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Granite base CNC router project

Bandit said:
Well it appears I made a "grave" mistake the other day when I was at the local picking transfer site (garbage dump), there were some head stones laying in the concrete junk pile. Replacements it appears as some corners were broke off, some edges chipped, etc. No, none had my name on them!!!
The thought did cross my mind,(didn't take long, small area), maybe a good simple close enough for some jobs surface plate. Alas, I did not even take one. Next time maybe.
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The biggest problem is the availability of a lifting device.

The second biggest problem is hoping its still there for next trip.
 
A buddy of mine works/owns a local monument shop. People upgrade monuments , they get damaged, etc. They deface the old granite and send them to the landfill. I was able to get a few damaged ones just for the asking, before they got to the landfill.
 
Shipping costs have been prohibitive. They come from Quebec. There apparently is a place in Sask that is now buying some of them. Some are too thin to take another slice from. So in the end it boils down to money.
 
There are a few shops in the GTA that cut and install granite and marble counter tops, they would be the first places I would go if looking for a usable flatish surface. They create lots of off cuts and pieces that may be chipped.
 
skippyelwell said:
There are a few shops in the GTA that cut and install granite and marble counter tops, they would be the first places I would go if looking for a usable flatish surface. They create lots of off cuts and pieces that may be chipped.
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I got my slab for reconditioning hand plane soles and blades from just one such shop. They aint throwing nothing to you unless you pay nowadays
 
So trying to figure out how to secure threaded inserts to a granite surface plate in precise locations, without causing any damage to the granite surface.

At the same time I am figuring out the least expensive yet rigid and strong internal structure to cast the epoxy granite around.

I think I have it figured out.

Five steel trusses made from sections of hex spaced 96mm apart, and 1/4" steel bar, and joined together by perpendicular steel bars to form 5 cross trusses whose centerline is 96mm apart.

A single long truss will look like this when assembled:

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But the trusses will be assembled into the mould from precisely aligned and secured subassemblies during the epoxy granite pour so that the upper portion of the structure does not interfere with the epoxy pour.

The lower subassembly (that will form the upper surface of the CNC router bed) will start with hex segments that are machined in the lathe 1 1/2" long, with 3/4" long stub threaded 1/4"-20 threaded on one end and a 1/2" deep hole on the other end threaded 1/4"-20. These will act as the threaded insert for fixturing onto the cnc router.

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The threaded hole of the insert will be protected with an acetal washer and flat head cap screw that have been dipped in wax. This will also ensure that the threaded insert sits slightly below the work surface.


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On top of the threaded inserts will be the long axis lower truss flange and on top of alternating inserts will be the lower flanges of the cross trusses. Then hex sections of two different lengths will be screwed onto the threaded inserts to securely fasten the flanges together.

1732245809884.png


The flanges will of course have 9/32" drilled 96mm O.C. on the milling machine. I'll likely shorten the long flanges a smidge.

The lower subassembly will be assembled on my workbench using the 96mm dog hole array. I'll turn 4 special dogs to screw to the four exterior most threaded inserts onto and then fasten the dogs to the work surface. This way my grid will be held in precise alignment as I assemble it and torque the middle hex web sections in place.

The subassembly will then be placed into the mould and positioned with a jig attached to the mould walls. The jig will have six plungers that align with the 4 outside threaded inserts and the two outside threaded inserts at the subassembly midline. The plungers will push the assembly into the surface plate to keep the inserts tight to the surface place as well as securely in position.

Steel plates will be bolted to the mould perimeter walls with bolts that will extend into the mould interior. When the epoxy is poured the bolts will act as tiebacks to securely anchor the steel to the finished plate even under heavy loads.
 
The first 3/4" of the mould will be filled with a sand and epoxy mix that will flow under the lower truss flanges. Then a sand, gravel and larger stone mix will be used to fill most of the mould interior.

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Then the upper flanges of the trusses will be bolted in place and the balance of the mould filled with sand, gravel and epoxy mix
 
drewnabobber said:
I have found Turtle wax from Canadian Tire to work incredibly well as mold release with both epoxy and UHPC concrete. I had no issues casting directly off a granite straight edge.
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Yeah I have seen a couple examples where the person cast right on granite.

I have two smaller plates, so to test mould releases I plan to cast a 1" thick sample on my 12x18 plate. If that works then I will risk my 24x36 plate. I used to work with moulds all day long, and my granite is so smooth I am confident I can add a few layers of release wax that will allow clean demoulding, and that will come off completely with a solvent afterwards.

