Gear cutting

[don]

What will be the outcome if I use a .8 module cutters instead of a .5 module cutters it calls for? Will my shaft center distance be off? the gears are the crankshaft and cam. a 36 and 72 teeth

 

[Eccentric]

The module of the cutter determines the shape of the cut. I think in DP so forgive me, a .5 module cutter is about equal to a DP of 50, that is this cutter will make a gear 1 inch in diameter with the proper shaped tooth when you cut 50 teeth around the perimeter of the blank. Likewise, a .8 module cutter is equal to about a DP of 32. So these teeth are much bigger as a 1 inch diameter gear will have 32 properly shapped teeth. Now if you used a .8 module cutter instead of a .5 module cutter on a gear of 1" diameter and cut the 50 teeth you can see that you would be cutting away too much material or cutting too shallow and not have a properly shaped tooth. Some times you can fudge a little, but a .8 module cutter and .5 module cutter are too different to do what you want to. sorry.

 

[MRA]

Hi Don
I have recently been making and using a rack-form hob - not a spiral, but a cutter which works like a stack of (in my case five) involute cutters whose flanks are straight - hence rack-form. It was not too hard to make and harden, and the idea is fully described in some web pages here.

http://www.helicron.net/workshop/gearcutting/method/
My hob is not quite bang-on, but I am making lathe change gears in aluminium and they work well enough that I don't yet want to make a better hob! It's fun - I am already on my fourth gear.
cheers
Mark

 

[mgbyrne3]

What will be the outcome if I use a .8 module cutters instead of a .5 module cutters it calls for? Will my shaft center distance be off? the gears are the crankshaft and cam. a 36 and 72 teeth

You can preserve shaft distance with different modules by adjusting the number of teeth.

pitch diameter is N x M, so PD =36 x .5 = 18

some modules that will have PDs of 18 with an even number of teeth are
PD = 45 x .4 = 18 PD = 90 x .4 = 36
PD = 36 x .5 = 18 PD = 72 x .4 Shaft distances remain = (18 + 36)/2 = 27 (but OD = (N+2)M)
PD = 30 x .6 = 18 PS = 60 x .6
PD = 20 x .9 = 18 PD = 40 x .9
PD = 18 x 1 = 18 PD = 36 x 1

So you can us some modules other than .5 for the same shaft distance.


You could consider .8M gears

20 x .8 = 16 (16+32)/2 = 24
25 x .8 = 20 (20+40)/2 = 30
30 x .8 = 24 (24+48)/2 = 36
35 x .8 = 28 (28+56)/2 = 42

.8M gears of 25/50 with 30mm shaft separation would probably work for a low RPM engine.

 

[Charles Lamont]

23 + 46 would give a centre distance of 27.6 mm, and 22 + 44 gives 26.4mm. These are as close as you can get to 27mm centres with 0.8 module gears (and a 2:1 ratio, obviously).

If you have to maintain the 27mm centres, and have to use a 0.8 module cutter, I think you could fudge the diameters, say using 22T & 44T, but pushing the PCDs out, and cutting the teeth slightly over nominal depth to get a good mesh.

If these are exposed gears, there is also the consideration of what tooth size looks right. If the face width is narrow, the larger teeth may look out of proportion.

 

[don]

Thanks for your replies, as you can tell gear making is new to me , I acquired the .8 module cutters years ago and haven't cut any gears yet. looking to buy a 0.5 set , but most have a PA of 20 degree, my print calls for a PA of 16 degrees will this make a big difference? by the way I'm working on the farm boy engine. Don

 

[Gordon]

I have done this several times and it works just fine. I use the MOD set which has a tooth depth close to the DP thread depth. We are building model engines etc. not moon lander modules. Also the pressure angle is not important in this context. Again we are not transmitting a lot of torque or too worried about the noise the gears are making. I have even made gears at a non standard size to get a desired ratio to fit in a given space, For instance I made a small rotary table and wanted a 10:1 ratio to fit a reasonable C/C distance. It worked just fine. It never had to fit a commercial gear. I have .5, .7 and 1 MOD and they have fit anything I wanted to make.

 

[awake]

PA of 16 is extremely unusual, and you are unlikely (probably impossible) to find any commercial cutters with that parameter. It is not that a PA of 16 cannot work; sure it can ... but for reasons of history (?), the two PAs in common use are 20° and 14.5°. I would not worry about matching the PA called for in the plans; I'm guessing this was based on home-made gear cutters. The important issue is the gear center to center distance, which as amply discussed above is a function of the module and the number of teeth.

 

[Grahn]

Thanks for your replies, as you can tell gear making is new to me , I acquired the .8 module cutters years ago and haven't cut any gears yet. looking to buy a 0.5 set , but most have a PA of 20 degree, my print calls for a PA of 16 degrees will this make a big difference? by the way I'm working on the farm boy engine. Don

Don,
you may want to go back and look at the plans again. On my set of plans (#465) it lists the gears as 14-1/2 Deg. PA . The box in the corner of that page in my set of plans lists the date as 01/08/09 sheet revision A.
Gary

 

[don]

Yes you are right, mine calls for 14-1/2 also , I ordered a set of 0.5 Mod with a 20 PA . thanks for your reply Don

 

[geo]

The farm boy gear cutters are #2 and #3 14 1/2 pa if you want to use mod a bloke on you tube called whale has done all the calcs for you.

 

[simonbirt]

Hello Don

By the way, the shaft centre distances on the Farm Boy drawing are incorrect (Revision A 01/08/09) I think the drawing gives 1.026" which is about .005" too tight. I used 1.034 to give a easy running fit. I found out the hard way after I had bored all the holes and made the gears. I sleeved and rebored the camshaft hole it is now fine.

 

[Gordon]

If you are interested in gears I recommend getting the Ivan Law book "Gears and Gear Cutting". There are several commercial sites selling gears which list all of the specification for standard gears like PD, OD, tooth depth etc. I use the Rush Gear site which also generates CAD drawings. I have also used the Boston Gear site in the past. Once you have the PD for the mating gears calculating the center distances is just a matter of half of PD1 + half of PD2 = CtoC

 

[AndrewW]

Hi
I concluded that the Farm Boy drawing was incorrect. The pitch diameter of the 48DP gears is as follows:
1.5" for 72 teeth
0.75" for 36 teeth
So the distance between centres is (1.5+0.75)/2 = 1.125"
The Farm Boy drawing shows the hole for the cam shaft being 0.45" below the centre of the crankshaft and 1.026" to the left of the crankshaft.
So the actual distance between centres in the drawing is:
Square root of ( 0.45x0.45 + 1.026x1.026) = 1.1203
So the distance on the drawing is 0.0047" TOO SMALL.
I used Mod 0.5 gears on mine.
I cover this in my YouTube video:


I hope that helps.
Andrew