theperfessor
07-16-2010, 06:05 PM
The sprue plate design I am using on my four and five cavity molds require two cuts to be made at 30* to the main axis. One cut is on the end of the sprue plate; the other one is the slot that fits under the hold-down screw opposite the pivot pin, where clearance is needed to open the sprue plate without binding.
When I ran the first batch of them I did all the drilling, profiling and slotting for the hold-down cut with the vise parallel to the X-axis. Then I had to swivel the vise 30* to do the other two cuts, in each case working off of different features of the sprue plate. While the end result was acceptable, accuracy was hard to maintain and of course indicating in the vise to 30* and then back to parallel is a real PITA.
I designed the fixture shown below. The four edges are parallel and square so I can mount it in a machinist's vise to use either the X or Y axis. (I'll probably use the X axis power feed to cut the end, since it cuts and feeds clear, and the Y axis manual feed to make the clearance cut, since that has to stop at a predetermined coordinate.)
The base is a piece of 1018 plate 3/4" thick and milled down just enough to take off the surface scale and make it parallel. The base is approximately 3.2" by 4.0". There are three permanently pressed in pins that are used for alignment. There are also three 1/4-20 tapped hole holes used for the clamp screws.
The hole nearest the centermost pin is drilled and reamed to 0.250" for 1/4" of depth, and then tapped to 1/4-20 the rest of the way. This is the locating hole for a four cavity sprue plate. I still need to make a screw in pin for this. This is the only part that needs to be removed/replaced to switch from four to five cavity plates.
In use, the fixture is mounted on parallels in a machine vise and the center pin is indicated in. Since all the other pin locations and part feature dimensions are known relative to the center pin it was easy to determine the coordinates for the necessary cuts. I have these written down in a folder that I can reference when I make the next run of plates.
I need to make one more modification for ease of use. I plan to open up one end of the strap clamp and put a bevel on it so that I can slide it on and off and limit how much I have to turn the locking screws for a part change. I'll also get a fender washer and bend two edges down 90* to straddle the clamp and prevent the open legs from spreading out.
When I ran the first batch of them I did all the drilling, profiling and slotting for the hold-down cut with the vise parallel to the X-axis. Then I had to swivel the vise 30* to do the other two cuts, in each case working off of different features of the sprue plate. While the end result was acceptable, accuracy was hard to maintain and of course indicating in the vise to 30* and then back to parallel is a real PITA.
I designed the fixture shown below. The four edges are parallel and square so I can mount it in a machinist's vise to use either the X or Y axis. (I'll probably use the X axis power feed to cut the end, since it cuts and feeds clear, and the Y axis manual feed to make the clearance cut, since that has to stop at a predetermined coordinate.)
The base is a piece of 1018 plate 3/4" thick and milled down just enough to take off the surface scale and make it parallel. The base is approximately 3.2" by 4.0". There are three permanently pressed in pins that are used for alignment. There are also three 1/4-20 tapped hole holes used for the clamp screws.
The hole nearest the centermost pin is drilled and reamed to 0.250" for 1/4" of depth, and then tapped to 1/4-20 the rest of the way. This is the locating hole for a four cavity sprue plate. I still need to make a screw in pin for this. This is the only part that needs to be removed/replaced to switch from four to five cavity plates.
In use, the fixture is mounted on parallels in a machine vise and the center pin is indicated in. Since all the other pin locations and part feature dimensions are known relative to the center pin it was easy to determine the coordinates for the necessary cuts. I have these written down in a folder that I can reference when I make the next run of plates.
I need to make one more modification for ease of use. I plan to open up one end of the strap clamp and put a bevel on it so that I can slide it on and off and limit how much I have to turn the locking screws for a part change. I'll also get a fender washer and bend two edges down 90* to straddle the clamp and prevent the open legs from spreading out.