# 25mm hole in 6mm mild steel plate?



## RobinBHM (7 Feb 2015)

I need to drill a 25mm hole in the centre of a 150mm dia steel plate 6mm thick.

Would a hole saw or blacksmiths drill be best? Itll be done on a pillar drill.

Super accuracy isnt required but it does need to be nice and round to fit over a 25mm shaft.

TIA Robin


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## t8hants (7 Feb 2015)

Do you know anyone with a mag drill and a rotabroach, that would be my first option, then a fine tooth hole saw as plan B


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## worn thumbs (7 Feb 2015)

It needs a pretty powerful pillar drill to turn a 25 mm drill bit and it is likely to snag as you break through.It takes a lot less power to turn a hole saw but the accuracy may not be quite as good.All things being equal- i.e. you need to buy the tool,I would be looking for a Rotabroach.The other alternative is to visit a machine shop.


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## kostello (7 Feb 2015)

Step drill?


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## kostello (7 Feb 2015)

A hole Saw with lube would do that I n a cordless drill no problem so in a pillar drill would be easy


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## RobinBHM (7 Feb 2015)

Many thanks for all the advise, I think I may try a hole saw if that will work in a pillar drill. The drill is a floor standing Axi so should have the grunt. I guess it will need cramping well to avoid any snatching.

Cheers Robin


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## CHJ (7 Feb 2015)

You really need someone with a metal lathe to bore it for you.


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## kostello (7 Feb 2015)

Don't forget to lube and keep the speed down


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## woodfarmer (7 Feb 2015)

If you will be using a holesaw vertically keep cleaning the swarf away.


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## dickm (7 Feb 2015)

Probably a good idea to check the size of the hole made by a holesaw in a piece of scrap. From experience, the nominal size is usually VERY nominal!

One other thought, if you have patience, might be a tank cutter. A lot of work, but could make a nice clean hole of the right diameter and would give you great biceps!


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## RobinBHM (7 Feb 2015)

Many thanks, I will try a holesaw first on scrap and see how it goes.

Im a wood butcher so anything metal is all a bit alien


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## Cheshirechappie (7 Feb 2015)

My first choice would be to use a centre lathe (as CHJ suggested). Catch the plate in the inside jaws of a three-jaw chuck, take the bulk out with successively larger drills, and finish by boring to a nice fit on the shaft.

If no lathe is available, and the pillar drill won't turn a 1" drill in steel, then there's a hand method that might work. Mark out the centre of the 150mm plate, and lightly centre-pop. Set a pair of dividers to 1/2", and scribe a circle using the centre pop to anchor one leg of the dividers. Lightly centre-pop round the circle at about 1/8" intervals (that's so that you can still see where the line was when you're nearly there - a circle of half-centre-pops is easier to see than a scribed line). Chain drill inside the line, using whatever size of drill the pillar drill will comfortably push into steel. Join up the holes by file to 'lose' the bit in the middle. Then, using a turnip half-round file, remove the bulk of the rest to near the line. Switch to a second-cut half-round file, and ease the fit until it almost goes onto the round bar. Then switch to a fine half-round file, and ease out the last few thous for a nice fit (interference, sliding or clearance, as desired) on the shaft.

May sound like a lot of work, but since it's only 1/4" plate, it won't be too bad, and you can control the fit between shaft and plate. You can't do that with a hole-saw, unless you 'saw' small and file to fit.


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## Hemsby (7 Feb 2015)

CHJ":11r2dklc said:


> You really need someone with a metal lathe to bore it for you.



+1
Would be my first choice, you can have whatever tolerance you choose. Piece of cake for a centre lathe =D>


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## graduate_owner (7 Feb 2015)

Definitely clamp it down. When the drill bit breaks through there will be a great deal of force from the snag. Do you know anyone with a milling machine? It would be an easy task with that, given suitable tooling of course.

K


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## DTR (7 Feb 2015)

Another +1 for boring it on a lathe. Second option would be a step drill.

If using the holesaw, drill a small hole inside the 1" diameter first. Position it so that the outer edge just touches the inside of the 1" diameter. This gives the swarf from the holesaw somewhere to escape to, rather than clogging the teeth.


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## Mooeee (7 Feb 2015)

A Mag drill with rotabroach bit will drill through in about 20 seconds and be perfect and as neat a cut as you would ever want.


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## RogerP (8 Feb 2015)

Given a pillar drill and the piece clamped in a vice on the drill table I'd just work through a few sizes of ordinary blacksmith's or jobber's drill bits. Start off with say a 5mm then 10mm, 13mm, 16mm, 18mm 22mm 25mm or whatever. Wouldn't take but a few minutes.


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## woodpig (8 Feb 2015)

Boring head on a milling machine is another option.

http://youtu.be/l_t8nqlLhH0


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## RobinBHM (8 Feb 2015)

I sometimes order structural steel beams and plates with pre cut holes and often wondered how come the holes are so clean. Ive looked up mag drills and rotobroach -actually not massively expensive, however I dont need much accuracy so I think I will stick with a hole drill, but maybe 26mm or 27mm as slack wouldnt matter.

I did think about working up in hss drill sizes, but dont think ive got any larger drill bits for metal.


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## nanscombe (8 Feb 2015)

Probably a bit naive but if you didn't have a large diameter drill bit could you make a number of smaller holes around the circumference, enough to allow you to pop out most of the metal, and then use a metal rasp to finess it?


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## woodfarmer (8 Feb 2015)

One of three shims I made 1mm, 1.5mm and 2mm







The smaller holes were drilled then sized using a taper drill (from screwfix).

Machining 1mm aluminium plate on a lathe is not entirely simple.


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