Drilling holes in thin sheet

[mielke]

We are having trouble drilling in 11GA 304 stainless steel.
We drill lots of closely packed holes 3/8" od. No matter how we fixture and clamp the sheet to the table the sheet is still warping and creating a gap between the sheet and the table causing vibration in the sheet while drilling. This causes chipped drill bits and eventually broke ones.

We're using a guhring drill specifically for the application and confirmed speeds/feeds w/ guhring.

Any ideas or help?

 

[unclesyd]

Is every thing ridged, both the drill press and work hold device?

What condition is the 304 sheet, like is it half hard, full or somewhere in between?

What you using for coolant?

You didn't post the precision needed so I have to ask, Could you use a perforated plate?

 

[MikeHalloran]

You can pack the holes a lot closer by cutting them with a laser.
They're expensive, but some outfits do nothing else but contract cutting.
There will be a thin HAZ, so the sheet will 'weld funny'. If that's important, you can cut the holes a little undersize and take off a few thou with a die grinder.

You can pack the holes just about as close with a waterjet, and with no HAZ, though the sheet will arrive covered with fine abrasive dust that you'll want to clean off.
Again, contract cutting is available.

Either way is probably much cheaper than drilling.



Mike Halloran
Pembroke Pines, FL, USA

 

[metman]

mielke

Try a "Lip & Spur" drill bit (AKA Brad Point). These work beautiful on sheet metal and on plastic.

The point is scalloped between the center and margin so that you have a center "point" to accurately start the hole and the lip at the margin cuts only the perifery at first making a clean round hole.

 

[racookpe1978]

If you have a lot of these to make, several companies can punch hole through 11 gage plate. What's the shape of your part, and how many holes are you punching/drilling? If you need enough parts, the whole plate can be done in one step with a pre-fabbed (but very expensive to make) plate press.

I've used a hand-held electric punch to go through Hastalloy-X plate 5/16 thick. That worked much faster than drilling.

If you only have a few parts to make - or don't want the expense of an elaborate setup, clamp plywood sheets on both sides of the plate, then drill through the backing plate and steel.

 

[mielke]

thanks everyone. We're considering laser or waterjet. punching is out of the question (holes are too close together)

Our work surface is as rigid as possible, we have a solid sacrificial plate underneath and we are using flood and thru coolant drills. We're clamping down but i guesse when the drill leaves the sheet its pulling and flexing it. We could clamp down very close to each hole and not have as much as a flexing problem but the vast number of holes we are doing, and them being closely packed, makes this impractical.

I am thinking this problem is happening because of the drill maybe pulling the sheet metal when it exits it and also a small bur on the back sides of the holes is happening which helps create gap between sheet and sacrifical plate. I am hoping another tool may be better, maybe a end mill specifically for drilling?

 

[ajack1]

Could you not just buy perforated sheet?

Here is one example but it can easily be sourced.

 

[mielke]

no we can't use perforated sheet.

 

[looslib]

How close is the hole spacing?
Have you tried a spring-loaded clamping device that presses down on the sheet around the drill?


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[mielke]

Like I said I can clamp down very close to the drill hole and get good enough results. But for the shear quantity of holes we are doing it is not pratical to move the clamps to each hole we drill.

We are doing 1000s of holes per sheet of 3/8"OD on a 29/64" triangular pitch.

 

[Compositepro]

Use a spring loaded drill bushing around your drill bit to hold the sheet down. I'll bet someone makes a fixture like that already.

 

[unclesyd]

How many holes do you need inside a 3" circle?

Given your dimensions I can get 30, but the web thickness for all practical purposes is too thin for drilling.

I would not even attempt this without making an indexable jig from thicker materials using drill bushings.

 

[emonje]

We usually sandwitch thin sheets between two sacrificial mild steel plates, clamp down & drill. Decent enough results for our purpose.

 

[KENAT]

Variation on the 'clamping between 2 sacrificial sheets' idea.

Why not have a 'pattern' piece above with slightly oversize holes, this could act to clamp the part being drilled over almost the entire surface. Obviously this pattern part will be even more tricky to machine, especially as it will probably need to be a good bit thicker gauge to have adequate stiffness - but you only need to make it once.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
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[KENAT]

Variation on the 'clamping between 2 sacrificial sheets' idea.

Why not have a 'pattern' piece above with slightly oversize holes, this could act to clamp the part being drilled over almost the entire surface. Obviously this pattern part will be even more tricky to machine, especially as it will probably need to be a good bit thicker gauge to have adequate stiffness - but you only need to make it once.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[bentwings1]

Possibly you can develope a heavy spring loaded 'stripper' on the spindle. It would press hard on the sheet/stack first before the drill started and stay pressed until the drill retracted. Similar to a punch and die. You could even use a guide bushing and guide the drill for even more accurate drilling.



99 Dodge CTD dually.

 

[dvd]

When you say triangular pitch, I assume you mean 60 degree staggered centers.

It sounds like you are stuck with what you've got unless something can compromise. What is this for?

I would vote for the perforated sheet: if you can go with a slightly larger spacing or a slightly smaller hole diameter or a thinner sheet then a perforator might be able to help.

Next choice is laser, last is waterjet.

 

[gadkinsj]

I didn't see mention of a Step Drill bit. Unless the Guhring bit you mentioned is a step drill.

I've done custom engine gaskets from very thin copper plate (much thinner and much angrier to drill through than this) and successfully used the sandwich method listed above. Put 2 pins between the sandwich pieces so that you can use it as a pattern.

 

[metman]

You really shoud try a lip and spur drill. Your material might be a lttle thick for it but surely worth a try. These drills work great on thin sheet metal.

 

[MikeHalloran]

The dimensions given put the web between holes at .0781", which makes drilling without distortion marginal.

You will kick yourself when you finally try getting a sheet laser-cut. You could probably push the web between holes down to .01", if open area is important to you.



Mike Halloran
Pembroke Pines, FL, USA