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Custom 4-diameter step drill - pecking in 6061 aluminum?

codeine01

Plastic
Joined
Sep 24, 2018
I ordered some custom-ground carbide step-drills from a reputable company, for a specific repeat use, to be used in a Haas TM-2P with 6k max rpm in 6061-T6 aluminum. The shank of the drill is 0.375", held in an ER32 collet tool-holder. This machine does not have through-spindle coolant, only flood.


I attempted to upload a PNG image of the tool, but even though it's under 800 pixels wide, I can't seem to upload it, so I'll describe it.


The holes to be drilled in the 6061 aluminum have diameters as follows: 0.285" (clearance for cap-screw head), 0.185" (clearance for cap-screw threads), 0.150" (bore size to form-tap 8-32), 0.100" (drain hole for anodizing acid to flush). Depth of holes/bores: 0.285" bore to 0.5" deep, 0.185" bore to 0.269" deep, 0.150" bore to 0.331" deep, 0.100" bore to about 0.040" deep (through-hole).


Thus, the custom-ground drill has stepped-diameters and lengths as such: 0.285" diameter, 0.750" flute length, 0.185" diameter, 0.269" flute length, 0.150" diameter, 0.331" flute length, and 0.100" diameter, 0.100" flute length (to allow for 0.100" holes deeper than 0.040"). The drill has a 118-degree included tip, and all steps between diameters have this 118-degree included angle, with the exception of the step between the 0.185" and 0.285", which is flat, to accommodate the head of a socket-head cap-screw.


So, after that long-winded explanation, I'm wondering if pecking will be a problem with this drill. Normally, with pecking, the sides of the hole/bore hold the chips into the flutes until the drill is retracted from the bore, and the rotation of the drill and coolant pressure help clear the tool tip of chips before re-insertion. However, with a step-drill, I'm imagining that the chips may simply fall into the hole as the tool is retracted. But without peck-drilling, that's quite a bit of material to be removed (relative to drill size) in a simple one-shot drilling operation.


Anyone have experience with this, peck-drilling with step-drills?
 

BT Fabrication

Hot Rolled
Joined
Nov 3, 2019
none in that specific style, but any type of step drill i have used is super slow.
id most likely drill the smallest hole all the way to depth, and try to remove as much material as possible to give the step drill (basically a reamer) a fighting chance to make a few good holes without you having TSC
 

codeine01

Plastic
Joined
Sep 24, 2018
Well, I tried 3000 rpm, 0.001 feed/rev, with 0.100 pecks, that seemed to give acceptable results, drilling blind. Could perhaps go up to 0.002 feed/rev, maybe.

Also, in a separate test, I pre-bored the 0.285" portion with a 0.250" end mill to depth. That helped with the bird-nesting by removing the counter-boring operation.

Admittedly slow feed, but better than using four different tools to create these bores.
 








 
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