher than the truck bed, so that was a no-go as well. Due to the weight, our driveway being sloped, and the stand being on wheels, the whole affair was deemed too sketchy - dropping 400 pounds could do a lot of damage to the surface plate, me, or the driveway, never mind having no way to pick it up again. I borrowed an engine hoist from my brother and, like when a 400-lb engine is hanging from the hoist, it was a little nerve-racking, but all is well and it's now safely on the stand.

 

[WobblyHand]

Okay, for fun, here are some arbitrary requirements:

• 91mm x 61mm (36" x 24") workspace
• Ability to cut aluminum without chatter and vibration
• Use a spindle cutter, not a router, for versatility

Building one would mean doing so in reverse, based on what's found to use as the side rails, gantry, and gantry pillars, as they need to be fairly robust to prevent vibration. Due to the mass, these bits need to be fairly large, which typically means expensive, so surplus material could be adapted. I'm not necessarily sold on using aluminum beams, as its stiffness/weight is no different than steel, but it is easier to machine, but harder to weld (granted, there would probably be no welding to avoid heat distortion). As of now, I don't know how stiff is stiff enough, and it's easy to "go big", and if large and cheap can be found, that's a win. Still just thinking about it for now.

If you can, you might want to edit those metric dimensions, they are off by a factor of 10.

 

[kb58]

Stopped by a local granite fabrication shop with grinding and cutting capability. The hope was to get a couple small pieces cheap, then have them cut to size. The two pieces would be glued and bolted to the sides of the surface plate, serving as raised platforms for the side rails. That was the plan...

When someone chooses a granite slab for, say, a kitchen remodel, they buy the entire slab regardless how much is used, so there are always leftovers. Pretty much everyone leaves those at the granite shop, and the shop just puts the smaller pieces back up for sale! It turns out that standard granite slab thickness is 3/4" (19mm). I was hoping for something thicker, 1-2" (25-50mm). Out of maybe 100 partial slabs, they had exactly one that was 2" thick ("$350"), and another $150 for cutting. Looking around, I strongly suspect what they tried selling me was "bluestone", not granite... and not cool. Regardless, since I'd only use 1/3 of it (them no doubt selling the leftovers - again), $500 was deemed too expensive.

Alternatives:

• Bite the bullet and do the above anyway

I have to decide whether I'm okay with a router weighing nearly 1000 lbs - probably. Also, there's the issue of ensuring how to machine the tops of the side plates so that they're both flat and level.

• Substitute metal for the granite side plates

3/4" steel is $180, aluminum, probably twice as much (getting back up to the price of the stone). Steel is cheaper, aluminum is easier to work with, and both are easier to machine than granite. Aluminum has a much larger coefficient of thermal expansion than granite, and I'm unsure whether that difference could lead to the glued connections possibly cracking.

• Switch to a fixed-gantry design, negating the need for long side plates.

Doing so simplifies things somewhat, but increases the footprint of the router because the bed moves beyond the base at each end.

There's also deciding which edges to run the side plates alone, the short 24" side or the long 36" side - there are advantages to both. The traditional way is running them on the long sides of the base, so the gantry is 24" wide. Because of where the router will be positioned in the garage, however, it makes accessing the bed slightly less convenient. Placing the side plates on the 24" sides means using less material (cheaper and lighter), makes access easier, but does mean that the gantry becomes 36" long, increasing the potential for more flexing, and increases pricing for the longer bearings and ball screws.

A parallel decision is to use steppers or servos. There are advantages to both: steppers being cheap, though closed-loop steppers, less so. They're somewhat noisy. Servos are all around better if price is ignored, but it's not an insignificant amount, on the order of 3-5X as much. I really appreciate quiet machinery, but the selling point of servos being quiet doesn't matter much if the sound of the cutter and dust collector are 10X as loud.

Then there's the spindle. I'll "probably" go with the popular import 2.2KW 220V water-cooled spindle that so many use, as it's hard to ignore the low price of the kits that include a VFD. "Real" spindles are so expensive ($$$$) that I just can't go there.

 

[kb58]

The search for 2" (5cm) thick granite continues. Two retail items that could be used are "edge coping" and "step treads." A third source may be gravestone suppliers... no kidding. Under "edge coping", I found a promising source close enough to warrant driving (again) to get two 12" x 24" pieces: https://www.wallandtile.com/gray-mist-f ... ping.html# These would work for the sides, leaving the one-meter gantry as more of a challenge because I'm in the wrong area of the country; there are many granite suppliers on the East Coast, but not out here, and shipping is prohibitive.

Another approach is to fabricate the gantry using ordinary 3/4" (19mm) countertop granite, maybe with an epoxy honeycomb core between two pieces, forming a composite structure about 2.1" (53mm) thick. This provides a number of advantages: availability, less weight, lower cost, and increased stiffness. Fabricating it is made much easier since I just happened to have a granite surface plate that it could be built up on!

Or, give up on a granite gantry and go with a large rectangular section of steel tubing? I have a mill for squaring it up, and a compromise might be to partially fill the interior with cement for harmonic suppression.

I realize I'm heading down my usual path of overdoing things. Part of the reason is the goal of once completed, it works well enough that I don't feel compelled to spend more money on it, replacing "just okay" parts with with good ones because I cut corners the first time. That said, even the linked video above shows the builder completely redoing the guides and ball screws later on. Some (many?) YT builders use the machines to either manufacture products, or generate YT revenue directly. There are a number of granite CNC router videos already and I don't see much point in adding another.

 

[kb58]

Visited several local stone yards and they recommended talking to stone fabrication shops. I first asked if they had any 2" thick granite - nope. Could they get it - nope. Did they have granite remnants - yes. Talked for a bit more and agreed that they'd cut three 40.5" x 7.5" (1030mm x 190mm) pieces of 3/4" (19mm) granite to size, and glue them together. That adds up to 2.25", which is slightly thicker than the granite design linked to above, but no two designs are ever the same, and having a bit more mass isn't a bad thing.