surface plate question (strength related)

dovidu · Jun 11, 2017

hey guys
i am currently making this granite surface plate based machine, as a concept machine & testing purposes.
i will jump right into the question.

i drilled 4 x 10mm diameter holes for hiwin hgh30ca LM guides
these rails use 4 x M8 bolts

so.. i currently have the rail bolted down with the 4 bolts and nuts(&washer).
accuracy of the rails around 2 microns (calibrated with test indicator).
side to side deflection around 7~10 microns when i put my full strength to it.
the rail it self is pretty much rock solid though.

i didn't take this into account when i designed it,
there might be a problem here because, the 4 bolts are applying quite a bit of force
i don't know exactly how much, by feel i am guessing around 200kg~400kg of force each bolt.
the granite plate is 8cm thick.

at first i didn't doubt the plate's strength, after all it's a tough piece of rock.
i was expecting tons of clamping force possible.
then i realized this thing could crack and break, and not so tough after all...

so has anybody else done this?
how much clamping force can a granite surface plate take??

in the future i am also planning to make combo lathe/mill based on 40cm X 60cm X 8cm surface plate, weighing around 80kgs
it's going to need like 20~25 holes drilled and components bolted down with hex bolt and nuts.

i would really appreciate some input. thank you in advance

Illinoyance · Jun 11, 2017

Are the bolts through bolts or did you glue in inserts? Please describe the fastening arrangement.

dovidu · Jun 11, 2017

it's through bolt with washers and nuts at bottom
on the top it's the linear rail.
so the force is better distributed on top
the washer on the bottom is around 2cm in diameter, for better distribution of force
maybe larger washer is needed

dovidu · Jun 11, 2017

i will post some pics if requested
i am new to this forum so haven't posted a pic yet

Illinoyance · Jun 11, 2017

I think glued in inserts would have less detrimental effect on the plate. I suggest you use a softer material like nylon between the washers and the granite.

Erich · Jun 11, 2017

Looks like compressive strength of granite is around 19,000 psi.

Multiply that by the area under your washer and you have an idea of the clamp loads it can take.
Use thick hardened steel washers so they don't bend and edge load on the id.

dovidu · Jun 11, 2017

excellent insight!
i examined the washers and they were bent into the hole
also the drilling is not perfect, especially at the end, so bit of chipping is inevitable
all my 4 holes have some chips at the end
one of the hole has severe chipping (the first try)
i either have to make solid washer 25mm diameter or greater 5~10mm thick washer or buy it
i think making one would be better, i will try 12L14
or to protect the plate, use the softer AL 6061 T6 30mm diameter

i goona work on the calculations right now

dovidu · Jun 11, 2017

if 19000 psi figure for the granite is correct.. then to my surprise
it is 1335.8322 kg/cm2
darn!!
that's... pretty strong isn't it?

one thing not sure is, that 19000 psi figure varies with thickness?

ignoring thickness of the granite plate, just the 19000 psi figure alone,
assuming my washer has surface area of 4.12cm2 (2.5cm diameter 7~10mm thick)
that's 5503.628664 kg or 5 1/2 tons... is this for real?
that's way greater than required for a single bolt

even if i under-engineer it and used 500kg force for each bolt, two LM guides totaling 12 holes equate 6 tons of clamping force.
which, from my understanding, is similar to 6 tons of mass excluding the coefficient of friction between components.

that's gonna be pretty hard to budge.. unless you take a hammer and smash it

seems like this may be feasible idea after all?

also, with all that force being applied, might have to take deflection of the granite itself into account

Milland · Jun 11, 2017

Don't use nylon, or any plastic - too much risk of cold flow deformation and loss of clamping pressure.

If you're going to make heavy washers, design them with a relieved (counterbored) face that mates to the granite, with a full flat side against the bolt head. Like a "[" with the right side against the stone. You could use a 25mm OD, 6-8mm thick, with a relieved diameter around 20mm about 0.3mm deep.

This will give rigid clamping without any edge loading around the hole in the granite. More resistant to stone cracking than a flat washer, no risk of tension loss as from using nylon or other plastic padding.

dovidu · Jun 11, 2017

thank you miland

you are pretty awesome haha

now that i think about this,
nylon between washer and the bottom of the plate may not be a good idea.
it will bend into the hole, just like the metal washer.
even aluminum will bend under 300~500kg force.

the best solution so far is to use as wide as possible and thick as possible washer
custom made with solid steel, counterbored, scraped flat or ground(doesn't take that long small piece), then heat treated to full hardness to minimize deflection.

one question though, don't we want larger surface area than the "ring" like support you've described here?
is it because it's gonna deflect anyways?

TDegenhart · Jun 11, 2017

You may want to consider a thin copper washer between the steel washer and the granite. It will cold flow a small amount to distribute the forces. Also remember that granite is a brittle material and local high stresses could create cracks.

