Tag Archives: cnc

Pocket NC power limit

I’ve been doing some machining on the Pocket NC lately to prototype some “design for manfacturing” improvements. Some time ago Q at Pocket NC posted that early versions of the v2-50 had a spindle power limit that they later decided wasn’t necessary. My v2-50 was pre-ordered at the launch, so had said limit. There was a procedure for removing it in later versions, which just required removing a single SMT resistor.

WOW! What a difference!

I’ve iterated such that with 4mm single flute tooling and the spindle extension, I was no longer having tool pull out issues, but the maximum MRR (material removal rate) I could achieve was still somewhat limited. My best recipe had 4mm step downs with a 0.2mm width of cut at 750mm/min (about the maximum speed of the Pocket NC). Given that in 6061, I basically always run at that 750mm/min, I measure the MRR as the cross-sectional area, or in that case 0.8mm^2.

With the spindle resistor removed, I’m able to get up to 2mm^2 cross sectional area! In practice, I’m mostly doing 1.6mm^2 to give a little margin, but that is still twice as fast. Granted the Pocket NC isn’t designed as a high speed VMC, but when I’m running parts that have an 8 hr machining time, getting those down to 4hr is a big win.

At this point, other things are the limiting factor. The stock enclosure I have needs emptying every 20 minutes of machining, and when removing large amounts of material across the full work area, you have to clear chips nearly continually from the X axis to keep it from blocking travel near the edge when the chips are flying in inconvenient directions.

Pocket NC windowed machining

When I first acquired my Pocket NC v2-50, I was planning on using it for rapid prototyping of small aluminum parts. I figured with 5 axes, I could do many things with a single setup just clamping from the bottom. However, I was initially thwarted in that plan and had to resort to more creative workholding solutions due to two problems.

First was the vice that came with the Pocket NC. It is serviceable, but provides very little clamping force if you want to hold something that is tall and skinny. For now, while it isn’t ideal, I’m making good progress with the wcubed vise.

Second was the range of Z travel. As shipped by Pocket NC, in order to reach the center of rotation, tools have to stick out something like 35mm. If you want to go beyond, that adds even more. This was a problem, as there aren’t that many tools that can achieve a reasonable material removal rate while sticking out that far, if they can do so at all. This, I’ve finally resolved with this Pocket NC “Q-Tip”:

With that modification, I got an extra ~15mm of travel, which means that I can reach the center of rotation with only 18mm of stickout which is completely reasonable for this class of tools.

Now I can finally “window” machine parts out of a few maximally sized generic blocks of stock with only a single setup. I’ve got 3.5″x3.5″ stock in a variety of thicknesses, which lets me do just about anything, if slowly, without having to worry about workholding.

That’s what it started with

And some video showing the cutting:

Recalibrating the Pocket NC’s X/Y home position

This summer I had to send my Pocket NC in for some service, when it came back, I immediately noticed that the X axis homing was very far off, something like 0.01 inches, as I was boring a hole in one side of a part, spinning it around the B axis, then boring a countersink in the other side. The two were very clearly not concentric. I suspect the homing mechanism shifted in transport or something, because the error was very consistent.

Pocket NC’s support was great as usual and I quickly received a screencast showing the location of the homing setting:

Step 1: Conf
Step 2: Server
Step 3: Launch in New Tab
Step 4: Configure
Step 5: Pick Axis
Step 6: Change value, select Save, then Sytem
Step 7: Restart LinuxCNC and Rockhopper

To calibrate the X axis, I just used the hole that was bored all the way through, and manually used the MDI to spin the B axes around and jog the end mill through the center of the hole. Then I used my calipers to measure the offset between the widest part of the mill and each side of the hole. Two iterations of that had the X error back to under 0.001″.

Fast forward a few months and I am running a part where the Y axis zero position matters. Sure enough, it is off too. Not as much, maybe only 0.004″ or so, but enough to make the part not work out. I tried a different technique this time, engraving an X axis line with a chamfer mill in two parts. One part with the B axis at 0, and the other half with it at 180. Any Y offset will manifest as a “jog” in the line.

Several iterations of testing
Checking it out under the microscope

I’m not sure if this was any more or less accurate than the boring method, but it was faster and seems to have also gotten me back down to under 0.001 inch of error in the Y axis.

