Understanding and Evaluating Machine Build Structures

Wade Anderson

By completing a quick online search for machine tools, an individual will quickly be inundated with a variety of options. As we all know, there is an increase in the need to produce more precise parts as profitably as possible, and as machine tool technology advances to meet those needs, so do the number of choices available on the market. The type of machine tool utilized can vary based on the size and complexity of the part, the amount of floor space available, the skill set of programmers and operators – honestly, the list could continue forever.

The purpose of this blog post is not to prescribe the perfect machine tool for your shop floor but to give a high-level overview of three very commonly found machine tool construction types: C-frame, double column (including bridge mill), and turret lathe.

C-Frame Machining Centers

CAD Machine

First, let’s answer the question, what is a C-frame machine structure? Very similar to the L-frame structures found on Okuma's Millac series, a C-frame is a traditional vertical machining center structure and is representative of a table-on-table stack up – meaning the X-axis is sitting on top of the Y-axis. Although the spindle can be mounted on the column or on a ram, the depth that is needed for the Y-axis to clear the spindle often causes the spindle to overhang significantly to ensure the Y-axis can travel sufficiently.

When evaluating a C-frame structured machine, be sure to evaluate the width of the base without taking the sheet metal into consideration. The actual base, or frame, of the machine is what provides stability as the machine is operating. As the X-axis travels, the narrower the overall base the more overhang the X-axis may encounter as it shifts from one extreme to the next - causing cutting characteristics to be different depending on where the X-axis and Y-axis fall within the cutting zone. In short, when choosing a C-frame structured machine, look for the widest frame that provides the widest support possible for the X-axis travel to ensure a sturdy-built machine tool.

Another consideration when exploring a C-frame machine tool is the guideways that are being deployed. There are two well-known guideways: linear guides and box ways. Linear guides allow for faster positioning with quicker acceleration and deceleration rates, where box ways provide rigidity that dampens vibrations, which can provide key benefits in hard-to-machine materials.

Double Column Machining Centers

Let’s define the physical features of a double column machining center (DCMC). Generally speaking, DCMCs are larger-size machines that have two stationary columns that support a cross-slide and a ram, and spindle assembly. The table moves between the columns under the ram. This allows for the separation of the X-axis and the Y-axis. Typically, the Y- and Z-axes are overhead, and the table travels in the X-axis.

A benefit of a double column machining center is the overall strength, rigidity, and support provided by the dual columns. Unlike traditional C-frame construction, the Y-axis travels overhead on the bridge cross-slide. By using two columns rather than a single column, the rigidity and stability of the machine tool are greatly increased to significantly reduce vibrations, tool deflection and, thereby, hold tighter tolerances while providing superior cutting characteristics. Because the table is fully supported (and therefore no overhang as discussed in the C-frame design) and the spindle is also fully supported, a part can be mounted anywhere on the table and have a full range of motion without any unsupported mass, achieving a harmonic-balanced signature and structural stability.

Almost all of Okuma’s double column machining centers use box way slides for Y- and Z-axis motion. However, the all-new MCR-S series is the first to utilize large linear rails for the Y-axis. The linear guides utilize linear roller bearings and provide a great advantage for high-speed, dynamic motion machining. With less friction to overcome, the acceleration, deceleration, and continuous feed rates are significantly increased. This is beneficial and makes it much easier to perform dynamic movements for high-speed machining due to the use of rollers that cut down on the surface-to-surface contact and friction as the table accelerates and decelerates.

A bridge mill is built with the same principle of a double column machining center, just on a smaller scale. Rather than having a cross saddle and ram assembly that are separate from the columns, most bridge mills have the cross slide and columns cast in as one piece due to the size of the structure being significantly smaller. The strength and support of the bridge structure are still superior over a C-frame design, again, due to its separation of the axes and ability to have full load-bearing support under the length of motion.


Lathes can come with a variety of configurations including single turret, twin turrets, and a variety of tool availability. Most lathes can be described as either a flat bed or slant bed design. The main differentiation is the elevation angles of the lathe. Okuma has utilized a slant bed, with a separate bed and base construction. The signature design feature for this construction is the motion system within the machine (where the X-axis, Z-axis, box guideways, ball screws, tailstocks, etc.) which is mounted onto a rectangular flatbed. This allows for equal heating and cooling of all components providing greater thermal stability and predictability. All the components including spindle, tailstocks, turrets, and sub-spindle, are mounted in the same plane so thermal effects are consistent. This flatbed is then mounted to a triangle-shaped base, which acts as a sine-plate, making it a true slant bed. This machine structure commonly possesses an air gap between the base of the machine and the flatbed component. This spacing keeps anything from the machining environment during the chip cutting process from making contact with the base – providing better thermal stability and support of the machine tool.

Benefits to any slant-bed construction include the reduction of the co-sine effect of errors, elimination of negative gravitational effects, and better coolant and chip flow. Going a step further and using a separated bed and base system provides superior thermal stability and tolerance control.

Partnering with Your Machine Tool Manufacturer

It’s great to have choices. Understanding the most common machine structure options is only the first step in realizing the potential available to your shop floor. At Okuma, we pride ourselves in adding to your shop’s productivity and helping you overcome any manufacturing obstacles, and that starts with our wide range of machines designed within each build structure category.

Do you have specific questions regarding an Okuma C-frame, double column machining center, lathe, or others? If so, remember, we are always here to help. Contact your local Okuma distribution partner.

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