1. Kinematics objective

The proposed project was to design and build an automatic loading mechanism for an existing keytag roundoff machine. A group of students designed and built a machine that takes a strip of plastic and makes small keytags with an embossed GVSU emblem. The existing roundoff machine takes the square keytags and rounds the corners using a routering operation. The keytag had to be manually loaded and clamped, after which the machine would router the corners.  The part would then have  to be manually unloaded. It was desired to automate the loading and unloading processes, which is the purpose of this project.

2. Design considerations

It was decided that a design that incorporates the following ideas would be desirable:

3. Design

3.1 Concept - four bar mechanism

The first step in the design was to brainstorm to come up with the initial concept for the loading mechanism. We talked for quite a while about different designs, and built quite a few prototypes in Working Model. After reviewing each of the initial prototypes and discussing the good and bad points associated with each, we decided to use two identical four bar mechanisms. One would be sufficient to provide the desired motion, but we felt that we should add another for reasons of stability (see attached drawings). In order to place the part on the level fixture, we felt that it would be best to pick up the part from a flat level surface and keep the part level through the entire travel of the mechanism. This would be best accomplished using a symmetrical (parallelogram) four bar mechanism.

3.2 Venturi device

The next phase of the design was to come up with a way for the four bar mechanism to actually pick up the part from a flat surface. After much time and debate we felt that the best way would be to use some sort of a vacuum. By using a vacuum we would not have to try to slide something under the part, and then remove it again when the part is placed on the small fixture. It was also decided that it would be best to set the part on the fixture instead of sliding it on. If a sliding motion was used the part would hit the locating pin on the fixture before it was in the desired position. We decided that a Venturi tube would be the best way to create the vacuum.

3.3 Driving mechanism

For the driving mechanism it was decided to use a disk and a driving link. When built to the correct dimensions this driving mechanism would give the desired rocking motion to the four bar mechanism. It was also decided to create a disk with multiple holes in order to have the ability to adjust the stroke of the driving link. After some testing, a different design was used. The new design replaced the disk with a driving arm that had a slot cut in it. This slot would enable the pick-up mechanism to have an extended dwell at the pick-up and drop-off points of the motion.

3.4 Unloading device

It was decided that it would be best to use a method of unloading the part that was separate from the loading. By doing this, the finished part could be unloaded while the four bar mechanism was picking up the next unfinished part and transferring it to the fixture. For this operation we would use a simple blast of air. The air would probably be directed through two tubes, one to lift the part from the fixture and the other to direct it out of the enclosure.

3.5 Controls

Now that the basic mechanisms have been decided on, we need a way to control the air supply to the Venturi and to the part discharge. We also needed a way to signal the PLC that the device has loaded the part and that the routering operation can begin. It was decided that the most simple way to do this would be to use cams. One cam would be used to trigger a microswitch that would signal the PLC when the part was loaded and the mechanism was clear of the fixture. The other two cams would be used to operate two air valves (one each), one for the Venturi and one for the discharge of the part. Each cam should be mounted on the drive shaft independently so that timings could be adjusted individually without affecting the other two operations. In order to avoid problems and difficulties in machining cam profiles, it was decided to use cams consisting of two materials. The cam hub and minimum radius would be made of a single aluminum piece. The larger part of the profile would be made by attaching plastic to the surface of the cam. The followers for the cams would be attached to the valves and the switch. These devices were purchased with rollers already installed on the follower arms in order to reduce friction on the cams and followers.

3.6 Design schematic

4. Building the device

4.1 Venturi

The first thing that we built was the Venturi device. We had to make sure that we could generate enough vacuum to pick up the part. The initial Venturi was somewhat crude, but needed only to serve as a prototype. After testing it was found that the Venturi generated plenty of vacuum to achieve the desired results. Once this was established, the Venturi that would be used on the machine was built. A block of aluminum was squared and faced, and a small diameter hole was drilled the length of the block. Then another small hole was drilled perpendicular to the first hole until they intersected. The first hole was then bored to a larger diameter in order to have an area reduction that would increase the velocity of the air traveling through the block, therefore creating a vacuum. The final step was to bore the holes just enough to tap threads for the NPT fittings that would be screwed into them.

4.2 Pickup mechanism

The first step in the manufacture of the loader was to make the top and bottom blocks of the loading mechanism along with four identical links. The blocks were machined and tapped to accept standard bolts. The bottom block was drilled so that it could be bolted to the base of the machine. The top block was drilled and tapped so that the actual pickup device could be bolted to it. The pickup device was made of plastic, with rubber gasket material attached to it to give a good seal on the part being picked up.

4.3 Driving arm and connecting link

The driving arm was machined from a piece of aluminum, with a slot for the connecting link, and a hole and a set screw for attachment to the shaft. We also needed columns to lift the drive shaft off of the base of the machine to make room for the cams and the devices that they would operate. The drive shaft was made of a piece of ¼" polished steel. The only machining done to this part was to make a flat for the setscrew on the driving arm, and it was then cut to the desired length.

4.4 Cams

The cams were made to a maximum diameter that was the same as the inner diameter of standard schedule 80 PVC pipe. A piece of PVC pipe was cut into small rings that were the same thickness as the cam. By attaching pieces of the rings to the round cams, the desired cam profile was achieved.

Once all of the individual pieces were fabricated, and all other materials were purchased, the devices were assembled and the final product was tested.

4.5 Final product


5. Initial testing of final product

The initial trial of the loader mechanism yielded excellent results. The pickup loaded the part without any trouble other than some minor misalignment. The bolt that secures the bottom block to the machine base was loosened, the block re-aligned, and the bolt tightened again. Another test was performed, and the results this time were perfect. The piece was picked up and placed on the fixture exactly as planned. The aligning pin on the fixture was centered in the hole in the part, and the part was sitting straight on the fixture. This test was repeated many times with the same results.