Deciding on Best Depth and Width of Cut When Milling

I got a note recently from a G-Wizard user who wanted to know how to decide on best depth and width of cut when milling. It’s a great question. Most machinists, I suspect, use rules of thumb and habit more than anything else unless the situation dictates something in particular based on the dimensions of the feature being machined. They’re used to using some fraction of the cutter’s diameter or some figure that they got to some other way through habit (40 thousandths or some such is what they’ve always used). Perhaps their CAM program has a hardwired default that is a percentage of the cutter’s diameter.

But these values, while they have worked over time, are not necessarily optimal figures with respect to material removal rates, tool deflection allowances, or a host of other variables we might choose to consider. What’s a more systematic way to approach the problem?

First thing is we have two variables (width and depth of cut), so it’s hard to make progress unless we can nail one of the two variables down and focus on the relationship of the other.… Read the rest

Cutting Chatter When Precision Boring

I just came across what looks like a great blog from Criterion, the boring head people. They had a great tip to reduce chatter: make sure your depth of cut is greater than the radius on your cutter. It makes sense, and should apply when turning too. Here is how they explain it.

Consider the cutting forces when the insert is cutting less than the radius:

See how the forces (arrows) are largely trying to push the insert out of the cut?

With a deeper cut, there are more arrows acting to stabilize the forces so they’re not all trying to get the insert to deflect and skip over the cut.

The result is a better finish and less likelihood of chatter. Having to increase depth of cut in this situation is just one of the many counter-intuitive situations we encounter when machining!… Read the rest

The Web is All About Sharing

The Web is a Social Place. It’s all about Sharing your finds with others. While search engines like Google are great, and I use them constantly, often the real gems come via friends who tell me what they’ve found and liked. For a long time I’ve kept a CNCCookbook page on Delicious of all the many interesting pages I come across. They have a tool bar for your web browser to make it easy to bookmark interesting things you come across. Check it out (click on that link above) to see nearly 2000 articles with my summary of what interested me as well as tags like “mill”, “lathe”, “cnc”, “tooling”, and “workholding” to help you focus in on topics.

That database of Delicious (love the name) links is my “newswire” for writing posts here. I grind through a lot of information so consider it sort of a Reader’s Digest. If you like CNCCookbook, you’ll probably like a lot of those articles too.

Meanwhile, you may have noticed all the CNCCookbook pages now have their own sharing bar up in the top left corner near our logo.… Read the rest

Of Pallets and Parts

I’ve always like the idea of pallets that can be set up with new parts while the CNC machine is working on another set of parts. Sort of like quick-change fixtures of a sort. You know it has to contribute to productivity, because when the machine stops you can swap the pallets real fast and the spindle turning again while you pull the parts of the finished pallet and set new workpieces in place to go back in. But how much productivity improvement is possible?

I was reading through a post on the LinkedIn CNC Machining and Manufacturing Network (pretty good group, actually) and saw some interesting figures being quoted by a former sales guy from System 3R. I look their web site and think large scale manufacturing and keep moving (I’m more interested in small operations and techniques that work for a lot of different parts), but what René van der Peet had to say was exactly on target for smaller shops:

- A machine, without any equipment, used during daytime in one shift, produces an average of 800 billable hours each year.… Read the rest

Ballnose Surface Finishes

It’s a small world, I guess. No sooner did I get done adding a section to my Turner’s Cube page showcasing Widgitmaster’s lathe-turned versions when I find him asking a question on CNCZone about how to calculate stepover to achieve a desired finish when profiling with a ballnosed cutter. As it happens, this function is built into G-Wizard like so:

Ballnose information appears below the RPM and Feedrate when the “Ballnose Cutter” choice is checked. It provides the effective diameter based on depth of cut and for a given scallop height will tell you the RA/RMS Surface Finish and required stepover. In this case, to achieve a maximum scallop height of a tenth (0.0001″) for an RA finish of 100, we need a 0.011″ stepover. That’s about 2.2% of the tool’s diameter.

Feeds and speeds for ballnosed cutters can be a bit tricky because the effective diameter of the tool is based on the cutting depth whenever you’re less than have the diameter of the tool deep. Think about it, the end of the cutter is a ball rather than the usual cylinder shape.… Read the rest

Interpolating Holes vs Twist Drills

They say nothing removes material faster than a twist drill. Just one problem, it only removes a cylinder of it, so it can’t really profile or pocket (the exception being pluge roughing, but I’ll save that for another time). Although, profiles and pockets often begin with the need to get the endmill down to proper cutting depth. Given that you know how long a tool change takes on your CNC mill, how many such plunges do you think are required before you’d be better off to use a twist drill to do the initial plunge and then let the endmill interpolate off of that?

It turns out to require fewer holes than I would have thought, and it’s pretty easily to calculate with a little help from G-Wizard to get the feedrates.

For a 1/2″ HSS 2 flute in 6061, GWiz gives a plunge feedrate of 4.96 IPM. A 1/2″ HSS Twist Drill can be fed at 15.28 IPM. That difference in speed, with the Twist Drill being a lot faster, has to make up for the toolchange time. In fact, let’s say we want to change twice–from endmill to twist drill and back.… Read the rest

I’m Back, Though Not 100%

People were starting to write and complain they hadn’t had their CNCCookbook “fix” in a while. Sorry about that folks–I was off in Cozumel, Mexico on a diving vacation. All work and no play makes Bob a very dull boy. At least now I’m back, though not 100%. I seem to have either a particularly bad cold or a mild case of the flu. It’s definitely on the mend, so I wanted to get back to writing some Cookbook posts again.… Read the rest

Drawbots

I love the idea of Drawbots!

What’s a Drawbot? It’s a little CNC machine used for drawing or painting. They’re typically 2D devices that are suspended on cables. If you’ve been to an amphitheater event any time recently that had a lot of video, you may have seen a video camera suspended on something similar. Here is a typical Drawbot:

In this case, the Drawbot hangs on the wall as a piece of kinetic art. It takes hours or days to finish a piece, but its always busy doing something, so is interesting to look at. Here’s a close up of the piece its working on:

There’s a fair amount of scoop on Drawbots over on the Makezine site. As I understand it, you can run a Drawbot with Mach3 by putting the correct math into the formulas for the axes. In this case, we have to simulate X and Y motions using the string droop.

I’ll have to try and make one some time. I’ve seen some set up for doing large commercial sign painting that seemed cool. Van murals, anyone?… Read the rest

Ferrari V4 Motorcycle

Crazy cool is all I can say:

Nothing like a good rendering to stimulate the appetite, but it leaves you feeling hungry, doesn’t it? How long before some intrepid CNC’er decides they can’t wait to have one, I wonder?… Read the rest