View Full Version : Small Fishtail?
I have the occasion to cut some relatively small internal threads, i.e. 5/16-32, so I decided to rifle through the old tool bits of my former lathe owner, and sure 'nuff I found some pre-fabricated cutters....one left and one right. They look OK, maybe needing a little cleanup, but then it dawned on me....how do you get a fishtail gauge on something that small??? How did he cut them to begin with?? Do they make a miniature version?? I'm missing something, I guess.
01-23-2011, 07:24 PM
Internal threading is painful enough, why would you wanna do micro internal threading? Use a tap :)
As far as your real question.. I have no clue. maybe you make your own fishtail guage, Or use a real thread guage as a fishtail gauge.
Something like one of these might work: http://www.dansmc.com/threadgauge.jpg
01-23-2011, 07:30 PM
What about cutting down one of the cheap fishtails? Definetly wouldnt want to cut B&S or other similar ones, but one of those $2-$3 ones.
01-23-2011, 07:33 PM
The previous owner may have used a projection comparator/shadow gragh to check the angle and centrality.
01-23-2011, 07:36 PM
The way I would do it is grind it on the surface or tool grinder and check it with the optical comparator. However, most HSMers don't have those choices. It looks like those were ground freehand. You can get small optical measuring loupes that have interchangeable scales to measure length, angles and diameters. They are a small round magnifying glass about 1-1/2" diam. and 2" tall. You hold the magnifier with the graduations over the part to compare the part or feature to the marks on the glass.
All I need to make is a one shot fixture, and naturally, I don't seem to have a tap, or anything close. But that problem is really secondary to my warped thought process, which is, "How did he do it to begin with?". He may have just freehanded it, but if so, it looks pretty good. I'd just like to know how to clean up what he has already created, and of course, how to do future tools, if necessary.
Black Moons: A thread gauge seems to be a hell of an idea. Never dreamed of that one.
01-23-2011, 07:49 PM
I would use a tap for smaller holes unless you are making a very odd thread count. You also have to worry about flex in the amount of overhang.
01-23-2011, 09:18 PM
My fishtail has a couple of notches for different size cutters. You could also simply cut an external thread on a bar to the pitch you need, using a very sharp bit, and use the bar for your gage. :D
01-24-2011, 01:32 AM
Put a (near) matching pitch thread/screw/bolt in the lathe. Put the internal threading tool on the back side of the screw etc. and visually align the tool to the thread (use the screw as a "fish-tail).
01-24-2011, 07:51 AM
As far as lining up the tool so that it is square with the work in the lathe. Use a boring bar and the compound to cut a 30 degree taper on the face of the bore. You need a chamfer anyway. Then put the threading tool in the lathe and align the leading edge of the cutter to the chamfer you just cut. Should be dead on.
01-24-2011, 07:59 AM
You don't align the cutter on the inside, you align it on the outside. If the shaft sticks out far enough put the fishtail on the side of the shaft and align the cutter in the correct size notch. If you can't use the side or the end of the shaft then align it on the face of the chuck.
EDIT: one of the things you have to do was check the end of the fishtail with a square to be sure it is square to the sides.
Great ideas. I knew I'd have to align the tool to the outside, but I had not given that a lot of thought yet, as I was still stuck on the initial cutting of the tool. I can certainly see how a bolt would work, as well as a chamfer (assuming the tool is cut correctly to begin with). On that note, I can also see that Black Moons' suggestion of a thread gauge on the outside of the stock should also work.....yes, no?
I was leaning in the direction of the thread gauge, as I could use a coarser thread to get a better visual. Trying to align that small wedge on a 32 TPI can be a bit of a challenge for these old, tired eyes.
01-24-2011, 10:11 AM
I bought a 10X loupe with 360 degree graduations for getting those angles right on. It helps to check you've ground the sides straight too for a novice like me.
Thanks for the all the help. It works!! Probably not the best looking set of threads I've ever cut, but internal threading on a 5/16" hole is no picnic. Thread depth on the compound should have been .023", but I wound up closer to .028. My limited threading experience tells me that this frequently occurs when you don't quite have your angle to the dangle correct. About the best I could do trying to hold a thread gauge on the outside of the stock and maneuver everything else in place....all while trying to keep your magnifying headset out of the way.
The important thing is......it works.
01-24-2011, 12:35 PM
Unless your doing this just for the experience it's a waste of time. You can get much better results with a tap. The only time to single point a thread like that is if it is a odd pitch.
01-24-2011, 12:53 PM
I'm missing something, I guess.
not really, but what you're concerned about just doesn't make to the top of pile very often:
- 99.99% of the time small internal threads are going to be cut by a tap, no issue there....and if they're really odd ball its sometimes easier to make the tap than cutting small internal threads (ie less than 1/4)
- When you do have turn them, and it happens when they're odd sized or multi start, you can make the V perfect to the body of the tool - method 1) turn a 60 degree profile on the end of some drill rod, file, mill, etc away everything you don't need, harden and stone....OR....do it on a T&CG or a fixture on the surface grinder so the 60 is perfect to the body. Use the body to align it rather than the V.
OR, if that's too fussy....use a free hand ground one and squint; in many cases it won't matter if the profile is perfect so long as the parts screw together.
there's always lots of rough and ready ways...grinding that tiny profile by hand and aligning with a fishtail is one of them, if the job demands exact I think you have abandon those and go with one of the above approaches. btw, you don't need the bit to go in the fishtail's V, with the fishtail against the end of the work or chuck just align one surface of the bit's V with the angle ends of the fishtail
01-24-2011, 01:59 PM
It Works!!! Fantastic! Looks like a real bugger to do. Great Fred
I never really thought about using drill rod at 60º, but it sounds reasonable. I assume, though, that you would have to have a flat, true surface adjacent to the point that you are trying to cut. That rarely happens on my grinder, especially when you're having to remove so much of the square stock to get that tiny point. That's why I was curious about a "V" device like the fishtail, only considerably smaller.
I wonder if I could take a small piece of flat stock, mount it in the mill vise at 30º, and take a small nick on one end with an end mill?
Carld: I agree...a tap is the way to go, but since I live in an area where such oddball taps have to be ordered online, and since I only needed one 5/16" -32 hole about 3/8" deep, single point is the quicker alternative for a one-time fixture, although I suppose a little experience comes along with the venture, nonetheless.
01-24-2011, 05:24 PM
I never really thought about using drill rod at 60º, but it sounds reasonable. I assume, though, that you would have to have a flat, true surface adjacent to the point that you are trying to cut.
The idea is you turn a 60 degree cross section on the end of a piece drill rod using the compound to get the two 30 degree sides. you mill/fill away all that you don't need including clearance on the one side. you remove material such that you expose the perfect 60 degree V. maybe this ascii pic will work....this is what you have after turning and before milling/filling. harden then stone, no grinding involved.
after turning, then remove material to get.....
Now that makes more sense. I thought you were having me build a gauge to check the angle I was grinding on the square stock. I do have a surface grinder available, so I'm going to have to think about that. Nice idea.