Model Engine Maker

General Category => Chatterbox => Topic started by: gbritnell on March 17, 2019, 03:21:02 PM

Title: Helical gear cutting question
Post by: gbritnell on March 17, 2019, 03:21:02 PM
 The other day I decided to build a small governor to go on the Maudslay Marine Engine. Basically cosmetic but I thought it would add some interesting movement to the engine. The design incorporates a helical gear set at 1.5:1 gear ratio. I opened up the helical gear chart that Don made and put in the numbers that I needed to get the specs. The crank gear is 50 degrees with 21 teeth and the governor gear is 40 degrees with 14 teeth. I only had a division plate for 14 holes so I had to make up another with 21 holes. I also needed to make the helical templates to create the helical teeth on the gear.

 When Chuck Fellows designed the fixture he used 1/8 aluminum for the template. My first template was made from .062 brass and as I worked with the fixture I found that .031 would work just fine, and it was easier to form.
 Ok I have the index plates and the templates made. Now I needed a cutter. The chart shows the required number involute cutter for each gear so being that the gears are tiny and they don't need extreme precision I decided to split the difference in tooth count. I made a layout of the tooth form in Autocad and came up with the number to cut the cutter from drill rod. I made the cutter, hardened and tempered it.
 
 I set the helical fixture up in the mill, clamped in the vise at the proper angle for the larger gear, 50 degrees. I then mounted the gear blank onto the arbor and tightened the cutter in the spindle.

 I have cut quite a few helical gears using this fixture over the past number of years and to be honest I don't remember exactly how I went about centering the cutter with the spindle. The gears were larger so maybe that aided in my eyeball calculations. Anyway I created a sketch to try and help explain what I need input for.

 As most of you know I have been machining for 50 some odd years and have been able to solve all of my machining questions. It's not that I can't come up with a way of doing this job it's just that I would like to know if there is an 'ACCURATE' way of doing it. By accurate I mean within a couple of thousands.

 I searched Youtube for helical gear cutting and there are a number of videos showing the process but none of them show a setup operation. My question applies to not only my fixture but on a horizontal/ universal mill using a universal dividing head.

 The question is: how does one find the center of the shaft/gear blank at the point of contact with the cutter? Maybe it's right in front of me and I can't see it but try as I might I can't solve this.
Remember the key word is 'ACCURATELY'
gbritnell


Title: Re: Helical gear cutting question
Post by: Don1966 on March 17, 2019, 04:12:59 PM
George I may not have this correct but this is what I came up with. Touch off the top of your blank with your cutter center. You know the length of your blank using the angle find the center which is cosine Times the length of the blank divided by two. My two cents!


Don
Title: Re: Helical gear cutting question
Post by: gbritnell on March 17, 2019, 11:12:07 PM
Hi Don,
My issue with trying to touch off the cutter to the blank is: 1. There's no way to guarantee that the edge of the blanks is sharp. If it has a slight radius this would change the dimension. I know we're being picky here but I'm looking for an actual dimensional method for doing this. On this particular job there is no way to get the cutter into the blank because the screws that hold the cutter and blank are in the way.
Here is my idea for doing the job. Again this leaves a little bit for error.
In the first picture I have inserted a pointed shaft into the arbor.
I then bring the cutter over to the shaft, split the tip of the cutter on the pointed shaft then touch off the edge of the cutter to the face of the shaft.
In the next picture I move the table over by the radius of the cutter.
In the last picture let's say that I don't have enough travel with the helical fixture shaft to cut the entire gear blank I could move the table and head by the calculated dimensions. This would put the cutter at a higher point but still on the centerline.
Like I said, it's close but only as good as your magnifying glass is. You can't really touch the edge of the cutter off to the top of the pointed shaft because the gear tooth shape is actually radial so there is not flat to touch off. Close but here again not dead on.
gbritnell

Title: Re: Helical gear cutting question
Post by: gbritnell on March 17, 2019, 11:17:03 PM
It's easy when cutting a spur gear. You just touch off the flat face on the involute cutter to the top of the gear blank and go down 1/2 of the cutter plus the radius of the blank and you're on center.
The tricky part about the helical touch off is the very tiny fragile edge of the gear blank.
gbritnell
Title: Re: Helical gear cutting question
Post by: Don1966 on March 17, 2019, 11:34:03 PM
I do understand what your saying George. I thought about using the pointer method you mentioned but it hard to convince myself that thatís the best way. My way the trick would be to make sure the edges are sharp on the blank. And use a disc the size of the cutter to touch off the top and knowing the thickness of the disc split it in half and calculate the rest. Just my ranting!


Don
Title: Re: Helical gear cutting question
Post by: Dave Otto on March 18, 2019, 12:17:55 AM
Is it possible that you may be able to use a tooling ball to locate your cutter?

FnEl6Kj-F-E
Dave
Title: Re: Helical gear cutting question
Post by: kvom on March 18, 2019, 01:38:57 AM
If I understand the problem you're looking for the X axis position such that the cutter is tangent to the inclined round rod, hence tangent to the cross-section ellipse.

How about mounting a test indicator in the spindle so that the tip swings horizontally.  You can adjust the X and Y until the indicator just barely moves as it swings past the stock.
Title: Re: Helical gear cutting question
Post by: Zephyrin on March 18, 2019, 08:17:04 AM
I can't see how to have an exact solution without having precise position of the intersection of the 2 axes...
I would suggest to use 2 half circular dummies for the cutter and the blank and finding the position where the 2 diametres are collinear...just a thought experiment not that easy !
by the way, the profile of the cutter may be a little too thin, as the apparent pitch of the inclined gear is larger than the real pitch (for spur gear).

the search of perfection in every details is amazing.



