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Drilling 12ga Sheet

jthornton

Aluminum
Joined
Sep 9, 2010
Location
Poplar Bluff, MO
I make a part with 7 holes out of 12 gauge hot roll sheet. I plasma cut the outside, grind smooth then put them in a fixture to drill the holes. I always get flash on the back side of the hole. Is there any way to reduce or eliminate the flash. Speeds, Feeds, Drill type?

Thanks
JT
 
Hole size?

I almost never drill holes in sheet metal, but the one time I did, I interpolated the holes with an endmill. Obviously it will NOT be the quickest method, but if you want to eliminate 90% of the burr, it is worth a shot.

They were larger holes (~.500") when I did them. If you have larger holes, you could get a "drillmill" that will punch the hole and interpolate to size.
 
Put them in a fixture? Using drill bushings? If so, you can grind your drill points past flat bottom, where the outside edges of the drill hit first, actually dished in the center. This turns the burr into a disc that pops off when you are drilling thru.... now you will still get some burr, but usually something you can get rid of with cordless drill and chamfering bit, or even a twist a burr or Noga type tool.

You do need to do this thru a drill bushing to guide the drills though, they would walk all over the place if not in a bushing. Drills won't last real long, so I don't know how effective in steel this will be. We used to use this trick on a small aluminum cover we made that had 18 holes around the perimeter, standard drill bit left a large blown out burr, doing them this way you had barely any burr and finishing them became the easiest part rather than a hassle.
 
Put them in a fixture? Using drill bushings? If so, you can grind your drill points past flat bottom, where the outside edges of the drill hit first, actually dished in the center. This turns the burr into a disc that pops off when you are drilling thru.... now you will still get some burr, but usually something you can get rid of with cordless drill and chamfering bit, or even a twist a burr or Noga type tool.

You do need to do this thru a drill bushing to guide the drills though, they would walk all over the place if not in a bushing. Drills won't last real long, so I don't know how effective in steel this will be. We used to use this trick on a small aluminum cover we made that had 18 holes around the perimeter, standard drill bit left a large blown out burr, doing them this way you had barely any burr and finishing them became the easiest part rather than a hassle.

I wish I could sharpen drills but I've never had the time to build a sharpener even though I have the plans for one. I use short bits and don't seem to have any problem with wandering. BTW, I'm doing this in a VMC.

JT
 
I ground one similar to the photo and it seemed to make a slight difference. I'm going to play with the SFM a bit and see what that does. BTW I'm using a 135° split point screw machine (short) bits.

JT
 
I tried 2800 rpm with 4.2 IPM for my 0.2031 bit and 2200 RPM with 3.3 IPM for my 0.257 bit and now I can chamfer the holes with a no flute in the cordless.

Any suggestions for RPM and IPM for stainless steel? I make a few of those too...

Edit: I failed to mention I use flood coolant.

JT
 
Have you considered just sending them out and getting them laser cut? Its seriously cheap!
 
Put them in a fixture? Using drill bushings? If so, you can grind your drill points past flat bottom, where the outside edges of the drill hit first, actually dished in the center. This turns the burr into a disc that pops off when you are drilling thru.... now you will still get some burr, but usually something you can get rid of with cordless drill and chamfering bit, or even a twist a burr or Noga type tool.

You do need to do this thru a drill bushing to guide the drills though, they would walk all over the place if not in a bushing. Drills won't last real long, so I don't know how effective in steel this will be. We used to use this trick on a small aluminum cover we made that had 18 holes around the perimeter, standard drill bit left a large blown out burr, doing them this way you had barely any burr and finishing them became the easiest part rather than a hassle.

Or you could use an endmill. Easier than grinding a 1/4" twist drill.
 
There is a special grind for sheet metal. I picked up a set at an estate sale, but can't find anyone that sells it online. Here's one link on how to grind it (found on an old PM thread via google):

http://qrpbuilder.com/downloads/sheet metal drills a.pdf

The discussion is:

http://www.practicalmachinist.com/vb/general/drilling-round-holes-sheet-metal-222049/

You would almost think it was a wood bit:

View attachment 119717

If you google "bullet point drill" you'll find several manufacturers of them. Dewalt even sell some to the big box stores.

I try to punch holes when working with sheet metal, works better and faster.
 
If you google "bullet point drill" you'll find several manufacturers of them. Dewalt even sell some to the big box stores.

I try to punch holes when working with sheet metal, works better and faster.

Seems all I can find is fractional sizes and one hole is F... I actually think I have a set of the Black and Decker bullet point drills around but was not impressed with their function.

Be nice to have an iron worker...

JT
 
It might help if I post a photo of the part I'm making.

20140811_104723.jpg

I cut them on my homemade CNC plasma so they are not perfect. I have to grind the OD after cutting them out. The holes are on an arc except for the mounting holes which are a metric spacing. I sell about 12 per month and make them in batches of 40-50. I really can't see building a punch to punch them out, but maybe I'm missing something. Just a quick google and I think the suggestion to use a drill press quill is way off the mark. It takes ~3 tons to punch a 1/4" hole in 1/8" steel according to the chart I found.

Other than drilling the only viable option seems to be laser cut but that cuts into my tiny profit that I make on them.

Thanks
JT
 








 
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