Annealed 4140 for project = DIY hardening?

Got started on the project yesterday. Cut 4 to size,laid out scribe lines etc.
I am drilling through holes (#3) for the 1/4 28, that went fine, I center drilled and ran a #28 pilot hole for the 7/16" -20 threads for the adjusters.
The book says to use a 25/64 drill. I broke 2 drill bits.
I'm thinking I need to gain on the size, too big of a bite or the bits are too hard.
This is annealed 4140, I am trying to avoid work hardening the material so I'm going in pretty heavy.
Ace had Milwaukee black oxide "Thunderbolt" 135 degree tip, Maybe the lightning made the bit too hard?
I wonder if they will take it back-exchange? I'm gonna try.
Your thoughts?
 
It might be annealed as purchased, but it sounds like you have work hardening. If you dwell with the drill rubbing, it will get harder than Kryptonite. But you probably already know that, Jeff. If that is the issue, it will need to be annealed or maybe use a carbide drill, though that can just ruin the carbide tool. What machine and speed are you using to drill the holes? I do not trust ANY hardware store drills any more, regardless of brand.
 
Last edited:
Janderso said:
Ace had Milwaukee black oxide "Thunderbolt" 135 degree tip, Maybe the lightning made the bit too hard?
I wonder if they will take it back-exchange? I'm gonna try.
Your thoughts?
Click to expand...

Review of these from Milwaukee's site:

Bit snapped in half drilling second hole
Used a 27/64 bit to drill a hole in an acrylic pen blank. On the first blank it drilled a nice clean hole. On the second blank the bit seized and snapped in two. Generally I have had good success with Milwaukee products. Right now I am considering this product failure a fluke. Too early to provide any type of recommendation, will wait to see how the remaining bits perform but I would say I am disappointed.



  1. Response from Milwaukee Tool:
    Anonymous
    · 2 years ago

    Thank you for your feedback. We would like to learn more about your experience and work with you to rectify this issue. If you could please contact us at socialmedia@milwaukeetool.com we would appreciate it.


 
Janderso said:
I center drilled and ran a #28 pilot hole for the 7/16" -20 threads for the adjusters.
The book says to use a 25/64 drill. I broke 2 drill bits.
Click to expand...

Are you doing this on a mill? If so don't drill a pilot hole. Spot it and drill with the 25/64 drill. Start at 500 rpm and if easy increase to 685 at most.
Don't get crazy about work hardening. Don't know if you have down feed but if so feed at 5-7 thousandths per revolution.
It's not stainless never had the work hardening with it as bad as with stainless.
 
Jeff, it sounds like you might be pushing the feed too hard out of fear of work hardening. #28 is a fairly small pilot, so I doubt the drill is self-feeding. Use cutting oil and drill like it's cold-rolled. As long as the drill is cutting you won't get work-hardening.
 
I'm running at 500 rpm, it just seems right.
I hadn't thought of no pilot. I ran an H (.266) cut like butter.
I'm thinking I'll just get a quality bit next week. I buy all my drill bits from McMaster Carr.
I've had good luck with their products and you can't beat the service.
I don't know, drill bits have to be tough enough but not too brittle.
The Norseman Magnum, ( Super Premium, Made in USA) letter set, is the best quality drill bits I've ever owned.
I'm still going to try to exchange the bit from Ace.
It's pouring rain right now. I may melt.
 
HSS drill at minimum. Cobalt even better. Harbor freight actually has a set of cobalt drills that are decent.
Tried looking them up and mine are standard point and if I remember $40.00 range.
All I find on their site now is split point and they are $65.00 item # 61885
May be decent.
 
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