Molding a concrete slab to use as a base for a bench mill.

[maxime.levesque]

I just received my Weiss32B mill, and my first project will be to build a cabinet.

I want the top of the cabinet to be as planar and rigid as possible, because the casting is kind of lightweight (the whole machine weights only 240kg), and I want to avoid any source of distortion. The mill base is the same as a PM30V (same manufacturer), the base rests it's rectangular contour.

A concrete slab for the cabinet top seems like a good choice, for it's rigidity and low cost.

I happen to have 24x18 granite plate (grade A accuracy), which could be used as the base of a mold for the concrete slab, with wooden walls around it. The slab should have one face pretty close to the granite.

The challenge is to not ruin the plate ! So I want to cover it with "something", to prevent cement from touching the granite surface. I might also put a film of oil on the granite for extra safety, because I have an almost emotional attachment to the granite plate ;-)

I would like to have suggestions of covering material that is both solid enough to not puncture, and that has a consistent thickness, so I don't loose to much accuracy.

 

[matthewsx]

Seems like Masonite would make a sufficiently flat base for a milling machine, I know folks use it for epoxy granite machine bases.

I wouldn't want to risk my grade A surface plate for something like this, presumably you will still be shimming between the machine base and table for the implied level of accuracy desired.

John

 

[whitmore]

A concrete slab for the cabinet top seems like a good choice, for it's rigidity and low cost.

There's a lot of lab tables (optical bases) made of concrete, usually a welded steel frame with a corrugated-steel
floor. The construction-materials industry has a wide range of corrugated steel sheets for this purpose,
as well as wire-frame reinforcing mesh and 'chair' items to keep the mesh in place when doing a pour.

Welding of steel isn't totally required (wire and pliers makes good steel-steel connections if they're gonna
have concrete surround) but it'd be handy. The 'floor' of a pour will be in contact with cement; that's sticky stuff,
best left stuck and undisturbed even months after the pour. I wouldn't want a floor for the pour to be
a surface plate; gravity and a float is plenty of flattening for the top side. Shims for leveling are always a good option,
and if there's a crack, you can fill it with grout.

 

[jwmelvin]

You’re looking for flatness overall among the contact patches. Having voids elsewhere isn’t a problem for alignment. So maybe waxed paper or a similar product would serve you well, despite overlapping seams. Or a sheet of plastic might be sufficiently even thickness?

 

[Larry$]

"Flat" concrete! Maybe, maybe not! Concrete will curl due to uneven curing and drying. As an attempt to prevent distortion, keep the concrete uniformly damp/wet during curing. ALL SIDES! Design strength is normally achieved @ 28 days when properly cured. It will continue to gain strength very slowly after that. I'd be inclined to just use the concrete for added mass and figure on shimming.

 

[MikeInOr]

Granite counter top rems/offcuts are incredibly cheap at granite suppliers and counter top installers. How would pouring a slab on top of a granite counter top rem then flipping it over when dry and leaving the granite attached to the cement work? You might need to scar the underside of the granite and roll on a layer of latex to get a good bond with the cement.

I installed my own granite counter tops and cutting them with a skill saw wearing a diamond blade using a garden hose with a trickle of water was pretty easy. I used a board as a straight edge to guide the skill saw.

 

[maxime.levesque]

The flatness of a granite counter top is unknown, I'd be surprised if it was better than an MDF board.

My grade A plate has a maximum variation of 0.00013" over the whole 18"x24" surface.

The distortion during the concrete curing might be the limiting factor.

Granite counter top rems/offcuts are incredibly cheap at granite suppliers and counter top installers. How would pouring a slab on top of a granite counter top rem then flipping it over when dry and leaving the granite attached work?

I installed my own granite counter tops and cutting them with a skill saw wearing a diamond blade using a garden hose with a trickle of water was pretty easy. I used a board as a straight edge to guide the skill saw.

 

[maxime.levesque]

Good point, my thinking was that curing shrinkage would be negligible if it occurred equally in all directions, but as you point out, there are precautions to take to get the best case scenario.

In the best case scenario the plate will become slightly spherical (given that there should be more shrinkage around the contour), which is not so problematic, given that the mill base rests on it's rectangular contour.

Perhaps epoxy granite, is a better alternative, I will investigate it.

"Flat" concrete! Maybe, maybe not! Concrete will curl due to uneven curing and drying. As an attempt to prevent distortion, keep the concrete uniformly damp/wet during curing. ALL SIDES! Design strength is normally achieved @ 28 days when properly cured. It will continue to gain strength very slowly after that. I'd be inclined to just use the concrete for added mass and figure on shimming.

 

[matthewsx]

With all due respect I think you may be looking for an advantage that really won't materialize. You have a lightweight bench mill. I know that many folks get very good results with stock stands or home built stands that are no where near the flatness you are trying to achieve. Having a well built stand is definitely important but I doubt the machine you've chosen will perform any better with a precision ground, ultra flat stand.

No matter what stand you choose your leveling process will likely require shimming. Once shimmed the flatness of any stand will basically be moot. It's unlikely the bottom of your mill will be even as flat as a decent piece of Masonite so having an ultra flat surface to mount it on is probably not going to make much difference.

I could totally be wrong here, but honestly I think your efforts will be better spent getting good measuring tools to ensure the parts you make are within the spec you need.

If you really want the flattest surface possible you might be able to find a used surface plate to bolt your mill to. You could even buy a new one if you like for fairly short money.

STM 255135 | 3" Thickness x 18" Width x 24" Length Granite Inspection Surface Plate

We distribute top-quality industrial tools and machine shop supplies at the best prices. View our collection of CNC machinist supplies and tooling packages.

allindustrial.com

But, I really haven't seen anyone else trying to achieve this level of flatness for mounting a machine on. Most want to be able to adjust the mounting as needed to achieve the desired tolerances at the machine, not the base.

JMHO,

John

 

[Chipper5783]

Reminds me of the saying, “making a silk purse out of a sow’s ear”. You will still have a light weight bench mill. Make a sturdy bench (per comments above), if you found a nice piece of granite or concrete counter top, it would likely work fine, as would pouring your own little slab, or any of numerous other designs. You’ll still secure per manufacture’s recommendation (flat mounting via shims is extremely common, level is not necessary, but is a convenience for set ups).

The way to gain rigidity is to get a bigger mill.

Sounds like you have not used the mill as yet. Consider setting it up per comments above and see how the machine performs (don’t try fixing problems that don’t exist). You will have lots to do, and tools to spend money on - focus on getting going on the projects for which you bought the mill.