Question About Drill/Tap Size

[bostonman]

Long story short, I had to replace a broken piece on my snowblower (see attached picture for a visual). The original piece had "self taping" bolts (although they don't resemble self taping). They are 1/4" - 20 x 0.5".

When I purchased the new piece, the holes were not threaded and that's where I learned the bolts used are self taping.

Rather than force bolts into it and risk them breaking and/or not being able to remove them in the future, I elected to tap the holes and also use harden steal bolts.

I have two questions. The first is regarding the chart in the attached:

https://www.montana.edu/jdavis/met314/documents/Drill%20Chart.pdf

The drill size for a 1/4" - 20 tap is #7. Are the other drill size numbers (4 through 12 - excluding 7) that fall within that section mean anyone of those drill sizes will work, or is #7 exclusive to a 1/4" - 20 and the other numbers are just showing the order of bit sizes?

My other question is regarding the replacement part. The holes were drilled at the factory to a size #1 drill size (I was able to slide a #1 bit into and through the hole).

This has me baffled because it tells me the hole is too large to tap (#1 versus #7, however, I taped all the holes with a 1/4 - 20 tap, and it was quite tight.

Why would the hole be drilled so large thus risking a loose fit bolt?

My guess is that the walls are thin, and, at the factory they use impact drills. If it was drilled to a #7, then maybe the metal would crack once the self taping bolts are forced in.

[jpanhalt]

If you have question s about threading or anything else machinery related, go to Machinery's|Handbook
https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Oberg/dp/0831137320/ref=asc_df_0831137320?

Mechanical things haven't changed much.  Older versions are just as applicable.  I use the 21st edition and have a later edition on CD, but rarely refer to it.

Usually thread charts will say what percentage of thread they are for.  75% is common.  The #7 is what's recommended for that, not the range shown.  (A quick way to get close using TPI is to subtract the reciprocal of the TPI from the nominal diameter.  Thus, 0.250 - 1/20 = 0.200.  A #7 is 0.201 .)  The tolerance of fit enters in to it too.

As for the hole size for thread forming, it is probably a little bigger than for thread cutting.  You can make the hole smaller by staking it, but I wouldn't bother.  Just tap it as is.  Or, get a proper thread forming screw.

EDIT: Here a chart for cold forming taps.  Note the larger holes: https://www.jarviscuttingtools.com/forming-tap-drill-sizes

[bostonman]

I have wanted to grab a copy of that Machinery's Handbook - it's on my list of things to buy when I want to splurge. Most often the charts online are enough because it's seldom I get involved in such machinery related projects; but the book will be nice to have.

Confirming that a #7 drill size is correct and not the range helped. The answer was somewhat obvious, but when looking at the difference in bit sizes, I began wondering if sometimes a certain +/- range existed for different reasons.

I saw different metals and TPIs called for different drill sizes, so I'm aware of that too.

Just to confirm, 75% is how far the pitch of the thread "cuts" into the metal (similar to the picture I attached)? So 75% means from the tip of the thread on the tap cuts into the metal 75%. A hypothetical 1% would barely cut the metal? Edit: I should have said: in this case a #7 drill size will (or should) provide 75% for 1/4 - 20 tap whereas if the hole was drilled larger, say #4, then maybe it would be more like 50%. Is this a correct statement?

I was unaware of 'cold forming' taps. Most of my experience is just locate the correct size tap and hope it doesn't break, but I should learn more about different types of taps.

Unfortunately my experiences are from an old job where 4-40 screws were used often but would sometimes break because the production department would cross thread them using a power screw driver. I learned about drilling and taping, but a good percentage of time I'd break the tap. I'd use oil, turn, back off, turn, back off, oil, etc... and without warning, they'd break.

Most recent I tried taping a 2-56. The tap seemed to hold up, things were going well, I tried un-threading it, and the tap broke. It held up the entire time under forces I thought would break it and then broke without any warning when trying to remove it.

Having said this, I lack confidence with taping and always reluctant to attempt it

[Stray Electron]

  First, I would not use hardened bolts unless there is a GOOD reason for it.  A lot of joints are purposely built with rather weak bolts so that the bolts will break before the more expensive parts. 

  Second,  + + + on the recommendation that you get a copy of Machinery's Handbook and that you read through it.  I bought my first copy, a 1939 edition (over 40 years out of date when I bought it), for $1 about 40 years ago and I still regard it as one of the BEST investments that I ever made.  They reprint these every years but for most basic mechanics such as we're talking about here, the old editions as just as up to date as a brand new edition.  And used copies can be found all over the place, I know that used book store of my local library has several copies and their price is $1 each for hard bound books.