Tom

dovidu · Jun 11, 2017

brilliant idea!
didn't take into account cold flow.
actually the steel washer itself may cold flow as well
so using copper washer to prevent that may make significant difference in the long run

dovidu · Jun 11, 2017

dovidu · Jun 11, 2017

i am sure all you guys get the picture

but i got you the real pictures , just figured how to post image

first pic the bolt being used, it's M8

second you can see the bent washers.. lol

third the nut being used

fourth hiwin hgh30ca pretty strong stuff

lastly the under side of the plate itself.

dovidu · Jun 11, 2017

thank you guys for all the input

i really did help me a lot, and i'm certain this would benefits others considering this.

i've reconsidered the whole washer design and decided:

the real objective here is to evenly distribute the clamp force as much as possible

the rail side, there's no problem here.

the problem is the bottom side where washer and nut is used that creates the problem

the design i concluded is to use hardened counterbored washer with considereable overengineered thickness and width exceeding the clamp force of the bolt

then since the granite is so brittle, softer material usage is inevitable as the granite will be chipped to irreparable point.

under the hardned washer a softer material such as AL or Copper will be used to transmit forces as even as possible.

since AL is softer than copper it may be a better candidate.

under pressure AL will deform and create perfect flat surface that will contact the bottom side of the plate.

even if it's not machined or ground perfectly, the pressure will "remachine" the washer

the small ridges and bumps will be replicated on the face of the washer itself, creating perfect mating surface.

well not sure how this is gonna play out, gonna give it a go in a day or two

Milland · Jun 12, 2017

dovidu said:
one question though, don't we want larger surface area than the "ring" like support you've described here?
is it because it's gonna deflect anyways?

Don't worry about using any other material than generic steel for the washer, the underside of the plate looks smooth enough than the minor discontinuities present will embed in soft steel, and as all steels are roughly the same stiffness regardless or hardness, you don't need to (and shouldn't) use hardened washers.

Stiffness comes from thickness (why surface plates have to be a minimum thickness to support a given load without deflecting more than specified), so an 8mm thick steel washer will have more than enough stiffness to handle the clamping loads you're putting on it. You can go larger on the OD, but I doubt you'll need much over 25mm to adequately spread the load to the granite away from the through hole.

TDegenhart · Jun 12, 2017

The asperities will not embed in the steel, rather they will be abraded away by small movements of the bolt/washer. It's a case of tighten, use, then re-tighten. If you want to use aluminum, make sure to use dead soft as in alloy 1100 or annealed 6061 or similar. Even with the soft washer you may need to re-tighten after use.

Tom

magneticanomaly · Jun 12, 2017

19,000PSI may be the compressive (crushing) strength of some particular variety of granite, but granite is a weak material. This source http://www.matweb.com/search/datasheet.aspx?matguid=3d4056a86e79481cb6a80c89caae1d90&ckck=1 gives a range of 1020 to 3600 PSI as tensile strength, less than 5% of soft steel. Allowable loads with safety factor will be half to a tenth of these. Also to consider is the stiffness or elastic modulus, given by various sources as 1 - 7 x 10^6 PSI, at most 1/4 that of steel. Stiffness may be more important than strength for a precision machine.

What are you building?

DavidScott · Jun 12, 2017

Drill half way through both sides meeting in the middle to avoid blowing out the hole as you exit. This will also allow you to use a shorter drill bit. When the drill starts to get dull it will really blow out the hole when exiting. Specs for "granite" are about as good a specs for steel, lots of variation depending on what flavor you have in hand.

DMF_TomB · Jun 12, 2017

dovidu said:
hey guys
i am currently making this granite surface plate based machine, as a concept machine & testing purposes.
i will jump right into the question.

i drilled 4 x 10mm diameter holes for hiwin hgh30ca LM guides
these rails use 4 x M8 bolts

so.. i currently have the rail bolted down with the 4 bolts and nuts(&washer).
accuracy of the rails around 2 microns (calibrated with test indicator).
side to side deflection around 7~10 microns when i put my full strength to it.
the rail it self is pretty much rock solid though.

i didn't take this into account when i designed it,
there might be a problem here because, the 4 bolts are applying quite a bit of force
i don't know exactly how much, by feel i am guessing around 200kg~400kg of force each bolt.
the granite plate is 8cm thick.

at first i didn't doubt the plate's strength, after all it's a tough piece of rock.
i was expecting tons of clamping force possible.
then i realized this thing could crack and break, and not so tough after all...

so has anybody else done this?
how much clamping force can a granite surface plate take??

in the future i am also planning to make combo lathe/mill based on 40cm X 60cm X 8cm surface plate, weighing around 80kgs
it's going to need like 20~25 holes drilled and components bolted down with hex bolt and nuts.

i would really appreciate some input. thank you in advance

.
.
when thick granite there is often a temperature difference and length difference. even if expansion rate of 2 materials is the same with increased temperature i have seen linear rail grow or get longer and bend cause its bolted on ends and extra length got to go somewhere. if you got many bolts rail can go S shaped
.
really thick materials can take a hour a inch to absorb temperature changes. so thick granite can take hours to catch up to size changes from temperature changes cause steel rail is thinner than the granite. temperature can change from 6am to 6pm quite a bit unless shop is temperature controlled

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