The Folded Cloth Trick (get truly round holes in sheet metal)

[Lo-Fi]

FFR: thin sheet doesn't drill well with large drills because the point breaks through and loses its guidance ability before the flutes engage and provide some stability. If the material is thinner than the axial distance point to outside flute lands, you're going to have a hard time. The rag trick is a neat solution out of a dilly of a pickle when you need to get the job done and does indeed work by filling the flutes to give some stability. Annular cutters work well, as do HSS hole saws.

 

[graham-xrf]

I prefer a fishtail drill for sheet metal.
We had machines that drilled holes in copper tubing during my apprenticeship and I sharpened thousands of those by hand.
Like this but with a point in the middle.

View attachment 448469

They look awesome, but as you say, will need the point in the middle. The image begs the question as to how those two came to be without the needed point.

I have seen drill bits intended (I think) for woodwork that look a bit like that, and I have some left over from one of those cheap hardware store sets. We do know that those which come with a point that is not just a point, but looks like the sharp end of a wood screw are definitely for wood, and will pull themselves through pretty well when turned slow with a auger.

My thinking is that while the cloth trick is intriguing, it still lacks the centre guide that disappears when the drill breaks through. Having a centre point guided drill sharpened like yours, with a backing under the sheet metal that can have the drill maintain the centre line, is what makes sense to me. I also think that my recipe of clamping between two bits of aluminium is pretty uncompromising, and the hole ends up centred where the pilot hole was put, without any "last turn re-location" effect.

I would never put you to new activities in making tutorial videos, but maybe you have a tip, or a link to one where sharpening like that is done?

 

[RaisedByWolves]

They look awesome, but as you say, will need the point in the middle. The image begs the question as to how those two came to be without the needed point.

I have seen drill bits intended (I think) for woodwork that look a bit like that, and I have some left over from one of those cheap hardware store sets. We do know that those which come with a point that is not just a point, but looks like the sharp end of a wood screw are definitely for wood, and will pull themselves through pretty well when turned slow with a auger.

My thinking is that while the cloth trick is intriguing, it still lacks the centre guide that disappears when the drill breaks through. Having a centre point guided drill sharpened like yours, with a backing under the sheet metal that can have the drill maintain the centre line, is what makes sense to me. I also think that my recipe of clamping between two bits of aluminium is pretty uncompromising, and the hole ends up centred where the pilot hole was put, without any "last turn re-location" effect.

I would never put you to new activities in making tutorial videos, but maybe you have a tip, or a link to one where sharpening like that is done?

I have no idea, they make no sense to me.

 

[woodchucker]

If depth isn't a problem the stepped cone drills are very good for sheet metal.

Stu

that's my choice.. works well on sheet metal, brass, plastic. I knew about the cloth, but the step drill is so much easier.

 

[RaisedByWolves]

I would never put you to new activities in making tutorial videos, but maybe you have a tip, or a link to one where sharpening like that is done?

Done.

Hand sharpening fishtail drills. A brief primer.

I mentioned making these drills in THIS THREAD and was asked if I would be so kind as to write up a tutorial. Since I can find no reference of this type of grind on the internet I figured why not. I started sharpening this style of drill tip to cut into copper tubing in a factory environment...

www.hobby-machinist.com

 

[graham-xrf]

that's my choice.. works well on sheet metal, brass, plastic. I knew about the cloth, but the step drill is so much easier.

I have a set of these, and they worked well enough on making holes in galvanized sheet metal in an aircraft hangar. The brake backplate modification I am doing is more precise. It requires 4 holes for stainless nut inserts located accurately enough that the assembled plate will fit the mounting holes on the brake caliper hub. This is something that must happen "one throw straight"!

I may try out a practice run on a piece of scrap. I do get it that I may be making more of this than it deserves, but I have to design and build as I go, during intervals in between when others will want the vehicle in running condition. One rear wheel at a time. The present condition has one wheel cleaned up to bling, and all the rest of the car looking like it just spent winter on a farm!

 

[Lo-Fi]

Why don't you do the job on the mill? Drill pilots with centre or stub drill, finish with an end mill of appropriate size. The first op is just so the end mill doesn't have to cut to centre. Working to coordinates I much nicer than trusting layout and punch marks
Cone drills in stainless are asking for a bad day in the workshop. One dull part and you'll rub and work harden. Ask me how I know

 

[graham-xrf]

Why don't you do the job on the mill? Drill pilots with centre or stub drill, finish with an end mill of appropriate size. The first op is just so the end mill doesn't have to cut to centre. Working to coordinates I much nicer than trusting layout and punch marks
Cone drills in stainless are asking for a bad day in the workshop. One dull part and you'll rub and work harden. Ask me how I know

I would do exactly that, except my mill is presently in the throes of being moved and set up in the new shop.
My Cormak HK-25L is only a "little" mill, looking like any number of others near clones of PM-25MV or G0704 types, but it would be quite up to the job if it were not presently parked on the shop floor awaiting the construction of some kind of stand/bench. Just safely moving it there was already a bit of a project!

I do get it about drilling into stainless. The advice here is to use a M42 cobalt bit, cutting paste, and go in hard, and don't let up. I know once you let it rub, that's immediately the end of the drill bit life until resharpened. AnchorLube, (green stuff + quite expensive) has lots of good things said about it, but trying to find a handy source in UK is like looking for rocking horse poo.

 

[graham-xrf]

OK - I have now tried the trick out, and managed to make up at least the first (of two) modified brake plates. I did start with 2.5mm (that's very near 0.1inch) pilot drill for the final 5mm in steel, and 3.5mm (0.138") as starter for the final 6.9mm in aluminium. The experience was interesting.
These drill sizes are not particularly "large", but still give problems with staying on course, and "snatching". The sheet metal is only 0.9mm thick, and the patch plate is 2mm.

I opted for aluminium instead of stainless steel for the repair plate, partly because both kinds do not rust, and the effort to work in stainless is make a thing of beauty overkill for an item that does not deserve it. The aluminum would be a whole lot easier to work. That said, the final job looks real nice, but on the way I discovered that the drilled holes (in aluminium) were able to re-locate themselves a little from where the punch mark started. This might have been "pilot error" at the first drilling. The holes themselves were beautifully round, almost looking reamed, and no sign of "snatch-burr". I still had to use the drilled plate as template to get the remaining holes in the right place.

You have to keep steady, and drill blind!
There is that moment of uncertainty. The cloth covers the hole, so you can't see to take aim with the drill press. You have to first line up as best you can, put the cloth in, and then come down on it blind, hoping you have not moved off aim. @Lo-Fi is absolutely right about the benefits of using a mill + DRO, and maybe a stubby bit as being the better way.

 

[Lo-Fi]

+1 on the stub drills, and I'd definitely recommend split point ground.