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Optimal hardness for gundrilling 4140 / 42CrMo4

scientizt

Plastic
Joined
Dec 27, 2009
Location
Netherlands
Hi guys and gals,

I was wondering if you could please share some thoughts with me on this one.

I am planning to do some gundrilling 4140 on the lathe. I have the tools and setup all ready to go (I've already made a chip box, a deep hole coolant through insert reamer using Mapal carbide inserts, a high pressure cooling / lubrication system, and I even built a diamond wheel toolgrinder, especially for sharpening carbide tools including gundrills).

But before I start drilling, I am wondering about what the optimal hardness of 4140 should be in HrC, for the very best drilling and reaming results. Should it be dead soft, or do carbide drills and reamers prefer a higher hardness?

I might be mistaken, but my gut tells me it should have some hardness to prevent tearing and clogging up the cutting edge. But what are the numbers?

I plan on using Gühring and ElDorado carbide tipped gundrills, mostly around 7mm to 10mm, with a stock N8 nose grind for as long as it lasts, and then probably regrinding the drill to an F8 facet grind. Speeds, feeds and lubrication etc. are also sorted out and should be alright.

I believe my 4140 as delivered from te supplier is around 32HrC, but I can heat treat it any way I like.

Thank you, best regards from The Netherlands,

Scientizt
 
Hi guys and gals,

I was wondering if you could please share some thoughts with me on this one.

I am planning to do some gundrilling 4140 on the lathe. I have the tools and setup all ready to go (I've already made a chip box, a deep hole coolant through insert reamer using Mapal carbide inserts, a high pressure cooling / lubrication system, and I even built a diamond wheel toolgrinder, especially for sharpening carbide tools including gundrills).

But before I start drilling, I am wondering about what the optimal hardness of 4140 should be in HrC, for the very best drilling and reaming results. Should it be dead soft, or do carbide drills and reamers prefer a higher hardness?

I might be mistaken, but my gut tells me it should have some hardness to prevent tearing and clogging up the cutting edge. But what are the numbers?

I plan on using Gühring and ElDorado carbide tipped gundrills, mostly around 7mm to 10mm, with a stock N8 nose grind for as long as it lasts, and then probably regrinding the drill to an F8 facet grind. Speeds, feeds and lubrication etc. are also sorted out and should be alright.

I believe my 4140 as delivered from te supplier is around 32HrC, but I can heat treat it any way I like.

Thank you, best regards from The Netherlands,

Scientizt

What hardness does the finished product need ?
 
Back in the days of trepanning and deep hole drilling, 41xx in the 28-36 HRC range machine great. Good chip formation, average tool life. Of course, the coolant had to be right, too! For most of the small ID's we drilled, a good gundrilling oil was used. Been too many years ago to remember the brand of oil used. Look at Mobil Mobilmet 462 or 766 for starters.
 
Thank you. Do you remember if it cut better on the low or higher end of that? I have found and bookmarked the Mobilmet oils. But I am going to try Eurol ST first, which is an affordable pure cutting oil, supposedly suited for high pressure cutting applications. I already know that it performs well for turning 4140, but I haven't tried it for gundrilling and reaming yet.
 
Most of that material we drilled back then was in the mid 30's in hardness. Most all was Q & T to spec. material, not stocked pre-hard as most of us buy today. In Houston, certain vendors carry 4140-4145 Q & T in the 285-341 BHN range for the oilfield.
 
Thanks everyone, much appreciated :cheers:

To be precise I also have 42CrMo4+QT, it's the most common available variant around here. I need to take care of some other projects before I continue with the gundrilling. But in time I'll let you know how it turned out.
 








 
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