Hi Goeman - To discuss your comment a little bit more:
Obviously machine designers need to understand if their machine is going to be stiff enough for the intended purpose.
Yes very much so
I'd just have zero confidence in using any general epoxy granite data or any specific epoxy granite suppliers results.
Zero is a bit harsh but it is good to be sceptical
One of the (many) things that makes resin different from metals is that stiffness is massively dependent on how it's cured and the conditions it will be used in.
Actually stiffness is partially dependent on cure but strength is very dependent on cure If you select a resin that can achieve a "crisp" cure at ambient then that's all you really need. The aggregate being the dominant stiff additive does most of the work
In general, the optimum stated stiffness and hardness is only achieved with high temp curing that follows a specific schedule. You then add a lot more variables when you use resin as part of a composite.
Optimum stiffness of the epoxy can be achieved at ambient if you pick one that does not need special post cure. By optimum I mean it is suitable for CNC machine parts. Having said that yes epoxy does benefit from post cure if you can do it
Re -epoxy curing. Heres the data sheet for an epoxy being used by one the forum members. It states it fully cures at 15deg C.
Trying to figure out how epoxy granite compares to steel or aluminum in a Young's modulus table would not be massively useful for a machine designer IMO. Epoxy granite is an entirely different material requiring a different design.
Could you propose how to design then if it requires a different approach?
For one thing, epoxy granite will not bend much before if breaks (unless you mess up the curing). If I was reproducing my Epoxy granite machine base to sell, the only data I'd have any confidence in is from stress testing my specific design.
All the companies making EG and UHPC machines regularly test their materials for strength and stiffness and use this information for machine design. Testing is done with representative samples that allow accurate stiffness results.
Testing how much force could be exerted on the center of the gantry before if snaps would be far more useful.
Breaking the gantry would not give anyone any useful information about the machine except perhaps how many elephants it may hold up .
Testing a small piece would not be that useful as it's performance is not linear. As it's mixed with stones you couldn't simply extrapolate that a 10" thick slab would be five times a stiff as 2" thick piece.
Testing the right size piece is critical to get accurate modulus results small is a relative statement and we have to presume the people doing these tests know what size is required for the purpose. For instance I do calculations on very large composite yachts, say 20-50m long or more and we design on coupons 25mm wide by 120mm long which are not very big compared to the yacht...
In the event that a design isn't strong and stiff enough, there are more options than simply increasing thickness with epoxy granite. It can be easily reinforced with steel tubes, for example, like they do with concrete.
Using steel in concrete is to control cracks and to improve the concretes tensile strength. Both are not needed in a CNC machine. Plus you introduce stiff loadpaths to the machine that may be counter productive
To conclude - The stiffness of any material used is needed to design a machine. The stiffness can be tested for and if the recipe is consistent and the casting is consistent then the resultant modulus is consistent. The rules or domain of static linear analysis apply and the strains in most stiff cnc parts is very small and nowhere near the failure or yield point of the material. If there is another approach to machine design, I and others would like to know about that so please share
Peter
attached is a dat6a sheet from an epoxy being used by one of the members. It states it fully cures at 15degC but it does recommend a post cure schedule of 40C for 15h. If you need a high Tg or HDT, which machines don't unless they are operating on ovens then it needs to be high temperature cured....