Measuring TIR, which measurement do I believe?

MrWhoopee said:
Unequal cutting edge length (point off center) is the culprit. A drill point gage is the tool, though it's only as good as your eyes.

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The ultimate test is drilling a test piece and measuring. A drill will always cut on size except when you need it to. Just like a drill press, twist drills are not a precision cutting tool. That's why we bore and ream.
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Why is it that a drill isn't a precision cutting tool? Is it a materials problem, insufficient rigidity, or something else?

While looking up drill point gauges, I found practically no hits on 135 degrees. I have some cobalt drills and they are 135 degrees. I did find a reference to Mechanix-Mate drill point gauge with 118 and 135 degree head, but it appears they are no longer made. Lots of 118 and the corresponding half angle of 59 degrees. Either that, or my google-fu is weak.
 
WobblyHand said:
Why is it that a drill isn't a precision cutting tool? Is it a materials problem, insufficient rigidity, or something else?
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For the very reason that you discovered, it's just too easy for them to cut oversize. A new drill will probably cut on size, once re-sharpened, all bets are off.

Here's an option for a gage.

and another
www.ebay.com

UNION DRILL POINT GAGE TOOL 711181146455 | eBay

Find many great new & used options and get the best deals for UNION DRILL POINT GAGE TOOL at the best online prices at eBay! Free shipping for many products!
www.ebay.com

and one more
www.ebay.com

(2) Drill Point & Hole Gages (INCH)Best Drill Gage Ever! 118 &135 degrees | eBay

This is a combination drill sharpening and hole gage. Plus,much more. Also, has a drill hole gage gage built in with sizes of 3.0,4.0,5.0,6.0,7.0.8.0,9.0, 10.0 (mm) and 1/8",5/32",3/16", 7/32",1/4", 9/32", 5/16" and 3/8" (fraction).
www.ebay.com
 
Hmm, must have hit the drill anti-lottery. It was a new drill, and it drilled oversized. C'est la vie... I'm coming to grips with the reality of big holes. I accept that the bits drill wide. Tough lesson for the newbie.

Still, I really wonder what the actual technical reason they cut wider. Is it a design defect? Tolerance issue? Rigidity? Is there a design that could work?

Since I mostly have new or slightly used drills, it doesn't seem like these drill bit gauges are that useful. The gauges sound more useful if I were regrinding the drill bits. Currently don't have a setup to grind the bits. In any case, couldn't one just use calipers to measure/estimate the lengths? What would be the allowable mismatch in length?
 
I'm guessing someone more knowledgeable will say, that lobe action yo see on the backside of sheet metal is the inherent problem. Cuts one way, kicks the other all the way down. I'm sure there are many possible reasons, but I'm leaning towards boring bars when I have enough size, but I need to get a mean set of reamers. That in itself is a whole other Oprah. Adjustable, hand, chucking and come in about every size to the thousandth. Ugh.
 
The only way to get an accurate hole is to bore it. In my limited experience.
An option that you may try is an end mill. You won’t find them in the wide range of sizes as you will drill bits Imho.
 
Using reamers kind of limits one to few standard sizes, just to protect your wallet.:dejected: Boring sounds like the way to go, but is darn difficult for smaller sizes. Above 3/8" seems to be doable, at least for "not too deep" holes.
 
No drill drills a hole on size, factory grind or not. On holes up to .500, i expect .002 oversize.
 
WobblyHand said:
Still, I really wonder what the actual technical reason they cut wider. Is it a design defect? Tolerance issue? Rigidity? Is there a design that could work?

Since I mostly have new or slightly used drills, it doesn't seem like these drill bit gauges are that useful. The gauges sound more useful if I were regrinding the drill bits. Currently don't have a setup to grind the bits. In any case, couldn't one just use calipers to measure/estimate the lengths? What would be the allowable mismatch in length?
Click to expand...

Think of it this way, for every .001 the center is off, the hole will be .002 larger. Any number of very small differences can cause one flute to cut more than the other. An optical comparator is probably the only practical way to measure drills precisely. You can't be certain what size the hole will be without drilling it. That's why there are still reamers, and even a reamer can be made to cut oversize.

Here's something dealing with the subject:

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Machinist Drilling Mechanical Tolerance Capabilities Chart - ANSI Size Drilled Hole Tolerance, ISO Metric Drill Sizes

This chart defines drill manufacturing process capabilities for ANSI size machinist drills. Drilled hole locations and size variations are cumulative of several manufacturing variables. Tool sharpness, accuracy, tool and machine rigidity, machine spindle bearings wear, general quality of...
www.engineersedge.com

Perfection is impossible, that's why we have tolerances.
 
Dabbler said:
No drill drills a hole on size, factory grind or not. On holes up to .500, i expect .002 oversize.
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Wish I could do that well. I'd be quite happy at 0.002 oversize. It's the randomness that's hard to deal with. (Standard deviation of hole size.) Some drills seem to do better than others, and chucking (is that a word?) tolerances (operator error?) seem to make the actual hole size vary more than is desirable.

Reducing this by upgrading to a mill, which will reduce TIR of the chuck and bearings, but the variation of the drill bits won't change. Are there known good drill bits that won't break the bank? Or is this a crap shoot where one slightly increases the likelihood of good drill bits by sprinkling more money about?
 
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