Monitor Steam Engine

[PStechPaul]

Speaking of drill bits, here's something I'm wondering about:  I used Marv's formula to figure the DOT at 65% for a 4-40 tap. If my figures were right, it came out to about .0910. So a #43 @ .0890 was about .020 to small and a #42 @ .0935 was about .015 to big. However, a 2.3 mm @ .0906 was "just right". So that begs the question of whether mm bits are sometimes used to bridge the gap between numbered bits or am I tuned into my own "fairytale" here?

Jim

Your math is a bit off, by a factor of 10. A #43 and #42 differ by only 0.0045" and the hole size actually made by drills can vary from -0.001" to +0.003". Larger number and letter size drills have greater difference in size, but they also have a wider tolerance for hole size, and the diameter for tapping is not as critical.

If you drill slowly or allow a drill bit to spin in a hole, it will tend to open it up by a couple thousandths, due in part to expansion from heating, and perhaps more because of runout, flex, and vibration and the side cutting action of the margins on the flutes.

[Flyboy Jim]

Speaking of drill bits, here's something I'm wondering about:  I used Marv's formula to figure the DOT at 65% for a 4-40 tap. If my figures were right, it came out to about .0910. So a #43 @ .0890 was about .020 to small and a #42 @ .0935 was about .015 to big. However, a 2.3 mm @ .0906 was "just right". So that begs the question of whether mm bits are sometimes used to bridge the gap between numbered bits or am I tuned into my own "fairytale" here?

Jim

Your math is a bit off, by a factor of 10. A #43 and #42 differ by only 0.0045" and the hole size actually made by drills can vary from -0.001" to +0.003". Larger number and letter size drills have greater difference in size, but they also have a wider tolerance for hole size, and the diameter for tapping is not as critical.

If you drill slowly or allow a drill bit to spin in a hole, it will tend to open it up by a couple thousandths, due in part to expansion from heating, and perhaps more because of runout, flex, and vibration and the side cutting action of the margins on the flutes.

Thanks Jason and Paul for your responses. Both the information and pointing out my math error.  :face palm:

Jim

[sssfox]

Speaking of drill bits, here's something I'm wondering about:  I used Marv's formula to figure the DOT at 65% for a 4-40 tap. If my figures were right, it came out to about .0910. So a #43 @ .0890 was about .020 to small and a #42 @ .0935 was about .015 to big. However, a 2.3 mm @ .0906 was "just right". So that begs the question of whether mm bits are sometimes used to bridge the gap between numbered bits or am I tuned into my own "fairytale" here?

Jim

The true size of the drill bit is really the system you use to measure it.  It doesn't make any difference whether it was built as a metric bit or letter bit or numbered bit or whatever.  If you are measuring in inches, measure it with an inch micrometer and it is the correct size, use it.

That being said, the accuracy of the sharpening is as least as important as the diameter of the bit.  Sharpening can make a bit cut an oversized hole, but never an undersized hole.  The bits I have purchased in the last several years all seem to drill oversized holes, at least 1/4" on up.  I'm assuming that is due to poor sharpening practices.  Note to self, I really need to come up with a good drill sharpener.

Many years ago, I had a shop teacher who had a session he called "Making Do".  I don't remember much about it other than he showed us that we could sharpen a drill bit off center to drill a larger hole if we didn't have one of the correct size.

As you have found out, there are different "correct" sized bits, even for one tap, depending on what you are trying to accomplish.  I'm sure there is a tolerance that a hole should be within for a given depth of thread, but I have no idea how to determine it.  As has been said before, this isn't rocket science, but the smaller the tap, the smaller the range. 

While almost every possible size of drill bit is available, there's no way one person can own them all.  In cases like this, the smaller the difference between sizes, the less critical the choice.  In your case, the 2.3mm bit may be the best choice or the #43 bit may be the best choice.  The #42 is probably too large.

