Thread restoration

[bmh]

So has everyone had a chance to read our new technical editors column in the latest issue? I have never met Red, but I do have issue with some of his recommendations on thread restoration in the last paragraphs of his article. Personally I have never "augered" a tap into a hole, whether it had been previously threaded or was about to be threaded for the first time. That would be improper use of a expensive precision cutting tool. I understand some have a preference for using a thread chaser for restoring threads rather than a tap (or die as the case may be) because the dirt and debris from used parts will dull your taps and could possibly even break a tooth. But as far as I can tell both will serve the same function. A tap will not cut metal from an already tapped hole every time it is inserted as Red states. Unless of course the threads in that particular area are deformed and would interfere with the engagement of the fasteners threads (or they were set up as an interference fit in the first place). And that would be why the threads were in need of restoration in the first place. A thread chaser would remove metal in these same areas to restore the clearance needed for the bolt to pass.

As far as cutting slits in a bolt and using it for a chase? Careful, if the threads already have the tendency to cross up you can wind up destroying whats left in quick order. Spend the money you will spend on Helicoils after on some proper tools beforehand. You can by a cheap tap set at Harbor Freight for very little. A tap has a good leed on it to help prevent this and a chase should actually follow the threads that have already been restored. That is not to say I am arguing against the purchase of things like thread files and chases and such. They are very useful additions to the toolbox. I've had this debate with others before and likely will again.

But I suppose the part I find the most disturbing is the suggestion that it is just fine to use valve grinding compound to restore threads. I really can't think of a better way to prematurely wear out and otherwise destroy a threaded engagement than to apply lapping compound to some contraption made out of a most likely used and beat up bolt and work it up & down the hole. This would be the indiscriminate grinding of all the threads, the good as well as the bad with absolutely no precision or control. Talk about a sloppy fit that won't hold very much torque. Not sounding like the type of person I'd want to build my motor.

[Ohio-Rider]

Based on forty years of machining experience I can assure you that every time you run a tap back thru a threaded hole it will remove/cut a tiny amount more metal until the thread is so oversized that there in nothing left for it to cut. By then your thread will have lost its intended integrity. What happens is all the crud in the threads pushes the tap around in the hole as we turn it in the hole the tap is pushed/wiggled around and cuts anything it touches making it over size.

There are a lot of factors as to just how fast this will happen though. Factors such as the type of metal or the method used for running the tap in and even what the pitch of the thread is. Re-chasing a thread by hand using a wrench is the worst way and guaranteed to remove metal. If it must be done by hand use a T handle. A better way is to chuck your tap up in a descent drill press and spin the spindle by hand into the hole. Even a sloppy drill press will do less damage to the treads than our shaky hands.

One last bit of advise for tapping a fresh hole or cleaning up an old one. Once the tap has a hold of the metal, do not keep turning it in and then backing it out. Keep turning the tap to the bottom in one smooth motion. There is no advantage of reversing direction to break the chip. All that will do is give you a freshly tapped hole that is already oversize. Sorry for being long winded -Steve

[T. Cotten]

Thread-rolling taps have their place, too!

....Cotten

[fabercycle]

There is always more than one way to "skin a cat".

I can tell you, from working 25 years in automotive Tool & Die, that I had many machinists that worked for me that should have been selling used cars, or digging a ditch.......but yet they continued on somehow.

The problem with talking about how to do something on these forums is, opinions are like a-holes, everyone has one. No matter how qualified you are in manufacturing or repair, you'll always have a plumber or an accountant, that knows how to do your job better than you.

Sometimes big boy tools should be kept out of the wrong hands.

[Ohio-Rider]

It a simple discussion Tom, not a pissing contest. That’s what makes this site different. Lots of different people just kicking around some thoughts in a polite manner. Its better than discussing the census.
-Steve

[Slojo]

Tap selection is important. I will not use cheap taps they tend to break or tear metal instead of having a clean cutting action. A spiral flute tap (looks like a drill bit with its spiral flute design) is the tap of choice in a bottoming hole condition because it draws the chips out of the taped hole instead of letting them bind in the flutes or ahead of the tap itself.
A spiral point tap (the flutes are ground straight and the leading cutting edge is ground in a left hand angular slant or negative leading angle) is best for through holes conditions it pushes the chips ahead of the tap and out the hole.
Thread forming taps and dies are an option except thread pitch availability and costs are issues. These devices are able to push or form a thread without cutting material, they have an out of round shape that allows lubricants to enter ahead of the radiused forming portion of the forming device weather it is a tap or die.
Many of our old machines are not standardized with SAE or USS thread pitch configurations so availability is a concern. Always generously use a good tapping fluid.
As for damaged threads re-tapping (cutting) is my last choice. The threaded part (inside or outside thread) can be repaired by pushing or forcing the damaged material back to its original location. This can be achieved by using an antiseize compound and a good mating thread device. It is best to use a device that is harder than the thread to be repaired such as a fresh or new grade eight bolt to reform a common material threaded hole. Clean both parts thoroughly then apply the anti seize compound to both threads and carefully thread them together. Minimal force may be required, when this happens I take a piece of aluminum rod and a ball peen hammer and lightly tap the item to be joined together. This pushes the defective threaded area away and allows the parts to be screwed together further where the process will need to be repeated until all damaged areas work together again. You are reforming the thread and no material is removed in this process. I have been doing this for many years.
When rebuilding close tolerance threaded devices such as ball screws and spindles the threads on these large heavy precision devices some times will become damaged by slight nicks in them that are not easily detectable by the eyes. The threads are so close fitting that they will weld together by forcing them with out lubrication to protect them. Always use caution when dealing with damaged threads a little patience will reward you with a successful repair without a trip to the welder or helicoil distributor. Good Luck
Joe

