Reasonably accurate holes in box section

[DAPPH]

I do this on 50 x 50 box with rotabroach cutter just like a hole saw , the pilot drill pierces the underside of the box , have to turn it over to drill that side
Plenty of cutting oil

One thing to discover & remember is that the tube will be a laser welded tube , the side that has the weld seam will have a heat hardened zone ( HAZE ) along the length of the weld . It frequently kills drills , hole saws and hacksaws quite happily or with anything else you try to mechanically cut through it .

So ensure you won't be drilling or cutting into the " HAZE " if at all possible .

Another tip is to cut your holes on the horizontal & use plenty of thin lube so any swarf can be washed down and drain away ,.
Cutting from above tends to see the cut ring fill with swarf , broken teeth & not only knacker the cutter but also often makes the hole over size . The slower & gentler pressure you use when cutting the better the hole will be .

If you use an airline to blow away swarf use , full eye protection , be aware of others and where the blown swarf will end up .. blowing it into an electrical control box , motor or welder etc, etc.is not the cleverest of ideas .

 

[cheb]

Why not a suitably sized blacksmith's drill? The reduced shank will fit into the usual sized pillar drill chuck.

 

[8ob]

You will get a more accurate hole using a holesaw through a jig plate,stops the blade walking. You wouldn’t need the pilot drill either. I would go for a bit of 10 or 12mm plate clamped to the tube.

Bob

 

[Turbo]

You can buy a mt adaptor for annular cutters also one for a jacobs chuck..

You can also grip the cutter in a 3/4" jacobs chuck but you'll not have a guide pin.

As somebody suggested, make an accurate centred hole in flat plate the width of your box section and use that as a jig, then you'll not need a guide pin and minimal marking out.

I have an annular cutter I use for making gate hangers and its only recently I bought a proper holder for it. I just clamped it in the chuck.

 

[Agroshield]

Step drill...

A step drill turns every square millimetre of the 20mm diameter hole into swarf. An annular cutter only turns the annulus swept by the cutter into swarf.

A power source of any particular specification will remove a certain number of cubic millimetres per minute. The fewer cubic millimetres of material you need to remove to produce the hole, the quicker the task will be completed.

On a 20mm dia. hole, assuming the teeth of the annular cutter are 3mm wide (so 14mm dia. slug), it is roughly 50% less material to remove. That is not insignificant over 240 holes. The percentage gets better as the cutter gets bigger. Another way of saying this is that even at a small diameter, where it is least efficient, the annular cutter is twice as good as a traditional drill.

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The idea of a jig is good for freehand drilling. It could be U-shaped so it drops over the box section. If it had a leg on it with a 20mm dia. boss attached, it would drop into the previous hole and locate the next hole.

On a drill press, that jig can be improved (speeded up). A fence (or two parallel fences) would assure front-to-back alignment, and a flag or flip stop that drops into the previous hole would index the work hole-to-hole.

 

[piman]

Hello Grim D,

how about a 20mm twist drill?,

Alec

 

[Rrobson]

@owas has already mentioned this, but laser cutting is your best option here if accuracy is required. You could specify a 20.05mm hole and that is what it’ll produce.
It certainly won’t be the cheapest option, but by the time you’ve shelled out for half a dozen different types of drill and cutter, you’d have been better just farming it out, plus it’ll take you a couple of hours to do that, that’s time back you can do something else with!

 

[PhillipM]

A step drill turns every square millimetre of the 20mm diameter hole into swarf. An annular cutter only turns the annulus swept by the cutter into swarf.

A power source of any particular specification will remove a certain number of cubic millimetres per minute. The fewer cubic millimetres of material you need to remove to produce the hole, the quicker the task will be completed.

On a 20mm dia. hole, assuming the teeth of the annular cutter are 3mm wide (so 14mm dia. slug), it is roughly 50% less material to remove. That is not insignificant over 240 holes. The percentage gets better as the cutter gets bigger. Another way of saying this is that even at a small diameter, where it is least efficient, the annular cutter is twice as good as a traditional drill.

I'm well aware, but most people will have a few sizes of step drill at hand already, less likely the exact size of rotabroach or similar without going and buying one for the specific job.

 

[grim_d]

Why not a suitably sized blacksmith's drill? The reduced shank will fit into the usual sized pillar drill chuck.

Hello Grim D,

how about a 20mm twist drill?,

Alec

By the time I've done pilot holes so my pillar drill can push a 20mm drill through id be just as well using a step drill, it's also double setup.

@owas has already mentioned this, but laser cutting is your best option here if accuracy is required. You could specify a 20.05mm hole and that is what it’ll produce.
It certainly won’t be the cheapest option, but by the time you’ve shelled out for half a dozen different types of drill and cutter, you’d have been better just farming it out, plus it’ll take you a couple of hours to do that, that’s time back you can do something else with!

