Well, after some distractions (a small contract for NASA, and struggling to learn solidworks (AARG)), I got back to designing this device, made major configuration changes to simplify, improve reliability and improve precision, and have almost finished now. Reasonable CNC quotes permitting, I'll have someone start making 2 or 3 prototypes in two or three weeks. So, last chance to add your two cents (or $64,000 worth) of ideas, and point out my worst ideas and design flaws.
I will attempt to post some photos here, and attempt to attach a PDF document that describes how it works. This is identical in principle to the original steps I outlined at the start of this thread months ago, but the configuration of the device has changed so the exact nature of many steps is somewhat different.
Sorry, but you're probably going to need to maximize your window to see the images. If you're willing to make a serious effort to critique the design, ask me for the solidworks 2010 files.
I usually think up really great, elegant names for things. This time I'm a total failure (!please help!), so the device is called "papec" for now (pick-and-place extra cheap). Sheesh.
First, here is the description of the device as currently designed:http://www.iceapps.com/papec_description_0003.pdf
The following is the overall view in solidworks:
The main components from bottom to top are:
#1: the PCB cradle
The PCB cradle
holds the PCB and slides around on the benchtop or tabletop to position the component template
surface under the camera
or vacuum pick-and-place tip
. The component template
is the 60mm square plate near the front edge of the PCB cradle. The one shown is for all BGA components with 1mm solderball pitch with an array of up to 51 x 51 solderballs. These component templates will be ~0.50mm thick blackened (?passivated?) stainless steel sheets with laser cut holes, squares, rectangles, etc. Beneath the component template is a bright white square of teflon permanently attached to the PCB cradle. The component template is held stationary by four 3mm hardened stainless steel pins press fit into the PCB cradle, and sticking through 3mm holes in the 4 corners of the component template. A sample PCB is shown (a simulation of the actual 5.8" square PCB that I need to assemble for my other project, but only including the connectors that stick a significant distance above, below or beyond the edge of the PCB.
#2: the base or stand
The papec stand
has four big fat sturdy legs to hold the main device at the appropriate distance above the PCB. As designed, components can stick upwards or downwards 36mm without hitting the bottom of the device or the feet of the PCB cradle (when the PCB is slid beyond the left or right end of the PCB cradle. I haven't added the leaf-spring clips that hold the PCB into the PCB cradle
, but the three M3 threaded holes that will secure them are visible just this side of the front edge of the PCB. At the far left and right of the top surface of the papec stand
are limit blocks
. When the rotary table is rotated exactly 180-degrees, these limit blocks
stop the rotation in exactly repeatable position. Because the limit arm
is so long, any tiny error in either limit stop position causes an error only ~20% as large in the camera
or vacuum pickup tip
. I have not added the "tension device" that gently pushes the limit arm
against the limit blocks
in the two limit positions. These might be spring steel leaf springs, or perhaps just loops of surgical rubber tubing attached to each limit block
for the operator to loop over the end of the limit arm
. That's right, "fancy rubber bands".
What you can't see in this image is an 8mm wide by 3mm deep groove in the top surface of the main papec stand
. In that groove, and in a similar groove in the 200mm diameter round rotary table
plate above it, up to 48 (forty eight) 8mm ball bearings act as a thrust bearing to precisely and securely support the weight of the rotary table
and the Z-axis assembly
that holds the vacuum pickup device
. The papec stand and the rotary table both have 56mm square holes the vacuum pickup device moves up and down through. When a component is lifted "all the way up", it stops in the square hole in the rotary table. Thus no component may be more than 12mm thick (oh, maybe 15mm).
