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I didn’t finish with the CAM part, but the CAD work only took 10 minutes and I didn’t use any shortcuts. It’s not hard to do if you want to play with plunge milling a bit. Programming something like this to clear a corner or two would be even easier.

First, does it do true 3D or just 2D plunge milling? 3D is obviously much more general while 2D will only work for flat-bottomed features.

MT-17F: (-0/+0.003" dia) (-0 length) 17g/cc density. 90% Tungsten, 7% Nickel, 3% Iron. High quality tungsten heavy alloy (W) rod in 12" lengths conforms to international standard ASTM B777, Grade 1, Type II & III. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is perfect for machine shops, labs, research and development, and DIY machinists. Tungsten alloy is much easier to machine than traditional pure tungsten. Other material names that match the same ASTM standard: HA170, HD 17D, HD17BB, SD170, CMW 3000, K1700, M3000.

One could imagine doing most of the roughing with an high-speed machining toolpath and an end mill that is much to large in diameter to get into the corners. Depending on pocket shape, this can clear most of the material without leaving much scalloping.

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One last special case for plunge milling would be corner clearing. When the depth of the corner is greater than about four times the cutter diameter that fits into the corner, rigidity issues develop.

That’s about all there is to it. You now have a CAD drawing that shows where the holes for the plunge strokes need to go.

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Older (or cheaper) CNC milling machines that have more slop in the XY axes, less precise interpolation, or slower spindle speeds may also benefit from exercising the Z-axis more via plunge milling. plunge milling seems tailor-made for the limited rigidity and performance of hobby CNC machines too, for example.

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As part of the final finish or an intermediate semi-finish pass, we use a much smaller diameter end mill to clear the area of the corner and then we can make an overall finish pass of the entire wall of the pocket or profile.

The technique worked and allowed a Bridgeport to open up a lot of material quickly. But, there are disadvantages. Twist drills don’t like overlapping holes and interrupted cutting too much, so mind your stepovers accordingly. No more than 40 percent overlap can be tolerated if the drill has a conical point, and even this much can lead to a lot more wear. This may or may not lead to good material removal rates, so this is another case where experimenting with a few scenarios in G-Wizard can really help refine your approach. Also carbide twist drills don’t like the shock associated with plunging too much. High-speed steel or indexable tooling may be a better bet.

I like the idea of plunge milling for lights-out-machining too. It seems inherently more conservative and less prone to problems like whether coolant is aimed properly or not.

How about a mill-turn situation where your live tooling is not nearly as rigid as on a pure milling machine? Here again, you may find plunge milling is just the ticket.

G-Wizard makes it easy for you to evaluate which one will perform best for your jobs given all the variables that are in play.

Second does the plunge cycle retract away from the wall during the overall retract? This reduces chatter and increases tool life when machining tough materials. Here’s the plunge rough retract style jointly developed by WorkNC and Ingersoll:

Just to keep things simple, I’m going to choose X and Y stepovers of 0.0625-inch for this job. Let’s check Feeds and Speeds with G-Wizard and assume we want to use a high-speed roughing strategy like Adaptive Clearing or VoluMill:

MT-17C: (-0/+0.003" dia) (-0 length) 17g/cc density. 90% Tungsten, 6% Nickel, 4% Copper. High quality tungsten heavy alloy (W) rod in 12" lengths conforms to international standard ASTM B777, Grade 1, Type II & III. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is perfect for machine shops, labs, research and development, and DIY machinists. Tungsten alloy is much easier to machine than traditional pure tungsten. Other material names that match the same ASTM standard: HA170C, HD 17, Dens21, CMW 1000, K1701, M1000

A general discussion on plastic cutting tool geometry can be started by dividing plastic into three general categories: hard, soft, and reinforced.

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Plunge milling leaves scalloped edges which may take a fair amount of cleanup or a semi-finishing pass before a true finish pass can be applied. Image courtesy Sandvik Coromant.

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Import it into your CAM package and treat just like trying to drill all those holes. It’ll generate the G-code which you can then further tweak as needed.

