CNC Notes & Product Selection Information

Which one to choose - Router or High-Frequency Spindle?

NOTE ShopBot Tools recommends and sells Porter Cable 3.25hp routers, Model 75182, for our CNC systems in which a spindle is not installed.  Mounting brackets and electrical connections are designed for this model router.  Other models may not fit or function correctly in this bracket and may thus be unsafe.

ShopBots can be fitted with a variety of tools/heads for cutting and machining. The most common tools are a standard woodworking router or a high-frequency industrial spindle. These tools are similar in principle; basically a motor for turning a cutting bit at high speeds.  A router (we recommend and sell a Porter Cable 3.25 hp router) provides the affordable solution for a CNC tool.  We also sell several high-frequency spindles that are the industrial grade solution for a CNC tool.  Power (usually 3-phase) for a spindle is routed through an inverter that converts it to a higher frequency for powering the spindle/motor.  Spindles have the advantage of being more powerful (note that hp ratings between routers and spindles are not comparable) and will maintain their full torque down to much lower RPMs than a router.  Spindles have precision bearings, which mean less run-out and thus smoother and more accurate cutting. These bearings also stand up to continuous use better than those of a router.  Bearings in a heavily used router might need to be replaced every three-four months, while a similarly used spindle should be good for a year or two between bearing replacements.  A high-frequency spindle is also much quieter in operation than a router.

3.25 hp Porter Cable Router on Z axis

4 hp HSD Spindle on Z axis

For many shops, the cost and power requirements of a high-frequency spindle will be prohibitive.  A ShopBot will certainly cut fine with a standard router.  It is also possible to get started in CNC with a router, and then fit your ShopBot with a spindle at a later date.  If you don't need a heavy cut, and noise is not an issue, then a router is a safe choice.  For heavy production situations, we recommend a spindle.  In addition, the full performance capabilities of a PRSalpha are only realized with a spindle because a router will bog down in heavy cutting at higher cutting speeds.

Conventional vs. Universal Vacuum Hold-Down

There are two general strategies for the use of vacuum in holding parts for cutting or machining with a CNC router. You can either create seals around specific areas to be vacuum clamped in a conventional manner, or you can draw air through a broad area of a permeable bleeder board to create a more universal vacuum.

Conventional:  The conventional solution is usually less expensive. It makes use of high vacuum (20-29”Hg) with a low flow volume. It works well for machining parts that are not cut out with through-cuts.  It works well if you are cutting repetitive parts for which it is practical to first cut a vacuum template that maintains the vacuum seal, even when the part is through-cut.  With conventional vacuum systems, small leaks will cause a large drop in vacuum, which is why the vacuum seal is so important to holding your parts securely.  When using conventional techniques it is often adequate to use: a shop-vac, compressed air venturi vacuum, small blower, or rotary vane blower to produce the vacuum.  Conventional vacuum solutions tend to be specific to the application or production process and are usually devised by each shop.  The plenum system, described below, could be used in conjunction with vacuum templates in a conventional system, even though it is designed for universal vacuum and bleeder board.  ShopBotters have experimented with a variety of conventional systems and with a number of accessories for improving holding.  You can find information on the Talk ShopBot Forum.  There are also links for vacuum accessories in the ShopBot CNC Resource List.

Universal:  In contrast to conventional vacuum holding, universal systems have an advantage in that you do not need specific sealed vacuum fixtures or templates.  You can just put a piece of material down on your table, apply the vacuum and cut, assuming there is enough surface area to the parts for the vacuum to hold the material while the parts are cut.  The universal system does not usually require a vacuum template.  Material can be placed directly on a bleeder board and sufficient vacuum can be maintained as long as the required airflow does not exceed the specifications of the particular blower.  The universal solution requires a lot of airflow and depends on relatively powerful and expensive industrial blowers/pumps that typically require 3-phase power.  We sell several different types of universal vacuum hold-down systems.  Each includes all the necessary plumbing to set up a multi-zone system for your tool.

For more information on 3d cutting

 Read > Ted's Introduction to 3D

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