Good luck scraping granite flat. If it's not flat when you get it, messing around with hand tools won't make it any flatter. Cast iron is hard to scrape, but at least it's possible. Granite - not so much...
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Good luck scraping granite flat. If it's not flat when you get it, messing around with hand tools won't make it any flatter. Cast iron is hard to scrape, but at least it's possible. Granite - not so much...
It's possible if you buy the right tools.... if you can remove material and you have (lots of) time and patience, you can make it flat but.... there is little point because granite is readily available already precision flat. Cast iron is not (although it can be found).
Given the choice, and all else being equual, granite is the supperior material for this purpose. The only exception is if a person is making a dovetail slide or box ways as cast iron has a unique ability to slide.
Definitely an impressive build, fox3d!
I have several questions regarding your machine, but I'll start with the size and machineable envelope. I would really like to try to build something similar, but I wonder if I could get away with using a smaller surface plate, as not everything about a machine like this scales down very well.
- Given the z-axis' plate width, what is the maximum travel along the width of the machine?
- Given the thickness of the gantry, 2 times rail and bearing thicknesses, thickness of the two aluminium spacer plates on the z-axis, and the radius of the spindle housing, what is the maximum machineable depth?
- What is the clearance between the gantry and the surface plate?
Given the z-axis' plate width, what is the maximum travel along the width of the machine?
granite surface plate is 900mm x 600mm Z axis plate with is 200mm so the travel is 700mm
Given the thickness of the gantry, 2 times rail and bearing thicknesses, thickness of the two aluminium spacer plates on the z-axis, and the radius of the spindle housing, what is the maximum machineable depth?
i guess it will depend on the length of your endmill. Z axis travel is around 160mm
What is the clearance between the gantry and the surface plate?
around 180mm.
Thanks for the reply!
I wasn't clear enough when I wrote that question. I was thinking of the depth along the y-axis. With gantry machines like yours there is usually some area in the back of the machine that the spindle can't reach. I guess if the gantry beam is 50mm thick and you use 20mm rails?(?) with a rail+block height of 30mm with two 16mm Alu plates the "lost" y-axis depth in the back is (50+2*20+2*16)mm=122mm. And that is before the spindle is attached.
If I were to build something similar but with a smaller surface plate I probably wouldn't be able to shrink that loss very much. I guess it is possible to offset the two side slabs of granite so that they stick out/overhang some cm towards the back. Do you think that would destroy rigidity and/or create a significant cracking risk on the side slabs?
Nothing really stopping a guy from putting a spindle on each side of that gantry beam. Not only could you machine the full bed length, but you could machine the ends of longer parts too. :)
Also if a job only required two tools, you are all set.
In video #4 you described how you aligned the gantry rails parallel to the surface plate, but how did you align the rails on the side slabs? I guess it is fairly straight forward to get the two rails to match up laterally but there is no guarantee that they are straight.
I was very impressed by your idea.. so i will copy you. I got 2 big slabs 4cm thick 15mm 1.5m linear rails and 1.5m 1605 ballscrews. Also i have 4pcs servo motors 200w 2 for y 1 for x 1 for z. For dimensions 1.5m x 1.5m what thickness granite you suggest for bed side pieces and bridge? For size 15mm rails & 16mm ballscrews will be enough or must go for 20mm rails & ballscrews ? 4cm thick can be stacked with epoxy glue so 8cm and 12mm can be done. Machine for cutting granite slabs exists allready.
Something i forgot to ask you..
What bolts diameter and inserts you used to joint granite pieces together ?
Any updates on this build lately? I really like what you did and hope to do the same one day!
I am working my way towards building one:
1. Engine hoists are inexpensive for moving plates. Think ahead and allow for slings to slip and the occasional dropped item (don't be under it; jump AWAY if it starts to fall).
2. fox3d posts occasional YouTube videos on this and associated projects, recommended!
3. Planning to extend the sides and Y rails beyond the bed to regain Y travel lost to the Y sliders length.
4. Surplus granite countertop material is inexpensive (Craigslist, remainders, cutouts free in dumpsters at places that sell granite countertops). If used for the bed would cost precision (won't be very flat).
5. Hard to find economical 5cm granite countertop; 2cm and 3cm are widely available (where I live). Perhaps epoxy lamination or basing machine size on the available surface plates.
6. Wet cutting controls dust and extends blade life. Best to use a GFCI or wet saw. Diamond blades are available for skilsaws. The Evolution DISCCUT1 has a 4" cut depth; stories about injuries and deaths due to kickback so appropriate care and precautions! Planning to add water blade cooling and always use a GFCI.
7. Suggest goggles and a ($40) half-mask P95 or P100 respirator when grinding granite. Look up silicosis; sharp particles cause scarring of lung tissue. Scars don't breathe well.
Hi Gaemon - You say granite is stiffer and stronger than steel. This is not true. At best it maybe as stiff as aluminium but I can't find any references that quote it above 60GPa stiffness. commonly are at 30GPa. Steel is 200GPa. Mild steel is 250MPa yield and it would be hard to find a granite with compressive strength above 200MPa. Its tensile is very weak <25MPa. Its benefits are : its very damp, low thermal expansion, relatively dense similar to aluminium and being hard wears very well in say surface plate applications.
For machine design you need to think of it more in terms of a less stiff aluminium.
Now if you make epoxy granite you can make stuff that's stiffer then granite by using alumina grit or silicon carbide grit. Alumina is 300GPa stiffness and SiC is 400GPa stiffness. If you achieve 50% grit fraction by volume which is about their bulk density and since particles are about 50% efficient in transferring strain through a matrix vs long fibres being 100% then if you use alumina you get 300x0.5x0.5=75GPa stiffness which is stiffer than aluminium same for SiC much stiffer than aluminium., So if you make epoxy/grit parts use these grits. Normal sand is SiO and is only 70GPa so its tough to get past 20Gpa with it. Sand is worn granite so has similar elastic properties to granite... For my next project I am casting parts using steel fibres and epoxy and alumina and epoxy.....Cheers Peter S