The rule I go by is use the drill bit I own that is closest to the needed diameter, period.  So the 2.3mm would be the right one, but if it is not sharpened on center, it may cut too large a hole.  The results may be so similar that it makes no difference.  I'm just getting into the small fastener arena.  I think I will proceed by drilling and tapping practice holes and once I find the correct drill for a given tap in a given material, I'll use it in the future.

Just be careful when you tap the hole.  If the hole is too small, the tap will jam and you could break it.
If that happens, before you break it, go back and drill the next larger hole.
If the hole is too large, it will probably still work, but won't hold quite as well.

Steve Fox

[mklotz]

Jim,

By now you've learned that any time you report a calculation in a thread it's a good idea to check it on a calculator.  I'm reasonably good at mental arithmetic and I still do that, just as I check myself on a calculator in the shop where an error can have more serious effects.

Nevertheless, your original question is a good one.  When you use my DRILL program to find the drill closest to an input hole size, it reports the closest as well as the two smaller and larger than the closest along with their difference from the desired size.  For example

(2.20 mm) with size 0.0866 (-0.0044)
(2.25 mm) with size 0.0886 (-0.0024)
(43) with size 0.0890 (-0.0020)
***** (2.30 mm) with size 0.0906 (-0.0004) *****
(2.35 mm) with size 0.0925 (+0.0015)
(42) with size 0.0935 (+0.0025)
(3/32) with size 0.0938 (+0.0028)

As you can see, it includes metric size drills.  A well-equipped shop should have a set of metric drills not just for this application but for dealing with metric machinery and metric threads.

[Flyboy Jim]

Jim,

By now you've learned that any time you report a calculation in a thread it's a good idea to check it on a calculator.  I'm reasonably good at mental arithmetic and I still do that, just as I check myself on a calculator in the shop where an error can have more serious effects.

Nevertheless, your original question is a good one.  When you use my DRILL program to find the drill closest to an input hole size, it reports the closest as well as the two smaller and larger than the closest along with their difference from the desired size.  For example

(2.20 mm) with size 0.0866 (-0.0044)
(2.25 mm) with size 0.0886 (-0.0024)
(43) with size 0.0890 (-0.0020)
***** (2.30 mm) with size 0.0906 (-0.0004) *****
(2.35 mm) with size 0.0925 (+0.0015)
(42) with size 0.0935 (+0.0025)
(3/32) with size 0.0938 (+0.0028)

As you can see, it includes metric size drills.  A well-equipped shop should have a set of metric drills not just for this application but for dealing with metric machinery and metric threads.

Thanks Steve and Marv for your input.

Yes, a mini-machining forum is a tough place to post a math error (actually, in my defense, it wasn't even a math error .......... it was a typing error).

Marv, I'll spend some time bonding with your program. It looks really handy. The more I work with this stuff the clearer it becomes.

Steve, I'll also spend some time looking at my various small size drill bits to see what they actually are. I suspect that's one of those basic things that everyone knows you need to do...............well, except us newbies!

My take away from this whole drill bit/tap discussion is that, in the end, it's kind of one of those "touchy/feely" sort of things. What I mean by that, is that after the calculations are all done, it still comes down to how it feels when doing the actual tapping. As Steve pointed out........if it feels like the tap might break, back it out and come up with plan "B"!

Well, the UPS lady just dropped off my drill bit order from McMaster-Carr. I wasn't sure what brand they carried, but I checked one and it says "C-L HS USA". So I'm assuming that C-L means "Chicago-Latrobe". They look really nice.......my first Tin-coated bits.

Also, gotta get all this drill bit info printed out and in my notebook for when that "CRAST" disease rears it's ugly head! 

Maybe I'll even get some machining done  and let Carl have his thread back!

Jim

[zeeprogrammer]

and let Carl have his thread back!

It's ze mob's thread now. 

Just want to thank everyone for all the information. Really good stuff.

What with Marv's reminder of an Enco discount, I hope I have time tonight to put an order together.

[10KPete]

I was taught to think not of the drill size (or reamer, etc. ) but of the hole size one desired. One then selected the weapon for creating
said hole.

