Gear Cutters?

[SMG]

Dumb question, what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter. I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.

Thanks
Steve

 

[GailInNM]

Steve,
The cutter number indicated the range of the number of teeth in a gear that that cutter can cut.
Gail in NM

• Cutter Number Cuts Gear from
• 1 135 teeth to rack
• 2 55-134
• 3 35-54
• 4 26-34
• 5 21-25
• 6 17-24
• 7 14-16
• 8 12-13

 

[cfellows]

To add just a bit more to what Gail said...

Gear teeth for for a given pitch are always spaced the same, you simply add more teeth as the gear gets bigger.

However, the shape of the sides the teeth changes as the number of teeth changes. As you decrease the number of teeth, the sides of the teeth become rounder or bulge more if you prefer... as you increase the number of teeth, the sides become flatter. With a rack gear, the sides are completely flat, with no radius at all.

So, you need different gear cutters to cut gears with different numbers of teeth. Theoretically, you should have a different cutter for every size gear you want to cut. But, since that's not practical, they make gear cutters that will cut a range of gear sizes. Not a perfect solution, but close enough for all but the most exacting applications.

Chuck

 

[SMG]

Thanks guys, That explains it really well.

 

[rhitee93]

I'm glad you asked. I didn't know this either. What's worse is that I didn't know that I didn't know it!

 

[AlanHaisley]

Chuck,
Could you elaborate:

... you need different gear cutters to cut gears with different numbers of teeth. Theoretically, you should have a different cutter for every size gear you want to cut. But, since that's not practical, they make gear cutters that will cut a range of gear sizes. Not a perfect solution, but close enough for all but the most exacting applications.

I assume that one of the numbers in the range is that of the number of teeth that should be "perfect" and that the other numbers of teeth in the range are off one way or another. For each number of cutter, do you know which number of teeth will be closest to perfect and which number(s) will be most off?
Alan

 

[gmac]

Alan;
This may help - the lowest tooth count for a gear cutter "range" is the most "perfect", the others less so. Can't tell you if that's particularly important in our hobby application.....

http://www.precisioncuttingtools.co.uk/gearshap.html

Cheers Garry

 

[kvom]

Here's a pair of mating gears with different # of teeth. The difference in tooth shape can be seen pretty clearly.

 

[SMG]

So here is another Question, Why would someone pick a 20deg pitch angle set of gears over a 14.5deg pitch angle? Does one run smothier than the other? What is the difference?

Thanks for your help.

 

[gmac]

Steve;
Top of the second page, Pressure Angle (PA).
http://www.xtek.com/pdf/wp-gear-terminology.pdf
Cheers Garry

 

[black85vette]

I have this book and find it pretty useful for understanding the basics;

http://www.amazon.com/dp/0852429118/?tag=skimlinks_replacement-20

 

[gus]

Dumb question, what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter. I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.

Thanks
Steve

Hi Steve.

Gus also going into gear cutting.Been reading text books and not getting any where. Planning to go into making simple clocks.

 

[gus]

Hi Guys,
I am following and reading all the expertise.
Did gear cutting in trade school 51 years ago.
I got good marks for cutting all 24 teeth. Some classmates goofed with the gear blank O.D. and ended up with shallow tooth height or sharp tooth. One chap did not set the compass diividers as taught and came up with 25 odd looking teeth.He manual count the no. of holes.Ha Ha Ha.
We were 18 year old kids then doing 2 years machining course.

 

[gus]

I have this book and find it pretty useful for understanding the basics;

http://www.amazon.com/dp/0852429118/?tag=skimlinks_replacement-20

Will buy this book to read up.

 

[WiKKiDWidgets]

I'm glad you asked. I didn't know this either. What's worse is that I didn't know that I didn't know it!

I have that problem all the time!

 

[GailInNM]

Why would someone pick a 20deg pitch angle set of gears over a 14.5deg pitch angle? Does one run smothier than the other? What is the difference?

