Gear cutting hob

I need to make a 32t helical gear to mate with my 8TPI acme leadscrew. I ha
ve a couple of questions for those here who know way more than I do.
I need to mill slots into my 8TPI acme blank. It's just an old Sheldon lead
screw that fits nothing I have. I figure I'll cut 4 or 6 groves on my mill.
My question is where should the leading edge be in relation to the center
line of the blank. My gut tells me that the cutting edge should on the cent
er line so the cutting edge is 90 degrees. If the center of the slot is cen
tered instead the cutting edge will not be at a 90 degree angle. Does this
matter?
Second, the diameter of the blank. The circumference of 32 teeth divided by
Pi gives me a dia of 1.27. Following a different method, adding three teet
h divided by DP of 25.13 gives me a diameter of 1.39
Color me confused. Can anyone here answer these questions please?
BTW the gear will be brass. I've made a jig to hold the blank in my tool po
st and hob will be in the lathe chuck. I intend to run the lathe as slow as
I can and feed the gear blank into the hob slowly until I get enough teeth
to operate my thread dial. I may do it in my mill, holding the blank in my
vice and hob in my collet holder, depending on which one rotates the slowe
st. Thanks in advance for any feedback/suggestions
Reply to
Gerry
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Complicated subject. You've got more questions you don't even know to ask yet.
Get the bible on gear cutting:
"Gears and Gear Cutting" by Ivan Law
I've seen a couple good You tube videos also.
Reply to
Karl Townsend
It doesn't absolutely have to be cut helical . Since there is a very light load , you can make the teeth a quarter inch wide and just cut them straight .
Reply to
Terry Coombs
I can second that . I bought the book and learned to cut gears for the lathe QCGB . You want workshop series number 17 .
Reply to
Terry Coombs
Karl is correct about Ivan Law's book. But if this gear is for a threading dial than you don't really need a gear that has traditional teeth. Just properly shaped pins would work because there is almost no load on the gear. It is after all just an indicator. Unless you cut a worm gear there will only be a line contact between the gear and the worm. The proper hob for the worm gear will resemble the worm, in your case the leadscrew. I have cut gears successfully using the info from Mr. Law's book. The book isn't very long but is packed with easy to understand information. It even has plans for making a tool for making gear cutters. The tool has a couple eccentrics on it that enable a 12 tooth gear cutter to be cut in the lathe because each tooth is backed off as the tool rotates. So there is constant relief for each tooth while at the same time the tooth profile doesn't change and sharpening only requires grinding the top of each tooth. Eric
Reply to
etpm
Don't you mean to sharpen you just grind the FACE of each tooth ?
Reply to
Terry Coombs
Thanks for the comments, All. Problem is I do not have the equipment to cut gears of any type. I am trying to cut one or two helical gears from brass or plastic. I don't intend on making other gears so I do not want to spend a bunch of money on an indexing table. I have tried to contact two gear man ufacturers but thus far they have not responded to my inquiries. Also, I ha ve been unable to locate another lathe that uses a 7/8"X8TPI leadscrew and a 32 tooth thread dial gear. This is why I asked two specific questions whi ch so far I have been unable to find the answers for. I feel certain that s omeone here, perhaps a lot of people here can provide the answers. Thanks a gain, Gerry
Reply to
Gerry
Gerry, as many of us have pointed out, this job doesn't require anything more than the crudest "gear." How adventerous are you about doing some experimenting? Especially if you make it out of plastic, you can produce a fine gear on your lathe with an 8 tpi bottoming tap and a jury-rigged "spindle" on your compound. I've made them out of steel, in much finer pitches, but this one should be no problem.
The hard part is getting the right number of teeth. That's because the method, which many here could explain, is based on letting the tap (which is held in a collet in your lathe, if you have the right size, or which you can even hold in a chuck if you have to) free-feed itself into the gear blank, and the number of teeth depends on how it feeds into the blank. But you can get really nice teeth.
That means you might have to make a few tries before you get it right. But, hey, you'r cutting plastic. If you like, after getting it right in plastic, turn a brass blank to the same dimensions and try it in that.
This is about the easiest kind of gear you could make. Give it a try. You may amaze yourself. d8-)
Reply to
Ed Huntress
have a couple of questions for those here who know way more than I do.
adscrew that fits nothing I have. I figure I'll cut 4 or 6 groves on my mil l. My question is where should the leading edge be in relation to the cente r line of the blank. My gut tells me that the cutting edge should on the ce nter line so the cutting edge is 90 degrees. If the center of the slot is c entered instead the cutting edge will not be at a 90 degree angle. Does thi s matter?
Probably makes no difference. If the cutting edge is on the centerline , t hat would be good. If the cutting edge is a little off , it should be so t he tooth are a bit sharper.
by Pi gives me a dia of 1.27. Following a different method, adding three te eth divided by DP of 25.13 gives me a diameter of 1.39
Is the larger diameter for the OD and the smaller diameter is based on the diameter being half way up the tooth? That is the 1.39 is the diameter of the blank. And the 1.27 being the diameter of the contact with the worm.
I like UHMW poly. It wears well and is easy to cut.
I've made a jig to hold the blank in my tool post and hob will be in the la the chuck. I intend to run the lathe as slow as I can and feed the gear bla nk into the hob slowly until I get enough teeth to operate my thread dial. I may do it in my mill, holding the blank in my vice and hob in my collet h older, depending on which one rotates the slowest. Thanks in advance for an y feedback/suggestions
If you wrap a piece of paper around the black and cut it so it makes exactl y one wrap. You can then mark it so you have 32 divisions, and use that t o gash the blank so you do not have the problem of having one tooth too man y or too few. See Ed's comment on self feeding.
Dan
Reply to
dcaster
Or -- you can cut them straight and wider, and mount the gear's arbor at an angle matching the angle of the thread on the leadscrew. A lot easier to do than cutting helical teeth -- especially without a universal lathe (one whose table can be set to angles other than 90 degrees to the Y-axis.)
Good Luck, DoN.
Reply to
DoN. Nichols
It depends on how you look at it. The top or face being the same surface, which is essentially flat, as opposed to the rest of the cutter which has a gear tooth contour. However, you are technically correct and I should use the correct terminology. Eric
Reply to
etpm

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