So really my greatest challenge is to ensure my inserts are pressed sufficiently into the granite surface that will be my mould bottom, at the precise locations I desire.

I wish I had a nice 3/4" thick piece of lexan to use as a mould top so I could depressurize the mould after everything is in place so I could vibrate and depressurize both to remove air bubbles.
 
I would not trust any release agent directly on your granite. If it sticks, you are in a world of hurt. As mentioned, I would get a film to act as an impermeable barrier. Release agent on the film..
Its always wise to do a small sample dry run, maybe some 6x6 tooling blocks that use the hardware & can serve some other purpose. I will bet you will learn some things that may not be occurring to us now ahead of the big pour day.

I see what you are doing with the lattice frame & the studs. Maybe you have thought this through but I see a lot of underside nooks & crannies that the mix may not flow & envelope. It will flow if its resin rich but you want as much inert material as possible which would be much higher equivalent viscosity, more than cement. The part I'm scratching my head is how to apply vibration to the frame without potentially shifting it off its exact stud locations. Vibrating the granite is going to absorb a lot of that energy.

Staged pours have their pro's & cons. By staging you get control over coverage & depth & consistency & exotherm & working time. But subsequent pours also have to occur at just the right time for bonding & other issues.
 
PeterT said:
I would not trust any release agent directly on your granite. If it sticks, you are in a world of hurt. As mentioned, I would get a film to act as an impermeable barrier. Release agent on the film..
Its always wise to do a small sample dry run, maybe some 6x6 tooling blocks that use the hardware & can serve some other purpose. I will bet you will learn some things that may not be occurring to us now ahead of the big pour day.

I see what you are doing with the lattice frame & the studs. Maybe you have thought this through but I see a lot of underside nooks & crannies that the mix may not flow & envelope. It will flow if its resin rich but you want as much inert material as possible which would be much higher equivalent viscosity, more than cement. The part I'm scratching my head is how to apply vibration to the frame without potentially shifting it off its exact stud locations. Vibrating the granite is going to absorb a lot of that energy.

Staged pours have their pro's & cons. By staging you get control over coverage & depth & consistency & exotherm & working time. But subsequent pours also have to occur at just the right time for bonding & other issues.
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I hear you, I'm listening and will heed most of your advice.

I bought a cheapy 12x18 slab to test theory by making replica. That will use wax release directly on the plate just to see. If any issues, it will be the film for sure.

The studs that will sit directly on the slab are currently 3/4" so the closest the structural elements will be to the slab will be 3/4". The first epoxy batch will be epoxy sand and 1/4" black gravel. I will make it less viscose than the remainder of the slab. With the long cure time I'll not let the mix set up before pouring the remainder of the epoxy.

My current thinking is to place the mould and slab on my work bench. The work bench will flex and vibrate under the combined weight and with the pounding of a rotary hammer. Then a separate concrete vibrator head right in the epoxy during the pour for first few minutes., then seal top and apply deep vacuum to the whole thing. I may instead put the whole thing in a vacuum bag if I cant find 3/4" plexi top cheap

edit:

I plan on making a jig that presses the hex studs firmly in place. But in hindsight maybe I should not drill the hex that will be the top of the slab when complete. That way I can drill using CNC post production and get exact placement. Then I dont need to pray for exacting tolerances during production
 
Something I've done on molds where I anticipate high adhesion is install kind of a jack screw fitting. Hopefully the sketch makes sense. Once the mold is cured you just tighten each set screw progressively by feel. It typically goes crack when release agent has yielded, maybe a thou or less. A downside in this project is the jack screw bolt end will be impinging on your nice granite in a localized area. You could use a nylon tipped set screw but I'm saying the stress could be pretty high depending on the weight of the pour+frame. But maybe worth a try on your test coupon.

1732685003118.png
 
I ran some rough back of the envelope calculations from which I concluded that it will be less expensive to purchase another 24 x 36" surface plate upon which to build the cnc router than to make an epoxy granite slab with all it's potential technical issues that required solving.

I am not in any hurry, we have surface plates come up for sale at affordable prices very often in the GTA, and I can even likely have a imperfect slab resurfaced to a grade B tolerance which will be more than adequate for this design.

What I desire most is stable, very rigid and very low vibration operation.

So the hunt is on for a used surface plate.


Or KBC has grade B surface plates new for $560 plus tax for a 24 x 36 x 4. That may just be the ticket.
 
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