Am I wrong here ? 3/4" bit = ?

[NUTTSGT]

Have I been doing it wrong all these years ? If I drill a hole in a piece of steel using a 3/4" drill bit (brand new bit, first hole) shouldn't I get a hole that measures 3/4" unless of course the drill bit is under size ?







The bit in question is a brand new Milwaukee 3/4" bit (48-89-4640) I just bought from Home Depot ($22.97) for a job.


https://www.milwaukeetool.com/accessories/drilling/48-89-4640


EDIT: pics removed from PB

 

[Techie1961]

Drill bits are very inaccurate. They rely on many factors to keep the drilled hole sized correctly. It could be sharpened wrong and off centre, the diameter could be worn, It could be too big for a thin piece, etc.

Most drill bits will drill slightly larger than their size unless it is sharpened very well.

 

[PoorOwner]

The drill has run out so they undersize it a bit. My hole saw is .72 and the hole comes out 0.749

 

[A_Pmech]

Twist drills have a small amount of back taper. They are slightly smaller at the shank end than at the cutting end. This makes it less likely that they will seize in the hole they drill.

Twist drills, as a rule, always drill slightly oversize holes. There are tables to predict the amount of oversize, which is dependent on the diameter of the drill.

 

[KnurledNut]

Drill diameters are slightly smaller than nominal.

 

[AndrewDouglasBird]

Only the cutting edge at the tip of the drill will measure to size. The rest of the drill will be under size so it doesn't rub the inside of the hole.

I'm assuming the hole ended up over size as drill will almost always drill over unless they are very sharp and used in a rigid machine like a mill. Drilling with a hand drill will be the worst as then the angle of the drill also plays a part.

Drilling with progressively larger drills, starting with one slightly larger than the margin of the 3/4" will result in the best hole.

If you need it more precise and don't have a mill or at least a drill press, a single flute uni-bit style drill will make a very nice, round hole.

 

[NUTTSGT]

Twist drills have a small amount of back taper. They are slightly smaller at the shank end than at the cutting end. This makes it less likely that they will seize in the hole they drill.

Twist drills, as a rule, always drill slightly oversize holes. There are tables to predict the amount of oversize, which is dependent on the diameter of the drill.

The first part I can understand, I measured at the back since it was a larger area for the calipers.


The last part, do you have a link to these tables. Looks like I'll need a bit larger than 3/4" for the rod to fit in the hole or I'm going to have to break out the die grinder and carbide to enlarge every one of them.

 

[KnurledNut]

^^^
How about a reamer?

 

[NUTTSGT]

Only the cutting edge at the tip of the drill will measure to size. The rest of the drill will be under size so it doesn't rub the inside of the hole.
That's the problem, the hole as drilled is too small for the rod.

I'm assuming the hole ended up over size as drill will almost always drill over unless they are very sharp and used in a rigid machine like a mill. Drilling with a hand drill will be the worst as then the angle of the drill also plays a part.
No, the hole is undersize, it's a sharp bit as it is brand new and I used my drill press.

Drilling with progressively larger drills, starting with one slightly larger than the margin of the 3/4" will result in the best hole.I'm guessing I need one larger as this bit fails to measure up.


If you need it more precise and don't have a mill or at least a drill press, a single flute uni-bit style drill will make a very nice, round hole.It's not a precise issue as I used my DP.

My answers in red.

Kinda hard to hold the calipers on the bit and and snap a picture with the phone.



The undersized hole.

 

[NUTTSGT]

^^^
How about a reamer?

That, I don't have. I'd be happy to have one right now or a bit that cut it slightly oversize as much as it cut it undersize.

 

[KnurledNut]

Or pick up a 49/64 drill...?

 

[fiemo weezap]

Check the size of the shaft. Hot rolled steel tends to run oversize. Also, you might try to intentionally re sharpen the drill to a non-standard configuration, ie. with unequal angles or unequal cutting lips. This will tend to make the drill drill oversize.

