I am looking to buy a 12mm reamer bit for my drill press. Should I use a 11.5mm, 11.8mm or 15/32 inch drill bit to pre-drill the hole? I have been googling for a while now to no avail.

I found this quite quickly:

https://www.hannibalcarbide.com/documents/pre-ream-drill-size-chart.pdf

I appreciate your help LeeJustice, I could have done without the lowkey derogatory statement though. I also found these charts but I do not know how to use them. Please can you tell me the difference between nominal, 2% and 3%? I am still very new to the world of metal working. I have heard it is easy to go either too big or too small.

  It would help to know what you are attempting to do.  Have you considered feeds and speeds.  I think Lee was trying to be of help.  What kind of drill press do you have?

Hey Nodebt! Simply trying to get a rounder hole than my drill bits can achieve in a 3mm mild steel bar. From what I have read, the 11.8mm and 15/32 drill bits should be fine, but I have also learnt that there is a very small tolerance between choosing a too small and too large. I know one of those drill bits will technically be a better choice over the other, I just want to be sure because I have to buy them in batches of 5. A bernardo tb 14T.

I know he was Nodebt, it was just how he said it.

LeeJustice may have use a different search engine or different search words, which could have given him faster or more complete answers.  He was kind enough, and helpful enough, to include the URL to the ream drill size chart so you could access the information. I do not see where this is an implication (subtly or otherwise) that you did not look very hard.  There was no deprecatory statement, (expressing disapproval or criticism),  just a direct link to the chart.   

I see Glenn. I am Sorry LeeJustice if I misinterpreted what you wrote, I assumed when you wrote ''I found this quite quickly'', that you were implying it didn't take much effort to find the information, and therefore, implying I didn't look very hard. If I am wrong, it certainly wouldn't be the first time!

Sort of like going to the scrapyard; I go and I'm tripping over automotive coil springs; but when I send others there to get one, they can't find any!

I hope schools nowadays are teaching how to create  good search engine queries;  I've made some real mistakes over time! (Though not as bad as a friend of mine who during lunch was researching getting a personal water craft and used a term easily misinterpreted!)

I know you are new to metalwork, but you still havent said why you want to ream your drilled holes. "Rounder" doesen't work. Your hole won't get rounder using a reamer. It makes the diameter more precise. Usually you use a reamer if, as an example, your drawing or specs calls for a particular tolerance, meaning say within a 64th. Reaming is more often done by a machinist, not a blacksmith. About the only time i ream out a hole is when I use an oil bronze bushing for the hinge pin to go into on large hinges. 

I am sorry I am confused. Wouldn’t a more precise diameter result in a rounder hole that a drill bit can achieve?

No it won't, it can not.

If your drilled holes are not round you are not drilling them correctly. There are a number of possible problems: Your drill bits may be improperly sharpened. You may not have the piece being drilled clamped securely enough in the vise. The vise may not be clamped down securely enough. Your center punch mark may not be strong enough allowing the drill bit to wander when starting the hole. There are more but those are the most common problems.

Feeding a reamer into a hole that is out of round will cause it to chatter, resulting in an out of round hole and potentially damaging reamer.

Can you post a picture of how you are set up to drill the holes so we can help you more effectively?

Frosty The Lucky.

Good Morning 9436,

Welcome to our world from the left coast of Canada.

There is a saying for a Blacksmith;

Measure with a Yard Stick (Meter Stick), Cut it with an Axe, Heat it in a Campfire, Beat it with a Hammer until it fits perfectly!!

Nobody knows yet what are trying to do? You are trying to make an accurate hole in what? Trial and Error is the best way to find the answer. Are you trying to make a Ribbon Burner? It is not critical for the hole size.

Neil

How thick is the material you’re drilling. If it’s real thin just clamp it on top of some thicker material. With me, if the drill point is longer than the material is thick it will drill a three sided hole. Clamping the pieces together adds thickness and you get a round hole.