Way back when, drawings would show something like " .375" Ream". One of the old guys used to grumble, quite loudly, "Don't tell
me how to make the damned hole, just tell me the size you want it!!"

Pete

[zeeprogrammer]

Back from a business trip to Canada.

Had to drive via Rochester, New York.

I could have sworn I caught a whiff of mint chocolate cookie.

I must have been real close to cookie-land...and its crumbs. 

Good to be home. Now I have a whole lot of catching up to do.

[crueby]

Back from a business trip to Canada.

Had to drive via Rochester, New York.

I could have sworn I caught a whiff of mint chocolate cookie.

I must have been real close to cookie-land...and its crumbs. 

Good to be home. Now I have a whole lot of catching up to do.

Should have let me know you were going by, would have handed you a bag of cookies on your way by!

If you came up to Rochester and caught the Thruway over to Buffalo/Canada, you were only a couple miles away.

Oh well, have to eat your portion for you...   

[zeeprogrammer]

Should have let me know you were going by, would have handed you a bag of cookies on your way by!

If you came up to Rochester and caught the Thruway over to Buffalo/Canada, you were only a couple miles away.

Oh well, have to eat your portion for you...   

Best be careful with the offers there my friend.
I suspect I'll be making that trip again (assuming I last long enough at my job).

In the meantime...enjoy my cookies. 

It wasn't until I was going through that area that I realized you might be close by.
If the opportunity comes up again I'll PM you.
(I try not to let people know on the forum when I'm on a trip. I don't so much worry about bad people. It's the pranksters on this forum that are the danger. I won't name them. I'm sure I needn't have to.)

Oops. Did I end that sentence correctly?

Initials are A, B, C, ... you get the picture.

[crueby]

Should have let me know you were going by, would have handed you a bag of cookies on your way by!

If you came up to Rochester and caught the Thruway over to Buffalo/Canada, you were only a couple miles away.

Oh well, have to eat your portion for you...   

Best be careful with the offers there my friend.
I suspect I'll be making that trip again (assuming I last long enough at my job).

In the meantime...enjoy my cookies. 

It wasn't until I was going through that area that I realized you might be close by.
If the opportunity comes up again I'll PM you.
(I try not to let people know on the forum when I'm on a trip. I don't so much worry about bad people. It's the pranksters on this forum that are the danger. I won't name them. I'm sure I needn't have to.)

Oops. Did I end that sentence correctly?

Initials are A, B, C, ... you get the picture.

Your cookies passed the QA test. Forgot to log the results, so had to retest with more. Yum.

[zeeprogrammer]

Your cookies passed the QA test. Forgot to log the results, so had to retest with more. Yum.

And I'll bet you forgot to log those results too. You're in an infinite loop.

Got my new taps and some tap magic.
Together with the fine advice from the members here...I'll never break another tap. Yay! 

I've just jinxed myself, haven't I?
Where's my lucky 'knock on wood' piece of wood?
Drat. Lost it. So unlucky.

And before you fine people get onto me...you'll get pics when I get machine time.

[10KPete]

I've just jinxed myself, haven't I?
Where's my lucky 'knock on wood' piece of wood?
Drat. Lost it. So unlucky.

You ain't lost it friend! It's parked right thar 'tween yer ear bones!!

Pete

[zeeprogrammer]

I've just jinxed myself, haven't I?
Where's my lucky 'knock on wood' piece of wood?
Drat. Lost it. So unlucky.

You ain't lost it friend! It's parked right thar 'tween yer ear bones!!

I'm doomed then.

Just read this thread for proof. 

[Tennessee Whiskey]

Well,  whilst in the wilds of the Virginia mountains,  it appears you've had nineteen pages of post and a couple of machining pics,  damn proud,  way to go  . Rochester,  NY, isn't that where the Dick Van Dyke show was supposed to be?  Hint on Tap Magic : pick up some syringes and use the to apply,  the crap will last forever.

Toodles,
Cletus