 

[xela456]

Just hand sharpen it. I will drill slightly oversize every time lol

On a serious note though it will because it's hard to get the cutter lengths exactly the same.

 

[Cruzan80]

All of these measurements, and nothing showing the size of the rod? Right now, I am reading the hole as being 1 tic over the the size of the drill bit at tip. How big does the rod measure?

 

[Hephaestus29]

Are you using a hand drill, drill press,
or mill?

If you're using a drill press or mill and can
get your hands on a milling bit it will
probably be right on size.

 

[larry_g]

All of these measurements, and nothing showing the size of the rod? Right now, I am reading the hole as being 1 tic over the the size of the drill bit at tip. How big does the rod measure?

And is the rod round? A bit oval and it will not fit.

On edit, What are you doing that requires such a tight fit? Me if it just has to go in the hole I would take the rod and grind the scale off of it till it would fit. If welding this joint the close tolerance is not needed. If a clearance hole then go with the /64 over bit mentioned above.

lg
no neat sig line

 

[NUTTSGT]

Are you using a hand drill, drill press,
or mill?

If you're using a drill press or mill and can
get your hands on a milling bit it will
probably be right on size.

Drill Press.

 

[Spudland_Dave]

Twist drills, as a rule, always drill slightly oversize holes. There are tables to predict the amount of oversize, which is dependent on the diameter of the drill.

+1
What does your 3/4" Round Bar measure out as? Is that rusty CR or is it Hot Roll, cause if its Hot roll, thats your problem...
999.999 times out of 1000 a drilled hole ends up oversize...I've never even measured a bit out. I just know a Bit marked 3/4" will drill a .752ish hole...which for me is close enough. If need perfection (rarely) I drill undersize and ream to finished size.

 

[Jim Diesel]

I agree with the other comments. Aim for under size hole, finish with a reamer.

 

[NUTTSGT]

And is the rod round? A bit oval and it will not fit.

On edit, What are you doing that requires such a tight fit? Me if it just has to go in the hole I would take the rod and grind the scale off of it till it would fit. If welding this joint the close tolerance is not needed. If a clearance hole then go with the /64 over bit mentioned above.

lg
no neat sig line

The stock is clean and appears to be round, not oval. It doesn't need to be a tight fit, I would have figured the hole would have been slightly oversize, not undersized.




I guess my fix will be hitting the rod with the belt sander and/or opening the hole up with a carbide. If I use a reamer, I'l have to order one as I'm not sure if anybody local would have any at all.


My thoughts were just simply, if I drill 3/4" hole, then a piece of 3/4" stock should fit in it. I suppose I'll just have to add some larger size drill bits and reamers to my collection.

 

[MoonRise]

A twist drill is not a precision hole maker.

Sometimes, it will make a hole that is GoodEnough.

Other times, the hole is too small, or too large, or tri-lobular, or some other shape that is not actually round, etc, etc.

Want 'precision' (size and/or shape)? Drill undersize and then use a reamer.

Just want to slip your 3/4" round steel stock through a drilled hole? Probably need to get a 49/64" drill bit.

Oh, and deburr your drilled hole before measuring it or trying to put your round stock into the hole.

Your stock is measuring 0.752" from the picture you posted, and your drilled hole measured at 0.744" (with the edge burrs ), which is actually pretty close for a "drilled" hole in my book. But 8 thou interference means you aren't putting the stock into the hole without either some (actually a whole lot of) thermal expansion and/or some pressing force (a rather large amount of force there too).

Even heating your steel plate to 1000F to thermally expand the steel won't get the hole diameter big enough to put the round bar into the hole.

A standard 'heavy' force fit on a 3/4" (nominal) diameter shaft/hole is about 1-3 thou interference. You've got 8 thou there.

http://www.amesweb.info/FitTolerance/FitToleranceImperial.aspx

Anyway ...

