928 Metal Gas Cap Ratcheting Pawl
12-18-2016, 01:01 PM
#1
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928 Metal Gas Cap Ratcheting Pawl
I have been posting in various other threads about the Gas Cap Pawl, but I thought I ought to bring this all together in its own thread rather than putting it back in the Gas Cap Tool thread, and since it is kind of secondary to that project.
Before, I had begun to develop these Pawls out of some recycled 7075 T6 3/4 inch round aluminum bar stock that I came across. I started a small batch of them, about 75 or so, and got them up to the point of doing the last three or four milling operations on them to have them fully finished. Then for some reason I set that aside and went on to other things.
When the topic popped up again recently I got back to it and thought that I had left the project waiting to do something different with milling the "teeth." I thought I had ordered and received some tiny tapered mill cutters to do the space between the teeth, but could not find them. I have realized that I never got around to ordering them; and have had quite a time finding and buying them even now.
Anyway, in the meantime, back at the ranch, while waiting for them to arrive, I took some time to more closely evaluate the geometry of the teeth on the pawl. Every time I tried to map them out it seemed to turn out different. I finally boiled it down to a formula based on a 27 degree angle and have now milled some more stock in strips to fit in the mill vice and have enough for an additional 150 or so more pawls.
Then I evaluated the first unfinished batch and find that some of them are way out of tolerance, but not too many, and the rest of them I will mill to my current standard before I proceed. Even the current batch of milled bars still need one more pass in the mill so that the teeth will both be ratcheting at the same time, since I now have the front tooth a bit too fat.
Before, I had begun to develop these Pawls out of some recycled 7075 T6 3/4 inch round aluminum bar stock that I came across. I started a small batch of them, about 75 or so, and got them up to the point of doing the last three or four milling operations on them to have them fully finished. Then for some reason I set that aside and went on to other things.
When the topic popped up again recently I got back to it and thought that I had left the project waiting to do something different with milling the "teeth." I thought I had ordered and received some tiny tapered mill cutters to do the space between the teeth, but could not find them. I have realized that I never got around to ordering them; and have had quite a time finding and buying them even now.
Anyway, in the meantime, back at the ranch, while waiting for them to arrive, I took some time to more closely evaluate the geometry of the teeth on the pawl. Every time I tried to map them out it seemed to turn out different. I finally boiled it down to a formula based on a 27 degree angle and have now milled some more stock in strips to fit in the mill vice and have enough for an additional 150 or so more pawls.
Then I evaluated the first unfinished batch and find that some of them are way out of tolerance, but not too many, and the rest of them I will mill to my current standard before I proceed. Even the current batch of milled bars still need one more pass in the mill so that the teeth will both be ratcheting at the same time, since I now have the front tooth a bit too fat.
Last edited by Jerry Feather; 12-19-2016 at 11:25 AM.
12-18-2016, 01:32 PM
#2
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Since there are 18 teeth in the later gas caps that these pawls are intended for I kept wondering why the angles on the teeth didn't somehow relate to 20 degree differences. I still haven't quite figured that out. What I did figure out is that the slope of the teeth that ratchet is not radial to the center of the cap, but rather sloped fairly significantly. That makes sense. Also the back side is also not radial, but rather angled so as to be in sort of barbed shape. That too makes sense. The end result is that rather than being based on 20 degree differences the seem to be more on a 10 degree basis.
With that decided I measured an original pawl and decided that the angle of the cuts as it relates to the body of the pawl, should be 27 degrees. With that I set the head of my mill at 27 degrees and with a straight mill cutter I cut the back side (the barbed side) of the back ratcheting tooth. The mill cutter I had was new and has a slight radius on the corner of each cutting edge, and I like that. Then I cut the front side of each blank with the side of that same cutter. That is the surface that I need to cut about 15 thou more, which I'll do later this morning.
The grove between the two teeth is the one that I have been struggling with. I decided that to match the other angles I needed to cut it with a 20 degree overall taper mill cutter with a 1/16th inch diameter ball nose tip. Those are the ones I had so much trouble finding and buying. I bought four and broke three of them in finishing up the bar stock shown in the picture. Fortunately I salvaged one of the broken tips and could put it in the collet and continue with it to the finish. I'll still be using it correcting the pawls that I have already cut off the bars from before.
With the mill head still at 27 degrees the 20 degree tapered cutter cuts the front and back surfaces at just the right angle to make both teeth match and still be oriented radially.
