Drilling accurately in 10mm steel


Old 07-04-18, 08:13 AM
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Drilling accurately in 10mm steel


I am currently doing an apprenticeship in mechanical fitting. Everything has been going great and i am on my last project where part of it includes drilling 3, 5 and 10mm size holes in 10mm thick steel with a hand drill. It has to be square and the sizes of the holes to a tolerance of 0.15mm.

Now i have no problem with the 3mm and 5mm holes but the 10mm hole is giving me all my days of trouble. Mostly the size comes out between 10.15mm -10.25mm. The 5mm holes come out nicely and consistently at 5.05mm-5.10mm same with 3mm.

Any tips on getting the size of the 10mm hole way down? I have tried many things but still no cigar😂

They are very strict and when i test i cant afford to fail because the 10mm hole is just slightly too big.
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Old 07-04-18, 08:16 AM
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The biggest problem is the 10mm drill biting at the end before it exits to complete the hole and that is where i think the hole opens up that slight bit too much
Old 07-04-18, 08:52 AM
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Exactly how are you drilling your 10mm hole? Are you trying it in one shot with the 10mm drill or are you pilot drilling and stepping up in drill sizes until you get to 10mm?
Old 07-04-18, 09:01 AM
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10mm steel is equivalent to our 3/8".
The 3mm is a good starter bit.
You should start with the 3mm and then the 5mm and then the 10mm.
Old 07-04-18, 09:20 AM
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You might even want a 7 or 8mm between the 5 and 10. Stubby bits will have less wobble. And you surely are drilling on low speed and using a cutting oil as you drill, right?
Old 07-04-18, 10:59 AM
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Yes i am stepping it up starting with 3mm, 5mm, 7mm then 10mm. Also drilling slowly. I am not using an oil though... Maybe that might make the difference.

Will any oil do the job? There is a canister of some sort of general lube oil i can try
Old 07-04-18, 01:59 PM
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Have you measured your 10mm drill bit?

Several years ago I purchased a 115 piece cheap Chinese drill set. Much to my surprise even though there were drills for every size (fractions, numbers and letters) upon measuring I discovered that there were groups of drills that were actually the same size. Just stamped a different size. So drills 40, 41, 42 and 43 are all the same size then drills 44, 45, 46 are the same size.

Another thing that can open your holes up above the drill size is not keeping your drill perpendicular when drilling. Especially a problem when breaking through. Also drilling too fast or keeping the bit turning after you've broken through and are pulling the bit out can open the hole size up.

Specialty cutting can do a better job lubricating a drill but in mild carbon steel just about anything will work. Regular engine oil (10w30 or straight weight oil) works pretty well in normal steel. Aluminum and more exoctic metals is where specialized cutting oils really shine.
Old 07-04-18, 06:12 PM
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Try a new 10 mm drill or sharpen the existing one . The cutting lips need to be symmetrical or the drill pushes to one side making the hole larger. Drill sharpening fixtures do a good job. Sharpening by hand on a grinding wheel is an acquired skill that few have mastered.

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