pilot hole drilling.......

[Shane123]

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.......

[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.

[idahoflyfisher]

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

[Shane123]

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.

[RICHARD ZASTROW]

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

Dick Z

[PriddyShiddy]

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.

[Shane123]

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....

[holbieone]

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