[bostonman]

I have some experience with that book because a co-worker at an old job referenced it when I approached him with some questions. At the time of seeing the material he referenced, I was blown away, so I fully agree with the recommendations from everyone.

[microwhat]

Threads of different percent depth exist to match the strength of the parts with the strength of the fastener. Most drill/tap charts give 75%-85% depth, which is only necessary for soft parts with short engagement (<1 diameter). If your tapped hole is more than 6 threads deep and the part and fastener are of similar hardness, >75% thread is generally unnecessary. Most of the parts I make have >1D thread engagement, so I usually aim for 65% thread depth. This makes tapping less expensive, and doesn't compromise strength. For example, I drill a 0.205" hole for a 1/4-20UNC tap.

"Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread. Often, a 55 to 60 per cent thread is satisfactory, although 75 per cent threads are commonly used to provide an extra measure of safety." (Machinery's Handbook, 27th edition, pg. 1924)

[bostonman]

That's some great feedback and good to know.

Unfortunately I lack experience using easy outs and taps (and dies). My experience is often times breaking 4-40 taps. Easy outs I've only used for a short time. My father has a set and I fear breaking them, so I give up quickly rather than apply a tremendous amount of force - I'd rather sacrifice the part in question rather than break his tools.

With taps, the ones I used were yellow, I assume Titanium, and usually I've gone into softer metal such as aluminum. As mentioned, at my old job the Production Department would cross thread cover screws (4-40) and the heads would break when I went to open a unit. I'd drill the broken screw, grab a yellow tap, attempt to tap the hole, and snap.

Maybe it was due to using oil instead of tap lubricant (I don't remember which I used at the time), maybe I didn't back off often enough, etc... At the time all I knew was a drill hole size for a certain size tap, a tap, and some sort of lubricant. Didn't know anything about different materials, the importance of lubrication, etc...

I watched a video last week. The guy claimed he uses high speed taps (?) and swears they are the best ones. He used a 1/2" - 20 tap into aluminum, Tap Magic, and only backed off maybe every 1/2 rotation.

It seems I've backed off more often and yet have broken taps - but I need more practice.

Until recently (and the help of this thread), I never realized how much science goes into simply taping a hole.

[Langdon]

"Tapping", not "taping".

[bostonman]

Good call.... thanks!

[microwhat]

Looking back at the original post, the existing hole is 0.228" where we would expect 0.201"-0.206". The larger hole corresponds to the size required for a thread forming operation (see https://guhring.com/media/support/Tapping-Drill-Size-For-Thread-Forming.pdf page 131). You don't need to buy a thread forming tap for this: the supplied bolts are probably hard enough to be reused once or twice. Put some lubricating oil on the bolt (NOT cutting oil), hold it as coaxial to the hole as you can, and force it in. I'd expect better results driving the bolt with a drill or impact driver.

Tapping that hole with a cut tap will give you a very weak thread.

In general, taps and dies that are purchased in a set are the worst possible quality. Individual taps purchased from a big-box "hardware" store fit in the same category. They may or may not be high speed steel (AISI M2/M42), might be a high carbon steel, might be somewhat shiny cheese for all you know. They are mostly so-called "hand" taps, with the end of the flutes ground square. If they are coated, it's for marketing appeal, and not utility.

Strictly speaking, for casual use, the material of which the tap is made doesn't matter as much. You can't turn the tap by hand fast enough to get it hot. High speed steel (M2/M42) and the fancier alloys are necessary when power tapping, but often coincide with a better quality tap.

"Hand" or straight flute taps must be reversed every turn or so when tapping a new hole to break the chip as it's cut. The flutes are ground so that the chip curls up and will block the flute if it isn't cleared. I never use these taps.

Spiral point taps have a gash ground at the end of each flute that pushes the chip further down the hole as the tap is turned. You cannot drive this type of tap all the way to the bottom of a blind hole. Depending on hole depth, it is not necessary to reverse these taps when driving them. I would not hesitate to drive a spiral point tap to 2 diameters deep before retracting it. Further, I recommend completely withdrawing the tap from the hole rather than backing off half a turn and continuing: this gives you a chance to clean the swarf off the flutes and out of the hole, and to apply more cutting oil.

