Granite base router

Hi all
After running my extrusion based router for the past year, I've decided to build a new frame using a granite surface plate as the base. I want it to cut aluminum. The one I built does a fair job, as long as you go slow. Very slow. I'm now in design paralysis when it comes to the gantry. The machine will be fixed gantry moving table layout. The base plate is a granite surface plate 24x36x4 inches. The gantry is on the long axis. I'm undecided as to the beam material, steel vs granite. If I go steel, it would be 8x8x0.5 inch square tubing. I think that would be stiff enough. I have most of the details worked out, but it will require some outside machining I would like to avoid, so I'm leaning toward granite. The thickest table top granite I can find locally is 30mm. What about laminating four slabs together with epoxy? Gantry support will be 5x5x0.375 inch sq steel tubing, with 0.5" plate welded each end. The unsupported length of the gantry will be 26 inches.

I plan on moving all the motion parts from the current machine to the new one. I have Hiwin 25mm HGH rails and carriages, 2505 screws driven by clearPath servos controlled by a Masso G2. All these parts work great, with NO issues.

ANY help you guys can give would be very much appreciated!

Ben

Have to you looked at the basic values for stiffness of the steel vs (laminated) granite beam?
That's where you should start.

How are you planning to mount the linear rails?

Hi pippin
I have played around with online calculators using Al values for the granite, but that probably is not correct for a laminated beam. The beam would be
215mmx120mm. Four slabs 30mm thick.

The rails will be mounted using anchors set in drilled holes with an Al2o3 epoxy mix. I will use the surface plate to insure alignment. Rail alignment on the extrusion
router was a nightmare!

I did a crude stiffness test on the present router using a spring scale on the spindle with all axis centered. I got deflection values of 0.0007" on Y and 0.0026" on X. This was a 22Lb (100N) load.
I would like to improve on this, a lot. Your suggestions are welcome. I have followed your writing for quite some time and have learned a lot.

Ben

This is what I think you want.

https://www.youtube.com/watch?v=gUFeoDf1YvM


The other option is to have the gantry move up and down for Z and have the spindle fixed to the X-slide. In that case I'd suggest steel as the gantry beam and just bite the bullet and pay for machining. The beam needs to be lighter if it's moving and I think steel would be better than a laminated granite beam in this configuration.

-Ralph

Hi all
I decided on using granite for the gantry and some progress has been made. I bought a drop from a local granite table top supply and had them cut four slabs 900x225x30mm. I picked the flattest one, using the surface plate, and used that as the face for the gantry. I put this side down on the surface plate and glued each plate one at a time to make it four layers thick. I used epoxy mixed with Al203. The mix was about 75% Al203, quite thick. After this cured, I tested the face for flatness using the surface plate, it was out by about 0.002 to 0.004". I then lapped the rail surfaces using a 2"x 4'" diamond lap. This took some time, and I quit when the worst error was about 0.0006". I may work on this again after the rails are mounted, and tested.
I made anchors using coupling nuts that I turned round, leaving a small hex to prevent turning. I knurled the round section to help with grip. I did pullout tests with anchors glued in with epoxy/Al203 mixed about 60%. The anchors would hold to around 200 to 300 Inch Lbs. I was surprised they held this kind of force, about 4000lbs. The anchor always pulled through the epoxy. The epoxy bonds WELL to the granite.
The gantry support tubes and plates are welded up.
Their is one part of the design that I have not decided on. Do I need two screws on the Y axis? The Rails will be 500mm apart with the screw in the middle. I'm trying to make this thing stiff. The Y plate will be, most likely, steel T-slot about 300x600x20mm thick running on 25mm HGH rails with the long carriages. Do you think it will rack?
Opinions welcome.

Ben

No drawings to share? I'm picturing something like this where X would be the direction of the moving table.
You will need a stiff table as the blocks are the only support.
Attachment 438380

Okay what I labelled 'X' you are calling 'Y'.
The gantry spans the longest dimension, or maybe I misunderstand the meaning of "The gantry is on the long axis", that's why drawings help.:)
The more the bearing spacing resembles a large square the more resistance to racking. Otoh the rails being used are hefty so a lot of resistance to racking is built-in.

Hi Mr Ben - Granite is only half the stiffness of aluminium but since you have made the gantry solid it will be very stiff and damp. Granite is about twice as damp as Al. Photos would be good, we like to watch. Tables are typically driven with one drive, no need for two unless you need more grunt. Peter

Quote:

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Originally Posted by peteeng

Hi Mr Ben - Granite is only half the stiffness of aluminium but since you have made the gantry solid it will be very stiff and damp. Granite is about twice as damp as Al. Photos would be good, we like to watch. Tables are typically driven with one drive, no need for two unless you need more grunt. Peter

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granite is more like 2/3rds as stiff as aluminum if its good quality and laminated with epoxy. figure 50Gpa. aluminum is 70 gpa, steel is 200. aluminum and granite are both about the same density, 1/3rd that of aluminum

however depending on the alloy, and the granite.. granite is on the order of 100 times better dampening than aluminum. almost all tuning forks are made out of aluminum btw... they ring for a very long time, say 20 seconds in air. cut a 440 hz granite tuning fork and you will probably break it before you can hear it ring.. if you are careful you may hear it ring for i'm guessing.. about one second.


my experience with synthetic granite counter top material (and i have several pieces of 1.125" 4 inch wide, 5 foot long pieces.. they don't have much dampening. they ring for a long time, at 20-40hz when you smack them with a hammer.

if you cut a piece of natural granite the same size, it will have a similar (maybe slightly lower) frequency but it won't ring long enough to hear it until you hit it hard enough to break it.

Attachment 440914

Hi all
Well I made a bit more progress. Most of the gantry work is complete, including the 74 anchors I epoxied in. That was a chore, very time consuming.
I also installed the anchors in the base for the support tubes. One thing was immediately evident, the base (surface plate) was much easier to drill.
When I was drilling the gantry I would hit spots that were very hard which increased drill time considerably.

Picture is the gantry with rails and ball screw mounts for a trial fit.
More pics later.

Ben

Iam collecting materials now to build a granite cnc router i have got 3 big slabs 3mx1.7m 4cm thick 2m hgr20 linear rails and 1605 ballscrews with end supports.
Also i have a surface plate 2.5mx1.3mx36cm for leveling.
What filler matterial can i use for leveling the rails and glue the granite slabs together ?
Also at the sides that will be glued iam planing to put make holes and put screws with glue for reinforcement.
What do you think granite needs re baring with galvanized steel bars potted with epoxy resin ?

Hi mrsa,
I used Epoxy mixed with 80 grit ALOX. The mix was about 75% ALOX, very thick. I found the Epoxy bonded well to granite.
I have no experience with Epoxy leveling, so can't help there.

Ben

You use 75% alox 80 grit / 25% epoxy for leveling the place where linear rails will mounted?
What you use as antistick agent so this mixture do not stick to leveling surface plate and damage it?

Hi mrsa
I used the epoxy ALOX mix for bonding the slabs only, NOT for mounting rails. I put the laminated gantry slab on the surface plate and used a diamond lap to true the faces where the
rails mount. I will try to get some pics up to make it more understandable.

Ben

I know that megapoxy is from the best glues in the market for marble/granite but costs about 200-250euros for 20liters