Title: Re: Helical gear cutting question
Post by: john mills on March 18, 2019, 09:37:00 AM
i would with the blank set up and the cutter set up i would go to about the centre of the blank just touch the od .if you move from just to the side of centre then move to the other side it should leave a mark with is on the centre line then you line the cuter up on that mark as close as you can ,it can come out close with practice.
try on a extra blank then move along to the starting position.
Title: Re: Helical gear cutting question
Post by: gbritnell on March 18, 2019, 04:56:39 PM
Gentlemen,
I have completed the gears for the Maudslay Marine Engine, made the mounting bracket and bolted everything up. The fit is very good so for the time being I won't worry about the setup. I thank everyone for their input and have had much the same ideas that are being presented. I don't know if there is going to be an exact way of doing it by I'm going to keep looking and thinking.
gbritnell
Title: Re: Helical gear cutting question
Post by: b.lindsey on March 18, 2019, 06:06:56 PM
They sure look nice George  whatever setup you used :ThumbsUp:

Bill
Title: Re: Helical gear cutting question
Post by: Kim on March 18, 2019, 08:29:34 PM
I said it on your Maudslay thread and I'll say it here too - those are just beautiful, George!

I like Dave's Tooling Ball idea.  I'd never heard of those before - found the video very interesting. I'm going to have to keep that in mind for these difficult-to-indicate setups.

Kim
Title: Re: Helical gear cutting question
Post by: Don1966 on March 18, 2019, 11:31:26 PM
George I donít know how you can get them looking any bettter then these they are great looking gears. Keep doing what your doing bud!..... :ThumbsUp:

One question George if your using my calculation sheet. Are you using the calculated cutter tooth count for your cutter?

 :cheers:
Don
Title: Re: Helical gear cutting question
Post by: gbritnell on March 19, 2019, 12:01:07 AM
Hi Don,
Because both gears fell in the mid range of the cutters I split the tooth count and drew up the tooth profile for that count. It's a little shy on the one and heavy on the other but they're so small it's hard to make the cutters exact anyway. To answer your question, yes, I did use your numbers.
gbritnell
Title: Re: Helical gear cutting question
Post by: toolznthings on March 19, 2019, 12:19:03 AM
Really nice work ! Way out of my skill level.
Title: Re: Helical gear cutting question
Post by: Don1966 on March 19, 2019, 12:26:22 AM
Hi Don,
Because both gears fell in the mid range of the cutters I split the tooth count and drew up the tooth profile for that count. It's a little shy on the one and heavy on the other but they're so small it's hard to make the cutters exact anyway. To answer your question, yes, I did use your numbers.
gbritnell
George can you post the cad drawings you did I would like to see them?

Don
Title: Re: Helical gear cutting question
Post by: Doc on March 19, 2019, 03:56:51 AM
Don I have used you spread sheet a few times easier than doing the math myself I like the way you have them set up. Thanks!
Title: Re: Helical gear cutting question
Post by: jadge on March 19, 2019, 11:34:28 AM
The question is: how does one find the center of the shaft/gear blank at the point of contact with the cutter? Maybe it's right in front of me and I can't see it but try as I might I can't solve this.
Remember the key word is 'ACCURATELY'

An interesting question; and I think the answer is, there isn't one. Or at least I haven't found it.

Before I cut my helical gears using a universal mill and dividing head it was a question that puzzled me. Quite a lot of professional machining books simply say centre the cutter over the blank and then swivel the table. As far as I can see that only works if the table pivot point is below the intersection of the two axes. Some books, like those from B&S, talk about taking a trial cut in a test bar, after sviwelling the table, and checking that the cut is symmetric.

Before I cut my gears I swivelled the table and then set the front face of one tooth on the involute cutter vertical. That was then moved to be midpoint of the gear blank, on the axis of the blank. I then used the trapped rule trick to set the cutter midpoint perpendicular to the gear axis. May not be "accurate", but the finished gears mesh well when placed on their theoretical centres.

Andrew
Title: Re: Helical gear cutting question
Post by: gbritnell on March 19, 2019, 02:05:30 PM
Andrew,
Thanks for the reply. Those of us that have been into machining for a long time know all the ways to set up jobs, wiggling, bumping, touching off, indicating and in some cases eyeballing. My question was posed more to see if there was something I was missing. There's always that old timer out there that says "oh you just do it like this." I have several books that I downloaded from the internet, one being the Brown and Sharp book and the other the U.S. Navy Machinists book. They both cover cutting of basically all types of gear forms with the attendant mathematics. The one thing they don't cover is how to pick up the center of a shaft that is at an angle to the axes.
In my years of machining I haven't come across anything that I couldn't solve, one way or the other. It might have ended up as some convoluted setup but it got the job done.
Thanks everyone for your replies.
gbritnell
Title: Re: Helical gear cutting question
Post by: 90LX_Notch on March 19, 2019, 11:26:43 PM
George-

I believe this will work well (if I described the math right).

If you mount an indicator on a mag base or surface gage and slide it on the mill table or vise, you should be able to zero to the top corner of your blank.  Then without adjusting the indicator use it to zero the top of your cutter.  One this is established, zero your Z dial.  Then it is half the cutter width minus* half the product of the cosine of the helix angle multiplied by the blank diameter.

A back plunger or drop indicator are better in this situation then a test indicator due to flat face of the tip.

-Bob

*Edit- "plus" changed to "minus".  Since the top of the cutter is being used instead of touching off with the bottom, 1/2 the cutter width needs to be subtracted.