# Drilling vs Milling hole tolerances



## WobblyHand (Jul 2, 2020)

Learned the hard way previously about over sized holes when drilling with ordinary twist drills and drill presses with poor bearings.  

What kind of allowance does one make when using a milling tool to make a hole?  For instance, in my shop I have a roughing mill that says it's 1/2".  Using a mike, it's 0.497", if I recall correctly.  If the hole is pre-drilled at 7/16" or slightly less, and then this roughing mill is used, what hole size could one expect?  I know no one knows the exact answer, but what would one typically expect?


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## Mitch Alsup (Jul 2, 2020)

My guess is that it will be dependent on how fast stuff is moving. a slow RPM plunged slowly into the hole with plenty of lubricant will end up only a few tenths larger than measured diameter. High speeds and a fast plunge and it might be a couple of thou. It is dependent on the cutting forces, the rigidity of the machine and the smoothness of the bearings.

But this is also a reason I have 115 drill bits from 1/32-through-1/2 all the letter and all the numbered bits.


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## WobblyHand (Jul 2, 2020)

I have the same 115 drills.  Can't say that all of those drills are straight or ground perfectly.  Certainly some of those twist drills have lead me astray.  If I find a bad one, it gets replaced.  

Of course, the hole size depends on the TIR of your machine, and the end mill tool.  I have no experience using end mills to open up a hole.  Hence, I posed the question to at least get a feel for what one might expect.  Guess it really is a beginner's question...


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## Mitch Alsup (Jul 2, 2020)

If the hole is "of reasonable size" then boring is the proper way to get an exact fit.
Where reasonable size is 1/4" and larger.


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## mikey (Jul 2, 2020)

Best way to answer this is to drill a hole and run your end mill in there and see what happens.


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## higgite (Jul 2, 2020)

mikey said:


> Best way to answer this is to drill a hole and run your end mill in there and see what happens.


And report back to us!  

Tom


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## WobblyHand (Jul 2, 2020)

mikey said:


> Best way to answer this is to drill a hole and run your end mill in there and see what happens.


Guess I'll run a test piece then.  Kind of hard to measure ID's of holes of that size.  At the moment all I have is a dial caliper, and a rod that I measured to 0.5005" using a micrometer.  Guess it will be a go-no go test.


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## pontiac428 (Jul 2, 2020)

Regardless of how the hole is made, if it has a tolerance or parallel spec you are trying to hit you might consider boring it on the mill with a boring head or on the lathe (if you can chuck the part) with a boring bar.  If it's just a bolt hole, use the drill and call it good.  You can also finish ream a drilled hole.


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## benmychree (Jul 2, 2020)

I would likely go with a machine reamer, it would be good to know what it needs to fit and how closely it needs to fit.


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## Janderso (Jul 2, 2020)

I'm glad you mentioned a reamer John.
I watched a video this morning where a guy made a floating reamer holder where the reamer could do it's work with no radial influence from the tail stock chuck or mill chuck.
I don't have very much experience with reaming but it did make sense.
Have you found they are required in some applications?


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## benmychree (Jul 2, 2020)

If the reamer runs true in the chuck, there is no need for a floating holder; I do not have one since I sold my business, and rarely used them there.  I do not generally ream from the tailstock, I use the Aloris tool holder and have adjustment vertically and horizontally to center the reamer or whatever tool that I am using; generally, one uses a fairly agressive feed with machine reamers to get them to cut to size.


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## Janderso (Jul 2, 2020)

benmychree said:


> I use the Aloris tool holder and have adjustment vertically and horizontally to center the reamer


Do you just clamp it in the holder or have you made a carrier for the chucking reamer? I never thought of using a QCTP method.


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## WobblyHand (Jul 2, 2020)

Found a piece of O1 rod that miked out at 0.5003.  Also the hammer handle that I turned to 0.5005.  The 1/2" roughing mill cut a 0.700" deep hole in a 12L14 test piece that just lets the 0.5005 rod go in.  The O1 rod goes in slightly easier.  No interference on the 0.5005" rod, but a tight slide fit.  If I pull out the rod (straight) then there is a vacuum pop.  

This only means the hole milled is close to 0.5005.  So for this end mill on my machine, the hole is not hugely far off.  Still don't know what size the hole really is.

This is just for a hammer that I am making.  Wanted a very close fit for the aluminum handle to the head.  The hole I made is close to what I wanted, but honestly, it was just beginners luck.  The right way is to bore it to fit.  Don't have a boring head yet for my mill. Maybe someone can make a suggestion for a boring head that would work on a PM25?  PM's boring tool is out of stock.


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## Aukai (Jul 2, 2020)

How thick of material are you cutting? I just did a test piece last week on a 1/4" wall thickness box tubing with a 1/2" annular cutter. It left a very nice hole, and a Brillo pad worth of swarf. I'll see if I can remember, and find it to measure it.


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## benmychree (Jul 2, 2020)

Janderso said:


> Do you just clamp it in the holder or have you made a carrier for the chucking reamer? I never thought of using a QCTP method.


I have a #3 and #4 morse taper holder, and use a drill chuck or use MT sleeves if the reamers are taper shank.


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## Winegrower (Jul 2, 2020)

I find these tools are incredibly useful to measure hole diameters closely, as in a press or sliding fit.


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## WobblyHand (Jul 2, 2020)

Aukai said:


> How thick of material are you cutting? I just did a test piece last week on a 1/4" wall thickness box tubing with a 1/2" annular cutter. It left a very nice hole, and a Brillo pad worth of swarf. I'll see if I can remember, and find it to measure it.


Hole is 0.600" deep


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