Have you got a tough job ahead, either because your machine is too lightweight or because of challenging geometry on the part (deep pockets, for example)? If so, maybe plunge milling (also called plunge roughing) is the answer.

See tool path diagrams on page 304. ... At 50% radial depth of cut (ae), the cutter runs along the pocket side with a tool engagement angle of 90 degrees.

MT-17D DieAlloy: (-0/+0.003" dia) (-0 length) 17g/cc density. 90% Tungsten, 4% Nickel, 4% Molybdenum, 2% Iron. High quality tungsten heavy alloy (W) rod in 12" lengths. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is specifically designed for die casting. It will extend your die life, make castings faster, and lead to fewer rejects. Tungsten alloy is much easier to machine than traditional pure tungsten.

Normally if your CAM software doesn’t support a particular kind of toolpath, you’re just out of luck. But plunge milling may be different, depending on how adventurous you are. A basic strategy for creating your own plunge milling toolpath works like this:

Retracting from the wall slightly during each plunge rough stroke can improve tool life by 10 to 15 percent, according to the developers of the technique.

Here is a list of the most popular software from our last CAM Package Survey that shows whether each package has plunge milling or not:

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MT-18C: (-0/+0.003" dia) (-0 length) 18g/cc density. 95% Tungsten, 3.5% Nickel, 1.5% Copper. High quality tungsten heavy alloy (W) rod in 12" lengths conforms to international standard ASTM B777, Grade 3, Type II & III. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is perfect for machine shops, labs, research and development, and DIY machinists. Tungsten alloy is much easier to machine than traditional pure tungsten. Other material names that match the same ASTM standard: HA180C, HD 18, Dens25, CMW 2000, K1801, M2000.

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The material removal rate isn’t bad at 3 cubic inches per minute, but deflection is way too high. This approach is going to chew up tools very quickly with almost 0.003-inch of deflection.

How about machining a tall thin wall? This is a notoriously chatter-prone situation that may be partially amenable to plunge milling. It’s not a total cure, because you’ll still have to manage a finish pass that removes the scallops, but it might allow higher material removal rates without chatter for the roughing pass.

G-Wizard’s Plunge Milling Mini-Calc lets us enter a Step Up (amount to move into material at start of each pass) and a Step Over (amount to move laterally from prior hole on the same pass) and adjusts the feedrate based on those parameters.

MT-175: (-0/+0.003" dia) (-0 length) 17.5g/cc density. 92.5% Tungsten, 5.25% Nickel, 2.25% Iron. High quality tungsten heavy alloy (W) rod in 12" lengths conforms to international standard ASTM B777, Grade 2, Type II & III. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is perfect for machine shops, labs, research and development, and DIY machinists. Tungsten alloy is much easier to machine than traditional pure tungsten. Other material names that match the same ASTM standard: HA175, HD 17.5, SD175, K1750.

Another way plunge milling can help overcome a machine’s limitations is when spindle speed is limited. Plunge feeds and speeds may be a little slower than straight up high-speed machining. But, if your spindle RPM is the limiting factor, you may not be able to take full advantage of high-speed machining. If that’s the case, plunge milling is even more likely to give you the best material removal rates.

MT-185: (-0/+0.003" dia) (-0 length) 18.5g/cc density. 97% Tungsten, 2.1% Nickel, 0.9% Iron. High quality tungsten heavy alloy (W) rod in 12" lengths conforms to international standard ASTM B777, Grade 4, Type II & III. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is perfect for machine shops, labs, research and development, and DIY machinists. Tungsten alloy is much easier to machine than traditional pure tungsten. Other material names that match the same ASTM standard: HA185, HD 18.5, SD185, CMW 3970, K1850.

The stepover to move deeper into the material for the next pass is limited by the insert diameter or the maximum cut width of any non-center cutting tool; 80 percent of that value is a good choice as well. Keep an eye out so there’s no skinny stalks sticking up in the corners between the holes. If you see stalks, you need less stepover in one dimension or the other.

Sandvik recommends starting with a stepover (Cut Width in G-Wizard) of 80 percent of cutter diameter for the sideways motion of a single pass.