Spiral flute taps have helical flutes, like those of a twist drill. This type of tap can be driven all the way down a blind hole, provided that there is still cutting oil for the tap to work in, and provided that there is no trash at the bottom of the hole for the tap to hit. These taps are often weaker than spiral point or hand taps, and will more readily break if they are not started coaxial to the hole. Their main advantage is that they will pull the chips backward out of the hole as the tap is driven. Unless you have a real milling machine or at least a drill press, I don't recommend these for casual use.

It is critical that the tap is guided coaxial to the hole. Taps with 3 or 4 flutes will go in the direction they are started, but a 2 flute tap can be rocked around to a limited degree after it has begun to cut. If you don't get the tap started exactly straight, you can take advantage of this to fix it before you get too deep. Larger taps can be aligned by eye if you are willing to accept some inevitable misalignment. Small taps will probably snap inside the hole if you try this. It is best to drill the hole in a machine tool or drill press, and have the same machine tap it. Given an existing hole in a part made by someone else, if I were unable to set up the part in a mill, I'd hold the tap visually straight, and send it.

use a proper tap wrench to drive the tap. Power drills allow you no feel of the cut. The tap wrench balances the force applied with each hand, transmitting torque to the tap and reducing radial force which tends to break the tap. Never use an adjustable wrench for tapping: you're pushing sideways on a fragile cutting tool and it will almost certainly snap for you.

In my opinion, coatings are useless in casual use. Tools are coated to allow better wear resistance at higher surface speed. When driven by hand, you can't turn the tap fast enough to justify any coating. Bostonman, your yellow taps are probably coated with titanium nitride (TiN), one of the earliest, and today one of the least useful tool coatings. It doesn't offer any significant benefit over uncoated tools. It's chiefly used on consumer grade drills and such because it's cheap and makes the tool look pretty. If you're tapping aluminum, the vast majority of coatings will never help. Aluminum is surprisingly abrasive and will wear the coating right off the tool.

Cutting oil is important. Don't use motor oil, hydraulic oil, transmission fluid, or any other kind of lubrication oil. Pipe threading oil from any half-decent hardware store will work. If for some reason you can't get any industrial metalworking fluids, bacon fat will work. Don't be parsimonious with the oil: it's cheaper than breaking the tap.

[alpher]

I didn't read the whole thread but for 1/4"-20 tap you need to drill what's clearly marked a 7 gauge drillbit. I don't know where you getting 4 to 12 range?
For soft materials you may go for a little smaller bit, but certainly not 12, 8 to maybe 9.

[bostonman]

Looking back at the original post, the existing hole is 0.228" where we would expect 0.201"-0.206". The larger hole corresponds to the size required for a thread forming operation (see https://guhring.com/media/support/Tapping-Drill-Size-For-Thread-Forming.pdf page 131). You don't need to buy a thread forming tap for this: the supplied bolts are probably hard enough to be reused once or twice. Put some lubricating oil on the bolt (NOT cutting oil), hold it as coaxial to the hole as you can, and force it in. I'd expect better results driving the bolt with a drill or impact driver.

I didn't know about a "thread forming tap". I've heard of "chasing the threads", but never saw a 'thread chasing tap'. So I thought only two types of taps exist; and differnet materials.

To elaborate a bit. The same taps I use to break at my old job (the yellow ones that I thought were high quality Titanium) were the exact ones in my toolbox and the one that snapped this time (2-56). If you haven't read the entire thread, the 2-56 tap was being used in hopes of removing the old broken 1/2 - 20 bolt. I drilled a small hole, an easy out didn't work (although I didn't know how much force to use), so I attempted to thread it in hopes of putting a 2-56 bolt in and turning the 1/2 - 20 broken bolt out.

I got lazy and used transmission fluid because I have a junk bottle that I use to dunk parts in if I want to lubricate something quickly - so I don't care about metal chips falling in it. The bottle of Tap Magic was in my basement and I figured lubricate is lubricant; but I was wrong.

Per your statement, you're probably correct about the new bolts being a one or two time use. Unfortunately (and fortunately) the part didn't come with bolts.

At this point I had three options: order the bolts and risk them breaking again should I need to dismantle the snowblower in the future, thread the hole to use quality bolts, or use a bolt and nut (but wasn't sure if I have enough clearance with the auger turning).

The part that baffled me was the oversized hole - which you've now explained.