Check Amazon, or ENCO http://www.use-enco.com/CGI/INSRHM or MSC (lots and lots or choices, usually have what you want/need), or McMaster or Grainger (almost certainly have what you want/need, but usually 'full' retail price, unless you happen upon a sale/clearance item) and order it.

ENCO, 49/64" drill bit with 1/2" reduced diameter shank, $15 to $40 depending on brand:

http://www.use-enco.com/CGI/INPDFF?PMPAGE=35&PMCTLG=00

Or 49/64" chucking reamer, $38

http://www.use-enco.com/CGI/INSRIT?PMAKA=331-1149&PMPXNO=2614413&PARTPG=INLMK32

 

[Spudland_Dave]

Just want to slip your 3/4" round steel stock through a drilled hole? Probably need to get a 49/64" drill bit.

IMHO I think the issue here is a perfect storm.. I cant say I've ever had to use a 49/64 bit to get a 3/4 shaft in...as it is, 3/4" bits always cut large. But you ARE Right the root of it (Obviously) is the 8 Thou interference...
I think the issue has less to do with the bit in question and more to do with what appears to me to be Hot Roll 3/4" Round Stock, which looks like it has mill scale, etc.. Hot Roll = Close Enough. I would wager deburring the hole and using 3/4" Cold Roll and the shaft would easily slide right in.

How thick was that flat stock you were drilling anyways? rule of thumb is bits larger in diameter then the thickness of the stock will tend to drill Tri-Lobular. I bet the hole isn't even round...quickly hit it with a half round file..

 

[2oolhound]

I'm not sure how far down the rabbit hole you want to go with the rod or what you're working on but…

1- fastest and cheapest is take the scale off the rod with your belt sander (carbide in the hole sounds messy)
2- 49/64" bit - should be easy to access locally and not too expensive.
3- adjustable reamer

 

[Kevin54]

Eric.....if you need a 3/4" reamer, I believe I have one I can send you to use.

 

[creativecars]

I guess chucking the bit in a 1/2 drill and wallering it around is out of the question??? Precision fit with manual adjustment..
How many of these do you need to do?

 

[Kevin54]

I guess chucking the bit in a 1/2 drill and wallering it around is out of the question??? Precision fit with manual adjustment..
How many of these do you need to do?

There is no precision with wallering

 

[creativecars]

There is no precision with wallering

That's what my wife says???

 

[NUTTSGT]

+1
What does your 3/4" Round Bar measure out as? Is that rusty CR or is it Hot Roll, cause if its Hot roll, thats your problem...
999.999 times out of 1000 a drilled hole ends up oversize...I've never even measured a bit out. I just know a Bit marked 3/4" will drill a .752ish hole...which for me is close enough. If need perfection (rarely) I drill undersize and ream to finished size.

Yes, it's hot rolled and I knew I would to knock the mill scale off, since I'm welding it in place. I was just thinking the bit would make a hole closer to what you mention, .752 not undersize like I have.

 

[NUTTSGT]

A twist drill is not a precision hole maker.

Sometimes, it will make a hole that is GoodEnough.

Other times, the hole is too small, or too large, or tri-lobular, or some other shape that is not actually round, etc, etc.

Want 'precision' (size and/or shape)? Drill undersize and then use a reamer.

Just want to slip your 3/4" round steel stock through a drilled hole? Probably need to get a 49/64" drill bit.

Oh, and deburr your drilled hole before measuring it or trying to put your round stock into the hole.

Your stock is measuring 0.752" from the picture you posted, and your drilled hole measured at 0.744" (with the edge burrs ), which is actually pretty close for a "drilled" hole in my book. But 8 thou interference means you aren't putting the stock into the hole without either some (actually a whole lot of) thermal expansion and/or some pressing force (a rather large amount of force there too).

Even heating your steel plate to 1000F to thermally expand the steel won't get the hole diameter big enough to put the round bar into the hole.

