Gear Cutting

[Ghosty]

Hi All,
I have plans for a gear reduction unit, but they do not list the DP for the gear cutters. they list the number of teeth and the OD of the gears, but not the DP.
Question, How do you work out the DP. The small gear is 20 tooth, with a listed OD of 1", the large gear is 30 tooth and a listed OD of 1.3125"
Any help would be grateful.
Cheers
Andrew

 

[Eccentric]

20 teeth gear blank OD of 1" is a DP of 22, 30 teeth 24 DP gear blank OD is 1.33333. 30 teeth, DP of 22 is a gear blank OD of 1.455"

Do these gears mesh? then they need to have the same Diametric Pitch.

What is the distance between the shafts and what is the reduction ratio you want? We can figure out the gear specs from there.

 

[Ghosty]

Eccentric, the ratio is 3-2, the 20 tooth gear can not be changed in dia, as it is on the engine, the 30 tooth, sits on an cam adj shaft so some adj for meshing is posable(90 thou of adj). The distance between the gears on the plans is 1", but can be adj either way as well.
I got the 22DP for the 20 tooth, but the 30 tooth was giving me problems, thought I was doing something wrong, just more problems with very old plans, they are from the 1940's
Cheers
Andrew

 

[Nerd1000]

22 DP is not a standard gear pitch, is it possible that the gear is meant to be 20 DP or 24 DP but the drawing was labelled incorrectly?

Might be worth checking what centre distance is required and figuring out a gear pair yourself.

 

[Ghosty]

Hi All,
After doing some drawings(love autocad) with 1" centers, and a 1"OD small gear and a 1.454" larger gear with the offset of the adj cam setup on the driven gear at max offset, it leaves 70thou intermesh. the DP22 gear has a 98thou depth of gear, so it will be well within specs to cut the gears and build the reduction unit.
Thank all for the help.
Cheers
Andrew

 

[Eccentric]

Andrew,

I was playing around with some numbers and found that if you use a 1 module cutter, very common by the way, a 20 tooth gear and 30 tooth would have thier shafts about 1" apart.

Just a thought.

 

[Ghosty]

Eccentric, I looked at that as well, have numbers written down for the gears, I also have the set of Mod 1 gear cutters, but as the DP22 gears will work with a 20/30 tooth gear set, I will just go that way, just have to get the cutters. For a plan from USA from the 1940's, they use BA threads through out the reduction unit. I have a full set of 0-10BA taps and dies so that is not a problem.
Cheers
Andrew

 

[timo_gross]

Andrew,

I was playing around with some numbers and found that if you use a 1 module cutter, very common by the way, a 20 tooth gear and 30 tooth would have thier shafts about 1" apart.

Just a thought.

(20 mm+30 mm)/2 to be ecxact . A module 1 gear has a DP of 25.4 , number looks familiar.
m = d/z
module = pitch circle diameter / number of teeth

Another way to adjust shaft distance is "profile shifting" (cutting a little deeper or less deep) or introducing a helix angle to the gears.

Greetings Timo

 

[nealeb]

There's something a bit odd here, just looking at the basic dimensions the OP has given. Unfortunately, I can't get a clear image of the jpeg to check.

Gear diameters are 1" and 1.3125". I'm not entirely sure if this is the actual gear OD, or the gear PCD. For simplicity, let's assume PCD. 20T, 1" PCD, means 20DP. Sounds reasonable - common size. For the second gear, 30T and 20DP gives PCD of 1.5". So the centre-centre distance must be 1.25", not 1" as suggested.

Let's try in the opposite direction. Centre-centre spacing is 1"; 30T gear PCD is 1.5x 20T gear PCD, so we end up with smaller gear being 0.800" PCD; larger gear being 1.200" PCD. That's 25DP, as it happens. Doesn't sound like a standard size, but the gear diameters are nowhere near those given, especially the small one. I reckon that there's an error on the drawing (and note that I am using gear PCD here - the actual OD is slightly larger). To find the OD of a gear given DP and tooth count T, it's:
OD = (T+2)/DP

However, if you make that correction for gear OD, 25DP is very close to module 1, as per previous reply!

 

[Ghosty]

There's something a bit odd here, just looking at the basic dimensions the OP has given. Unfortunately, I can't get a clear image of the jpeg to check.

The dimensions given in the plans are for the OD, he driven gear is on an adjustable shaft for the intermeshing of the gears, so there is enough adjustment to get the two gears to work with the 22DP profile. The OD of the driven gear(30 tooth) will be 1.454" dia.
Cheers
Andrew

 

[nealeb]

I guess a lot depends on whether you can find the gears you want ready-made or plan to cut them - and if you can get the cutters off the shelf or have to make them... At that date, availability of ready-made gears might have been very different! Good luck, either way.

 

[Ghosty]

Hi All,
Just done another drawing in autocad, using Mod 1 gears, with the 20 tooth at 0.868" dia OD and the 30 tooth at 1.260" dia OD, with 1" centers, the mesh is 0.064", the depth of cut for the Mod 1 is 0.085, I can cut the gears with Mod 1 cutters(that I have), and with the adj cam on the driven gear(30 tooth), I will get full mesh of the gears.
I will now go this way as I don't need to hunt down the 22 DP cutters.
Cheers
Andrew

 

[Ghosty]

Hi All,
Just an update, back has been good the last couple of days, got out and made some chips today, took all day but turned up two gear blanks for a reduction drive. Machined out of 4140.
Felt good to do this. still have 6 #8BA holes to tap in the larger gear front. Will see how I feel tomorrow.
Cheers
Andrew

 

[Eccentric]

Looks good Andrew. What speeds and feeds are you running the gear cutter on the 4130?

 

[Ghosty]

What speeds and feeds are you running the gear cutter on the 4130?

Will look at that when I get to that stage.

Hi All,
Drilled and tapped the gear(8 BA)
Gear arbours done, nephew will be up after lunch, I will get him to lift the dividing head up on the mill

Hope to have at least one gear done this arvo.
Cheers
Andrew

 

[Ghosty]

Hi All,
We have a 20 tooth gear, see how I feel tomorrow to weather I do the 30 tooth one.
Cheers
Andrew

 

[timo_gross]

Hi All,
We have a 20 tooth gear, see how I feel tomorrow to weather I do the 30 tooth one.
Cheers
Andrew

It is good to see you are getting things done, I did not get around to cut some gears quite a while. I bought two 2nd hand hobs few days ago. They seem to be in O.K. condition, at least no broken teeth. (how to resharpen them?, I will have to find out when dull )
How long does it take to cut one gear? I feel always soooo slow.

Greetings Timo

p.s. tail stock helps a lot with surface quality, does it? I saw you champfer the gear blank before cutting the teeth, does that help against burrs?

 

[Ghosty]

How long does it take to cut one gear? I feel always soooo slow.

It took me most of the arvo to do this one, but I have serious back problems at the moment. I would normally be able to cut this in about 30mins.
Cheers
Andrew

 

[timo_gross]

It took me most of the arvo to do this one, but I have serious back problems at the moment. I would normally be able to cut this in about 30mins.
Cheers
Andrew

Back problems sounds awful, hope it is not too serious. I felt I should test the 2nd hand hob today.


I seem to be lucky with this one, it sound good. See what comes out. Five passes took 30 min. Gears roll sort of, but the tip of the teeth is quite big, the gap narrow. I guess radial depth was too less.

Greetings

 

[Ghosty]

Have both gears done.
Cheers
Andrew