I didn't know a tap/die set is poor quality. I thought if you buy a name brand set that they'd be high quality. Also, I didn't know a drill bit/tap set such as ones sold at Lowe's were cheap. I expected them to be high quality since they are purchased as a single item meaning: people buy individual taps because they are such good quality that a complete set of them would be too expensive, so they are sold individually thus allowing people to afford high quality.

Does an order of tap quality exist and/or what do you recommend? If you're suggesting an entire set is low quality, then that means they need to be purchased individually.

Thankfully you're making me feel a bit better for breaking the tap because I thought it was a high quality Titanium tap broke that should take quite a bit of force to break.

If I remember tomorrow, I'll take a picture of the Titanium tap I broke and post it.

[alpher]

Titanium is just a bunch of BS.
Proper taps don't need any bs coating, but they cost money. Use only high sped steel taps for ordinary jobs and cobalt steel for these 304/410 jobs.

[bostonman]

This may be a tough question to answer, but how much force is enough to apply for a tap, die, and easy out, to prevent them from snapping?

Is it a matter of feeling?

When I watch videos of CNC machines, they spin at a high rate with lubricant spraying and don't need to back off periodically (or they are and the video is too fast to capture).

From those videos and reading, it seems taps are quite strong. From my personal experience, they are as delicate as a toothpick.

[CatalinaWOW]

They are quite strong, and also brittle.  You develop a feel for how they are cutting.  Read, reread and think about the advice above on alignment.  Cutting force goes up incredibly with misalignment.  It takes a very minor misalignment to make it impossible to finish tapping a deep hole without breaking a tap.

You might want to think about why the manufacturer used a self tapping screw in this application.  It might have been cost, but might also have been to limit motion between the bolt and the receiving part.  Either as a poor man's locking feature or for some functional reason.

[bostonman]

Just to give a visual, the location of the part is where I placed the red arrow and the part the three bolts screw into is in the other picture.

The cylindrical section goes inside the auger shaft so when the auger is turning, this piece keeps it in place.

[microwhat]

It looks like that connection isn't heavily loaded, so oversize holes being tapped shouldn't be a problem. If you want new screws, I suggest SAE grade 8, or alloy steel socket head cap screws (ASME B18.3 / ASTM A574). Grade 2 and grade 5 probably aren't hard enough. You're bolting through sheet metal, so the strength of the screw is irrelevant to that aspect of the joint.

Taps are quite strong provided they are only loaded in torsion. They will not tolerate significant bending force. I'm being deliberately nonspecific because it's hard to quantify exactly what a tap will tolerate. Learning how much torque to apply to a tap requires the experience of breaking a few. I can look at my spindle load meter, but that doesn't help you.

I won't try to convey the extra torque required to drive a misaligned tap. It's more obvious to feel each flute grab on the deep side of the cut, and then *snap*. A CNC mill such as you have seen in videos can hold the drill and tap dead nuts straight. There is no misalignment. Peck tapping (feeding the tap a small amount, retracting, feeding more etc) is a thing, but will be hard to see in most videos.

I recommend Guhring 4402 series taps: we use them at work in annealed S7 and H13 tool steels, among other materials. They will tap steel up to 32RC without quarrel, and can also be used for aluminum and plastics. They will probably tap harder steel, but we usually threadmill or EDM harder parts. They are real HSS, and have a steam oxide coating which may help to hold cutting oil on the tap's surface. This tap is on my reference list: https://www.mscdirect.com/product/details/42793935

There's a whole world of industrial tooling beyond what's sold at big box stores. Hardware stores sell taps to people who need to clean up one or two holes. Tap and die sets are either for maintenance, chasing existing threads but not cutting new ones, or people who don't know what they actually need. Just because $hardwareStore will sell you one tap at a time doesn't mean it's any good.

Lubricants don't all behave the same. I don't know the chemical reason, but lubricating oils resist metal to metal contact, and cutting oils promote it. Transmission fluid is closer to a lubricating than a cutting oil. WD-40 is neither a lubricant nor a cutting oil.

[bostonman]

Attached are pictures of the same brand tap I broke.

[microwhat]

As far as I can tell, that tap might have been made by Widia (not trash), but the markings don't really narrow it down. Do you know who made it, or can you get a picture of the package it came in?

Regardless, a #2 tap will absolutely break if it's not dead nuts coaxial with the hole, or if the hole is curved (drills are flexible and can walk, leaving a curved hole), or if there was trash in the hole before you presented the tap. What kind of wrench did you use to drive the tap?

[bostonman]

Good call, forgot I had the package it came in.