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[4]

How large a bit before needing to predrill?

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Jake2465

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#1
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.
 

rwm

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#4
I'm curious to see the answers. I almost always predrill. Less work for the final drill.
R
 

kd4gij

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#6
On a BP, I have drilled 3/4" with out pre drilling.
 

markba633csi

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#7
I have found over the years that predrilling, especially in steel, makes the larger bits last longer.
Mark
 

kd4gij

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#8
In aluminum I never pre drill and Have drilled 2" holes. In soft metals and plastic pre drilling or step drilling can lead to bits grabbing.
 

mikey

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#9
I don't know about you guys but I don't predrill unless I go above 1/2" but that's when I hold the drill in a drill chuck. On my lathe, I use morse taper drills and can go up to almost an inch without predrilling. Drilling a hole with the main drill is always more accurate for me and prevents any grabbing.

Jake, a 5/16" hole in aluminum without predrilling is no problem - just spot it and drill it.
 

rock_breaker

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#10
I will go along with mikey on this one since it is aluminum and a 5/16 bit. I predrill most holes in steel and above 1/4-5/16 in aluminum. It's more of a habit than anything.
Have a good day
Ray
 

Giles

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#11
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.
 

rwm

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#13
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
 

P. Waller

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#14
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
 

Janderso

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#15
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.
 

benmychree

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#16
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.
 

Jimsehr

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#19
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.
 

Suzuki4evr

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#20
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.
That's how I do it.....don't know about the rest of society
 

P. Waller

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#21
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 are designed for producing rough holes for the most part, if one were to always pilot at the chisel edge diameter as the drill size becomes very small the pilot would approach zero, what is the chisel edge width of a .020" diameter twist drill?

As mentioned above, drill straight through with the largest tool that the machine will push. The power of the machine is the limiting factor, other then this there is no good reason to step drill a rough hole if it can be avoided.
 

f350ca

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#22
I hand sharpen my drill bits, its difficult at best, impossible for me to get the flanks exactly equal. If I need a hole close to the required size without boring or reaming, drilling an undersized (pilot) hole close to the required size first followed by the required dia gets me on size. If the cutting edges aren't equal and no pilot hole is drilled the bit will swing around the chisel point and drill oversize with the radius of the hole set by the longer cutting edge.
You can see the result when a bit breaks through and leaves a reduced dia hole at the end.

Greg
 
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