A question about ideal drilling speed

Adam Buttry

Adam Buttry

Joined
Nov 10, 2010
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575
I was wondering if anyone could tell me what the ideal drilling speed for my application would be. I am drilling through 1/8" and 5/32" stock only (for now). My drill press range of spindle speeds is 140,260,320,380,480,540,980,1160,1510,1650,2180, and 3050 RPM. Hole size will vary from 1/8" to 1/4".

I would like to use one speed for both thicknesses if possible. Given the correct feed rate, which RPM setting would be best? Thank you for your input.

Adam Buttry
 
To answer that, the material, and the type of bit would need to be known. I am assuming its steel, and stainless steel, but it could also be aluminum and brass as well. Non ferous metals should use a slightly modified drill bit to prevent bite and shear forces from damaging the work or user.

If its steel and you are using a run of the mill bit, run it around 300 rpm, and try to achieve a uniform, clean chip. Discolored chips are too hot, small chips are inefficient and will lead to overheating, uniform, and unborken metal cuttings are best. Try to control the output rather than the input, and listen and look for the red flags of improper feed and speed, or dull tooling.

metalshavings.jpg


These cuttings are uniform and clean.
 
RPM doesn't vary based on material thickness.

It's based on surface feet per minute of the cutting edge which changes with tool diameter, as well as type of material, type of cutting tools


Rule of thumb for drilling in steel, with HSS drills

1/2" hole is 0.500" run it at 500 rpm (reaming starts at half that)




half that size, double the speed

1/4" use 1000 rpm



Half that size, double the speed

1/8" use 2000 rpm



That's all based on maximizing production in mild steel
You can always go slower and should is knife steels

You can find calculations and charts for more specific numbers too.


Watch the colour of the chips, Blue may be getting too hot slow down.
 
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Don't forget that feed is important too... to little feed, even at a slow spindle speed, in materials which can work harden such as carbon knife steels, can result in rubbing and a work hardened hole. You can normally feed pretty aggressively in mild steel, it's good to get a continuously curled chip. It is true that if the chip is blue, you need to slow down.

I have good success with 1000 rpm all the way down to 300 rpm for small holes in steel like that, just watching my chip and feeling how the bit is cutting. Squealing is not good!

Cutting fluid is another thing to think about. I like to use soluble cutting oil, with water, in a 1:10 ratio of oil to water. The particular product you get will have mixing instructions. The soluble is oil is really cost effective, you can make a quart last a long time.
 
Thank you for your replies. My apologies for not mentioning the steel type or drill bits. I am drilling through annealed 52100 with 135 degree split point, black oxide HSS bits that are screw machine length. I will be using cutting fluid. I will also be sure to pay attention to the signs for correct feed rates. Hopefully this info will not change your recommendations. If so, please let me know.

Thank you
Adam Buttry
 
I drill a lot of 52100, and it can work harden for sure. Make sure to keep the bit biting. If it does work harden on you, re-sharpen your bit or use a new one, go with the slowest speed possible (I use a hank crank drill press for this) and feed real hard to get right past the hardened skin at the bottom of the hole. If you can pierce through with your sharp bit and heavy feed, you should be able to finish the hole OK. If not, torch anneal by heating to very low red, cool to black, quench in oil, three times. Then it should cut OK again.

Is this commercially anneal stock, or home annealed? 52100 can be difficult to home anneal, ashes, a forge cool, or vermiculite hardly work. Thermal cycles are better.
 
Salem,

It is annealed bar stock from Aldo. I will be outsourcing the heat treatment to Peters. I am kind of new to this and will be using the stock removal method only, for the time being.
 
You need to run slower SFM in 52100 than you would in mild steel, and the feed window is narrower. Short answer, a good RPM for a 1/4" bit in your application is 600 RPM, you can also turn your 1/8" at 600 RPM though you can raise it to 1,200 RPM to get the job done faster.

Within reason, nothing bad happens if you go slower than recommended with your RPM, but the chip load cut per revolution needs to remain correct regardless. You should aim for long curly chips. This is one of those times when "easy does it" does not apply. You need enough feed pressure to get the bit cutting or you'll dull the cutter quickly. It is important to let off this pressure as the web of the bit exits the back side or you'll over feed it at the end of the hole and wipe out the corners of your drill.
 
Dang, had no clue as to the 'specs' involved in simply drilling holes. Didn't do 'shop' and have no machinist's experience, I have a lot to learn....good thing enjoy learning :)
Great forum.
 
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