Gear design/selection is a big game of trading one advantage at the expense of another. Since there are a lot of variables that can changed on a gear it can get to be very confusing. Nothing is sacred. Even the tooth form may be changed to suit certain applications so the involute form may not be the best in some cases.

Generally smaller pressure angle gears produce less pressure on the shaft bearings as there is not as much pressure trying to force them apart. Run a little bit quieter also as I recall. But the teeth are not as strong, particularly at low tooth count, as higher pressure angle gears so they can not transmit quite as much power.

For model engineering most of this will not make a bit of difference. Even though there are model engine designers who are quite capable of doing the math required to optimize gear selection in a design, I don't know any of them who actually do. The TLAR (That Looks About Right) approach is probably the most common selection method. And TLAR is further modified by other mundane things like what gears are available, either on hand or for purchase. If making our own gears what tools are available. You know the story. 32 DP gears would be ideal but 48 DP cutters are on hand and will work just fine.

So the selection of pressure angle for model engines is mostly going to come down to practical matters.

If you are going to purchase gears then 20 degree PA are probably more commonly available and would be my choice. In metric (MOD) sizes 20 deg PA are very predominant.

If you are going to make your own gears you have more choices available. If you are going to make your own cutters to make gears then it is as easy to make any pressure angle you desire. If purchasing commercial involute cutters the situation is changed a little bit. In recent years fewer suppliers are carrying as big a selection of involute cutters. In particular imperial diametrical pitches of 32 and finer are getting harder to find. A few years ago 14.5 deg PA fine pitch cutters were 1/3 to 1/2 the price of 20 deg PA. And this had been the case for 25 or more years. No apperent reason -- just the way it was. So, many of us who got into gear cutting back then went with 14.5 degree just because we were cheap. As I have complete sets of gear cutters in the sizes I use I have not looked prices recently. Of course the gears I make and use are because I have cutters for those sizes. Practical matters again.

Gail in NM

 

[lensman57]

Dumb question, what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter. I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.

Thanks
Steve

Hi,

Unless you wish to make gears for commercial purposes then I would strongly advise you to learn to make your own cutters, the best ones being a home made hobb. This will approximate the involute gear shape if multiple cuts are taken and it is a hell of fun to make. They are well documented if you Google for them. Here is one in German but the photos are self explanatory.

http://www.metallmodellbau.de/GEAR-CUTTING.php


Regards,

A.G

 

[ELM6061]

Thanks for posting this question SMG, so many of my own have just been answered here, and to you Lensman, a great link, and as you stated "even though it is written in German". I have to follow up with; A picture paints a thousand words, and with 46 detailed photos & drawings, it is like reading a book.

One question I do have from that link, if you were going to cut your own HOB'S; What metal/steel would be best to use?

I do have an electric kiln & quenching oil, though apart from Tech school 30 odd years ago, with an oxy torch, I have never tried to heat/soak/temper/harden.

Eddie

 

[Maryak]

Annealed, harden and temper after machining.

US - Drill Rod
UK - Silver Steel

Best Regards
Bob

 

[lensman57]

Thanks for posting this question SMG, so many of my own have just been answered here, and to you Lensman, a great link, and as you stated "even though it is written in German". I have to follow up with; A picture paints a thousand words, and with 46 detailed photos & drawings, it is like reading a book.

One question I do have from that link, if you were going to cut your own HOB'S; What metal/steel would be best to use?

I do have an electric kiln & quenching oil, though apart from Tech school 30 odd years ago, with an oxy torch, I have never tried to heat/soak/temper/harden.

Eddie

Hi,

As Maryak has already mentioned use either silver steel and or drill rod , once the teeth have been cut heat to cooked carrot colour and quench and then heat again to about 250 C to temper , I just wack the oven up to its max and leave the tool there for about an hour as I find it difficult to bring the whole tool to straw colour at once. You may have to do a bit of maths to get the top land dimension for the gear and hence the cutter you have to make to cut the silver steel into a hob. Here is another link for making cutters per specific module or DP.

http://www.metalwebnews.com/howto/gear/gear1.html

Regards,

A.G