[T. Cotten]

Well, You guys made me stop and read it.

Having previously proof-read some of RF's prose for another web forum, I must understand him better: I read the word "auger" to be used to emphasize that it was not a desireable thing to do.
Read it again, and think sarcasm.

The rest of the paragraph goes in a circle to say that ingenuity is a virtue. And indeed, the loss of self-reliance and resourcefullness of the average rider is, in itself, a great burden upon vintage motorcycling today.
So kudos to TechCzar RF for attempting to address it, but the valve grinding compound idea should have been edited, lest it be taken seriously!

....Cotten

[Paps]

All of the replies have their merits. Regarding valve grinding compounds....compound is to used when threads are to close to one another. By this...I mean...mated near a zero clearance tolerance, when one needs a thread to fit smoothly, yet freely. Zero tolerance threads have their own application but most motorcyle applications will not call for them. Taps and dies can create zero tolerances. WHY ? taps and dies have tolerances as well. There are under and over sized taps and dies which are not specified on any label when one purchases a tap or a die. Many machine shops have taps re ground. Re grinding reduces tap size. Feel is vastly important when repairing a thread with a tap or die. Experience pays off as well. Repair of threads will always depend on thread application and tolerance of mating threads needed. Paps

[Slojo]

Taps come labeled with a GH or GL value (GL is uncommon). A GH 3 would stand for a Ground High tap GH3 = +.0015" GH5 would equal +.0025 a greater clearance. A GL 3 would correspond to a ground undersize - .0015, each incremental value is equivalent to .0005" increments over nominal pitch diameter measured over thread wires. Common over the counter taps are typically GH3
Joe

[Paps]

Taps come labeled with a GH or GL value (GL is uncommon). A GH 3 would stand for a Ground High tap GH3 = +.0015" GH5 would equal +.0025 a greater clearance. A GL 3 would correspond to a ground undersize - .0015, each incremental value is equivalent to .0005" increments over nominal pitch diameter measured over thread wires. Common over the counter taps are typically GH3
Joe

Yes...when purchased as new. When re ground, no labeling. You mentioned the nominal pitch diameter over wires. This is good that you brought this up. Such measuremnet is the correct and only accurate means of measuring a thread, minus the use of thread mics. To many people measure the thread major diameter when determining the correct tap size. Thread profile will not show up measuring this way.

[Slojo]

Yesterday while working on my 1914 Cannonballer bike I ran into a thread problem. The front axle is a non-standard 7/16X24 thread. The axle was a long axle for a latter fork than the 23 fork I am using. I cut the axle to length and then single point threaded the axle in the Monarch lathe. when attempting to install the nut on the new thread resistance was found about half way through the engagement of these two parts on both ends of the axle. Knowing the thread I just cut on the axle was of proper pitch diameter, the nut was determined defective. These castle nuts are harder than the axle so using the axle to "push the threads around" was not an option. A call to MSC landed a purchase of a 7/16X24 tap. Next day UPS delivery of the tap allowed this nut to be repaired without removing excess material. A perfect fit between the axle and the nut, problem solved. In some cases a re-thread with a tap is necessary.
Joe

[fillibuster]

Well, You guys made me stop and read it.

Having previously proof-read some of RF's prose for another web forum, I must understand him better: I read the word "auger" to be used to emphasize that it was not a desireable thing to do.
Read it again, and think sarcasm.

The rest of the paragraph goes in a circle to say that ingenuity is a virtue. And indeed, the loss of self-reliance and resourcefullness of the average rider is, in itself, a great burden upon vintage motorcycling today.
So kudos to TechCzar RF for attempting to address it, but the valve grinding compound idea should have been edited, lest it be taken seriously!

....Cotten

ditto, cotton! RESOURCEFULNESS being the key word. I've use most, if not all methods mentioned, including valve grinding compound, depending on availability. On the side of the road, you know, barbed wire may be the cure!