A standard 'heavy' force fit on a 3/4" (nominal) diameter shaft/hole is about 1-3 thou interference. You've got 8 thou there.

http://www.amesweb.info/FitTolerance/FitToleranceImperial.aspx

I was expecting it to be slightly oversize or even tri-lobular like you mention. Honestly, I bought this Cman drill press second hand and did some clean up work on it. I believe I posted some pics or a thread about it. I never checked run out on it but after drilling this hole, I'm guessing that I should be mighty happy about the run out my drill press has on it.

I hadn't deburred the hole yet, it will get done but I figured I shouldn't need to in order to get it to fit. I was wrong. This first hole will get cleaned up, enlarged and the mill scale will be cleaned off the rod.

 

[NUTTSGT]

IMHO I think the issue here is a perfect storm.. I cant say I've ever had to use a 49/64 bit to get a 3/4 shaft in...as it is, 3/4" bits always cut large. But you ARE Right the root of it (Obviously) is the 8 Thou interference...
I think the issue has less to do with the bit in question and more to do with what appears to me to be Hot Roll 3/4" Round Stock, which looks like it has mill scale, etc.. Hot Roll = Close Enough. I would wager deburring the hole and using 3/4" Cold Roll and the shaft would easily slide right in.

How thick was that flat stock you were drilling anyways? rule of thumb is bits larger in diameter then the thickness of the stock will tend to drill Tri-Lobular. I bet the hole isn't even round...quickly hit it with a half round file..

Dave, if we were dealing with wood, even square peg/round hole, the fit would be easier to deal with....



The stock I'm drilling through is c-channel and the bit did cut nice as it was a brand new bit. I was happy with the way it cut...just not the size. I will remediate that tomorrow.

 

[NUTTSGT]

I'm not sure how far down the rabbit hole you want to go with the rod or what you're working on but…

1- fastest and cheapest is take the scale off the rod with your belt sander (carbide in the hole sounds messy)
2- 49/64" bit - should be easy to access locally and not too expensive.
3- adjustable reamer

It's not a big problem for me, I've made adjustments before. After buying what I thought would be a great bit, I just wasn't happy with it last night. The carbide won't be an issue, I've done it many a time to get something to fit.

 

[NUTTSGT]

Eric.....if you need a 3/4" reamer, I believe I have one I can send you to use.

I'm good Kevin but thanks for the offer.

 

[NUTTSGT]

I guess chucking the bit in a 1/2 drill and wallering it around is out of the question??? Precision fit with manual adjustment..
How many of these do you need to do?

In a piece of steel like I have, I'd prefer not, too easy to break a bit.

There is no precision with wallering

Kev, you've never seen me work.....I'm the king of wallering on the fire ground, AKA pigpen. Come by a fire scene some time and you'll be able to tell which one I am, I'll be way dirtier than everybody else.

 

[Fcvapor05]

Your expectation was just unreasonable.

Slip fit on a steel part would tend to be a thou or two oversize. This also assumes that your drill press table is perfectly perpendicular to the work (which it isn't) and that a two flute drill bit drills a round hole (which it doesn't) and that your drill press has zero runout (which it doesn't) and that the hole is perfect and free of burrs (which it isn't) and that the stock is perfectly round (which it isn't) and that the stock is exactly sized (which it isn't) you get the idea.

If you're welding it, who cares. Hit the inside of the hole with a sanding drum or flap wheel and go on about your day.

 

[NUTTSGT]

I'd like to thank everybody for the comments and advice, it's greatly appreciated.

To resolve my issue, I'll use a carbide bit to clean the hole out and the mill scale off the rod as I'm welding them in place. My work doesn't need to be precise as it'll be going through both sides of some square tube, both getting welded.

I ordered this from Grizzly this afternoon. Since I just signed up, I got a $25 off shipping, which gave me UPS 2 day shipping. The 13/16" bit should work for me.

http://www.grizzly.com/products/Silver-Deming-8-pc-Bit-Set/H7993?utm_campaign=zPage



Much appreciated.