MR-1 BASE DESIGN

The Importance of a Rigid Machine Base

If you have spent some time reviewing the MR-1 content on our website, we hope you have been left with the following impressions when it comes to machining metals:

1. MR-1 has the rigidity necessary to produce exceptional side wall and floor surface finishes.
2. MR-1 has the rigidity necessary to repeatedly produce dimensionally accurate parts.
3. MR-1 has the rigidity necessary to take large roughing cuts (as high as 10.5 cubic inches per minute in aluminum and 2 cubic inches per minute in steel) without failure of the machine or tools.
4. MR-1 has the rigidity necessary to ensure respectable tool life.

The key term from the list above is the word rigidity. As machine designers, we hold rigidity as the single most important attribute of any metal cutting machine.

Machine rigidity starts with the base. It is the foundation for the rest of the machine. Without a rigid base, it is impossible for a machine to be rigid because every other component either directly or indirectly is tied to the base (the Y-axis rails, the machine base plate, the gantry beam etc).

The Search for the Ideal Base Material

During the early phases of MR-1 development it was increasingly clear that in order to achieve sufficient rigidity, the base structure would have to be hundreds of pounds in weight to reach the required level of stiffness and mass damping. During initial proof of concept testing we constructed a base from thick steel tubing and plate but found that it did not have sufficient stiffness or sufficient mass damping to be feasible. This lack of stiffness led to unacceptable workpiece vibration, tool chatter, and poor surface finish and accuracy.

The next logical step was to look at developing a cast iron base structure, but that presented numerous other challenges as follows:

• Cast iron is extremely expensive to manufacture, finish-machine, and ship at the required size. Only a handful of foundries have the capability to produce castings that would be large enough to ensure acceptable performance.
• The castings would weigh 400-500 pounds each in order to meet the required rigidity needed for our performance expectations of this machine. This would present unique assembly challenges due to the fact that either a forklift, crane, or engine hoist would be needed. Again, MR-1 was designed with the garage hobbyist in mind.

From its inception, the goal of the MR-1 project was to introduce CNC metal cutting to a much broader audience. To do this, we needed to be able to provide the machine at an affordable price point. Given the challenges and expenses of a cast iron base design, we believed that it was not cost feasible to offer a high performance CNC milling machine at a price with mass affordability unless a better solution to the construction of the base could be reached. The solution would need to offer exceptional stiffness, mass damping characteristics, and dimensional stability for minimal cost.

How the Construction of Industrial Gantry Mills influenced the design of MR-1

Our previous collective experience on large platform gantry mills (ones that have travels expressed in feet instead of inches) showed us the near universal use of concrete as the base structure material. A monolithic concrete foundation that is many feet thick supports the relatively compliant Y-axis rails and the table/bed of all large platform gantry mills in industry. It is the concrete foundation on these machines that provides the stiffness and vibration damping characteristics that are required for peak performance.

We took a closer look at concrete as a potential base material for MR-1 and immediately identified many benefits:

1. It is extremely strong, stiff, and durable.
2. It has superior vibration damping compared to cast iron.
3. It has excellent long term dimensional stability once cured.
4. At only ~ $0.05 per pound, it is extremely affordable (~$20 USD in concrete needed per machine base on MR-1).
5. It can be purchased in 50-80 pound bags from any home improvement store making it easy to manage and lift.
6. It forms an extremely strong composite material when cured around steel (as with rebar).
7. Only water is needed to start the curing reaction and no special tools are needed. If you have a bucket and a shovel, you can make concrete.

Once the many benefits were established, we set out to build a prototype. While the initial prototype was merely a rudimentary proof of concept, we knew from the immediate success of the first cuts that the concrete base structure would form the core of our product. Since that first concrete prototype there have been many design iterations and refinements that have made their way into the production MR-1. Let's take a closer look at how it works.

A Closer Look at the Base Structure of MR-1

Let's take a closer look at how the MR-1 base is created. Prior to pouring the concrete, the Y Axis Rails are supported by the Front and Rear Y Axis Rail Mounts. The mating surfaces of these components are precision machined to guarantee a high degree of coplanarity between the Y Rails once assembled. The Rail Mounts are supported by three points of contact on the bottom so that the precise coplanarity between the rails is not compromised by its own weight when resting on the relatively uneven surface of the Chip Tray during assembly. The large square rectangular pass through holes in the Rail Mounts are to guarantee good mechanical interlocking between the concrete and the Rail Mounts.

Stabilizer plates attach to the inner and outer edges of the 1” thick steel Y Axis Rails. The downward arrowhead geometry of the stabilizers ensure a strong and stable connection to the concrete once cured. The job of these plates is to transfer the stiffness and mass damping properties of the concrete base to the Y Axis Rails.

The base plate assembly consists of two 10”x10” billet aluminum plates that are .700” thick. In addition, a total of 52 steel anchors protrude from the underside of the base plates to ensure a stiff and intimate connection to the concrete base structure. Concrete completely encapsulates the base plate assembly on 5 sides to ensure maximum vibration damping performance.The steel anchors were designed so that the base plates could be replaced if needed, even after the concrete has cured.

The two base plates are positioned relative to the Y Axis Rails using the supplied Base Plate Positioners. These tools suspend the base plates at the correct height during pouring of the concrete to guarantee excellent planarity (within 0.030” once cured) to the XY machine plane. Once the machine is commissioned after assembly and alignment, the base plates are skim cut by the machine in order to achieve parallelism to the XY plane of 0.002” or better across the entire travel envelope.

Next, the flood coolant corner drain funnels, drain lines, and applicable fittings are all added prior to pouring the concrete. Much like the plumbing in slab foundation homes, the drain lines are completely encased in concrete. The four large drains are located in the corners of the Chip Tray and include stainless steel mesh screens to keep out machining debris when cutting. Even if you decide not to purchase the Flood Coolant Kit with your machine, we recommend at least purchasing the drain kit so that it can be installed when pouring your base.

Adding the concrete is as simple as mixing (7) 50 pound bags of concrete according to the manufacturers recommendations and then filling the chip tray until roughly half of the baseplate edges are exposed. It takes the average user about 1 hour to mix, pour, and smooth the concrete base and the only tools that are required are a bucket and a shovel.

The concrete takes approximately 5 days to cure before it has reached sufficient dimensional stability for use. While it is not required, we always recommend the application of an epoxy topcoat (can be purchased from Langmuir Systems) to seal the surface and to provide a laser flat surface that is exactly level with the corner drains. This will ensure that there are no low spots that puddles can form in. Epoxy also carries the added benefit of being easy to clean since it does not absorb liquid like regular concrete and it also creates a great looking machine base surface.

After curing and epoxy coating (if applicable), the machine can be fully assembled, aligned, and commissioned for use. See below for images of an MR-1 machine base being poured.

Concrete is poured to fill the Chip Tray

Concrete Base after curing

MR-1 Machine Base after Epoxy Top Coat

MR-1 Machine Base after skim cut with Fly Cutter

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