View Full Version : drilling large diameter holes

09-18-2003, 07:22 PM
A lot of my work has been drilling large diameter holes in round stock. An example would be a 2.5" hole through a round bar of 4" in diameter. I have a mill as well but I tend to use the lathe most of the time. I'm at present and for the past 30+ years using large diameter twist drills to take out most of the material before I start the boring process. Lately I have been considering either investing in indexable drill bits or I have noticed "spade drills". In addition to this is the "Multi-tool system" which uses what appears to be a flat blade in a slot of a round bar. Has anyone tried either of these types of drills? The "multi-tool would require a pilot hole and I'm not sure I like that. Any opinions?

09-18-2003, 07:58 PM
I tried spade drills for making large holes in aluminum, expecting a bad result. Instead, I was surprised how much material could be removed in a short time without a big overheating or jamming problem. I think the largest one I tried was a 1 inch. There does need to be a pilot of some sort, and I did experiment to see how small of a point could be used, cutting the usually 1/2 long or so tip down to about 1/8 long, and 45 degree lips. Worked fine. Larger sizes, I don't know. The spade bits where the flutes extend up into the blade somewhat are best for this, as they allow the newly created tip to cut like a normal drill bit, where the chips can escape up the flutes.

Forrest Addy
09-18-2003, 08:19 PM
If you're on a lathe, drill the largest convenient size (say, 3/4 of the quill diameter) and single point bore from there. Goes surprisingly quick if you take cuts that work the lathe a little.

09-18-2003, 10:13 PM
Avoid the drills and bore it out. Drill to about 1", then buy a $60.00 bore bar for 3/4 inch or more bore sizes. You will have a tool that is quite versatile, and you will have no limit to the sizes you can make.

Spade drills are good, but lack of versatility over a bore bar. However, they can have different size "spades" put on them. I like them, wish I could invest in them more, but alas, my budget leaves me with a few bore bars to do many holes.

Another method - I have used two flute end mills as bore bars in the past, and boy they remove metal good and fast. You can "off center" the end mill and use a single cut tip to do well. Anyone else do this? This is a good way to use your busted two flutes where one flute has busted right off, and you have but one cutting edge remaining. I can discuss this more if asked.

Good question, I am looking forward to more responses so I may learn more.

09-18-2003, 11:22 PM
I have in the past and still do make my own single point piloted drills,its just a HSS tool bit stuck through a piece of cold rolled and held with a setscrew,the toolbit is bored through the center of the rod and not offset,this provides a shearing cut that is effortless in CRS,a little oil on the pilot,speed as usual and let it rip.The biggest one I made for steel was 3.5" with a 1-1/4"pilot,I even made one with a 3/4"pilot for use in the mill,it would handle a 1-7/8"hole no problem,hot,fast and cheap!

09-19-2003, 07:18 PM
You guys sound as out of date as I am. I still use the boring bar method and will use it for the last cuts to insure accuracy but I was trying to remove steel more efficiently. That's why I was wondering about other methods. Drilling aluminum is no problem but a lot more of my work is stainless and hot rolled . Maybe I should go all out and see if I can find a 100 ton press on ebay cheep. http://bbs.homeshopmachinist.net//biggrin.gif

09-20-2003, 02:53 AM
The APT "multibore System" works very well. Infact with the larger set, the pilot hole (3/4") and the spade drill can be done in one step up to about 2" then larger bushings are used. I have used these on mills, lathes and magnetic drills. You can buy a fixture to regrind the blades or to make custom diameters. You can buy the shanks with and without coolant feed and also rotary coolant adapter for lathes.

I find it is more fun to use a big boring bar on a big lathe = big chips! http://bbs.homeshopmachinist.net//biggrin.gif

To use indexable drills you need big HP - 15HP for 2" indexable drill and high pressure coolant (1500 psi) plus a big drill like this will set you back $2k and you will need a 2" mounting hole in a turret or NT40/50 shank for it. Other wise forget the indexable drills (unless you can score one cheap on eBay).

09-23-2003, 02:10 PM
What about those cored drill bits? They don't go too deep but since you only cut out a ring of material instead of the entire hole, they work faster.

09-23-2003, 03:04 PM
Where can I find some info on the APT multi-bore system?

09-24-2003, 04:36 AM
<font face="Verdana, Arial" size="2">Originally posted by INTERPOLATE:
Where can I find some info on the APT multi-bore system?</font>

KBC definately sells them
www.kbc.com (http://www.kbc.com)