1. ## pilot hole drilling.......

is there a standard percentage/equation for determining your pilot hole size for preparing for a larger drilled hole when just using standard hss drills? just curious, as i usually stick around the 3/8" or 7/16" for pilots when drilling larger holes.......

2. As far as I know the only reason for using a pilot drill is to remove the material that would otherwise have to be squished away by the web of a large drill going in without a pilot. So your pilot drill diameter should be a bit larger than the distance across the web on the large drill.

But you also have to consider that for a deep hole it may be necessary to use a larger pilot drill simply because the depth to diameter ratio is too large for a small one.

However, if you use a drill with a split point grind a pilot hole is not really necessary.

Althoug I suppose if you are looking at a 3" drill maybe it is.

3. Originally Posted by Geof
As far as I know the only reason for using a pilot drill is to remove the material that would otherwise have to be squished away by the web of a large drill going in without a pilot. So your pilot drill diameter should be a bit larger than the distance across the web on the large drill.

But you also have to consider that for a deep hole it may be necessary to use a larger pilot drill simply because the depth to diameter ratio is too large for a small one.

However, if you use a drill with a split point grind a pilot hole is not really necessary.

Althoug I suppose if you are looking at a 3" drill maybe it is.
At our shop we usually only drill a pilot on a hole over 1". And even up to about 1 1/4" only a starter (kinda hard to get centered up on a center punch mark), not a full pilot. But I've always heard it's better to go about 1/32" smaller than the web thickness to minimize drill bit "walking".... just my \$0.02

4. ok, was hoping for a nice general equation/fraction/percentage to use as a basic figure, but i guess not, eh? I know that with your larger drills (5/8 & up) you will put too much pressure on the tip aka webbing, but was wondering if someone had worked out a good equation that put just the right amount of pressure on the wings instead of going too big with the pilot and burning up your corners of your drill.

5. Maybe ther's a standard web to dia. ratio that could relate to pilot drill size.

Dick Z

6. a while back I asked the same question on PM I think. I was drilling 9/16" holes in crs and wondering because of my mill size... and the fact that it was a customers part.

the consensus was 10% larger than the web of the final drill. it was explained that up to 30% of the total forces are from the web "pushing" metal. what size to start at depends on rigidity of the machine imo. an X1 mill would be scarry drilling a 1/2" without a pilot... I've drilled 1" on a bp with no pilot easier than I can drill a 1/2" WITH pilot on my x3.

I have used the 10% rule without issues and have run 7/8" holes on my little x3 with no problems.

7. yah, sometimes i have found better holes when going no pilot, but thats mainly been in the lathe, and only up to 3/4.

thanks for the help guys. the reason i was asking was to standardize a few things around the shop for training noobs. seems like the first thing i train them is how to run the drill sharperner and life gets a little easier that way anyways, lol....

8. it mostly has to do with how much h.p. you have to drill with

I've drill 2 7/8" holes 5" deep in one shot with a 50 h.p. spindle