I started writing this with the naïve thought this could be done quickly and in one page. That isn’t happening unless I speak of one material and one bit- not in a constructive way.
Because of this, I’m guessing making it a series will be better?
In that spirit, this one is going to cover ‘basics’ and nothing more- but with hopes to whet your apatite to share your own thoughts and experiences so we can all become better at our craft.
The anatomy of a CNC ‘tool’, aka Bit:
When we set forth to cut something there are considerations to, uh… consider.. which makes the job easier on tooling, the machine, time, and quality. I’m sure there are more, but these are what I’m gonna play with here.
Depending on the project you have a bunch of different tools to pick from- and each has a purpose. They’re all made in the same basic way, though, and yes this includes a V bit. The two most important aspects are the diameter of the shaft as that dictates which collet you use- which is often a constraint of the spindle’s set up, and the cutting diameter.
We’ll get into ‘how’ it cuts shortly.
One of the biggest if NOT the biggest discussions in our trade is Speeds and Feeds. Speeds are how fast that bit twirls, as in RPM… Feeds are how fast you shove it through the tool path. Shoving that tool through material with too much force while the bit doesn’t twirl fast enough to cut and clear the chips? – you’re going to break that bit. Going too slow, and you risk not only burnishing the bit but starting fires.
There is an optimum speed and feed for your tool and your material. It’s that simple- but what’s not simple is discovering what works best for YOUR setup. Bit manufacturers offer charts and they’re useful, but they aren’t perfect. They’ll often get you started- but so will this:
In most instances you’ll want to cut to a depth HALF the diameter of the tool. (this is in contrast to manufacturer settings). In most cases you’ll want to start your experimentation at manufacturing recommended RPM’s. If your software offers chip load calculations based on speeds and feeds, and most do, you’ll want to aim for a window of .008 and 0.01. This is a conservative range that will save you money and time when setting up your tool data base.
Please, however, don’t stop reading here- there are exceptions.
EXCEPTIONS with Speeds and Feeds
Really small flute bits are fragile. They can’t be pressed to extremes, and the starting point recommended settings above will break them. These are bits with cutting flute diameter of 1/8th and below. Slow down the feeds to a crawl. Pick up the speeds. As a for instance and using my Phantom S-Series mill with 6hp spindle and DSP controller, I run a 1/16″ up cut spiral long reach (cutting length around an inch) no faster than 80IPM, at a depth of 1/16″ (again in contrast to above settings) and at an RPM of 16k in most woods. That is what “I” run it at. Your mileage may vary.
Compression bits are a different sort- they start out as upcut bits, which if you stand the bit on it’s non-cutting end will have the flutes walking “upstairs” left to right, and transition to downcut geometry (if you have the bit standing on it’s end the flutes walk “downstairs” observing left to right)- the upcut clears material out of the kerf (the line the bit leaves in it’s wake– the material removed) and by that mechanic it leaves a pretty cut on the bottom or a clean cut at the depth it was ran, whereas the downcut presses into the material and leaves a pretty cut on top. This transition between upcut and downcut is GENERALLY at the diameter of the bit in length from the entering cut to the larger portion of the flute. Compression bits are made to run at least as deep as twice their diameter and cut through the material in one pass. Compression bits absolutely shine when cutting sheet goods especially plywood.
Spoil Board Bits, Planing Bits, Flattening/surfacing Bits are NOT to be shoved into the material as others. It’s a damn fine way to destroy your equipment. They are made to skim at relatively shallow depths. Duh… The plunge rate on these is tiny- like, 10 inches a minute tiny.. and they’re better served running slower in RPM’s- such as 12k RPMs… the depth is where it gets tricky for these. Some will peel off as much as a quarter inch of material per pass, while others will scale down to a 32nd of an inch or less in effort to save as much material as possible. Things to consider when twirling a surfacing bit: those things are heavy, they’re wide, and they may not be expertly balanced. If the spindle is twirling at 12k rpm and you’re running a 3″ surfacing bit (cutting diameter of 3″), how fast is the outside edge twirling? Hint: a lot faster than the spindle... and it’s easier to leverage against the spindle with that size of cutter and work it too hard needlessly. So…. take shallow passes… as a frame of reference, I twirl a surfacing bit which is three carbide “insert” cutters and 2.5″ in diameter at 14k RPM and 1/32″ deep and push if (feed) at 500IPM on MDF (spoilboard) without any issues. I run the same bit over massive planks of mahogany at depths up to 1/8th inch at the same 14kRPM but at around 275-300IPM. Surfacing bits run shallow and with slight plunge rates.
V-Bits and tapered bits should always consider the outside diameter of the cutting/flute area. On a V bit, if you don’t specify a flat bottom, it will attempt to cut to the depth of the bits diameter. These are generally not spiraling bits so they will chatter at deep passes causing visibly choppy lines. And, they can be tricky to learn as each has a purpose and a required set of considerations. Tapered bits are similar as you enter the cut at the tip of the tools diameter, but the deeper you cut the larger the kerf/opening is- and is dependent on the tapering geometry of the bit. The rule of thumb with these geometry bits, V and Tapered, is to cut deep enough to use the flutes on the side of the bit but not so deep as to have too heavy a chipload.