It doesn't need to be that accurate, also by the time I contact, faint at the price, take the section to them then subsequently collect them id have been able to do it myself.

The whole objective here is to spend my time rather than my money. If time was so critical id just have bought some drying racks for the hilarious price they charge.

I'm well aware, but most people will have a few sizes of step drill at hand already, less likely the exact size of rotabroach or similar without going and buying one for the specific job.

I don't know about the larger sizes but my experience of annular cutters is that any time saved in drilling anyway is spent trying to eject the damn slug!

 

[Shiny Steve]

Try a starret tct hole saw they give really nice results

 

[carbonarc]

Would it be possible that the drilled holes could be sized to receive neoprene grommets to lessen dimensional errors and ensure fixity of the 20 mm diameter tubes.

 

[grim_d]

Would it be possible that the drilled holes could be sized to receive neoprene grommets to lessen dimensional errors and ensure fixity of the 20 mm diameter tubes.

Like cable grommets? I like your thinking

I had thought about machining some sleeves but the thought and cost didn't make it viable.

 

[daedalusminos]

Mark out one side
Centre punch one side
Pillar drill:
Drill pilot holes on one side
Go all the way through with large jobber
Open out from each side with step drill if jobber not large enough

Don't over think it!

 

[Agroshield]

Like cable grommets?

How far does the tube need to slide through the hole? Unlubricated, if you push it too fast, it can push the grommet into the inside of the tube. It can happen with T&E on a socket, which is way smaller in diameter.

 

[Kram]

Most holesaws are rolled from sheet, its common they are not round but the result is they take a bigger kerf, the resulting hole should be round and possinly oversize. If you spin one up and grind it, that will improve roundness, kerf and get the hole to the correct size.

 

[timgunn1962]

What "cheap hole cutters" did you try?

Most of my use has been holes for glands and buttons/switches/lights in electrical enclosures, but I've certainly found the single-piece carbide-toothed holesaws, like the SM-series from Starrett, seem to make much rounder and less-oversized holes than the bimetal ones that thread onto a separate arbor. I haven't actually measured how the hole size compares to the nominal size though. The SM-series are for sheet metal but there are deeper-hole versions too.

The cheapies from Amazon/ebay seem to perform pretty much the same as the Starrett ones, at least for my purposes.

If I had to do the job, and the Carbide holesaws are good enough, I'd make a jig to get a pilot hole right through the box section, straight and in the correct location, then go from both sides with a Carbide holesaw. I'd probably also swap the pilot drill in the holesaw for a bit of silver steel, just to eliminate the possibility of wallowing out the pilot hole and causing the hole to wander.

 

[owas]

When I used to do a similar job (25mm hole every 50mm on 75x75x3 over 2200mm) I didn't really find a better DIY method than a mag drilll.
I knocked up a stand with a top thick enough for the mag drill to stick to, and just slid and clamped the tube underneath it.

 

[Brad93]

tube laser all the way

 

[grim_d]

Mark out one side
Centre punch one side
Pillar drill:
Drill pilot holes on one side
Go all the way through with large jobber
Open out from each side with step drill if jobber not large enough

Don't over think it!

It's not me overthinking it for once. But that's 120 drill changes...there's a better way.

How far does the tube need to slide through the hole? Unlubricated, if you push it too fast, it can push the grommet into the inside of the tube. It can happen with T&E on a socket, which is way smaller in diameter.

All the way through the. About 15mm out the other side.

What "cheap hole cutters" did you try?

Most of my use has been holes for glands and buttons/switches/lights in electrical enclosures, but I've certainly found the single-piece carbide-toothed holesaws, like the SM-series from Starrett, seem to make much rounder and less-oversized holes than the bimetal ones that thread onto a separate arbor. I haven't actually measured how the hole size compares to the nominal size though. The SM-series are for sheet metal but there are deeper-hole versions too.

The cheapies from Amazon/ebay seem to perform pretty much the same as the Starrett ones, at least for my purposes.

If I had to do the job, and the Carbide holesaws are good enough, I'd make a jig to get a pilot hole right through the box section, straight and in the correct location, then go from both sides with a Carbide holesaw. I'd probably also swap the pilot drill in the holesaw for a bit of silver steel, just to eliminate the possibility of wallowing out the pilot hole and causing the hole to wander.

Just ones from Rennie... they are truly garbage.

 

[grim_d]

Got a Karnash step drill from cutwell @ £42.

Changed my spacings a bit so it ended up only being 192 holes but it chewed through them with relative ease, biggest issue was belt slip on my old meddings.