[nicad]

I have no idea if this would work well or not. I am looking at using a "Granite Inspection Plate" as the base of a desktop mill. From what I understand, they are typically flat to 0.0005"(for the lowest/cheapest Grade "B" plates") and resist thermal expansion, rust, etc.. And they are cheap (~$100).

I have some concerns tho-
#1) How to mount to it? If the plate is 3 or 4 inches thick, I do not think there are drills to drill granite that deep. It would be possible to drill only 1" deep or so and insert threaded inserts that lock into place.. but any granite drill bits are expensive and a PITA to use from what I hear (I would need about 30 mounting holes in it).
Another option would be to bond (yes- GLUE.. Epoxy!) a plate of Mic6 aluminum plate (24" x 8" x 1") to the granite surface and drill/tap/mount my rails, motors, screws, etc to that aluminum plate. Again, no idea if this would work well or not. Anyone have any experience with bonding aluminum to granite? Needs to be a permanent bond and resist things like coolant, some heat, and forces in every direction. I would imagine a product like LocTite Hysol 9430 doing the job.

Concern #2) How well the granite would absorb vibrations? I have only run small desktop mills that are made of aluminum, and it squeals+howls when doing any amount of "work". I know Iron is the "best" material to construct for a mill due to its absorption characteristics, but getting any "ready-to-use" Iron is not easy or cheap. Granite is dense, but so is steel and steel does not absorb very well. I don't know...

Lastly, not having to do with granite directly, is the use of one of those "Right angle Irons" as the Z axis column. I have seen them in the 8"x10"x16" size range, ground to 0.0005" square over 6". Again, somewhat cheap ($200) for a decent looking chunk of somewhat square cast Iron.

I plan to use ball linear ways (already have them) ballscrews (don't have them yet) and 300oz/in servos (already have them) in a configuration something like this (image from minitechCNC): http://www.minitechcnc.com/VMTT1.jpg

Any input? Am I crazy?
Thanks

[wizard]

I have no idea if this would work well or not. I am looking at using a "Granite Inspection Plate" as the base of a desktop mill. From what I understand, they are typically flat to 0.0005"(for the lowest/cheapest Grade "B" plates") and resist thermal expansion, rust, etc.. And they are cheap (~$100).

Not at all a bad idea. This concept comes up from time to time. You should know that some of the cheaper plates are synthetic.

By synthetic we are talking about plates that are cast out of a granite/quartz and epoxy mixture. The ingredients are in part a geuss, but something you should be aware of as you might get something that is not a real stone.

By the way that is not a bad thing either. Many a machine base is cast out of the same materials.

I have some concerns tho-
#1) How to mount to it? If the plate is 3 or 4 inches thick, I do not think there are drills to drill granite that deep. It would be possible to drill only 1" deep or so and insert threaded inserts that lock into place..

In the synthetic machine bases the inserts are cast in place. Epoxying inserts in palce is doable. As you note the drilling is a pain, best to place the inserts and then drill the precision holes after the inserts are in place.

As to the epoxy the construction industry uses special adhesives to set bolts in concrete. This stuff may be worth looking into.

but any granite drill bits are expensive and a PITA to use from what I hear (I would need about 30 mounting holes in it).

As state above go for prcision after the inserts are in place. If you are doing linear rails yes that is a lot of holes. Another option is to bolt conventional ways to the surface.

Another option would be to bond (yes- GLUE.. Epoxy!) a plate of Mic6 aluminum plate (24" x 8" x 1") to the granite surface and drill/tap/mount my rails, motors, screws, etc to that aluminum plate. Again, no idea if this would work well or not. Anyone have any experience with bonding aluminum to granite?

No experience but I'd have to caution agianst it. An Aluminum plate of any length will expand signifcantly and if there is a big differential in expansion stress the glue line signifcantly. The other issue is that you blow the precision of the plate out of the water with the glue.

Needs to be a permanent bond and resist things like coolant, some heat, and forces in every direction. I would imagine a product like LocTite Hysol 9430 doing the job.

Nevr heard of it.

Concern #2) How well the granite would absorb vibrations? I have only run small desktop mills that are made of aluminum, and it squeals+howls when doing any amount of "work". I know Iron is the "best" material to construct for a mill due to its absorption characteristics, but getting any "ready-to-use" Iron is not easy or cheap. Granite is dense, but so is steel and steel does not absorb very well. I don't know...

There are a number of reasons why granite is used on so many tools, but there are better solutions for damping vibrations. Many a precision lathe or grinder has a base made of cast "granite". That is the composite materail alluded to above.

Lastly, not having to do with granite directly, is the use of one of those "Right angle Irons" as the Z axis column. I have seen them in the 8"x10"x16" size range, ground to 0.0005" square over 6". Again, somewhat cheap ($200) for a decent looking chunk of somewhat square cast Iron.

I suppose they aren't that bad if what you are looking for can be accomplished with the angle. You will be limited in height and Z-axis motion with such a plate. Futher such a plate would be rather flimsey for some usages. Probably would work well for a PC board router or similar limited horse power router. Either way I suspect that you would need a serious shim under the plate.

Try Carr-Lane or one of the other tooling supply houses for other ideas. Tombstones might be a better choice.

I plan to use ball linear ways (already have them) ballscrews (don't have them yet) and 300oz/in servos (already have them) in a configuration something like this (image from minitechCNC): http://www.minitechcnc.com/VMTT1.jpg

Any input? Am I crazy?
Thanks

Ar you crazy? How do I know!

