How large a bit before needing to predrill?

About what size would it become necessary or at least advisable to predrill with a smaller bit? I have a little project that needs a 5/16" hole shot through about .750" aluminum plate. I would like to just pop one of my HSS screw machine bits in the collet and do the entire hole with it and forgo the predrill.

In most cases, I predrill any hole in any material from 3/8 and larger. Mainly because it is easier with less heat and stress on drill bit.
In your case, I would use a split-point drill bit.
Just be aware that hole will be slightly larger without predrilling.
 
Important point to remember. Pre-drilling with my lathe often causes the MT2 tailstock taper to release or spin. If you are having this problem DONT pre-drill.
Robert
 
Haven't found the limit with this machine yet, have pushed a 3 1/2" twist drill through 304 stainless and it didn't even groan (-: Also do not stand to close to it as breaking such a drill is a shrapnel laden affair.

The limiting factor is the power of the machine being used, if drilling manually from the tail stock on a lathe for instance the limit is how hard you are willing to work to make time, I will only drill from the TS on an engine lathe as a last resort.

This is a 2 1/2" spade drill straight through (no pecks) 4" of 6061 aluminum in about 2 minutes, a pilot hole it is not recommended when using spade drills.
21halfspadedrill.jpg
 
I'm no expert but shouldn't you at least pre-drill to the outside of the end drill size web diameter?
The web is the center point called the chisel edge of the drill, the web gives the bit it's strength.
Just my 2 cents.
drills-101-2.gif.aspx
 
It really all depends on the machine that is driving the bit; on a BP type of machine, drilling a 1/2" hole, pre drilling is not necessary, just spotting it to eliminate the tendency for the drill to wander. On larger lathes like my 19" Regal, I'd likely predrill for holes (in steel) over about 3/4", when holding the drill in the Aloris holder. I have drilled holes nearly 4" on diameter with spade bits about 3 1/2" through steel, using a 8 ft arm radial drill with mist coolant through the tool shank, this was through base plates for the San Mateo Bridge towers.
It is true that the pilot holes if used should be no larger than the thickness of the drill's web. Stepping out is to be avoided, if possible, do it only if the machine's power or rigidity is not sufficient to do it in one shot.
 
When you split your drill point you cut down on the force you need for the web or chisel edge of the drill to penetrate the material.
 
I'm no expert but shouldn't you at least pre-drill to the outside of the end drill size web diameter?
The web is the center point called the chisel edge of the drill, the web gives the bit it's strength.
Just my 2 cents.
drills-101-2.gif.aspx
That's how I do it.....don't know about the rest of society
 
Back
Top