With that decided I measured an original pawl and decided that the angle of the cuts as it relates to the body of the pawl, should be 27 degrees. With that I set the head of my mill at 27 degrees and with a straight mill cutter I cut the back side (the barbed side) of the back ratcheting tooth. The mill cutter I had was new and has a slight radius on the corner of each cutting edge, and I like that. Then I cut the front side of each blank with the side of that same cutter. That is the surface that I need to cut about 15 thou more, which I'll do later this morning.
The grove between the two teeth is the one that I have been struggling with. I decided that to match the other angles I needed to cut it with a 20 degree overall taper mill cutter with a 1/16th inch diameter ball nose tip. Those are the ones I had so much trouble finding and buying. I bought four and broke three of them in finishing up the bar stock shown in the picture. Fortunately I salvaged one of the broken tips and could put it in the collet and continue with it to the finish. I'll still be using it correcting the pawls that I have already cut off the bars from before.
With the mill head still at 27 degrees the 20 degree tapered cutter cuts the front and back surfaces at just the right angle to make both teeth match and still be oriented radially.
Last edited by Jerry Feather; 02-05-2017 at 12:15 PM.
12-18-2016, 03:32 PM
#5
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Second!
12-18-2016, 04:12 PM
#6
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Oh, good. Now I have a list of 9 and Randy is at the top. I took the names out of the other threads--as many names as I came across. There will be plenty of pawls to go around. I think I will end up being about 250 more or less out of this venture. At some point I'll be needing addresses which can be sent to me by PM.
I think I still need an additional piece of tooling, consisting of a 7/16 inch straight end mill. That turns out to be kind of an oddball size. I am hoping to get one that has the very small radius on the corners like the half inch one I cut some ot the teeth with, but that is not critical. That bit is to cut the recess in about the middle of the pawl for the lock.
When I finish doing the front of the ratchet side of the first tooth I'll get these bars of material cut and trimmed to size for individual pawls. Then I have three or four mill operations to finish these up.
I think I still need an additional piece of tooling, consisting of a 7/16 inch straight end mill. That turns out to be kind of an oddball size. I am hoping to get one that has the very small radius on the corners like the half inch one I cut some ot the teeth with, but that is not critical. That bit is to cut the recess in about the middle of the pawl for the lock.
When I finish doing the front of the ratchet side of the first tooth I'll get these bars of material cut and trimmed to size for individual pawls. Then I have three or four mill operations to finish these up.
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12-19-2016, 11:35 AM
#8
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I have the front of the ratcheting tooth milled a bit narrower now and spent the entire football game with emery cloth polishing the bar stock before I start milling and cutting the bars into tiny pieces. I found the 7/16th inch mill cutter I need on eBay and will order it shortly. I couldn't find one in my bunch of cutters.
After these pieces are cut and milled to size all I have left to do is to mill the lock recess in the top, mill the two sides at the top a bit narrower, then bore the hole for the spring in the back. By the time I have the individual pieces cut and milled true the cutter ought to be here and then I'll finish these up.
After these pieces are cut and milled to size all I have left to do is to mill the lock recess in the top, mill the two sides at the top a bit narrower, then bore the hole for the spring in the back. By the time I have the individual pieces cut and milled true the cutter ought to be here and then I'll finish these up.
12-19-2016, 01:02 PM
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12-19-2016, 03:26 PM
#12
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12-19-2016, 03:51 PM
#13
I have an older metal pawl.... but i NEVER lock my gas cap.... so I'm still debating. i know i will break it trying to make sure it does not break... LOL
12-19-2016, 06:03 PM
#14
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The only other metal pawl I know of is the one made by JP. If that is the one you have you may find that it is not finished. If it is not you may be best off with one of these when they are done.
I think the gas cap for the 928 is unique, that is, nothing else in the world that I know of to replace it with; and original new or even used ones are pretty expensive.
Having the older metal pawl says one thing, but not having finished it or put it into service says another. So, it may be like the leak in your roof--it only leaks when it rains and you can't fix it then; and it doesn't leak when it is not raining, so it doesn't need to be fixed then either.
I think the gas cap for the 928 is unique, that is, nothing else in the world that I know of to replace it with; and original new or even used ones are pretty expensive.
Having the older metal pawl says one thing, but not having finished it or put it into service says another. So, it may be like the leak in your roof--it only leaks when it rains and you can't fix it then; and it doesn't leak when it is not raining, so it doesn't need to be fixed then either.
