Widening holes in steel plate

[unionworkeruk]

Just taken delivery of steel stringers for staircase. The 6mm thick plates have four holes each to take bolts to support 35 mm wood treads. The holes were drilled at 8mm instead of 12mm. I have had nothing but trouble from the company so want to drill the holes to 12mm myself.

I have used a cone drill and widened 24 holes to 10mm without too much effort using an antique 40 year old Black and Decker drill with a cone bit. The recess in the orginal hole helped a bit. But I am having problems widening them to 12mm.

The cone no longer seems to cut after 10mm so am using a Ryobi RPD1010 drill with an Erbauer TiN HSS-G 12mm bit but that is having no effect at all. Whatever speed I use it just throws out a bit of dust with the drilling paste.

I am a painter and decorator not a fabricator so I just need the easiest and best drill piece to widen the 10mm holes to 12mm in 20 6mm plates. Cone or non cone I dont care. Just want it done quickly so I can get on with my own work. Thanks

 

[EddieM]

Drill a 12mm hole through a bit of scrap steel, clamp over the steel with the 8mm hole, centering the 12mm hole with the 8mm hole, then still through, the 12mm hole in the top bit of steel will act as a guide for the bit.

 

[Burnerman]

If the brackets are mild steel then you should be able to drill them without too much bother....use new HSS bits, go up in 1mm stages. Drill at a slow chuck speed, some oil or WD40 can't do any harm!
John

 

[Harry Bloomfield]

Might these be stainless steel? SS work hardens when you try to drill it, producing the effect the OP is reporting.

 

[JohnD]

Would a HardPlate drill be suitable?

Cobalt alloy steel I think.

Edit, just looked, seems to be TCT now.

 

[unionworkeruk]

The company who made the stair stringers usually make fire escapes and these are a variation of them so I suspect they are stainless steel. The drill bit, although new does not cut even at the slowest speed but skids around the rim of the 10mm hole without cutting into it. I have tried it on a couple other thin metals and it is through in seconds.

 

[Harry Bloomfield]

Would a HardPlate drill be suitable?

Cobalt alloy steel I think.

Edit, just looked, seems to be TCT.

I will pass on answering that, I just know it work hardens from trying to drill it myself, to make a motorbike lift from stainless steel box section I happened to acquire.

 

[Harry Bloomfield]

- explains how to go about drilling it, but doesn't explain how to go about drilling where it has already work hardened.

 

[Tigercubrider]

Step drills are pretty crap and really only suitable for thin metal odd jobs

I would use a 10 then 12 cobalt drill on slow and some cutting fluid

 

[Mottie]

If you knew someone with a pillar drill, it would be a piece of pee to go straight through them with a 12mm bit.

 

[cdbe]

Use 10mm fixings.

 

[unionworkeruk]

Problem solved. Bought a 12mm HSCO drill bit which as directed used slowly did cut through the steel. Only problem was jambing and nearly taking my arm off a couple of times. But more practice and 20 more holes to go I will master it.

I also found drilling a 12mm hole in the wood gave me more leeway for matching the 10mm hole with the steel. These are rough cut planks with waney edges so not regular shape or size which is a problem in marking up for drilling

Thanks for all the advice

 

[opps]

Only problem was jambing and nearly taking my arm off a couple of times.

In my late teens I used to work for a lift engineer. In those days we only had 750W Metabo corded drills. These days I would only consider using a drill with a slipping clutch. Alternatively, by all accounts, if you regrind the drill tip to a more shallow angle it reduces the "grab" as you pass through the metal