[lukahr]

I've been back at CAD work for the past few days and started seriously considering getting a granite surface plate for my worktable. I found one in 1000x600mm with a flatness tolerance down to a few microns.

My plan would be to drill mounting holes on the top and bond steel plates to the bottom with a strong epoxy or maybe a diamond polymer compound. I could then mill these plates for mounting surfaces for rails and ballscrews.

Do you think this would be a solid approach? Or should I stick to using a cast iron T-slot plate and consider casting nanodur on top for a "sandwich" setup? Also, would an epoxy or diamond polymer bond be strong enough to hold up under load, or would it eventually give out?

With the granite plate, I'd start with a completely flat surface, which should make alignment easier since I'd have one guaranteed straight, flat reference.

[peteeng]

Hi Luka - What is "diamond polymer" ? Since the granite is flat mount directly to the granite! Any mixed media build has to be considered carefully. 1) The nanodur will not bond to the CI plate so over time they will come apart 2) epoxy bonding done correctly is a well understood and reliable process 3) the surface plate is not guaranteed flat! you have to support it correctly to get it flat. Gravity still sucks! 4) several machines in the forum start out on granite slabs. Its a great start. Peter

[joeavaerage]

Hi,
am I to understand you want to use the granite plate to 'underly' the machine base?

which should make alignment easier since I'd have one guaranteed straight, flat reference.

The problem with granite is that its not hugely stiff, about 70GPa, whereas steel is 205GPa ans cast iron is 110GPa.
The likelihood is that you'll 'pull' the granite to the bow/warp/twist of the steel or iron rather than 'pull' the steel/iron down flat to the plate unless the granite plate is really REALLY thick.
The probability of having the granite stay genuinely flat is not good.

I would suggest getting the steel/iron surface ground. There are companies around that offer service of that description. In Auckland (New Zealand) for instance there is a company that has two huge Proth surface grinders,
one being 2.7m x 1m x 0.7m capacity. Costs $140NZD/hr....bargain!

Craig

[lukahr]

The problem with granite surface plates is that they are precision-lapped only on the top side; the bottom remains rough. That’s why I would glue steel plates for rail carriage mounting and grind them flat.

I made a mistake ‘Diamant Polymer Solutions’ is actually a company that produces special epoxy resin glues, primarily used in machine building and construction as precise gap fillers. They also offer specialized, strong, steel-filled glues. (Moglice or DWH compound is from them)

Yes, with Nanodur, I would need to use negative geometry on the steel plate, like anchors or adhesive, as recommended by the experts who manufacture Nanodur. They cast steel T-slots directly into Nanodur.

No, the granite surface plate would actually serve as the milling table. The base will be cast from Nanodur, with linear rails mounted on steel inserts that are ground flat. On those rails, I would place a precision surface plate, 120–140 mm thick, to prevent warping, and it’s rated to handle up to 400 kg loads.

[lukahr]

Yes, with Nanodur, I would need to use negative geometry on the steel plate, like anchors or adhesive, as recommended by the experts who manufacture Nanodur. They cast steel T-slots directly into Nanodur.

I found a photo in the documentation they sent me. This shows how they do it. There are likely anchors with negative geometry on the backside of these inserts to secure them firmly in the concrete.
This way, it can be cast without any major machining operations beforehand, requiring only precision grinding on a large grinding machine after casting.