Spot weld drill bit sizing

I need to drill out a few spot welds on my Miata (mainly the radio bracket support while doing a half-dash). Does anyone have input on what size to use? 5/16? 3/8?

"They Say" this size: http://www.harborfreight.com/3-8-eighth-inch-double-sided-rotary-spot-weld-cutter-95343.html

Yes, the HF tool works quite well surprisingly, and seems to last a while. It's a good way to go.

This is the style I am looking at (only because I can get them through work for cheap).

Many moons ago, I drilled out a crap load of spot welds using a standard 3/8" drill bit.

I have the hole saw type, and I went through bits WAY too quick. One tooth breaks, and it's all over. You can still sort of use it, but much cussing ensues.

People use hole saws on metal!?

GameboyRMH wrote: People use hole saws on metal!?

Yes. I used to drill 2" holes in sprinkler risers for alarm switches. Learned that speed is your enemy. Have used hole saws for all kinds of steel things, wire entrys, etc.

Having just done this on another car with a 3/8," I would recommend the smaller bit. The spot weld itself doesn't cover much area and the smaller bit will do less collateral damage to the metal. That may be important to you as you may want to leave as much metal as possible to effect the repair.

Here's the "hole saw type," and, yes, it is pretty worthless:

http://www.harborfreight.com/3-8-eighth-inch-double-sided-rotary-spot-weld-cutter-95343.html

Oops, sorry double link post-- same as Dr. Hess.

Have an Eastwood set, works OK. Good placed center punch w/ 1/8" pilot drill and follow up w/ correct drill size for the spot weld will work as well, might need to wobble a little once planted, just don't go too deep or through, you'll get the hang of it.

I have drilled out thousands of spotwelds and dont bother with the specialized bits.

Center punch the spot

1/8" bit down to the bottom sheet

5/16" bit till it hits the bottom of the 1/8" hole; you will feel it hit the bottom if you pay attention.

I have been known to sharpen a bit with a flatter tip when doing this job but it is not really necessary.

In many cases, depending on planing and luck, you can just drill through both panels since the new part will form the bottom of the new spot weld.

NOHOME wrote: In many cases, depending on planing and luck, you can just drill through both panels since the new part will form the bottom of the new spot weld.

In my case I can't drill all the way through, this is for interior brackets that will not be replaced by anything.

Dr. Hess wrote: "They Say" this size: http://www.harborfreight.com/3-8-eighth-inch-double-sided-rotary-spot-weld-cutter-95343.html

I had one of those years ago and I could not get it to drill anything.

spitfirebill wrote:

Dr. Hess wrote: "They Say" this size: http://www.harborfreight.com/3-8-eighth-inch-double-sided-rotary-spot-weld-cutter-95343.html

I had one of those years ago and I could not get it to drill anything.

I had one and I think I scratched a circle around a spot weld with it before the "cutters" were nubbed down to nothing. Someone then told me I needed to use a much lower rpm than I was. I think it was on the order of a drip eventually wearing away a stone...

your answer is 5/16"

or 42

NOHOME wrote: I have drilled out thousands of spotwelds and dont bother with the specialized bits. Center punch the spot 1/8" bit down to the bottom sheet 5/16" bit till it hits the bottom of the 1/8" hole; you will feel it hit the bottom if you pay attention. I have been known to sharpen a bit with a flatter tip when doing this job but it is not really necessary. In many cases, depending on planing and luck, you can just drill through both panels since the new part will form the bottom of the new spot weld ^This^ I drilled out many, many spot welds taking apart my Sprite. I bought the special drill from Eastwood, but ended up using stub-length cobalt 135 degree split point drills. 1/8" to pilot drill, 5/16" the get through. And I had a piece of 3/16 x 1-1/2 mild steel about a foot long I made a "splitter" out of. Drive it in, whack it on the back edge w/ a hammer, splits panels easily.

There is a high end version of the HF unit that works really well. I used it on my 98 Eclipse GST to convert it to a GSX, tons of spot welds to drill out and it did them like a champ.

Buy extra blades, if one tooth chips it will still run, but if two teeth chip, you are SOL.

The high end version is called Rotabroach. It comes with a center punch that is profiled to make a dent that fits the spring loaded guide pin. Making a good punch is key to keeping it from wandering.

I have cut thousands of spot welds doing panel repair on cars. I have access to snap on drill type cutters but have much better luck with the cheap HF hole saw variety. A stout spring loaded center punch and an air drill makes short work of it.

There were at least 250 individual spot welds that I cut using one HF cutter on the repair of this Ferrari 250GTE. I never even had to flip the bit. At $2.99 per cutter I don't feel bad buying one per job.