Need help with threading

[haggard616]

Me again. I've really gained some traction on getting my lathe together with all of the advice from the members here. So I would first like to say thanks!
I have the odd ball 8" SB 408-Y. I have not yet disassembled the cross slide to see what I would be getting into to upgrade to large dials- but that's definitely on the list.. As long as there's no milling operations required.
Finally, my question- now that I have my change gears all ready and I've been doing a little threading, I've been wondering what's the formula and if someone can explain it for me, pertaining to the major and minor diameters. For example, the project I have in mind is, I want to make knobs to replace the nuts that hold my change gears in place. The change gears use three different sized wrenches to change the gears. It would make life much simpler to have knobs so I could easily change the gears quickly by hand. I can measure the major diameter of the threads on the end of the gear shafts but don't exactly know how to find the diameter I should bore the center of the knobs in order to thread them? Hopefully the shafts aren't some strange diameter/thread combo and I'll be able to just use a tap for the knobs. Any and all help is definitely appreciated guys. I've made it a good ways so far with all the help.

[dly31]

There are formulas but what is easiest is a "tap drill chart" giving recommended drills for standard taps. The recommendations will vary somewhat depending on type of material, percentage of thread desired, and whether number and letter drills are to be considered. Just do an internet search for "tap drill chart" and take your pick.

Another method is to just see what size drill fits nicely inside the nut you take off.

[Harold_V]

The diameter you seek is the very information typically provided by those small plastic tap and drill charts. You can also find that information inside many of the drill indexes. You'll see that, under specific sizes, they list tap drill diameter and clearance drill diameters. For small diameter threads, tapping is the accepted method, as the geometry of threads makes it very difficult to produce small diameter inside threads by the single point method.

Do you own a copy of the machinist's bible (Machinery's Handbook)? If not, get one. I have no clue what edition is current, but mine was purchased new, way back in the late 50's, and is the 15th edition. In it there is a wealth of information that will help you to understand manual machining, materials and processes, plus there are extensive charts that provide all manner of specifications, including those for threads.

I don't really recommend the current editions, as the basic information hasn't really changed, although they now included CNC operations, which would have little to do with the machining you most likely will deal with. So then, buying an old used copy is more than acceptable. With luck, you can land one for less than $25.

Harold

[Hopefuldave]

+1 on Machinery's Handbook, mine is a little older at 13th edition, but covers all I've needed so far, thread forms, tapers, gear profiles etc. haven't changed since WW2!

[earlgo]

The later editions of the MH have metric information that the earlier ones don't. I can't tell you when it was added but it is not in the 18th ed.

Whilst you are rummaging around in the old book stalls, you might try to find a copy of South Bend's "How to Run a Lathe". The last edition, #56, was printed in 1966. Might be available on-line as a freeby, but I haven't looked.

--earlgo

[mklotz]

The general formula for tap drill size is:

TD = MD - 0.013*DOT/TPI

where:

TD = tap drill size
MD = major diameter
DOT = depth of thread expressed as percentage
TPI = thread pitch expressed as threads per inch

As an example, consider the common 1/4-20 thread with 75% DOT desired. Then

TD = 0.25 - 0.013*75/20 = 0.25 - 0.04875 = 0.20125

Examination of a numbered drill chart will show that the closest drill to that size is a #7 so you would use that drill as the tap drill.

You may encounter references which ignore the explicit use of DOT and show the formula as the somewhat simpler:

TD = MD - 1/TPI

Effectively, this formula sets:

0.013*DOT = 1

or, equivalently,

DOT = 1/0.013 = 77%

which is fine for many materials. However, tough to thread materials are best tapped with lesser DOT to minimize tapping effort and risk of tap breakage. Here's a chart from the DRILL program on my website which shows some typical recommended DOTs.

Code: Select all

MATERIAL                                          % DOT

MILD AND UNTREATED STEELS                         60-65
HIGH CARBON STEEL                                 50
HIGH SPEED STEEL                                  55
STAINLESS STEEL                                   50
FREE CUTTING STAINLESS STEEL                      60
CAST IRON                                         70-75
WROUGHT ALUMINUM                                  65
CAST ALUMINUM                                     75
WROUGHT COPPER                                    60
FREE CUTTING YELLOW BRASS                         70
DRAWN BRASS                                       65
MANGANESE BRONZE                                  55
MONEL METAL                                       55-60
NICKEL SILVER (GERMAN SILVER)                     50-60

[haggard616]

Good stuff mklotz! Thank you very much. I'll copy all that info down and put it into practice.
As for the other suggestions, I do have a drill/tapping chart.. I also wanted to learn the formula to be able to do the calculations without always needing a chart nearby.
I also have a digital copy of South Bends books. Those are a good read. I'm looking for a copy of machinerys handbook or American machinists handbook, I know they can be had on eBay, but I'm bound to find one at an estate sale or the like for pennies eventually.

[haggard616]

Oh, and as far as the tapping chart, I have an old stainless steel Craftsman one like I said.. But what if the diameter of the shaft isn't standard to the thread it has? For example, what if the shaft were to be 1/4", and the thread 16tpi. I doubt my gear shafts are something goofy like that, but I'm sure I'll eventually be faced with strange parts that the chart won't be able to help me with.

[spro]

It really doesn't hurt to buy one when it isn't, "pennies". Don't know about where you are but many of the old shops closed long ago. You/I could go to a plethora of sales with ONE item in mind. That would be the day NONE of them are around. I would always buy something that I could use to make it a worthwhile trip. It could have been the neatest stand or book or part which fits. An entire box of connecters or great coat. Some thing that someone I know could use.
It has happened that a brother or friend reciprocates by being at the most out-of-way place and finds something for me.

[Harold_V]

Oh, and as far as the tapping chart, I have an old stainless steel Craftsman one like I said.. But what if the diameter of the shaft isn't standard to the thread it has? For example, what if the shaft were to be 1/4", and the thread 16tpi.

The plastic charts typically cover only NC and NF threads, along with pipe threads. If you encounter bastard threads, the charts in Machinery's Handbook will cover many of them, although not all. In such a case, Marv's information can prove to be invaluable, although if you happen across a set of the H28 manuals, published by the government, you'll find all of the formulas for solving the unknowns.

With rare exception, the vast majority of threads you'll encounter, especially if the product is made in modern times, will usually be the accepted standard threads (NC or NF), although one finds metric as well, but bastard threads aren't common. The exception can be manufacturers who hope to hold the consumer hostage and use bastard thread sizes so the consumer must turn to the maker for repair parts. In such a case, my advice is to avoid buying their products, as the parts can prove to be unnecessarily costly. I take exception to those who willingly rape their customers. Sort of brings to mind the $3 aspirin dispensed to patients in a hospital.

Harold

[spro]

It is still a 3.00 pill. It becomes a 6.00 pill when everything becomes involved . Everyone else has to pay for that $6.00 pill. That is the way it works.

[ronm]

The Atlas lathe manual has a chart called Double Depth of Thread, I'm sure the MH has it too, but I've used the Atlas more...shows the infeed needed to make a complete thread for all 60 * pitches, regardless of diameter.