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Advice about a void under our concrete floor


Dec 11, 2021
We've got two machines and old Haas VF-OE and a newer DS30Y. We ran these two machines ~3 feet from each other in our last shop and everything was fine, apart from tripping over ourselves. So we moved to a new shop down the street and have way more room. Problem is when the lathe winds up it will shake the floor enough to cause chatter in the mill! While we could try to improve the vibrations of the lathe or tighten up the mill we know that there's an environment issue as well. We've had a couple of different people out to look at the floor and there's definitely a void under the slab. How much of a void is a great question that doesn't seem easy to answer though. I did reach out to a NDT company who said they could inspect it for ~$1500 and tell us where the void is but not how deep it was. The other option is to have a company put fancy expanding foam in through a series of holes and support the slab. They do this a lot to correct slabs that have failed and bring everything back into level-ish. I am the property owner so for better and worse this is on me to correct. The foam company gave me some references that all checked out but they haven't done this for someone running machines and looking to cure vibrations before. The cost to do the foam work is ~7k so it's not bad IF it solves the issue. The alternative is to cut it all up and repour the slab. which sounds awful, disruptive and expensive but once it's done it would be nice. I haven't had that quoted yet.

Anyone been through this before by chance? Any other words of advice apart from buy a different lathe that runs smoother?
I have seen the foam work done to lift sinking floor in a shop before, it worked and lifted it, seemed solid, but I don't know long term how long it has lasted and stayed level. Forklifts where driving on it though and seemed solid when I was there.

I would ask for some references for work they have done in other shops or places handling similar weight.
Some 20+ years ago I was involved with a couple projects where slabs over poor subgrade were not stable enough for good machine performance. A company was hired to do what they called "slab injection". A grid of holes were drilled in areas that were bad and a cement-like grout was pumped into the holes. The end result was very good.
I would be very apprehensive about having concrete sawn in the same building as my expensive equipment. I have seen it done, with a ridiculous amount of "abatement" which still resulted in a fine layer of dust.
Like vancbiker was involved with I was on a project that had a void under the slab , probably due to not properly compacted under slab fill, they core drilled some holes that fit the nozz;e of a grout pump hose and just pumped in grout until it was full.
Slab jacking is a similar process if you want to find more info....
I have a similar problem in one of my shop floors, I had both foam and cement slurry quoted and in my case the cement was cheaper and the guys that quoted have done this exact process for the same type of applications and have claimed good results.

In your case, and I suppose mine there may be a third option to consider. Cut out only under each machine a rectangle that represents the area you intend to have a machine placed on in that spot. Then you can dig down much deeper and pour a very thick block under each machine that will sit on it's own prepped subgravel and then not only will it be much more rigid and aid in damping, but it will decouple the vibrations of the machines from each other. I wouldn't be surprised if this doesn't cost much more than getting a crew in to pump cement under the floor if you do it yourself, because the blocks themselves will not really take that much concrete compared to replacing the entire floor and the prep to do it will not be that bad, you don't even need forms. I read up on this process years ago and there was a material you could put around the perimeter to prevent direct contact but I can't recall what it was.

Once you have cut out the areas you will see the voids as well and likely they can and will be filled when you pour the large areas anyway. Also, you can do one at a time without major downsides other than the additional concrete delivery costs, which would be not major anyway. Doing one at a time might make it much easier to manage while keeping your shop going. The biggest pain would be sawing the concrete and containing all that dust. The dust will be extreme and it would be best to lay plastic everywhere Dexter-style before you start.

I think I've just convinced myself to do this. I have in floor heating which will create a bit (A lot) more hassle but I have a thermal camera so I know where the lines are, I can cut around them and break the concrete off the lines after removing the rest. A pain, but not impossible.
I think the old adage "Do it right, cry once. Do it wrong, cry multiple times." should be stated.

We had a customer cut corners on a foundation. Instead of following the machine manufacturer reccommended slab XY dimensions for their 65k lb. multi-pallet HMC, they just decided on their own to add 3" to the machine's perimeter, 6" of concrete, no rebar, and no gravel... because "it's good enough... Machine Tool Builders go way overkill on those slabs anyway...". Maybe they do. Then again, machine accuracy begins with a solid foundation, so there's that.

