To Pre-form a Large Diameter Hole Through Thick Material.

Create a center slug of material suspended by thin webs of stock between holes. Then knock out the slug, and mill/bore the hole to a finish size.

Rather than "saw" out the hole with an endmill (where the slug can drop in on the cutter) or machine the slug into costly chips, drill a circle of holes and then remove the resulting slug.

Drills are cheap and easy to sharpen and have a high metal removal rate. Use 135" split point parabolic polished flute screw machine stub drills. For heavier material, spot drill first then use jobber drills or inserted drills.

Any size drill can be used. But using a drill the same size as the finishing endmill, or smaller, works best. 5/16", 3/8", 7/16", and 1/2" drills work well on small millers. Larger diameter high-rpm high-penetration carbide drills, or inserted drills, work best on machining centers.

No special tools or software are required. 

Pre-forming a large hole by drilling with high metal removal rate (MRR) drills
is about as fast as "sawing" out with a roughing endmill, (without all the inherent problems) and about 175-200% faster than "spiral" milling the entire hole into chips.

Gas or plasma cutting, particularly in high carbon steel, requires an annealing or stress relieving process. Drilling out the hole eliminates this problem.

Large hole drilling is tough to do on small knee type millers, and drilling is easier on the machine tool spindle than is milling. 

In high horsepower, high RPM machining centers, solid carbide high MRR drills can blow thru a ring of holes in no time. Pre-forming an 18" dia. hole thru 3 or 4 inch steel would be no big deal here.

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First choose the finished hole diameter A , and the preferred drill size B.

Next, determine the information needed for the bolt circle program.

C = number of holes; D = the diameter of the bolt circle; E = angle of the first hole;
F = angle of the last hole. ( default zero )

Fill in the blank brackets:
( or go to the auto-calculator below )

 

A = finished hole dia. ( less .050 [ 1.5m ] )  - - - - - - -[_______] = A

B = preferred drill size: - - - - - - - - - - - - - - - - - - - -[_______] = B

C = no. of holes: - - - - - - - - - - - - - - - - - - - - - - - - - - [____]  = C
      ( A - B ) x 3.14                                                 (drop the decimal )
                    B

D = dia. of bolt circle: - - - - - - - - - - - - - - - - - - - - - -[______] = D
      ( B + .005 [.13m]) x C  
                 3.14

E = angle of first hole:  360   - - - - - - - - - - - - - - - - - - [______] = E
                                    C

F = angle of last hole: zero  - - - - - - - - - - - - - - - - - - - - - -[ 0 ] = F 

 

 

Note:    Due to rounding off the "no. of holes" value, the actual rough hole size
            may differ slightly from the original "finished hole - .050".

             B + D = actual rough hole diameter.

            The .005 [.13m ] in equation ( D ) refers to the web thickness between
            holes. This can be altered if you wish.

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Click Here to go to the Auto-Calculator.
No numbers to crunch!
Just enter the known values and click the COMPUTE button
to solve the equations that are shown above.

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Created: 06/16/2001
Last Rev: