26 replies to this topic

[Yoda]

I like this. just spoke to a ex sheet metal worker and he said to drill the hole under size and then use a cone cutter.

 

Exactly what i do, but if you drill a pilot hole first and go up in drill size in stages, you should get the same result. You cant expect to go straight in at say 13 mm and expect a perfect round hole due to the way a drill cuts. It constantly tries to take two opposing curved cuts so pulls itself out of round. (simple explanation, scientific one coming soon LOL)  

[tiger99]

Yoda, I await your explanation with considerable interest, as I have, many times, seen drilled holes try to go almost triangular, especially in ali, but sometimes in steel. I have actually not had too much trouble drilling steel myself, but few of the holes were large.

 

All the previous advice seems to work, speed as high as possible for the drill diameter being a good start. Pilot hole always seemed to work for me. I think that the step drill is a wonderful device for flat sheet, but not so useful in curved bodywork, And, I have drilled and filed to size more than I care to remember. All you can do if you don't have a suitable drill, or a high speed grinder or burr. Nowadays I might try the Dremel.

 

On flat sheet, you can't beat a Q-Max punch for a neat result, and you can get them for rectangular holes too. But first you need to drill the hole for the bolt. It that ends up oval, it does not matter too much, as the punch removes that bit.

 

http://www.buckandhi...CFbIPtAod4B0AJw

 

But in most circumstances, the "best" method depends on what tools are immediately accessible to you.

[Yoda]

The quick scientific explaination is that if you lok at the end of the drill bit, you will see a small flat straight edge before the cutting edge starts and then curves away to the edge of the bit. when you  start the drill. it wants to slip sideways and then takes a bite, it then wants to slip again and take another bite. i still want to find out why as Tiger says, it always seems to end up triangular, especially if running a fast speed!

 

The reason for first having a small pilot hole ( as explained to me by my college lecturer many years ago "definitely old school" ) is that the pilot hole allows the flat centre of the drill bit to enter the hole therefore allowing the cutting edges to bite instantly rather than trying to slip sideways.

 

The reason for going up in size of bit in many slight stages is that when the drill takes its bite, the curvature of the cutting edge is trying to push the bit sideways in two directions (drills usually have two cutting edges) therefore in theory cancelling out the "push". this is pretty true on "new" bits, but those of us that hand sharpen their own bits, soon find out how important that each side of the drill is sharpened at exactly the same angle, length and clearance or this sideways push effect will be seen to be more prominent, as one side cuts keener therefore having more force available for the push!

 

If you dont understand, or agree with this theory, take maybe a 1 inch drill bit and experiment, with it.

 

And the final factor for wandering bits in sheet steel, is the fact that once the hole is started, there is nothing behind the sheet to stop the bit wandering. imagine a drill going into a solid block rather than a sheet, the bit is using the pilot hole to start, then the bit is forming a cone in front of it to keep itself on track and true. you may have seen this when starting a hole in a solid block. The drill bit will chatter around until it has formed a small conical shape to settle into, then it will smooth out and cut true. 

 

To assist cutting sheet, a block held firmly behind it will allow this conical shape to form behind the sheet and keep the bit centred. This takes us back to a previous post recommending a block held behind the workpiece. 

 

 

I am sure an Modern up to date engineer will be along soon, to put me right, but i am just passing on some theories given to me when training as a lad at college.

 

 

To state the reason why i use a cone cutter, once a small pilot hole is drilled, the cone cutter then uses just a narrow part of its cutting edge in the sides of the hole and its shape replicates the cone you would normally see in the backing block or solid part being drilled into, therefore centring itself once again.

 

BUT it is important to keep the cutter at 90 degrees to the workpiece, or one side will see a different cutting angle and will start to push the hole to one side.

 

Any arguments to these theories will be welcomed, as will any better theories and explanations.

 

May the force be with you.

 

YODA.

[Rog46]

OK here's a tip I was given nearly 50 years ago . I still don't understands why it works but it does ! Holes often go out of round when one flute of the drill grabs and the other flute tries to rotate around that rather than around the centre .

When drilling thin sheet metal fold a piece of cotton cloth into 4 and lay it on the metal . Now drill through this and into the metal. The drill doesn't grab, doesn't punch through before it's finished and doesn't go out of round.

I've never heard of this since and don't know why it works, if anyone has an explanation I'd love to hear it !

[tiger99]

I have heard of that too, and have not the slightest clue as to why it works! Yoda, as always, makes some interesting observations. Sharpening drills correctly is not easy, and I don't, as yet,have the equipment to do it properly.

[Rog46]

Sharpening drills correctly is not easy.

I stopped trying to sharpen drills when I realised that the time spent doing it, plus the time wasted when they didn't work properly was more than the cost of a new drill. The biggest reason for trying to sharpen one now is that you haven't time to wait to get a replacement.

[Yoda]

I have heard of that too, and have not the slightest clue as to why it works! Yoda, as always, makes some interesting observations. Sharpening drills correctly is not easy, and I don't, as yet,have the equipment to do it properly.

 

 

You dont need anything other than a fine stone bench grinder, a good eye and a steady hand!

 

knowing what angles work best with each material you are drilling helps a lot. How many times have you bought a standard hss drill bit and assume it is blunt, when you actually bought a bit cut for a different material.?

 

Oh and patience!

[Yoda]

OK here's a tip I was given nearly 50 years ago . I still don't understands why it works but it does ! Holes often go out of round when one flute of the drill grabs and the other flute tries to rotate around that rather than around the centre .

When drilling thin sheet metal fold a piece of cotton cloth into 4 and lay it on the metal . Now drill through this and into the metal. The drill doesn't grab, doesn't punch through before it's finished and doesn't go out of round.

I've never heard of this since and don't know why it works, if anyone has an explanation I'd love to hear it !

 

 

This works because it reduces the sharpness of the cutting edges for the first part of the cut. therefore reducing that "sideways" push i spoke about earlier.

 

Another way to achieve this is to take a pumice stone and after Sharpening your bit, use the stone to just take the edge off it again. this reduces the extreme sharpness but allows the bit to cut enough to do the work required.

 

A third method is to use a slightly blunted used drill bit to start off the hole centre and than change over to a sharpened bit to continue.

 

Again, any comments or arguments are welcomed, it is never too late to learn something new.

[Rog46]

You dont need anything other than a fine stone bench grinder, a good eye and a steady hand!
 
knowing what angles work best with each material you are drilling helps a lot. How many times have you bought a standard hss drill bit and assume it is blunt, when you actually bought a bit cut for a different material.?
 
Oh and patience!

You forgot the 10 years practice / experience

[sonikk4]

When we install a specific type of fastener on aircraft structure we normally drill a hole at a specific size under the required dimension. We then use a piloted reamer to enlarge the hole to the correct size. Now on thin ali this becomes very difficult to achieve even at high speed and and with a new reamer.

The same applies to countersinking thin material. We have to ensure we do not knife edge the countersink as this will cause weakness issues.

 

Now there is also the issue of chatter with thin material so the use of a backing material helps to alleviate this problem. Like anything if you are on a workbench then you can usually get something behind what you are drilling whereas in situ this becomes more difficult due to access.

 

So it boils down to experience and ability, using the correct type of tools, brand new drill bits where possible over sharpened ones. And the reason why i say this about drill bits being sharpened is we had a more than capable engineering workshop that could resharpen drill bits however we found they blunted very quickly so ultimately it was cheaper to buy new quality drill bits over sharpened ones.

[Yoda]

 

You dont need anything other than a fine stone bench grinder, a good eye and a steady hand!
 
knowing what angles work best with each material you are drilling helps a lot. How many times have you bought a standard hss drill bit and assume it is blunt, when you actually bought a bit cut for a different material.?
 
Oh and patience!

You forgot the 10 years practice / experience

 

 

Hmmm do i dare say, more like 35 years !!!!!!!!!!!!!!

[1984mini25]

 

Or use a step drill

 

 

 

like these... https://www.aldi.co....drill-bit-sets/

 

I bought a set a few years back (mine were in a little wooden box, but the same as the link) and are great for drilling clean, round holes from 4mm up to 32mm.

Edited by 1984mini25, 23 October 2013 - 09:27 PM.