Fast forward a few weeks, concrete is allegedly cured, machine is placed, leveled, and geometry checks completed. Service Engineer gives the thumbs up. Machine is in spec. I come in to calibrate and test the tool measurement system. At first I run things slow. "Never be in a hurry to crash." All good, something "feels" weird. Whatever. I continue testing. I go full rapid and I felt like I was standing on jello. I'm looking around thinking we're having an earthquake. But nothing else is moving. No lights swinging or anything. Servive guy looks at me and says WTF? We could actually see the slab moving as the machine was moving. Not just a little. Like 15mm. He put his dial indicator on it... fluxuating between13-14mm! Crap. We tell the custome, he says we're "...exaggerating, and it'll be fine....".

One week later, customer tore that slab up, kept the outer perimeter the same, but went with the manufacturer spec on gravel, depth, rebar, grout, etc... After 30 day cure, they placed the machine, level, geometry check, etc... movement was nearly eliminated... down to about .5mm... You could feel it, but barely.

Machine manufacturers make foundation requirements for a reason. And no they are not in cahoots with concrete companies. It is not only for vibration isolation, but for machine geometric integrity as well.
How many square ft is the floor? Rather then 7k in foam I would consider cutting the slab where the machines are and repour a proper pit.
Or isolate a new slab under lathe.
A local shop had a brand new DNS/Doosan Horizontal mill that was chattering badly, in even light cuts. Turned out that there was a huge void under the floor, where the
HMC was located. They hired a contractor to drill a hole and pump in concrete. Problem solved.
The existing slab is 6" by print and when we cut into it to add a mop sink we found that it was a little thicker than that. So it's up to the task of our machines and tolerances (+/-.001 is about as tight as we get). The lathe runs fine as it is honestly. While cutting out spots and pouring pads for specific machines would be ideal, it's overkill for our application and it's not going to be any less disruptive than repouring a large section of the floor. The section of the shop in question is ~2500 sq/ft, about 1/3rd of the shop. If we had a horizontal or a big dual turret lathe I would be on board with that kind of solution.
If you do not isolate the machine on its own pad I doubt filling the void will do much for the vibration. Been there done that.
We ran these two machines on a common slab for 2 years, 3 feet from each other with zero issues.
You said you own it, I personally plan on some day owning a HMC, So I would either pour machine slabs or re-pour the entire area.
I know depends on price.
what is cost difference of a few holes and filling with concrete underneath(dont know I trust foam) vs pour slabs or re-pour entire floor,
need to calculate down time loss also unless you scooch the machines to a different area while the pour.

good luck brotha :cheers:
I ran multiple machines on a monolithic 12" thick 600 sq. ft. floating slab, think parking garage.
The huge mass had such good vibration reduction there were no issues.

Later moved same machines to a 5" floor shop, with each areas cuts so deep and the floor so thin you could see that they cracked and were no longer connected to each other(isolated)
this was bad new, surface problems.

just a reference.
Some 20+ years ago I was involved with a couple projects where slabs over poor subgrade were not stable enough for good machine performance. A company was hired to do what they called "slab injection". A grid of holes were drilled in areas that were bad and a cement-like grout was pumped into the holes. The end result was very good.

I have also seen this done successfully with 2 machines that are much heavier than a DS30Y.
……need to calculate down time loss also unless you scooch the machines to a different area while the pour.

This was a big factor in the decision to slab inject in both instances I posted about before. One of the projects had 5 50 taper Okumas over the bad slab and at the other plant it was a pair of laser cutters with 5’ by 9’ travels. No small job to move that much stuff around to pour new floors and wait the cure time.
If you want to be an absolute cheapskate just go rent a walk behind diamond saw and cut all the way through the slab all the way around the mill. Then caulk the cut with polyurethane.

If the slab isn't connected you likely won't have anymore vibrations.

And I've cut concrete floors inside a 1500 sq ft shop I rented with a wet diamond saw. I didn't get any dust on anything. I cut two rectangles 3ft x 6ft to bring the slab up to spec to install a 9K 2 post lift. It was a one day job to cut, break out, wheelbarrow mix and pour those two holes 12" deep.
I isolated a small machine like this years ago, was next to a bigger cat50 ruffing machine. Had decent results. Could be worth a shot.