New machine day: Vibratory tumbler

To date with my machined parts, I’ve mostly left everything in an “as-machined” state.  As I get ready to make some servos where I care at least a little about how they look, I decided to invest a little in surface finish options.  I started using some Scotch-Brite, which gave passable results for some components, but it was hard to be consistent and the final results were always somewhat anisotropic.

Thus, a new vibratory tumbler!

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This is designed for polishing ammunition cases, but works fine for any metal parts that aren’t too large.  I’ve been able to fit entire back housings from the mk2 servo into it, although at that size the polishing isn’t super efficient.

The resulting parts look pretty decent for features that the media is able to reach, definitely better than my hand attempts:

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Left: as machined, Center: Hand Scotch-Brite, Right: Tumbler

 

mk2 leg knee stud

One of the parts on the original quad A0’s leg that was prone to failure was the “knee stud”, a little cylinder that acted as the mating interface between the upper leg and the lower leg.  It directly attaches to the upper leg, and has bearings that ride between it and the lower leg.  The entire tension of the leg belt is born in shear by this part.

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In the mk1 leg, this part was 3d printed with heat set inserts used to form the threaded holes.  This mostly worked, although occasionally the stud could shear along the 3d printed lamination lines.  Thus, for the mk2 leg, I’m making this part out of 6061.

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The first op takes a 0.875 inch cylinder, and does all the work on one of the sides.  That includes roughing it down to length, getting the outer diameter that the bearing rests on accurate, and drilling and threading the holes.

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5x knee studs with the first op done

At that point, the part is turned over and bolted into a 3d printed fixture.

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Then, all the tool paths are repeated on the other side, as well as the middle being cut away.  I didn’t really worry about surface finish on the middle section, since it will never be seen.  This of course would be much easier on a CNC lathe with live tooling, but hey, you use what you’ve got!

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All 5 that I made

wcubed vise for Pocket NC

Just because I’m generally looking for workholding solutions for the Pocket NC, I recently picked up a vise designed for it from wcubed.co.

vise.jpg

Unlike the stock vise that comes with the PNC, this has two movable aluminum jaws.  It can probably hold with greater force than the stock vise, since there is a larger contact area, although the screw mechanism doesn’t necessarily apply the force all that uniformly.  Also, since both jaws are movable, you have to take some care to either manually center things, or do some edgefinding, which isn’t terribly easy on a PNC.

What it does allow though, is clamping narrow things.  The stock vise bottoms out at around 0.5″.  This vise can go all the way down to 0.

That came in handy with some recent moteus servo parts that I wanted to do a “5-axis” style toolpath from 3/8″ thick bar stock.

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Is it really that tall?

The vise provided plenty of clamping power to hold and machine at the tip of this awkwardly long bar.  This cut does chatter like crazy, but that’s about what you would expect.

 

Preparing stock on a CNC Bridgeport

As mentioned previously, I made up some soft jaws to hold 4in round stock in a 6″ vise.  My goal was to prepare stock for workholding on the Pocket NC v2-50 to machine prototypes of the front and back housing for the reduced weight moteus servo mk2.

Now, I’ve used those soft jaws to trim down both pieces of stock to the correct length, bore a center hole, and in the case of the front housing, remove a bunch of additional material in a more expeditious manner.  There’s not much more to it than that, so here’s the video:

First soft-jaws for CNC machining

While working to build the reduced weight moteus servo mk2, I got tired of hand machining the first operation on a manual mill and lathe for the front and back housings.  It was necessary, primarily to enable workholding on the PocketNC v2-50, but also because it allowed me to remove much of the excess material more quickly than could be done on the PNC.  So, I got trained up on the AA CNC Bridgeport and went to town.

The manual work I did on the mill used V blocks to hold the round stock, but for this I wanted something that was more repeatable and would offer more gripping power.  Thus I decided to try my hand at soft jaws for the first time.  I got some blanks from MonsterJaws which would fit the vise there and got started.

For the CAD/CAM, I grabbed a random 6″ Kurt vise model from the interwebs and stuck my part in it.  Then I added the vise blanks and used a “combine” operation to subtract out the stock from the blanks.

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Then, when doing the CAM, I just ran a 3d adaptive followed by a finishing contour pass:

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When I ran the actual toolpath, I messed up and had the spindle running about 1/3 of the speed I wanted, which made for some nice chomping noises, but it did cut.

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Blanks ready to cut

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Stock mounted!