Drilling vs Milling hole tolerances

WobblyHand

WobblyHand

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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?
 
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.
 
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...
 
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.
 
mikey said:
Best way to answer this is to drill a hole and run your end mill in there and see what happens.
Click to expand...
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.
 
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.
 
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.
 
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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