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Frequently asked Questions- Welding vs. bolts

Is welding better than bolted construction?

Welding would on the surface seem like a superior joinng method than bolted together construction. After all doesn't the term "all welded construction" give a feeling of solid well built machinery.

But welding causes its own set of difficult problems. Welding can impart distortion and stress after the high temperature necessry for welding steel (and especailly aluminum) as the metal cools. This necesitates that the entire stucture be stress relieved via a process called "nortmalizing" where the entire machine frame is heated to a high enough temperature  to anneal the metal to reform the internal grain structure of the metal to be stress free. As you can imagine a very large expensive oven is used. Many cheaply made welded CNC routers simply skip this step since the manufacturer may not have an oven large enough to heat the entire machine in.

Even after normalizing there is still the problem of warpage and distortion. The surfaces that the linear ways are mounted to must be either ground or machined at very high cost to make them true and parallel. All of this can introduce cost and manufacturing delays., And keep in mind that bolts must still be used to mount the hardened and ground linear guideways since welding would soften their hardened surfaces! And even the spoil board is bolted to the machine bed. So much for all welded.

You also may be at the mercy of the skill (or lack therof) of whoever welded your machine together. Once welded together, cutting and rewelding a machine frame is a daunting task at the machine's site.

A welded frame where the metal is fused together also tends to act like a giant "tuning fork" under the vibration of cutting. High end CNC manufactueres are aware of this and will use cast metal which absorbs this unwanted vibration. But lower end welded CNC machine manufactueres don't have the equipment to cast a bridge structure and instead use welded steel tubing

Also, remember that, with the high cost of gasoline these days, shipping a monolithic welded together large cutting format CNC machine can be very expensive indeed.

Bolting a machine structure together on the other hand has many hidden advantages over welding. In the case of aluminum extrusions which are very straight right from the extruder (think of a panel saw carriage), bolting is definitely a low termperature, low distortion assembly method.

If there are any mismatch in the parallism or perpendicularity of any axis, the extrusion can be simply unbolted, fine tuned, and then rebolted together - something that is simply not possible with a welded together machine.

Bolting also has a very good vibration damping properties which are very desirable in any machine tool. As the bolted together surfaces micro-shift against one another at a microscopic level the friction caused by the nut and bolts connection actually absorbs and disipates the vibrational energy.

And finally, a knocked down table construction allow huge cost savings since the entire table can be shipped in  a compact form (trucking companies charge by the volume of freight they can get on board as well as the weight) and can be bolted together at the shop. This analogy is RTA furniture which is so much more affordable than high end fully constructed furniture.

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