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

[maxime.levesque]

keep in mind that concrete floors and wall move with ground movement and any attempt to use the wall/floor intersection to create a solid 90 deg joint (in the specific intent of machining) is doomed.

My concrete basement was built in the 1950s before every inch of the world was ruled by accountants, and the walls are 12" thick with lots of rebar, and I'd be surprised if the ground slab was not equaly robust.

That being said, If I ever chose to implement this "wall anchor", being of the curious/paranoid type, I will monitor the movement of the column relative to the wall with an indicator, when I loosen the bolts, every week or so, and If I see needle movement, I will keep the anchor detached, and only attached when I need maximum rigidity for critical cuts.

There are chances that temperature changes can cause movement of the base relative to the wall, having the anchor detached, by default, and attached only when needed might be required.

 

[verbotenwhisky]

My concrete basement was built in the 1950s before every inch of the world was ruled by accountants, and the walls are 12" thick with lots of rebar, and I'd be surprised if the ground slab was not equaly robust.

That being said, If I ever chose to implement this "wall anchor", being of the curious/paranoid type, I will monitor the movement of the column relative to the wall when I loosen the bolts, every week or so, and If I see movement, I will keep the anchor detached, and only attached when I need maximum rigidity for critical cuts.

There are chances that temperature changes can cause movement of the base relative to the wall, having the anchor detached, by default, and attached only when needed might be required.

None of my machines are bolted to the slab, if you build your cabinet/base heavy and add mass with concrete rigidity should not be an issue. My mill and metal lathe are large and heavy so no issues, the wood lathe was boarder line dangerous and simply adding mass solved its issues.

 

[Eddyde]

I agree that a concrete block would make an excellent base for a bench mill though, I think trying to get it "Surface Plate Fat" would be a wasted effort. As mentioned above, the base of the machine is probably nowhere near as flat as a SP and you will need to shim it anyway. That being said, I doubt casting concrete on top of a surface plate would do it any harm as long as there was some sort of barrier, oil, silicone or plastic film.

 

[MikeInOr]

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.

I personally value rigidity, stability and vibration dampening over absolute flatness for a machine base. It is a machine base, not a reference surface. IMHO "0.00013" over the whole 18"x24" surface" is not needed for a machine base. I would build/make a heavy solid vibration dampening mass then bolt the machine to it. If you want to improve the performance of the machine look at using epoxy and granite aggerate to make the base out of. It should do a great job minimizing vibrations.

vibrations = chatter

 

[Provincial]

If the walls were poured separately from the floor (you can tell if there is a seam around the edge of the floor) then you must consider them as two completely separate structures. Even if there was rebar stubbed to carry through to the next pour, they do not have structural integration.

I've told this before, but a fellow I know took a 1940's Craftsman (Atlas-built) 12" lathe and formed up two pyramids to cast concrete tied into a concrete floor. The pour for the pyramids was carried up high enough the capture the feet of the lathe bed, with long anchor bolts using the existing mount holes in the feet. This set-up was used to make 1" Acme double lead screws for a paving machine. The spindle was modified to a roller chain drive, and the threads were cut in three passes. The pyramids were like adding 1,000 pounds to the weight of the machine, in terms of rigidity.

 

[maxime.levesque]

From my understanding, if a machine is bolted to a non flat surface, the surface errors will distort even the most rigid machine over time, unless:

1. shims are used to distribute the load appropriately over the contact area
2. the machine rests on adjustable jack screws, and a torque wrench is used to achieve the load distribution prescribed by the machine's design. In some designs, levelling the machine is sufficient to ensure that the load is appropriately distributed, most 4 legs machines fall into this category.
3. the machine is designed with a 3 legs system.

I personally value rigidity, stability and vibration dampening over absolute flatness for a machine base. It is a machine base, not a reference surface. IMHO "0.00013" over the whole 18"x24" surface" is not needed for a machine base. I would build/make a heavy solid vibration dampening mass then bolt the machine to it. If you want to improve the performance of the machine look at using epoxy and granite aggerate to make the base out of. It should do a great job minimizing vibrations.

vibrations = chatter

 

[verbotenwhisky]

From my understanding, if a machine is bolted to a non flat surface, the surface errors will distort even the most rigid machine over time, unless:

1. shims are used to distribute the load appropriately over the contact area
2. the machine rests on adjustable jack screws, and a torque wrench is used to achieve the load distribution prescribed by the machine's design. In some designs, levelling the machine is sufficient to ensure that the load is appropriately distributed, most 4 legs machines fall into this category.
3. the machine is designed with a 3 legs system.

I agree, all my machines have adjustable leveling feet, I like the machine to be leveled at the floor. If I were building the base I would put adjustments at the foundation/floor and level the base first.

 

[dabear3428]

My concrete basement was built in the 1950s before every inch of the world was ruled by accountants, and the walls are 12" thick with lots of rebar, and I'd be surprised if the ground slab was not equaly robust.

That being said, If I ever chose to implement this "wall anchor", being of the curious/paranoid type, I will monitor the movement of the column relative to the wall with an indicator, when I loosen the bolts, every week or so, and If I see needle movement, I will keep the anchor detached, and only attached when I need maximum rigidity for critical cuts.

There are chances that temperature changes can cause movement of the base relative to the wall, having the anchor detached, by default, and attached only when needed might be required.

ok but keep in mind that for builders 1/16th of an inch over 3 feet is no movement, where for your use it is intolerable. and long walls / floor slabs will move a lot regardless of rebar and thickness because the wall/floor is very long and lots of leverage is applied. not to mention temperature variations.

 

[MikeInOr]

From my understanding, if a machine is bolted to a non flat surface, the surface errors will distort even the most rigid machine over time, unless:

1. shims are used to distribute the load appropriately over the contact area
2. the machine rests on adjustable jack screws, and a torque wrench is used to achieve the load distribution prescribed by the machine's design. In some designs, levelling the machine is sufficient to ensure that the load is appropriately distributed, most 4 legs machines fall into this category.
3. the machine is designed with a 3 legs system.

Inexpensive bench mills do not come with a bottom that is machined perfectly flat to begin with. Just the act of placing the mill on a perfectly flat base, or any base, is going to cause it to to flex and contort. Further bolting it down to said base, flat or not, is going to cause it to distort more.

No matter what you do the bottom of the mill will distort. I have never seen a procedure for placing mill and removing all distortion from the mill/base interface. Maybe I am just inexperienced???? I bet distortion is just a fact of life and bolting a bench top mill to a heavy ridged stabile surface gives you more distortion BUT also gives you a consistent repeatable starting point that will also add rigidity to the mill while also dampening vibration.

IMHO "0.00013" over the whole 18"x24" surface" for a base will buy you absolutely nothing in the ability, functioning and repeatability of the bench mill. Rigid, stabile and vibration dampening, in my opinion, is what you should be chasing... not flatness. If you are going to chase flatness you had better disassemble the mill and take the base to someone that has a Blanchard grinder to flatten the bottom of the mill if you want your flat base to have any positive affect.

 

[MrWhoopee]

TL;DR, apologies if it's already been said. Suggest you make a steel framework with raised anchor points for the mill, position inverted in the mold and pour the concrete.