I don't like the BlairCutter style of bit as it's slow and you still have to grind away the center. Plus you don't get anywhere near the amount of cuts per bit that you do with a drill bit style spot weld cutter
Here's a little tip that will save you money and make the job faster. On panels that you are replacing you don't need to drill out the spot welds. Faster and easier to use a 3" weld grinding wheel on a die grinder and grind the spot weld away. Just grind and just when you notice the metal start to blue, stop. Move on to the next one, and so on, and so on. When you have them all ground use a carbon scraper like these:https://www.tooltopia.com/mayhew-tools-60001.aspxand a hammer and separate them. Sometimes you may have to grind a little more but once you get the hang of it, it's far cheaper and faster to grind them out rather than drill them. Another tip, keep the flat end of the scraper facing the panel you are removing. Keeps the scraper from damaging the panel underneath.
I've used this technique for years and it saved me a lot of money especially when doing heavy collision repair (Insurance Salvage) as oftentimes I'd have to remove hundreds of spot welds (sometimes more than that) and you could easily go through $50 or more worth of spot weld bits on each job.
the pin was spring loaded and would only stay on center if you punched it first, but the point of these is trying to cut around the spot weld, to the metal that was not heated and hardened. Thats why they go bad so fast, because you dont always find the outside softer stuff.
For the most part, its just a short 3/8 cobalt bit that ends up finishing the job. 2896A34 5.61
Best thing you can really do is learn to properly sharpen drill bits, get a diamond cutter to square up a grinding wheel or hope your drill doctor actually does the job right.
I have used the Blair spot weld cutters in the past and Chris is right about them.
You center punch every spot weld and then start the cutter at an extremely slow speed until you see a circle being formed. Otherwise the bits will chip and brake if they hit any of the weld itself.
I tried a few different types of drill bits but not the ones recommended above. They seem to work much better.
These types of spot weld removers are best when you plan to reinstall the panel and maintain a "factory look" to a restoration. As pointed out above, if replacing the panel you can save yourself a lot of time by grinding the welds down.