10 Things to Consider When Buying machine way covers

Having previously been a Manufacturing Engineer I crave facts, statistics, and an understanding on why one way is better then another. Coming out of college working in Manufacturing I learned that getting these facts is often difficult to come by on CNC machine Construction. I felt too often I was getting a sales pitch rather than facts. For this reason, I will be writing on some of the construction techniques used in the manufacture of CNC mills. Why they are used, and what difference this make to you-- the machinist, or shop owner. First will be Way systems, specifically Box ways versus Linear guide way systems, as this is one of the most frequent questions I am asked.

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The way system is what a machine moves on in X, Y, and Z directions on most CNC machines. Two primary styles are utilized in construction of modern CNC equipment. The first is called Box way construction. In this style rail is meticulously ground and heat treated portion of the machines casting. This style of machines then run metal-on-metal or Metal-to-Polymer on a thin film of oil. The second style is a Linear motion guide, a product with several different varieties. I will elaborate on the two most common. The linear guides consist of two components: the rail, and the block. The Block slides along the rail. While the rail is secured to the machine casting, and the blocks are secured to the rails.

When should I consider a Box way machine? They are ideal when you need high weight capacities, and rigidity and dampening are critical. Box ways generally have superior dampening rigidity characteristics. Simply put, they allow for greater surface area contact, thus getting the least bending, deflection and deformation when under-load and stressed. This creates the ability to take the heaviest loads and heaviest cuts. Since the Box way is part of the castings, the principles of mass in dampening takes effect, providing the best tool life. Finally, due to the robustness of construction, this style of machine is less likely to have accuracy affected if crashed. 

Its down side comes in speed, in general box ways will not have the same rapid or acceleration rates of a Linear guide machine. The second deterrent is cost. To precision grind and heat treat box ways is a costlier manufacturing process. If you do damage a Box Way, repairs are extremely costly and labor intensive.

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Linear guides are more complex subject due to the fact two distinct types are commonly used in CNC manufacture. These are the Ball type of Linear Guide (below top) and the Cross Roller type (below bottom). Regardless of style, this type of Way System has less friction and in general will have faster rapid and acceleration rates when compared to a Box way machine. They are also more simplistic to assemble to a machine, often leading to lower costs.

Lets now talk about the two main types of linear guides, first the Ball type Linear guide, are the most common type of way used in CNC construction today. The chain of balls ride in two grooves in the linear rail designed to handle multi-directional loads. Each making a point contact, so your surface area is limited to a single point of contact of each ball. In the Cross Roller style, you typically have 45 degree angle of contact of rollers (think a cylinder) acting on the two surfaces on each side of the linear rail. This makes your surface area dramatically larger when compared to the ball style as your contact point is a line equal to the height of your roller. Ultimately what this means is on the same size linear rail, a Cross Roller style Linear Guide will have 20-40% greater Dynamic Load capacity and a 40-60% greater Static load capacity. More importantly Cross Roller Style of Linear guides will have 25-45% greater resistance to moment bending (image below) on the linear block which is critical when you are doing work where accuracy and precision are at a necessity.

Beyond the different types of Linear guides, there are a variety of linear rail sizes. The most commonly used are from 30mm-65mm, and each of which will have different capacities. Finally, you must consider the number of blocks in each axis, and the length and width of these blocks. This is important as this will directly correlate to the surface area, and rigidity of the overall system. In summary when you are looking at a linear guide style of machine it is important to understand the type (Ball or Cross Roller), size (Both of the Rails and Blocks) and quantity used in each axis. Generally speaking, Cross roller style with larger rails and blocks are seen in higher cost machines.

All these systems have their place. Ask yourself which is best for you based upon your needs. What are your driving factor on your next machine? If speed and lower up-front cost are important look at a machine with ball type linear guides, if rigidity, weight capacity, and tool life are of paramount importance look at Box way machine. Finally, if you need a blend perhaps a Cross Roller way is right for you. Asking the right questions based on your needs will assure you get the best value from your next purchase!

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