# question on involute gear cutters



## werowance (May 3, 2013)

i need to cut a change gear for my lathe and want to purchase a set of gear cutters on ebay but i have a question and also want to make certain i am getting the correct kit or at least ill purchase a single cutter.

i need to cut a 40 tooth change gear for my chinese lathe,  it measures 1.647,  i believe the pa is 20.  so i think what i need is a module 1, #3 with 20deg preasure angle - correct?  what i dont understand is why do some list the dp and some do not?  i thought dp was pretty much the depth of the cut you will make?  i know the math to calculate the dp would be 40+2 / 1.647 correct?  but im still a little confused about if dp is all that important on selecting gear cutters and also question "module number"  is module just the size of the kit?  ive seen some tiny ones that easily fit in my hand and some that looked like they would cut gears for a battle ship,  so bigger the module = bigger the size?

thanks for any help 

werowance


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## Jasonb (May 3, 2013)

DP is the imperial system, MOD is the metric.

DP is not "depth of cut"

Depth of cut is Addendum plus Dedendum (1xmod) plus (1.4xmod) so in your case 2.4mm


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## jasonh (May 3, 2013)

DP = Diametric Pitch  = Ratio of the number of teeth to the *pitch* diameter.

BTW - I assume you have just put some calipers on the gear. That's not the pitch diameter, that's the outside diameter, which is slightly larger.
outside diameter = pitch diameter + (2 x addendum)

The diametric pitch of a spur gear can be approximated by adding 2 to the number of teeth on the gear, and dividing by the outside diameter of the gear.

In your case: 

PA = 20 degrees
DP = (40 +2) / 1.647 = 25.5 ~ DP 26
And you want a #3 cutter (35-54 teeth)

BTW:
DP (teeth per inch) is similar to TPI (threads per inch)
Module (mm) is similar to Metric pitch (mm per thread)

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


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## werowance (May 3, 2013)

ok, so i need module 1.4  and a #3 cutter?

man now i really dont know what im doing.  how did you determine the gear was metric?  because i said it was a chinese lathe or by my measurement?  i was assuming standard/imperial since i cut imperial threads with these gears.  but i guess with the right ratio i could cut imperial threads with metric gears.


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## jasonh (May 3, 2013)

No- Module is the inverse of DP and expressed in mm.

In your case:

Module = (1/26) x 25.4 = 1mm (as you originally stated )

Someone with more direct practical knowledge might want to chime in, but I suspect it doesn't make much difference if you use a metric/imperial gear cutter so long as the outside diameter, tooth depth, number of teeth, etc. is correct on the resulting gear. I say that because gear cutters only approximate the correct involute form over their range of use.

Make sure you get the pressure angle right. If you don't do that you will violate the fundamental law of gearing!


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## Jasonb (May 3, 2013)

You want a 1 Mod cutter as your far easter lathe will have metric profile gears and the number 3 will cover your 40T gear.

Turn the blank to 42mm ( 1.654") touch the cutter on the edge and zero the handwheel dial or DRO then your depth of cut from there will be 2.4mm or 0.094"

J


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## werowance (May 3, 2013)

thank you both,  this does explain alot of the questions and seeing the math laid ot like that is good.  i very much appreciate the help and explinations.

w


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## mc_n_g (May 12, 2013)

Sorry late too the party, but....

If you ever have to make a gear and the DP or Module is not available the other method is to make your own cutter.  A little more work involved but works well if you have the correct tools to make it.
The book mentioned is an excellent resource.
This is also an excellent resource  http://www.astronomiainumbria.org/a...ca/easyweb.easynet.co.uk/_chrish/geardata.htm

Here are a few picks of make a rack-style cutter.

http://i10.photobucket.com/albums/a140/mc_n_g/machinery/DSCN9757_zpsca606afd.jpg
http://i10.photobucket.com/albums/a140/mc_n_g/machinery/Making_hob_zpsd2c3dcbf.jpg
http://i10.photobucket.com/albums/a140/mc_n_g/machinery/Cutting_flutes2_zps43446c03.jpg
http://i10.photobucket.com/albums/a140/mc_n_g/machinery/Cutting_18DP_teeth_zps8d836b29.jpg
http://i10.photobucket.com/albums/a140/mc_n_g/machinery/cut_gears_view2_zps9c1024fe.jpg
http://i10.photobucket.com/albums/a140/mc_n_g/machinery/gear_zoom_facets_zps585ffafa.jpg

Using the rack method does not cut perfect involutes but segmented cuts approximating the involute shape.


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## Mainer (May 13, 2013)

For that matter, an involute gear cutter is strictly correct for only one tooth count even though it is used for a range of tooth counts for which the error is "small." A gear cut with a rack hob, on the other hand, will always have the correct involute form for the tooth count, it will just be lumpy. So pick your tradeoff.


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## werowance (May 13, 2013)

mc_n_g,  that looks like a gear hob,  correct?  how did you grind your bit to cut the grooves in the rod?  just grid it to fit your gear?    i have also tried with some success the fly cutter method,  but it just doesnt like cast iron or steel.


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## enfieldbullet (May 13, 2013)

Mainer said:


> For that matter, an involute gear cutter is strictly correct for only one tooth count even though it is used for a range of tooth counts for which the error is "small." A gear cut with a rack hob, on the other hand, will always have the correct involute form for the tooth count, it will just be lumpy. So pick your tradeoff.



can you please elaborate as to why gears cut with hobs are lumpy?

EDIT:

nevermind, i see it now.

but that doesn't mean you can't lap or grind them smooth , right?

shouldn't the profile of a hobbed gear be closer to the right one even with those deviations?


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## mc_n_g (May 13, 2013)

It is the easiest as you are cutting a rack of teeth.  The angle is a constant.  The distance between teh teeth is a constant.  Take a look at the first reference link.  All you need is the spacing, pressure angle and depth of cut to space the teeth.
If you know the DP (or Module) all can be calculated.  The only other important factor is the pressure angle.  The one I cut is 18DP with 14.5 pressure angle to fit a South Bend.  If you do not know it you approximate it by fitting to the teeth of your known gear. But remember the straight teeth cut the 'involute' form over multiple cuts.  You do not make the cutter involute (or curved), only straight teeth.

*For the 'purists' it is not a correct hob.*  It is 'hob-like' but serves the same purpose.

Lumpy because the rack only cuts sections of the curve and not a true curved surface.  Take a look at the last picture you can see the very small flats from the mulitple cuts.  This is much more pronounced on a larger DP.  At 18DP I had to magnify the image to see them.
The curved surface (involue) is approximated by a series of straight line cuts.


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## CrashedAgain (May 26, 2013)

What lathe?  
http://mikesworkshop.weebly.com/change-gear-pressure-angle.html  has determined that the plastic gears on the mini lathe are module 1, 20 degree pressure angle. His website also has some info on making your  own gear cutters and I have cut these gears using a home made  flycutter.


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