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mjcann

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Digging about in the back of the shed, I found a box of drill bits that havnt seen the light of day for 25+ yrs,
all sorts of sizes and shapes, but one type that made me wonder what material they were used to drill.
They have what I would call a slow twist, largest is standard length about 12mm but flute only one turn end to end, at smaller end of selection is 2mm dia 150mm long and flute does one turn in about 60mm.
Martin
 

jasonB

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Slow spirals can be used on Copper, brass, bronze and plastics as they are less likely to grab the work
 

mjcann

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Thanks for reply, I can understand that having been in the electrical industry, I have broken my fair share of bits drilling brass etc, I wish somebody had told me there was a bit made for the job 50yrs ago, it would have saved a lot of bad langauge.

Martin
 

Normancb

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The helix angle of a drill controls the rake angle at the cutting edge and the cone angle creates the clearance angle. For brass it is usual to use zero rake tooling as positive rake angles cause the tool to pull inwards and grab. Slow spiral drills are a compromise. You can get zero helix drills to give zero rake but they have no lift for the chip.

Engineers drilling a lot in brass will modify a normal drill. You take a stone or a diamond lap and run it across each cutting edge, parallel to the drill axis, to create a tiny zero-rake face on the edge. Ruins it for steel but kills all that grabbing in brass and easily ground off to get back to normal. Some folk keep two sets of drills, one ground for steel and the other "blunted" for brass.
 

Spindle

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Hi

All of the above is correct but I'm surprised that no one has mentioned the other use for slow spiral, low rake bits - they are very useful when drilling thin sheet material as there is less to no tendency to grab - this holds true for all thin sheet material including iron and steel.

Regards Mick
 

rafezetter

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Spindle":z6amv8ei said:
Hi

Just buy a standard set:

http://chronos.ltd.uk/acatalog/info_CG76_.html

and stone to zero rake - will take about 30 seconds per drill

Regards Mick
how do you do that as I drill thin sheet metal sometimes and have issues with it trying to take too much off and getting wedged (it starts to cut, but too much and even removing the drill bit and reoffering it, soon as the flutes hit the partial cuts, the bit stops spinning - nearly always as it's about to cut through) - is this what you mean by grab? Happened the other day trying to enlarge steel washer holes to 8mm from 6mm - took waay too long as it happened time and again, even with the washer wedged in an engineers vice.
 

Spindle

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Hi

Yes - a zero rake drill bit will be far less likely to grab, it will however require more pressure to get it to cut.

This is how to stone a drill to zero rake - you only have to stone the cutting face, a 0.020" length at zero rake is more than enough.

0 Rake Drill.png


Regards Mick
 

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Spindle

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DTR":1r0ry74c said:
For drilling thin sheet, brass or otherwise, you might find a step drill cuts a lot easier without grabbing:
Hi

That's because step drills are normally zero rake :wink:

The draw back is that step drills are limited in the sizes they can drill, usually 2mm increments

Regards Mick
 

rafezetter

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DTR":7dgcczb3 said:
rafezetter":7dgcczb3 said:
how do you do that as I drill thin sheet metal sometimes and have issues with it....
For drilling thin sheet, brass or otherwise, you might find a step drill cuts a lot easier without grabbing:

http://www.toolstation.com/m/part.html? ... tAod6QMARg

I'd wondered about those - I've seen them used in metal bodywork fabricators but always thought they were for enlarging holes only.
 

Spindle

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Hi

Step drills will cut their own hole from scratch and continue to enlarge it to the drill's maximum, cone drills, depending on their design may or may not need a pilot hole. Both are only really suitable for thin sheet metals - both will prove challenging to sharpen.

Your best option is to stone some zero rake drill bits in the sizes you need.

Regards Mick
 

Normancb

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If you are going to drill many holes in sheet metal there are two other things that can help:

1) Get a set of bullet point drills. Piranha brand are petty good (though not easy to sharpen). Basically the bullet point locates the centre but the outer rim of the hole gets cut away before the supporting material is cut. They are the metalworker's equivalent of a woodworker's brad-point drills and work very well in thin sheet.

2) Try the emery trick. Locate the drill point (normal drill) over the desired spot. Take a bit of fine emery or wet-and-dry and fold it in half with the abrasive inwards and drill through that into the metal - works surprisingly well, especially if you want to enlarge a pre-existing hole
 
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