Center drilling in steps or 1 pass?

Use the biggest drill that the machine will drive, spade drills are excellent for large rough holes when your goal is to remove the maximum amount of material in the shortest possible time followed by a finishing operation.

Spade drills seem to work best without a pilot hole, spot and go leaving generous room for finishing, for example, for a 3 1/4" bore I would rough with a 3" drill straight through. As mentioned this requires a machine capable of doing so.

As a side note, if needing a 3" hole 6" through a blank and the machine already has a 2" drill set up I use that because I'm lazy that way, changing tools may take far longer the the actual drilling of a single part. A 2 1/2" X 6" deep hole through 304 stainless should take 15-30 minutes each or so, changing drills would take longer. Remember that 15 Minutes times 50 parts is 12 1/2 hours or roughly $1500.00 in machine time or more.
 
Okay, wait ... drilling a 3" ID hole ... most of us are lucky to drill a 1" hole on our machines but a 3" hole on a home shop machine is, well, heroic.
 
At work drilling a 3" hole in one shot, piece of cake. At home not so much.:einstein:

Now for bronze, plastic and aluminum I never step drill at home. Made that mistake once.
 
Last edited:
I used to spend days pushing a 3 1/4" drill through a piece of some sort of stainless just to rough the part out for the CNC lathes finishing the parts. Some threaded jack tube for a big jet landing gear as I recall. The tubes were nearly 2 feet long. Two things happen. You learn to sharpen big drills, and you get strong arms. Orders for 50-100 at a time used to come in. I was wishing for a spade drill, but never happened in that shop. One lathe the operator ruined the nut in the tailstock on our Colchester pushing it so hard.....but he was using a cheater bar in the wheel. He never learned when or how to sharpen the drill. So much for the "good old days". Those I don't miss, even a little bit.

I might add a small tip on drilling to help stay straight. Once you get about 1 diameter deep with a slightly smaller drill, bore the hole to close fit the next drill size. It will drill much straighter on the next step. I'm one of the guys who never step it up though, other than that, if I have to drill close to finish size. If the piece is short, and the hole is undersized to the point that the hole needn't be all that true, I just punch a "dirty hole". Drills generally are the most efficient method of material removal for ID work. There are all sorts of things to help keep the hole from drifting, but that's another day, another thread.
 
I just found out this. We had to drill close to a 1 inch drill inside our piece. First I started with a #5 center drill. Then I went to like a 1/2" drill. Then I moved up to the 1 inch drill. I can honestly tell you it was a breeze cutting the rest of the material inside. Once I hit the wall where I stopped with the .500 it was a lot harder to cut. Using the .500 was easy too. I fully believe in stepping up in drill sizes is a good idea and practice..puts less wear on the drill bit too I would think..
 
We were taught in shop class to step drill but not proper stepping.

For sheet stock the step drills do well but back to big drills.
Center drill with the largest one yiu have with tail stock or spindle as retracted as possible to get a good start.

Look at the Web in the center of the bit.

That is the diameter of the drill to use as a pilot.

Your limit is the pressure you can apply and hp of your machine.

Final size should be via a boring bar

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 
mikey said:
Okay, wait ... drilling a 3" ID hole ... most of us are lucky to drill a 1" hole on our machines but a 3" hole on a home shop machine is, well, heroic.
Click to expand...
I began with "the biggest drill that the machine will drive" fully understanding that one is limited by the equipment at hand.

Merely wanted to point out that drilling in steps is not required if drilling to size in one shot is possible, this saves a good deal of time.
 
I am not a huge fan of step drilling, and minimize it as much as possible.
Step drilling chowders the heck out of conventional drills. You can actually buy special drills meant for step drilling.
Start with a spotting drill, then push the biggest drill that your machine can handle into it.
If you don't have spotting drills, use only the very tip of a center drill - do not use the larger diameter meant for registering a center. Use the biggest center drill you have, followed by the largest drill whose web fits into the spot.
Then push the next largest drill your machine can handle through that hole.
The largest drill your machine can handle is close 2x to the maximum depth of cut you can make while turning. If your lathe can cut 0.250" doc, it can handle a 1/2" drill. In theory, it can go right from a 1/2" to a 1" drill, but this is actually limited by the rigidity of the tail stock.

If your pilot hole drifted (it did), then all the following drills will follow that drift + what ever drift they impart from uneven sharpening. in other words, the more drills you run down the hole, the worse the hole gets. To make matters worse, the more you use a drill for step drilling, the worse that drill dill perform as the cutting edges deteriorate unevenly.

The fact is that a drilled hole is not round, on size, or concentric.
You have to consider where you are going with the drilling operation. Are you going to run a tap through a 1" hole? Not likely unless you are a pro. For most of us large hole drilling is to make space for a boring bar, or single point threading.

All that said, there are plenty of lathes that cant drill a 1/2" hole in steel. Same rules apply, biggest drill you can use, followed by the next biggest. Worth considering drilling 1/64" undersize then making a finish pass onsize for taping stuff in the 1/2" size range.
 
DAT510 said:
For me... I step drill. Starting with a center drill. I had been taught by an old machinist, that each of the drills should be slightly larger than the width of the chisel point of the next large drill you will be using in the step. So, for example: if my final bit is a 1/2" drill. I look at the width of the chisel point and pick the next smaller bit so that it's diameter is slightly large than the width of the chisel point on the 1/2" drill. For a 1/2" hole, I'd personally do it in at least two steps depending on material (Center Drill, Intermediate Drill, 1/2" Drill).

If the accuracy of the ID needs to be close, I drill it close and then ream to final size.

If Concentricity is important, I drill, then bore, and possibly ream (if the ID is reasonable for me to ream)

Hope this helps.
Click to expand...
What you say is the best practice if one has a machine capable of doing it in terms of power and ridgidity; If one drills holes using sizes of drills that do not use the whole cutting edge, it invites wobbling of the drill and out of round holes
 
You must log in or register to reply here.
Back
Top