When discussing dimple quality, saying that a hole is under dimpled is not meaning that the dimple is not formed enough to properly accept a rivet. It could mean that, but it would be a severe case that I think even most beginners would detect on there own.
When using the under dimpled statement, I am talking about the condition of the skin in the immediate area surrounding the dimple. As already mentioned by others, evaluating this is best done by analyzing the reflection on the skin surface. Looking at the reflection of florescent light tubes is a good way to evaluate the condition of the skin because they are long straight objects.
A properly dimpled skin will have a reflection that is pure and undistorted except for right at the point of the dimple. An under dimpled skin will have distortion in the reflection within about a 1-1.5" diameter circle (depending on the degree of distortion) area centered over the dimple. This can be seen when looking at the reflection in some of the OP's photos.
A good way for a beginner to learn to detect this is to dimple (with the vinyl removed) a single hole out in the middle of a large skin and then lay it perfectly flat on a table and look at it with your eyes positioned so that the reflection of an overhead light tube aligns parallel and over the top of the row of holes where one is dimpled. If it is a good quality dimple, the quality of the reflection in the area directly adjacent to the dimpled hole, will be similar to the reflection at all of the other holes that are not yet dimpled.
Try this with a thinner (.020) skin and then a thicker (.032 or .040) skin and see what the difference is in the results with the method you are using to dimple in the middle of skins.
Looking for scuffing in the zone right under the footprint of teh dimple dies is often used as a benchmark for a properly completed dimple. It is a good start
There are a lot of variables that can have an influence on what the final result is.
- What dimple dies are being used.
- How large the hole is that is being dimpled
- What tool the dimple dies are being used in.
- The technique of the user
- How thick the material is
- Adjustment of the tool (how well the dies align with each other, how much force is applied to the dies, etc.)
It may not be obvious, but the thicker the skin is, the more force that is required to get a nicely formed dimple. Some tools may do just as good of a job as others, on thinner material, but then not so well on thicker ones.
Same goes for hole size. The bigger the dimple, the more force that is required to properly form the dimple. In some combinations of dimple size and material thickness, it may not be possible to get a well formed dimple with certain tools.
An additional factor that should be mentioned is that in situations where skins were dimpled by a builder to the best they could be done, the processes and techniques used in riveting the skins can turn something that would have been beautiful (in the eyes of RV builders anyway
), to something that looks like a mess. Point being.... that perfectly dimpled skins does not in itself assure a perfect finish when done riveting.
Because of all of the above, besides only using high quality dimple dies, I prefer to use a C-frame tool for any dimple that is on an exterior skin and will be visible on the completed airplane. The reason being that it is capable of applying the needed force regardless of skin thickness or dimple size.
For dimples on any sub structure I use what ever method is most efficient and convenient for the location as long as it looks like the dimples are being formed reasonable well.
This response is not meant to say that only certain tools should be used.
I fully understand that there are specific benefits to dimpling with a compression tool. A big one is when noise is a major consideration. I do not agree that it adds insurance against punching extra holes. I have seen it done at an equal level regardless of the tool being used
I will finish by saying that I have talked with thousands of builders over the years, and looked at lots of finished RV's, and have learned that everyone has a different eye for what is good and proper. There is nothing wrong with that, because in the end, as long as a builder is satisfied with the finish quality they achieved, that is what matters (as long as all of the work at least meets acceptable standards). A high level of quality in dimple finish is not an airworthiness factor. If the dimple it self is fully formed, the work done meets acceptable standards.
Having said that, I have looked at a lot of RV's where the builder was convinced that the dimple finish was as good as was possible, when to more experienced builders, it clearly wasn't.
So a lot of different tools and methods will work. Some will work just as well as others, depending on the variables (dimple size, or skin thickness) involved in that particular situation. But in certain situations, I think there are methods that are superior, but each builder has to decide what process provides a result that meets
their standards
This is my opinion based on a
lot of years doing this as a daily vocation. I have tried pretty much every tool and method that can be used (some that the average RV builder has probably never even thought or heard of). Not everyone may agree, but hopefully it is helpful to some.