Aside from special cases, some shops report that plunge milling allows them to do roughing on older machines in the shop that might otherwise go unused. They keep their newer machine centers busy on jobs the older machines can’t do, and over shop spindle utilization goes up. That means more profit.

Here’s a case where plunge milling shines: our material removal rate is back in the three cubes a minute territory of the original high-speed machining scenario, and while the Deflection error is still red, we can ignore it because we’re plunging and there will be no significant deflection while doing that.

Now we need feeds and speeds. Our G-Wizard Feeds and Speeds Calculator has special features to help with plunge milling.

The graphic shows a typical plunge milling operation for a square pocket, courtesy of BobCAD. As you can see, a chain of holes are plunged vertically with the tool to rough out most of the pocket area. A subsequent finish pass will complete the pocket.

There are certainly embellishments. For example, you could do the fancy wall retract move with a little bit more hand-coding. You may also need to deal with entering the pocket, though you could use just have your CAM package generate its entry and then cut and paste that G-code to create a starting point for your Plunge Milling routine.

Let’s work through an example that shows the strengths of plunge milling. Suppose we are handed the task of roughing out a pocket that has 1/8-inche corner radii, which dictates and end mill no larger than 1/4-inch in diameter. Next, suppose that pocket is 1 1/2-inch deep.

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One shop found that button cutters (round insert indexable tooling) worked especially well for their needs. Normally, Plunge Milling Specialty Cutters use flat-bottomed inserts, but the round inserts were much less prone to chatter than the flat-bottom inserts.

I can already hear the groans out there in the audience–a pocket that deep with such a small end mill is likely to be a bear!

One thing to be careful of when Plunge Milling is any cutting near the center of the tool. Many indexable tools can’t cut in the center and many end mills are also not center-cutting. That means you’ll have to plan your stepovers so no center cutting is called for. You may need to create a starter hole with a drill (indexable or otherwise) to avoid center cutting issues.

I call plunge milling a “secret weapon” because it can really save your bacon in some situations. It is designed to take advantage of two important properties:

With a little bit of G-code programming familiarity and some decent chops with your CAD software, this is not hard nor does it need to take very long.

And speaking of a lack of rigidity, plunge milling can be ideal for those really deep pockets where side forces are causing so much tool deflection you can hardly make progress at all. Sandvik says plunge milling is advantageous any time overall tool stick out is more than four times tool diameter.

Let’s start by taking a look at what plunge milling is. The idea, is to rough out a pocket, profile, or 3D surface by plunging either a twist drill, an end mill, or a special-purpose tool straight down into the material. Here’s a typical example:

Plunge drillbit

Another issue with twist drills is the holes are not flat-bottomed. There are versions available that are flat-bottomed, both indexable and solid, so consider those. Otherwise, you either want a through situation (a hole not a pocket) or you’ll have quite a lot of finish work to do on the scalloped floor of the pocket.

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Perhaps you have a relatively lightweight or less rigid machine. By taking advantage of the greater rigidity your machine will have in the Z direction, you may be able to get higher material removal rates. Or, you may be able to overcome a chatter problem that’s due to a lack of rigidity.

MT-18F: (-0/+0.003" dia) (-0 length) 18g/cc density. 95% Tungsten, 3.5% Nickel, 1.5% Iron. High quality tungsten heavy alloy (W) rod in 12" lengths conforms to international standard ASTM B777, Grade 3, Type II & III. You will receive a complimentary material cert in the safely packaged shipment. All material is lot controlled, stress relieved, and formed through high temperature pressing and sintering processes. This alloy is perfect for machine shops, labs, research and development, and DIY machinists. Tungsten alloy is much easier to machine than traditional pure tungsten. Other material names that match the same ASTM standard: HA180, HD 18D, SD180, CMW 3950, K1800, M3950.

A quick click on the “Cut Width” label has Cut Optimizer taking us all the way down to a Cut Width of only 1.4 thousandths and there’s still a bit too much deflection. We could probably live with that, but MRR is down to a lousy 1.1 cubic inches per minute.

As far as differences in plunge milling toolpath quality, it’s worth checking on whether your CAM package supports two capabilities.