Work hardening while drilling

[martik777]

I am trying to drill a 9/32 hole about 3" deep (800rpm) between two 1/4" cold rolled plates sandwiched together. It drilled easy and had good chip flow for the first 1 1/2" then just stopped cutting. It just squeals and heats up, no amount of force will start it cutting again. I have tried brand new bits (1/4 and 9/32), tap magic, oil etc with the same result. So, did I do anything wrong or could there be some hard material in the metal? How would you suggest I complete the hole? Carbide / masonry bit?

 

[cg 2005]

I use Monroe Prime Cut at a 16:1 water to oil ratio as a coolant for deep cuts or drilling. Cooling may be more important than lubrication. Prime Cut can be mixed as high as 33:1 water to oil and still be very effective.

 

[Bill Gruby]

You didn't say what the drill looks like? Workhardening while drilling will destroy the end of the bit. If it has indeed workhardened it will be difficult if not impossable to restart that hole. I also use Prime Cut to cool.

"Billy G" :thinking:

 

[Rbeckett]

Yo may be able to turn it over and restart the hole from a non hardened side. Work hardening is very prevalent in carbon and other more exotic mixes of steel. Also a rotobroach cutter may do the job too. 800 RPM seems kind of fast too for 1/4 plate and the hole size. I turn just fast enough to pull out a nice curly chip that doesnt turn blue or gold when it gets ejected. If cutting slows and it starts ejecting fine chips I change bits and slow down some more. And I try to circulate enough coolant/oil to keep the hole cooled to ambient temp the whole way through the cut. If your using AR400 or work hardening plate then all bets are off, use a torch or plasma cutter. We used to fab dozer and scraper blades in the shop I worked in. And we did all the holes with a die and punch on a huge press with hundreds of tons of force. Couldn't hardly grind on it let alone even consider a drilled hole. Just some thoughts from the gallery.

Bob

 

[george wilson]

I wonder if you really know what your metal is,or did you just pick it up at a scrap yard? Mild steel doesn't have enough carbon to work harden like that. I HAVE had 01 harden from filing too vigorously,and scratch the file. It only hardens a few thou. deep,but the heat of filing has made it do that several times in the past.

I keep several masonry drills,as well as some straight flute carbide drills for drilling hard stuff,like spring steel. They are a necessary thing to have on hand. I even use them for drilling SOLID HSS power hack saw blades,which I have made kitchen knives from(the solid ones are a bit hard to find,most just have HSS teeth welded on). They are still made if you want to pay the price for new ones.

Another possibility is that your mild steel is Chinese. Then,no telling what inclusions(like tossed in ball bearings) you might find in the metal. This steel has ruined many a cutter according to the other forums.

 

[kd4gij]

I agree a little more information will help! Re-reading is post is a bit confusing 3" deep between two 1/4" plates sandwiched together?
Frank

I think he has the standing on end and drilling down the seam. I agree with outhers probly has a chip in the bottom of the hole ( hate when that happens )

 

[martik777]

Yes, they are standing on end, (how else would I drill 1.5" deep? ) and I am drilling the seam lengthwise. The drill bits are HSS and have worked with the same material without issue. Does not seem to be close tolerance. I did frequently eject the bit to remove chips and cool as I anticipated problems (peck drilling?). There were no signs of overheating, chips were short, not long and curly or blued. It is possible, the bit was a lesser quality of HSS - I just can't remember but the bit still looks sharp. Material was from s scrap dealer. I will take the plates apart and have a close look at the area and post it here.

Thanks for the tips and help

 

[November X-ray]

If you are able to separate the two pieces, have you considered using an undersized ball endmill to make the initial half hole on each part (accurately positioned of course) and once thats done place the two halves back together and finish drill / ream. This might be a better method if accuracy is an issue.

 

[DMS]

One thing that has happened to me in the past is that I have broken part of a HSS drill bit in the hole, and the fragment has gotten buried in the material. Switching to a new drill bit just ends up destroying the new bit until you get the broken pieces out of there.

 

[brucer]

I've never seen cold roll work harden...