I think you have some good ideas here. But you really need to put some thought into how all of this is going to go together. I'd suggest full size cut outs of the parts that you intend to use. If you are like me you are on a budget that is tight, the stuff you do purchase you want to get right and not drain the funds.

Also it might help if you could narrow down what type of milling are we talking about here. If you are doing a vertical mill tell us, because if we are thinking something different things might become confused.

Dave

[2muchstuff]

Drilling holes in granite takes a little longer than concrete but it can be done. Use a good (real) hammer drill not one of those combo hammer/drill units. The use of a good quality bit is also essential.

After drilling your holes, epoxy your inserts in place or some studs. The epoxy's they have out today are quite remarkable and are chemical and coolant resistant.

[miljnor]

Allot of the reasons people use other materials than steel, are false assumptions, based off of either a false conclusion from previous data, or someone else’s false assumption.


The material composition of steel to cast iron is very similar, and, so are most of the characteristics. Any kind of dampening capabilities that might or might not be attributed to cast iron, are probably do to structure, and not composition (i.e. air pockets, shape, impurities ect...) The only deficit you really cited with steel is, its absorption properties. These can be dealt with after construction, by carefully placing crossties, or adding some sort of absorptive material, like sand.

And steel is by far one of the easiest/cheapest materials to work with. It welds, machines, and is strong and long-lived.


PS: I have a 600.00 hammer drill and it is still a Hammer/drill unit. JFIP

[debogus]

Granite is best drilled with a water cooled diamond bit. (pricey)
Carbide can be used sure, sharp new bit lots of presure and slow rpm.
Use a scrap bit as a center punch.

Softer stones marbles and lime stone ,most the time, just a good ole twist bit works just fine.

Trick keep the bit sharp. heavy presure low rpm .

Been stuck in this stupid job playing with rocks for 20 yrs. (chair)

Dave

[Builder4wd]

What kind of inserts are appropriate? What kind of epoxy is best suited for bonding the inserts to granite?

[mdreitzusa]

here are some dimond bits at a good price.
http://www.cyberrockhound.com/lapida...ond_drills.htm
just keep them wet and apply a steady pressure.

[NEATman]

Nicad-
I've posted many time s before on this subject. As far as the inserts, here is a link showing what one of the leaders in granite tecnhology is doing.
http://www.tru-stone.com/pages/tech_assistance.asp
Granite is outstanding for high precision machines. take a look at these machines:
http://www.precitech.com/
http://www.doverinstrument.com/vvLinearProds1.html

I agree gluing slugs into the granite is tedius, but if you drill and tap them after they are glued, you will get much better accuracy. Also, if you make a steel plate template overlay first, you can use a magnet based milwaukee drill to drill the holes accurately.

Neatman

[frank02189]

Hi
I have mounted a Taig cnc mill on a granite surface plate 12x9x3 40lbs.
An easy thing to do.
I mounted on the bottom of the plate the bad surface if you can pick one up
check the surface for a clean cut.
Use the mounting configuration for a template and mark the holes on the plate.
Drill the plate with a mason drill carbide tip. I used 5/8' dia drill 1" deep
You can use a hand drill or drill press would be better I also turned the shank to make that work.
Make your inserts pref stainless but I used aluminum drill and tap but not thru.
Attach inserts to machine mount.
With holes drilled in surface plate and inserts mounted to machine base place on plate recess the inserts below surface plate
get an idea of how is all centered and if enuff clearence for bottom of inserts.
I used fiberglass epoxy and put just enuff in the surface plate holes to lock the inserts in place let cure. Then add more to capture the rest without the machine base attached
Now you want to do this with the inserts attached to the machine base otherwise they will never line up or be vertical.
I also added thin sheet of rubber mate between the plate and machine cuts down the vibration.
This was all done cheap the biggest cost was the surface plate $25. drill $6 epoxy (evercoat resin) boat store or auto store
$5 insert made myself rubber mat auto store you may want to use the mat to protect the $$ surface too.
Take time and consider the weight and size of the machine and surface plate but it can be done and I think you may be happy with your results I am
thanks
frank

[Builder4wd]

Where did you buy the inserts? Also what type of epoxy is optimal?

Thanks!

[cbass]

I don't think I'd use a hamer drill. Think about the shock treatment you'd be giving that stone. If there's even the slightest weakness in your stone, you might end up with lots of little stones. As someone else mentioned, diamond bits and lots of water would probably give a better result.

For the epoxy question give granitecitytools a call (they can help with the diamond drilling question too). Though again someone has already mentioned you would be fighting the coefficient of thermal expansion between three materials (stone, steel, and epoxy). No small feat.

http://store.granitecitytool.com/store/Main.aspx



Also, someone once posted a link to machine design classes from MIT

http://pergatory.mit.edu/2.75/2-75_Lectures.htm

There is some pretty interesting stuff here, even a section on damping. A perfect resource for curious people. A lot of it is rocket science but may make sense to mortals like us (you are a mortal, right?).

Good luck, and keep us posted!

Carlo

[virtualmentor]

Maybe you are crazy...
My ex wife said i'm crazy..

And I had EXACTLY the same idea
epoxy / stainless inserts

I have wondered often for all the glory of flat acuracy of a surface plate...

Just how parallel / flat and 90 deg plumb the sides really are..???!!!
since that would be an important part of the total solution...