Drilling 4140

[Wreck™Wreck]

I make a Stainless Steel part with a 14mm bore and an OD which tapers from 16mm down to 14.4mm over 5 inches, I use an ordinary drill bit and a finish reamer, I do not use a pilot as I don't find that it helps when drilling deep holes. I do not scrap any parts ;-)

Never said that it can not be done merely that it rarely ends well, no pun intended, with twist drills.

 

[Airpirate]

Perhaps you may have been running too slow and tended to feed too fast, 4140 is not all that hard, even in the commercial heat treated condition if it is annealed, not much speed reduction compared to CRS 1018 is required; one thing you could do is to drill the pilot hole as deep as is appropriate, then drill a larger hole to a depth less than the pilot hole to allow chips to work out and coolant/ cutting oil to work in; also it is good practice to withdraw the drills frequently to clear the chips out and re oil the drill bit If the drill is making any noise, it is definitely time to pull it out. Depending how large the finished hole is, I would grind a drill with a split point and not use a pilot hole, only a center or spotting drill to start the hole.

I may be drilling too slow. I checked a drill speed table and mistook SFM for RPM. A table I am referencing suggests 40-85 SFM for drilling.
I found a formula that specifies, (3.8197/drill size)X SFM = RPM, so for a 9/64 and 40 SFM I come out with 1046 RPM. This seems high can someone who has experience weigh in please? Pardon my ignorance.

 

[Dan_S]

I may be drilling too slow. I checked a drill speed table and mistook SFM for RPM. A table I am referencing suggests 40-85 SFM for drilling.
I found a formula that specifies, (3.8197/drill size)X SFM = RPM, so for a 9/64 and 40 SFM I come out with 1046 RPM. This seems high can someone who has experience weigh in please? Pardon my ignorance.

I'd want to see the exact chart, before making a definitive statement.

However, a lot of the formulas listed in old texts, are for industrial work where they are trying to maintain the maximum material remove rate to tool wear ratio. They assume you have more than enough power and rigidity for the task at hand.


For a 9/64" jobber bit, I'd run it in the 600 to 700 rpm range personally.

 

[cg285]

What is the final size hole you need"

+1

 

[kd4gij]

I would drill a 1/4" or 5/16" pilot hole clearing chips often. then 1"

 

[Airpirate]

I'd want to see the exact chart, before making a definitive statement.

However, a lot of the formulas listed in old texts, are for industrial work where they are trying to maintain the maximum material remove rate to tool wear ratio. They assume you have more than enough power and rigidity for the task at hand.


For a 9/64" jobber bit, I'd run it in the 600 to 700 rpm range personally.

The chart I am using is found at www.hannibalcarbide.com/documents/reaming-speeds, this isntt the entire URL but it is current and the formula was found on several drill manufacturer's web pages.
I really appreciate your feedback and will drill at the speed range you state as it makes sense.
Thank you.

 

[gasengin]

When the speed is too slow, it is really easy to break small drill bits. Slow is not better. I personally had a bad teacher on this when I was young (not a machinist). He thought slower was better. I broke lots of small bits until I learned about surface feet and how it relates to rpm.

 

[Airpirate]

When the speed is too slow, it is really easy to break small drill bits. Slow is not better. I personally had a bad teacher on this when I was young (not a machinist). He thought slower was better. I broke lots of small bits until I learned about surface feet and how it relates to rpm.

So if the formula for the recommended SFM states 1046 RPM stick to that speed or close to it?