I am working on a hobby project and therefore I have to drill a total of 68 holes of 3mm in Inconel 625 plate of 0.4mm thickness. I bought some titanium drill bits: link, but I have trouble to find an appropriate RPM to drill with. Is there a way to calculate this or should I just try some RPM and see what cuts best?
3 Answers
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SPEEDS AND FEEDS
RPM is not the number you should be searching for. You should be searching for surface feet per minute (SFM) required for drilling for Inconel and that number has to be for the drill material are using. That's the number that exists for materials since it is independent of drill diameter. Then you convert that number to RPM based on your drill diameter.
Also, you ask about RPM but not feed. That makes me think you don't have a machine that can do this. SFM, and by extension RPM, is not that critical as long as you do not exceed the maximum. You can run well below the recommended SFM and RPM for the material. You just need to a machine that can produce enough torque at the reduced RPM.
Feed per revolution matters a lot more than SFM because if you don't feed fast enough per revolution then the tool won't take a sufficient cut and rubbing and shoving occurs which produces excessive heat, wears the tool, and work hardens the material. If you feed too fast you break the tool. But you can't control feed without a mill.
People routinely run their tools, especially carbide tools, well below the recommended SFM simply because their machines are not powerful enough to get maximum productivity out of their tool. You just keep the feed per revolution the same (reduce the linear feed per minute so that with the reduced RPM the feed per revolution remains the same).
DRILLS:
First, the drill you posted are crap. You need machinist-quality tools, not hardware store junk. Drills of the quality you posted don't even have a titanium coating worth having. That's why a titanium-coated drill set from the hardware store might cost \$50 but a titanium-coated drill set from a machine shop might cost \$400.
The fact they have hex drive also indicates you aren't using a mill...and probably not even a drill press. This is not happening with a hand drill.
For Inconel, a carbide drill would be best BUT you cannot use a carbide drill on anything less than a CNC mill. Too much vibration, poor alignment when feeding, or inconsistent feed will shatter the drill because they are so brittle. That's the price that was paid for them being very heat and wear resistant. Even a manual mills tend to not be rigid enough which means drill presses are hand-drills are out completely.
I'm assuming you only have access to a drill press at best, since if you had access to even a manual mill you would probably be referring to more appropriate terms and tools. And if you only have a hand drill...well this is not happening with a hand drill. That means you are limited to either high-speed steel (HSS) or cobalt drills. You can use HSS speeds and feeds for cobalt drills. Cobalt drills technically should be able to handle a but more SFM than HSS but as previously described you can always run lower if your machine produces enough torque and power at those lower speeds. You do not want to push the SFM with Inconel in any case.
I have a pipefitter friend who needs to drill Inconel in the field with a hand drill but he's not drilling the large number of holes you are. He says he needs to lean into the drill pretty hard, use coolant, and sharpen the drill after every hole. Your holes are 3mm which means they need to more accurate than larger holes and are also too weak for you to lean into the drill. This isn't happening if you are using a hand drill.
On a drill press, without referencing tables (you should reference the tables first ) I think it would be 30-50SFM. 50SFM tends to be what is used with high-speed tools on work hardening stainless steels like 316 and Inconel is work hardening but worse. But can your drill press actually be geared down to produce enough torque a that low RPM? I would put on safety glasses and additional shields and feed hard and break some drills on purpose to see when they break. Then I would feed a bit less hard than that since you don't that rubbing and work hardening. Being too gentle is really bad here. It's not like your HSS or cobalt drills are going to last more than few holes anyways. You also definitely need coolant or good cutting oil. I would use coolant personally but it's a lot messier than oil because you need more of it. Coolant and cutting oil have different objectives.
Why do you even need Inconel?
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$\begingroup$ Thanks for the long answer, I do have access to a milling machine, I am not doing this with a press drill or hand drill. The hex drive does fit in the mill. It is not a cnc mill though. I need Inconel because it is heat resistant, it will deal with 500-800° c and high air pressure together. I wanted to use stainless steel, but I have gotten the Inconel sheet from a friend, so I am going to use that. Also to mention, the mill does not have an antomaticly feet on the Z-axis. $\endgroup$– BartCommented Jan 22, 2023 at 20:35
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$\begingroup$ @Bart In that case you might try and go with a carbide tipped drill. I mean...you could try solid carbide and since it is a drill it might fare better than a solid carbide endmill since the force is being applied axially rather than radially. But at 3mm...I think there is still high probability of shattering. Use stub or machine screw length if you can find it. $\endgroup$– DKNguyenCommented Jan 22, 2023 at 20:51
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$\begingroup$ @Bart Calculate the linear feed and practice rotating the hand wheel at the appropriate turns per second to get that feed. But inconsistent feed is probably what will shatter it rather than not getting the feed bang on to your calculated number. Or vibration but vibration is kind of beyond your control. $\endgroup$– DKNguyenCommented Jan 22, 2023 at 20:51
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$\begingroup$ Only issue is, the Z-axis is very heavy to turn around, with the mill I use, needs me to press the wheel in, and then turn it around, which is quite heavy, so I am afraid the feed will be slow anyway $\endgroup$– BartCommented Jan 22, 2023 at 21:06
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$\begingroup$ @Bart Slow down your SFM I guess to compensate and do the shatter tests...with cheaper HSS drills to start, $\endgroup$– DKNguyenCommented Jan 22, 2023 at 21:07
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With an unlimited number of HSS bits , I decided Inconel was undrillable. I had a somewhat softer Inconel 600 but about 2mm thick. Ti means a layer of less than 0.001 in. of Ti nitride. Likely to be worn off quickly in Inconel. Hopefully your bits are actually HSS with the Ti coating. Low speed, HIGH pressure, high sulfur cutting oil and drill in stages, eg, 1 mm, 2mm,3mm. If the bit does not cut a couple revolutions' ,it will cold work the Inconel making drilling more impossible. How much do you want 625 ? Certainly great material for glycerega and for 2000 F corrosive atmosphere , do you need it ?
answered Jan 21, 2023 at 16:40
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$\begingroup$ Yeah, it need to withstand temperatures of around 500-800° c and high air presure, I think indeed it is a titanium coating. $\endgroup$– BartCommented Jan 22, 2023 at 20:27
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$\begingroup$ 800 C is easy service for standard 309 and 310 ( or HH and HK). Titanium is worthless at 500 C. $\endgroup$ Commented Jan 22, 2023 at 21:35
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$\begingroup$ It is the Inconel I use, the drill bit itself is Titanium $\endgroup$– BartCommented Jan 22, 2023 at 21:37
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Everyone is being pretty negative so far - you can Drill one or two holes in Inconel pretty easily (yes, even with a hand drill) using Carbide Tipped 'Multi' bits. These are like masonry bits, but they have an actual chisel tip.
I use these bits for emergency repairs of all sorts of tricky hardened and work-hardening materials when making repairs to combat robots in the pits.
https://www.screwfix.com/p/bosch-expert-straight-shank-multipurpose-drill-bit-3-x-70mm/271ky
answered Jan 23, 2023 at 8:17
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