Tap Drill Bit Size?

[Swamp Buck]

I had been watching the Ron Ehlert muzzleloader building DVD and he explained that even though the recommended drill bit size for an 8-32 tap is #29, he prefers and recommends the #28.

Does anyone know why - he doesn't say why in the video.

 

[ricklandes]

Threads deal with major and minor diameters. Major the outside points, if you will and the minor the inside bottom groove. He may feel that the slightly undersized bit allows a better thread in the end as the tap will have to cut completely cut in the thread to both major and minor diameters rather than to just cut the thread to the to the drill diameter. In effect you will end up slighlty enlarging the hole with the smaller bit hole being tapped as to just cutting in the threads with the larger version.

I hope that makes sense. Often there are several bits, fractional, letter or number that will do the job...often dependent on the end use of the threads IMHO...

 

[Jim-Iowa]

There are many thread size charts available on the internet.One in particular, and (I don't have my copy available here, its in the shop and I'm at work) lists something like 50% & 70% thread pilot hole sizes.Often it lists a fractional size for 50% and number for 70%. Using a slightly smaller pilot hole would give you slightly deeper and stronger threads. A 20%increase would be a significant improvement, especially if those threads are under a load. So I'm assuming that is his reasoning. That much tighter might take more care in taping(to prevent tap breakage), but could be worth the extra effort. I'm sure one of or engineers can and are welcome to correct me if this is incorrect.

 

[paulvallandigham]

I think both Rick and Jim have it right. By going to the smaller pilot hole, you put more work load on your tap( so be very careful as you cut those threads), but you cut a deeper thread, which therefore can take a greater load( pressure).

Being careful usually means tapping the hole by hand, and doing at MOST, a half turn, then backing it out 1/4 turn, then going another half. On some hard steels, I have reduced that movement by half, so I was cutting 1/4 turn, backing out 1/8 turn to break the chips, then going on 1/4 turn. Clear the chips frequently by backing out the tap and brushing it clean. Then oil the tap and the existing threads in the hole to guide the clean tap back into the hole. If you have ever seen a commercial production line, using constant flow, water soluable oil to cool their taps, you begin to understand the need to constantly put oil on your tap flutes as you cut threads. When you use the undersized drill size, you need to use oil even more.

Cutting metal generates a lot of heat, and heat is bad for sharp edges. More speed generates more heat, but also increases the risk of breaking a tap off in the hole when you hit a void, or worse, and vug( hard spot consisting of some foreign material) in the steel. Although taps are cheap today, due to mass production, they are fine edged tools, and should be handled and treated with great care to make sure they last, and complete the job.

You can sometimes succeed in cutting threads fast, but the price usually paid is a thread that has cracks, or tears in the teeth. In a shop class in High school, my instructor insisted on threading a piece using a lathe, but he had it set to turn too fast. The threads looked and worked terribly, but I was stuck with them. Later we did a second project, and I tapped those threads myself, and by hand. I took my time, and the threads came out right, smooth, unbroken, or cracked, and you felt like you were putting a screw in a swiss watch by comparison to that job done by my instructor. Just because some guy is hired to teach doesn't mean he knows what he's doing. :hmm: I learned to use dies and taps from my father, and when Dad saw the screw that my instructor cut, he was more than upset. He also knew instantly what the guy had done wrong. Dad had worked in plants in the 1930s, fixing machines, and repairing electric motors, and had tapped thousands of new holes, or threaded stock to mare replacement screws, and bolts.

 

[Vaino]

Sometimes the drilled hole can be quite a bit oversized than the drill when the drill has been incorrectly sharpened. This can be corrected to some extent by first using a small drill that's slightly larger than the drill "web". Fine threads on #6,#8 and #10 sizes are shallow and the tap drill hole is critcal and using a smaller drill ensures a deeper thread......Fred

 

[Dalekg6]

UHH,guys,

A #28 (.1405) drill is larger than a #29 (.1360).

Sometimes I also like to use a slightly smaller drill than called for but it is harder to get the tap started and sometimes it's too hard to turn. Perhaps he recommended a larger size so things would go in smoothly and straight with less struggle.

Dale

 

[mazo kid]

Dale, you beat me to the punch. Yes, the #28 bit IS larger than the #29 which is the usual bit specified for an 8-32 tap. I'm wondering if he mistakenly said #28 when he really meant #30, that drill is smaller and thus would result in a tighter fit? If making your own screws, you can open the die up a little and make very snug fitting threads on the screw itself. Emery

 

[Vaino]

You're correct asre the #28 vs the #29....getting old I guess.I still think a slightly undersize hole is preferable. When tapping, I use the drill press for a perpendicular start. Align the hole and tap, oil the tap, turn the motor on and then off and while the spindle is coasting, engage the hole and when stopped, un-chuck the tap The few turns that the tap has cut ensures that the tap and hole are in alignment and the remainder of the hole is tapped in the vise......Fred

 

[Old40Rod]

I use the drill press to get things started right on tapping the lock plate for bolts, but I NEVER turn it on.... just turn it by hand. In fact, I unplug it so I won't be tempted :grin:

 

[Zonie]

IMO, the small threads like a #6 and #8 both have a bad habit of breaking taps.
By going with a slightly larger tap drill, there is less material for the tap to remove so there is less likelihood of breaking the tap.

With a #8 tap, going with the larger tap drill only removes .0045 more material.
As the others have mentioned, that makes the thread somewhat weaker however this is usually not a problem as these smaller screws are not loaded very high.

As a side note, there is another class of threads which are used extensively in the aircraft industry. These are the UNJ series of threads and they are made for high stressed critical applications.
The male thread in this series has a controlled root radius which is larger than a standard UN thread to lessen the chance of a stress related failure.
Because of this larger root radius the female thread must have a slightly larger minor (tap) diameter than the standard UN series threads do.

The minor diameter for a #8-32UNJ-3B thread is .1336-.1417. You may notice that the maximum size is very slightly (.0002) larger than the #28 drill.
This tells me that the slight loss of material in the minor diameter caused by using the slightly larger drill bit should not be of great concern to the average gun builder.