Drilling 3/8" plate

[billm]

Hi, I want to drill a 12.7mm hole in 3/8" steel plate. I have drilled a pilot 3mm hole to begin with.

My question is #1 on the amateur scale of dumb questions ....

should I drill gradually going up in size, eg 4, 6, 8, 10mm finishing off with the 12.7mm or go straight onto the 12.7mm drill bit but take it very slowly and with cooling fluid ... oil ?

The reason I ask is if going up in drill sizes will the hole stay centred or could it jump / elongate the hole ?

thanks

PS I'm only using an ordinary drill in a DIY drill press attachment, not a bone fide drill / bench press

 

[Burnerman]

Use as many steps as you can Bill - much easier on you and your gear that way.
Slow the speeds down the larger the drill, and any lube like oil or WD40 can only help.
The holes won’t go off centre.
John

 

[billm]

Use as many steps as you can Bill - much easier on you and your gear that way.
Slow the speeds down the larger the drill, and any lube like oil or WD40 can only help.
The holes won’t go off centre.
John

Great, will do, many thanks

 

[opps]

For 13mm it is recommended that you drop down to about 420 rpm. Don't forget that as the drill bit is about to exit through the metal, it will "grab" and try to spin the metal that you are drilling. Ideally you will clamp the metal.

Over the years I have spoken to metal workers who grind their drill bits to a different angle to reduce the "grab".

After my A'Levels, I spent 6 months working as a lift engineer's mate. We spent a lot of time drilling through mild steel with corded drills that had no torque control. The first thing I was taught was to anticipate the grab and make sure that if the drill span onwards, I had an "escape path" for my hand so that the spinning drill body didn't slam into my wrist.

edit- if your are patiently working your way up through drill sizes, the grab is less of an issue.

 

[Burnerman]

I remember when I first started to grind my own drills.......they would cut a perfect figure of eight
I'm not that much better now!
John

 

[opps]

I remember when I first started to grind my own drills.......they would cut a perfect figure of eight
I'm not that much better now!
John

Yeah, I can't regrind drills. It is a skill that I lack. When a bit becomes blunt, I bin it, which admittedly is a waste.

During the lockdown, I spent £300ish for a wet wheel to sharpen my chisels- drill bits however are relatively cheap so I didn't want to pay proper money to maintain a cheap item. When it comes to drilling out rounded screwheads, I now use TCT bits. I have however learned that you need to work your way up when drilling through a pozi/phillips head, if you use the wrong size, the inset (read: screw head hole) will snap the tct head off.

 

[Notch7]

Use low speed and consistent pressure

If you can clamp the work each drill size will automatically be centred

If it was I’d go straight for the finalsize drill, 1/2” through 10mm is no biggie

 

[billm]

I need to buy a 12.7mm bit as biggest bit I have is 10mm. I'll be drilling upto 8 of these holes.
Will a Jobber bit be ok if lubricated, or should I buy Tungsten. Doubt I need Cobalt ?
Its a 1-off job not a regular workshop thing.
Thanks

 

[Burnerman]

For drilling mild steel plate, an HSS jobber drill should be fine.
Tipped drills are great for stuff like stainless steel or chilled cast iron - I guess it depends on what is available plus the costs.
I’d prefer to buy two drills, just in case one ends in grief!
Good luck with your project.....can we ask what you are up to?
John

 

[billm]

thanks John, I never really understood why they called them Jobber bits other than their lifespan was probably limited ?

The job: I'm repositioning wheel studs in the hub of a classic car. Some egit removed the original studs, elongated the holes and loosely put the studs back in so he could fit wheels with a different PCD to the original wheels !

So, I have bought 4 new studs (cos he'd shortened them too) and am going to drill 4 new holes in the hub placing them at 45 deg offset to the current holes into the nice untouched part of the hub. The studs are 12.7mm diameter and vertically "serrated" at the very end to stop them turning. I'm hoping with a nice clean hole they will be a push fit and I can whallop the last serrated 10mm into the hole as I can't think of a way of serrating the hole to match those on the stud.

That's the plan so far

 

[carmanmemoranda]

Those holes need to be accurate and specific. Is the plate separate to the hub? Otherwise it sounds like you might be weakening the hub, too much.

If it's separate, take it to a machine shop and pay for the precision.

 

[Munroast]

I need to buy a 12.7mm bit as biggest bit I have is 10mm. I'll be drilling upto 8 of these holes.
Will a Jobber bit be ok if lubricated, or should I buy Tungsten. Doubt I need Cobalt ?
Its a 1-off job not a regular workshop thing.
Thanks

I suppose it depends what you are drilling into - I guess its mild steel, and if so the HSS bits that jobbers are normally made from will be fine, and if used properly at a sensible speed and plenty lube will last a surprisingly long time. You should be aware of it cutting the metal when drilling and if it isn't then stop, no use just going faster and faster and hoping for the best, that just destroys the drill.

if the accuracy of the hole is critical, centre punch it first and start the hole with a stubby centre drill bit.

 

[billm]

thanks guys, the hub is off and using maths & a mic I have scribed an arc from the outside edge where the PCD should be. I am going to refit the hub back to the axle and spin it to check the arc is correct with the mic from the centre of the axle just incase the outer edge of the hub isn't perfectly round which is very unlikely.

I centre punched the 3mm pilot hole and will do same on the 3 others then offer up the wheel. I should be able to see all 3 marks through the holes in the wheel and they should be central.

If they look good, then on to the bench with the jobber bits and drill all 4 holes then stick weld up the elongated holes

 

[Burnerman]

Wow - that's quite an undertaking!
Normally we would use a vertical milling machine with a dividing head to give exact measurements, and the divisions of 45 degrees was accurate because of the lathe type compound screw adjusters.
I guess the new wheel studs have locating splines?
Good luck with your project - we'd love to see a pic or three!
John

 

[freddiemercurystwin]

I'm hoping with a nice clean hole they will be a push fit and I can whallop the last serrated 10mm into the hole as I can't think of a way of serrating the hole to match those on the stud.

That seems like a rather crude method, I'd have thought it'd be better to pull it through by tightening up the wheel nut on some washers or other spacer?