Tips & Tricks

[Under development]

Working with a CNC router provides many challenges.  The combination of hardware and software, and the usual tasks associated with managing a computer adds to the skills required.  For the hobbyist, or small workshop operator, it is likely that a range of CNC machining tasks will be attempted, each with their specific challenges. While there are many online resources, few are able to provide sound advice based on real experience.  Care is required to filter out the talkative out from the experts.

Here we attempt to offer sound advice based on real experience - enjoy.

The topics are in order of creation, not any specific suggestion of importance.

Index

  1. Choosing Router Bits
  2. Feeds and Speeds

Choosing Router Bits

Router bits are central to a successful machining task.  The basic ones tend to be spiral fluted end cutters, either with a flat end or ball end.  Flat end cutters are used for a range of machining tasks, including:

Ball end cutters are typically used to cut curved surfaces, both convex and concave.  With the appropriate computations, the ball ended cutter can produce quite accurate curved surfaces providing the depth increments are kept small.  They can also be used for machining grooves with curved bottoms, especially if a cutter of the required diameter is available and the groove can be cut by following a single line, perhaps with several increasing depth passes.

Other special shape cutters can also be used (as they are in a hand-held or table router) where special shaped cuts are required.  The obvious example is for engraving text where a Vee shaped engraving is required - naturally a Vee shaped cutter is used, with a particular angle of cut.

Some example and common CNC cutters:

It is often possible to use cutters intended for the table router in a CNC machine.  Straight-edged cutters can be used, while other shapes can be used for engraving tasks.  Some table router bits come with a bearing to limit the depth of cut (meant to follow a prepared edge).  In the CNC these bearings will probably need to be removed.

Most spiral-fluted bits have a clockwise spiral (when viewed from the cutting end).  The purpose of this is to feed the swarf (chips) away from the cutting edges and clear of the stock. This is described as a up-cut bit.   While this is probably quite effective it increases the possibility of tear-out at the stock surface, possibly leaving a rough edge to the cut.  For this reason, if the quality of the surface is important, then it might be useful to use a down-cut bit. For the down-cut bit the flute is counter-clockwise, which has the effect of forcing the swarf down into the cut and away from the top edge.  As a result the top edge of the cut is less likely to be damaged, but at the expense of a less efficient clearing of the swarf (chips).

Down-cut bits are commonly used for inlay work (where the quality of the finished top edge is important).   Naturally if the cut goes all the way through the stock then the opposite problem will occur at the bottom edge.  The up-cut bit will produce a better bottom edge finish than the up-cut bit!  For such situations (which are common when machining plywood) it is possible to get special cutters, sometimes referred to as compression bits, that combines both down and up-cut elements, like these:

One strategy, which is quite workable for cutting plywood, is to create two cutting paths (typically as profiles); the first cut (perhaps to 25% of the depth) using a down-cut bit, then a second cut from this depth to the bottom of the stock using an up-cut bit (same diameter).  This requires a bit change, and a little more time, but you will be rewarded with good quality edges, both top and bottom.

Quality router bits can be expensive.  The quality of the cut will depend on the cutter maintaining its cutting edges and you may be prepared to pay a premium for this. However, for the novice the chances of breaking a cutter by attempting a too large/deep/fast cut, or by running into a hold down clamp!, might suggest that using less expensive cutters might be a wise starting point.  These are often more readily available from internet sources (especially from China).

The optimum performance of any particular cutting task will depend on:

Setting of feeds and speeds is a bit of an art, some tools are available, but experience an practice also counts (see elsewhere for Speeds and Feeds)

Speeds and Feeds

For the novice CNC user the selection of the appropriate speeds and feeds is a daunting task. Remember:

It is the combination of these two settings that is critical, and they are not independent of each other.  The optimum settings also depend on the material being cut and the depth of cut, as well as the power of your spindle and the robustness of your your machine.

Where to start?  There are some software tools that can be purchased that might provide a good starting point.  Once such tool is the G-Wizard Calculator from CNC Cookbook (for a fee).  There are also others, some free, that are available from the web sites of companies that manufacture/market router bits.  While most of these are set up for machining metals, some offer "wood" options.  It is also true that the properties of wood varies considerably so any tool is likely to give you a starting point only.

From my experience, and after some initial trepidation, it has been some experimentation and practice that I now rely on.  Here is a little series of steps that one might follow when tackling a new task with wood.

  1. Speed:
  1. Cutting depth:
  1. Feed Rate:
  1. Try a test cut:
  1. If your spindle shows a noticeable slow down when cutting, then you will need to choose a higher speed.  Some spindles may not produce much power if running too slowly.

After some trial and error you will begin to develop an intuitive understanding of the optimal combinations.  Always remember, listen to your machine and that if your machine sounds happy it probably is.

[More coming]