flion
Well Known Member
Every so often someone raises the question of countersinking structure that will be overlaid by dimpled material. A case in point are the RV-10 spars, which are countersunk to take the dimpled wing skins. Van's instructions call for using a test piece but it seems like everyone wants more concrete instructions. If only it were that easy. Variances in countersink cages and technique make it impossible to tell you how many 'clicks' deeper to go, much less what setting to begin at. With that in mind, and as I am currently on the wing spars, I thought I'd share my method with you.
First, I find a flush rivet depth. This is done on a piece of scrap; I drill an appropriate hole and then countersink and test with a rivet. If it is too deep, I start again shallower. If too shallow, I 'sneak up' on the setting a few clicks at a time until it is right. I then proceed to the work piece and countersink with a fair degree of confidence - but I check each hole with a rivet anyway. Sometimes my technique is less than perfect and I find a hole that needs to be a bit deeper, usually meaning I angled the cage slightly. Another pass with the countersink usually takes care of it.
I make a test dimple like Van's recommends. It's .032 and is long enough to grip, with a dimpled hole in one end. The edges have been deburred both to prevent slicing my fingers but also to allow it to lay flush if it can. You can try it in a hole dimpled for a flush rivet and you can easily see it does not lie flat against the work. Again, we sneak up on the setting. While I already have a good idea of what it will take on my cage, I start out with four clicks. Not enough, so I do four more, which is better but not quite. Two more iterations of two clicks each and I'm happy with the setting. That's a total of twelve clicks but I wouldn't count on it to work with your equipment. Here's a picture of my gauge labeled 'AN3 Dimple'; I've made ones for AN4, #8 screw, and #6 screw as well.
Now I can countersink all the holes for dimples. I still, as with the flush rivets, test. I can test using the gauge or with a rivet, once I've seen a rivet in a hole tested with the gauge. Below is a picture of both the gauge and a rivet in a deeper countersink; you can see the ring around the rivet. It's not that hard to 'eyeball' using a rivet to at least determine whether you really botched the countersink or not, though the gauge will give you better results.
Anyway, that's how I do it. Knowing that I went 12 clicks from flush to deep-for-dimple means I can easily revert to flush - at least until I change countersinks. When I get rich, I will invest in a cage for every cutter size I use. In the meantime, this works well and is easy enough to do with accuracy and consistency.
Edit: Oh, yeah, the blue tape. That's used to keep chips out from between the spar doubler and the spar web. Right there at the web/flange curve is a gap that seems to like to collect aluminum chips.
First, I find a flush rivet depth. This is done on a piece of scrap; I drill an appropriate hole and then countersink and test with a rivet. If it is too deep, I start again shallower. If too shallow, I 'sneak up' on the setting a few clicks at a time until it is right. I then proceed to the work piece and countersink with a fair degree of confidence - but I check each hole with a rivet anyway. Sometimes my technique is less than perfect and I find a hole that needs to be a bit deeper, usually meaning I angled the cage slightly. Another pass with the countersink usually takes care of it.
I make a test dimple like Van's recommends. It's .032 and is long enough to grip, with a dimpled hole in one end. The edges have been deburred both to prevent slicing my fingers but also to allow it to lay flush if it can. You can try it in a hole dimpled for a flush rivet and you can easily see it does not lie flat against the work. Again, we sneak up on the setting. While I already have a good idea of what it will take on my cage, I start out with four clicks. Not enough, so I do four more, which is better but not quite. Two more iterations of two clicks each and I'm happy with the setting. That's a total of twelve clicks but I wouldn't count on it to work with your equipment. Here's a picture of my gauge labeled 'AN3 Dimple'; I've made ones for AN4, #8 screw, and #6 screw as well.
Now I can countersink all the holes for dimples. I still, as with the flush rivets, test. I can test using the gauge or with a rivet, once I've seen a rivet in a hole tested with the gauge. Below is a picture of both the gauge and a rivet in a deeper countersink; you can see the ring around the rivet. It's not that hard to 'eyeball' using a rivet to at least determine whether you really botched the countersink or not, though the gauge will give you better results.
Anyway, that's how I do it. Knowing that I went 12 clicks from flush to deep-for-dimple means I can easily revert to flush - at least until I change countersinks. When I get rich, I will invest in a cage for every cutter size I use. In the meantime, this works well and is easy enough to do with accuracy and consistency.
Edit: Oh, yeah, the blue tape. That's used to keep chips out from between the spar doubler and the spar web. Right there at the web/flange curve is a gap that seems to like to collect aluminum chips.
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