Need Help!- Pilot Drilling Holes ???'s

[CORBIN92087]

My name is Corbin and im from Hillco Technologies. I am sort of re vamping our machining practices here, and was wondering about pilot drilling for holes. Right now for holes ranging anywhere from 3/4 in. to 1 3/8 we use a half inch drill bit. Just wondering if there is a formula or general rule for finding the right size pilot for a given hole size. hope someone out there has an answer!

[Geof]

What material are you drilling, what type of drill and on what type of machine?

Using HSS drills with a split point grind a pilot hole is not really needed for any size up to around 1".

If you are drilling larger than 1" maybe the best choice is an insert drill and again a pilot hole is not needed.

[CORBIN92087]

Hey Geof! We use a lot of A36 and A572 material. Any holes that range from 3/4 in to 1 3/8 we use spade drills with spade inserts. And even with 1/2 in pilot holes they still scream through the whole cut. Our typical RPM for the larger spade bits (1" to 1.375") is around 400 with the tool feeding in at about 1 to 1.5 IPM. Our machines are a HAAS CNC and Bridgeport CNC.
Thanks for the reply

[davereagan]

At 400 rpm and 1.5 inches per minute, that's only ~.004" per rev. That's just rubbing for a 1" Allied spade drill with a 1/2" hole pre-drilled. I would think you'd have plenty of thrust to go .010"-.015" per rev easily and I bet it would stop screaming. Better chip breaking too. Counter drilling never sounds as nice as drilling from the solid.

[Geof]

The screaming is possibly because you are not giving them enough cut keep them steady, spade drills do not get any support from the sides of the hole and will chatter. However, with a spade drill the thrust the machine get generate may not be limiting but the torque needed to make it cut could be limiting; insert drills are freer cutting.

My suggestion is to figure out how to move away from pilot drilling; as I said below 1" it should be a simple change to split point drills above 1" have a tooling supplier come in and trial some insert drills.

[CORBIN92087]

Thanks for the reply Geof! I just want to make sure I got this right. So your saying with spade bits we shouldnt have to pilot drill? Even holes up to 1 3/8? All we need to do is center drill then spade drill? We do all our larger holes in the Center drill, pilot drill, spade drill fashion and order. what do you reccomend for spade tools??? lets use a 1" spade for example, how would you set the RPM and feed rate? typically we run em at 400 RPM and 1 to 1.5 ipm.

[Geof]

Originally Posted by CORBIN92087 Thanks for the reply Geof! I just want to make sure I got this right. So your saying with spade bits we shouldnt have to pilot drill? .....

No.

I am suggesting up to 1" you use a regular drill with a split point grind and no pilot drill.

For larger than 1" I am suggesting you use an insert drill and no pilot drill.

I would not use a spade drill.

[ctate2000]

If you do not have through coolant you should stay away from indexable drills. One chip can kill a $400 drill body.

I have used annular cutters with great success on large holes. Go to the website and check them out. They are very easy on your machine as they require very little horsepower compaired to drills. If you are not going deep <2.0 inches try helical interpolation with an indexable mill. One size would do many holes.


http://www.hougen.com/cutters/cutters_index.html)

[davereagan]

I do not have coolant through the spindle, but I have been able to run 1-1/4" and 1-1/2" traditional Komet triangle insert type insert drills in mild steel in depths up to 2". Up to 3/4 of a diameter of depth, I don't need to peck. After that, I run a peck with the most minimal retract (just .050" I think) with a peck increment of .035" I can run a 1.5" insert drill at 1000 rpm and .004" feed per rev with just flood coolant in mild steel. If I had a stronger Z servo, I could probably push .006" per tooth. I blew up a 1" insert drill pretty quickly, like the ctate mentioned. I wouldn't dream of paying $400 for one though. $150 seemed steep enough. Indexable tools 1" or under are pretty delicate in most forms. There is just not much meat behind the pockets.

[PixMan]

A number of years ago I worked in making downstream filters for plastic extruders. For the adapters and bodies, I often had to drill holes ranging in size from 1" to over 8" in hot-rolled (1020, A36) steel.

Using HSS spade drills, I would push up to 4-1/2" diameter drills with NO pilot hole. Anytime I had to open-up a hole, I bored it. Only once did I try to enlarge a hole with a spade drill, and that once was enough for me to know.