Help cutting a gear

[lugnut]

OK so I have a 4'" rotary table with a 14 and 28 hole dividing plate. My question is,,,,, how do I cut a 50 tooth gear. My chart with the RT skips from 48 tooth to 54 tooth. I'm old and dumb and can't figure this out? I sure don't want to buy a new dividing plate for 1 stupid gear, so is there anything close? 😐

[Randy]

Assuming your RT has a 36:1 ratio (most of the four-inchers I see on Amazon do) each step is 36/50 of a crank turn. That simplifies to 18/25 and no further, so you need a 25 hole dividing plate. Without that you'll have to do it by angles rather than with a dividing plate. Tedious and error prone, but do-able. Prepare a table of angles at 7.2° steps, and do a dry run marking with a sharpie before cutting.

[lugnut]

My plate is a 72 to 1,, I hope this helps.

[BCRider]

I think that's a classic case of "you can't get there from here....". Also with gears "close enough" is never "close enough". It's either 50 or it's some other number.

It's also odd that you only have 14 and 28 holes on the plate. Why not JUST a 28 hole circle since you can get the 14 out of the 28.

The plate you need would depend on the ratio of the hand wheel worm and from there you could find out how many holes you need in the new plate.

For example my own RT has a 90:1 ratio which is 4° per turn of the hand wheel. 50 teeth means 360/50= 7.2° per index. In MY case that would mean one full turn of 4° plus 3.2/4= 0.8 of a turn more. And that looks like it would be 16 holes around a 20 hole ring. Alternatives would be 8 holes on a 10 hole, 24 holes on a 30 hole. 32 holes on a 40 hole. Or some other where it works out to an integral answer.

In your case if you have the same 90:1 ratio 0.8 x 28=22.4 which is not an integral number. You would need a 20 hole or 40 hole plate.

[lugnut]

I don't think there 9s a positive answer, just wanting a close answer... I'm not planning to the moon and back, just wanting to make the rest of gears mesh. A little slop would suffice.

[BCRider]

I don't think there 9s a positive answer, just wanting a close answer... I'm not planning to the moon and back, just wanting to make the rest of gears mesh. A little slop would suffice.

If you're eyes and the vernier scale on your handwheel will let you use the vernier closely enough to do it directly with the degrees and fractions and resolve to within plus or minus a tenth of a degree then I'd say that's close enough.

What about making an index wheel? there's a lot of us CAD types here and drawing a 20 hole pattern is child's play. This one took me about 60 seconds. Depending on the ratio of your table it would not be tough to figure out what you need and produce a pattern you can use to make a new wheel. And then you'll have it for next time. Just print it, cut and paste to some metal, drill and you're good to go.

[Randy]

My plate is a 72 to 1,, I hope this helps.

Well, that makes it 72/50, or one full crank turn plus 22/50ths of a turn, or 1+11/25. Same problem.

[RMinMN]

While people gave several suggestions, nobody asked what the 50 tooth gear was to be used for. That information is just as important as the rest. Is the gear for power transmission like in a truck transmission or is it more for getting a proper gear ratio for cutting threads on a lathe? It doesn't take much gear strength to cut threads and a 3D printer combined with most any CAD program will have a 50 tooth gear spit out in less time than this discussion has taken.

[Mcgyver]

Using coordinate drilling, you can make an accurate plate with any number of divisions you want, e.g. make 25 hole plate and move each division 36 spots. The Handbook gives tables, not sure if up to 25, but it wouldn't hard to do a quick excel table. A DRO makes it easier still, just do a 25 hole bolt pattern. Or BC's idea, drill to a pattern. Inaccuracies are minimized by the 72:1 gearing.

[Baz]

I think you implied your table has a 72:1 ratio. This gives a substantial reduction in any errors in your hole plate so you can use a cruder index plate. randy above has pointed out that you want a 25 division one. I used the word division not hole because you don't have to have holes, just marks you can align the pointer to. Maybe you have a changewheel with 25 teeth.
Otherwise there are loads of low-tech ways to make 25 marks in a circle.
One of the easiest is to mark 25 marks at 1/4 intervals in a line on a paper strip - that's 6 1/4 inches long. Turn it into a loop making sure you have 25 marks not 24 or 26. Put that round a wood, plastic, or even cardboard disc diameter 6.26/pi = 2in near enough. Mount that eg by gluing to your existing index plate.

[David Powell]

I want to make a gear with 74 teeth. I mentioned this to a friend with a 3 D printer. The next time I visited he gave me a 6 inch diameter disc with 74 neatly cut triangular notches, ready to take a positioning pin and a 3/4in diameter centre hole. As far as I can tell it is true within 3 or 4 thous., plenty good enough for my needs. Just got to make the gear now. Regards David Powell

[mikey553]

To make just one gear and not a precision one I would not buy or make any additional tooling. Make and print out a table with all 50 divisions calculated as angular degrees and minutes of a one full turn. 360/50=7.2 degrees. So your table will look like 7.2, 14.4, 21.6, etc. If your table is graduated in degrees and minutes, convert decimal degrees into degrees and minutes.

Zero the table and make the first cut. Cross out the first line of the table and advance to the next position. This method should be fairly accurate even on the 4" table. The chance of making a mistake exists, but you can minimize it by paying close attention to your table.

50 teeth gear is a fairly large one unless you are making a clock gear. Are you sure your little table is up to the task? How are you planning to hold the gear blank?

[RSG]

Make and print out a table with all 50 divisions calculated as angular degrees and minutes of a one full turn. 360/50=7.2 degrees. So your table will look like 7.2, 14.4, 21.6, etc. If your table is graduated in degrees and minutes, convert decimal degrees into degrees and minutes.

The chance of making a mistake exists, but you can minimize it by paying close attention to your table.

This is what I'd do! It's the fastest and simplest way to get it done! Just have to be careful!

[Stepside]

If we had a description of the gear we would know what gear cutter was needed. There is a probability that one of us has the tooling and the time to make the gear.

[lugnut]

Thanks for all the Ideas guys. I started this project about 10 years ago and I had to put it way back on the back burner, but now I decided give it another go. The gear is only 1.625" in diameter - 1.250 PD 3125" bore it has 32 pitch and 14-1/2 PA. and I need 3 of them. here is a link to what I am attempting. http://brassorrery.blogspot.com/2009_09_01_archive.html
I like the idea of not using the the degree wheels instead the dividing plate. I made a cutter using one of John Stevens articles on gear making, and will probably try using it.
When I get a little ways along I will post my progress with some phots. Thanks again for all your ideas.