#3: rotary table
The rotary table
is the round flat plate on the center of the papec stand
. As described above, the bottom surface of the rotary table
has an 8mm wide by 3mm deep groove on near its outer periphery to act as a thrust bearing surface. A 1mm thick nylon or teflon sheet with 48 holes of 8.01mm diameter retain up to 48 hardened stainless steel balls and assure they remain equally spaced. The rotary table
also has an ABEC-7 sealed radial ball bearing in its center that is bolted to the center of the papec stand
to provide for rotation but prevent any lateral motion. The Z-axis assembly
is bolted to the top surface of the rotary table
#4: Z-axis assembly
The Z-axis assembly
is a linear stage that moves the vacuum pickup device
up and down about 80mm. Four linear bearings are bolted to the Z-axis assembly plate, and two 20mm diameter hardened stainless steel linear shafts
slide up and down about 80mm through those linear bearings. A thin spring steel compliance plate
is bolted to each end of the two linear shafts
, and the rectangular vacuum pickup device
is captured and precisely aligned by rectangular holes in the compliance plates
Any one of 3 or 4 vacuum pickup tips
can be screwed into the 16mm x 1.5mm thread in the bottom end of the vacuum pickup device
to pick up components within a certain range of sizes and masses. The rectangular vacuum pickup device
has a vertical 10mm hole from the vacuum pickup tip
and upwards through about 1/3 of the length of the bar, where an M6 hole from the backside of the bar connects to a barb fitting
, which connects to the vacuum tube to deliver suction to the vacuum pickup tips
When a vacuum pickup tip
presses against the top of a component, the thin spring steel compliance plates
flex, but remain in perfect alignment (no x,y displacement or rotation) due to the geometry of the device. However, the top of the rectangular vacuum pickup device
presses against a tiny pushbuttom "limit switch", which triggers a vacuum solenoid to toggle between "vacuum suction to the vacuum tip" and "light puff of positive air pressure to the vacuum tip" (depending on whether the component is being "picked-up" (from the component template
) or "placed" on the PCB.
Not visible in this view is the downlooking camera
, which is located 0mm ~ 50mm above rotary table
plate, looking downward through a 55mm diameter hole in the rotary table
plate. Someday maybe this will be a super high-resolution 5 megapixel camera (that's I'm developing), but for now I found a very nice $10 webcam with 2 megapixels, adjustable brightness LEDs, and fairly sharp glass lens that focuses even closer than I need (50mm ~ 100mm, which images 25mm ~ 50mm square components).
That's the description. I'll add a few more images below to help make sense of the above descriptions.
In the above image you can see the PCB cradle
assembly, the component template
at the front-center edge, and a sample 5.80" square PCB. The legs of the PCB cradle are cut very low so the PCB can slide off the left or right end of the PCB cradle
without any component on the bottom of the PCB hitting the PCB cradle
. This maximizes the width of PCB that the device can place components on.
In the above image you can see the groove for the thrust bearing, square hole in the papec stand
for the camera
to look through and vacuum pickup device
to pass through, plus the limit blocks
In the above image you can see the groove for the thrust bearing, the shoulder-shaft-bolt that bolts the radial ball bearing to the papec stand
, and barely just a tiny bit of the radial ball bearing that is held in the 22mm diameter recess in the top surface of the rotary table plate
The above image shows an exploded view of how the rotary table
, teflon retaining ring
, 6 of up to 48 balls
, and papec stand
all fit together.
The above image shows the vacuum pickup tip
designed to pick up largest components (virtually all BGAs and QFNs).
Okay, everyone. Time to point out my stupidities before it is too late. Come up with some really great ideas and get one at cost once I get them into production.
Oh, and if anyone out there has a quality CNC setup and wants to make my prototypes, pipe up! I am especially interested in finding someone who wants to make batches of 20 to 100 units once I finish the software, create a video to show how to operate the device, and announce them as available for sale. I'm doing this much more as a service to the poor "left out in the rain" [potential] developers of really cool electronics devices who can't now prototype with BGAs, QFNs, 0201s, 0402s, etc. The way I see it, all the really creative new developers and startups are pretty much out of luck at the moment (or need to pay through the nose for PCB assembly services). Note: I got prices to assemble my prototype PCBs for my robotics vision camera device and the cost was $7200 for several sets of PCBs. I want to be able to sell this device for under $1000, so people can assemble their own. Compare $7200 to $1000, then realize that's just for one freaking project! PS: I was going to have 25 of the 5.80" PCBs and 50 of the 2.80" PCBs (which have only a few components), but that's still just insane if you ask me. And yes, I already have all the components, so they don't need to do anything but assemble. Oh, and that included them assembling ZERO of the through-hole components on the PCB. Their quote to run only ONE set of PCBs was $2000, which is still more than buying one of these suckers (CNC machining costs permitting). Obviously if the volume is significant, it should be cheaper to manufacture by designing some components as sand castings, die castings, or some other intermediate-cost castings. I've sorta lost track of what relative casting costs are these days. Comments welcome on every aspect of this project.