Thank you for the replies. 

I am trying to drill some 12mm holes so this 3mm flat bar can be hung on a wall using mounting screws. As you can see in the image, the circle is not quite up to snuff.

 

20 hours ago, Frosty said:

The vice may not be clamped down securely enough.

I suspect this might be the cause Frosty, as the vice is not actually clamped down at all.

 

20 hours ago, Randy Griffin said:

Clamping the pieces together adds thickness and you get a round hole.

Great advice, if it does not help me this time, I am sure this information will help me in the near future! Thank you Randy!

 

20 hours ago, swedefiddle said:

There is a saying for a Blacksmith;

Measure with a Yard Stick (Meter Stick), Cut it with an Axe, Heat it in a Campfire, Beat it with a Hammer until it fits perfectly!!

This made me laugh Swedefiddle.

That's the problem alright, clamp your vise down and place a piece of wood under the bar stock so you don't drill into the vise. Flat smooth wood, Yes?

Or you can clamp the bar directly to the table with the hole location over one of the holes in the table or on a piece of wood so you don't drill holes in the table. Yes?

As it starts to break through the other side ease off the pressure so the bit doesn't grab. The hole will turn out smoother with less burr to clean up. Drill the small hole first!

Frosty The Lucky.

Amazing! Thank you for helping me.

You're welcome, it's my genuine pleasure to be able to help folks, it makes me feel good. A win win.

Frosty The Lucky.

Mounting holes generally do not require close tolerances and extra work on a project impacts profit margins!

  I'm wondering how much value a reamer would have on that drill press for anything. They usually have a lot of run out.  I would save your money.

8 hours ago, ThomasPowers said:

Mounting holes generally do not require close tolerances and extra work on a project impacts profit margins!

I agree, using a reamer to make a mounting hole is overkill, but this will be my first product, I was just looking for the best result possible.

 

7 hours ago, Nodebt said:

  I'm wondering how much value a reamer would have on that drill press for anything. They usually have a lot of run out.  

Do you mean the drill press or the reamer has a lot of run out?

  I mis-spoke, sorry.  I think that drill press will work fine for your needs.

Hi LA, sorry I am a bit late getting back to the discussion.  I want to male clear that my answer was in no way intended to look down on you.  Sometimes things are more easily done from one person to the next, no matter what it may be.  Many people have a natural ability for certain things.  I did not have the answer for your next question, but I knew that there are plenty others here who would.

5 hours ago, LA9436 said:

best result possible.

Unless you are doing a lot of holes as in production work, why not just clean up the hole with a what we call a rat tail (round) file. File work has always been a part of blacksmithing.

Also "best" is meaningless without the criteria; if you are trying to speed up production then using a reamer is a BAD thing; not an improvement.

Remember the old story of a new mechanical engineer designing a simple bracket and when he goes to the shop to price it they tell him several hundred dollars apiece.  But when an experience engineer makes a slight change they tell him 50 cents a piece to build them.  The difference was the experienced engineer adding a reasonable tolerance to the design...

Ah HAH, a first project! As good as you can get it will change as you  make more, drilling holes is about as basic a skill as reading a scale or using a hack saw. We all had fantasies about our first project being some sort of wonderful. In reality coming out suitable for the job is the correct metric. A clean hole where you need it is a success. I think you know what the basic necessities are, turn the drill speed down turning fast is not a good thing, add one or two drops of oil to help the cuttings clear the hole and keep a steady pressure on the bit. Ease off the pressure when you feel it starting to break through. You WILL feel it. 

What I'd recommend is you pick a length of that stock and drill holes through it full length for practice. That way when you drill a hole you NEED to be right you'll have practice. 

Worry about perfection when you're making watches or space craft. The rest of humanity does just fine with good enough.

Welcome to the club, I've been working with metal for more than 60 years and learn new things all the time. Makes it worth getting up in the morning.

Frosty The Lucky.