Gear cutting on mini mill

[Nikhil Bhale]

I have a Seig X2.7 mini mill. I have to cut a Mod 1 gear with 63 teeth in aluminium. I have commercial Mod 1 gear cutter set.
I checked the internet for gear dimensions and indexing calculation.
Mod 1 gear.
63 teeth
OD 65mm
Root dia 60.5mm
Pitch dia 63mm.
My rotary table has 1:72 ratio. I will use 28 hole indexing plate. Increment will be 1 turn and 4 holes.

I have some questions about the operation as this is my first time.
1. What should be the RPM?
2. Should I do full depth of cut (2.25 mm) or do in steps?
3. Should I do conventional or climb milling?
4. Feed will be manual and I will try to be as slow as possible.

Any other concern or trick that I can use.

Regards
Nikhil

 

[Jasonb]

1. Speed will depend on the diameter of the cutter and material being cut
2. Full depth in one pass
3. Conventional
4. Better to work out a decent feed rate based on 1 and number of teeth on the cutter so you don't let it rub which will blunt the cutter

 

[Jasonb]

Oh just saw you said aluminium.

Assuming cutter is 50mm dia 5-600rpm feed of around 75mm/min.

 

[Nikhil Bhale]

@Jasonb
Thanks for the help.
I was able to cut the gear.
I read in your post somewhere that mod 1 gear depth is 2.4mm. I kept the same.

This was my first time cutting a gear.
Now I am a little less scared to use a rotary table and indexing plates.
I now have to cut more gears, 42T, 30T, 16T and 10T.

One question
The minimum number of teeth mentioned on gear cutter are 12. So does it mean that if I have to cut a 10 tooth gear it is not possible? Or is there some workaround method that I can use.

Regards
Nikhil

 

[Jasonb]

When the number of teeth gets low they need to be undercut which can not be done with this type of cutter. If the load is not great you can play about with the depth of cut and also shifting the cutter up and down slightly combined with an adjustment to the angle of the gear which is similar to the "parallel depth" method of cutting bevel gears.

 

[RM-MN]

You haven't said what the gears are for which might help us guide you into how to make the best use. If the gears are for a lathe to cut threads, that is different from gears to fit into a gearbox to use on a power mower.

 

[timo_gross]

When the number of teeth gets low they need to be undercut which can not be done with this type of cutter. If the load is not great you can play about with the depth of cut and also shifting the cutter up and down slightly combined with an adjustment to the angle of the gear which is similar to the "parallel depth" method of cutting bevel gears.

I am confused. I thought the undercut will be there, but it is not really. The tooth gaps are still opening. (pressure angle 20° Modul1 and 10 teeth).

Is the undercut only a repeated myth? Or is there something wrong with the CAD software (found several mistakes in gear macros, so I would not vouch for it)

With six there is still no "undercut". Suspicion rising!
Do I miss something?
The tooth gap changes, but I do not get the promised undercut.

Greetings Timo

 

[Nikhil Bhale]

You haven't said what the gears are for which might help us guide you into how to make the best use. If the gears are for a lathe to cut threads, that is different from gears to fit into a gearbox to use on a power mower.

The gears are for a model traction engine I am trying to built.
They are not super critical dimensionally or otherwise.

Regards Nikhil

 

[danallen]

You may have to play around with the depth or maybe rotate the blank a smidge either way to get it to fit. I would try it if was me.

 

[Klrskies]

I am confused. I thought the undercut will be there, but it is not really. The tooth gaps are still opening. (pressure angle 20° Modul1 and 10 teeth).
View attachment 154938
Is the undercut only a repeated myth? Or is there something wrong with the CAD software (found several mistakes in gear macros, so I would not vouch for it)
View attachment 154939
With six there is still no "undercut". Suspicion rising!
Do I miss something?
The tooth gap changes, but I do not get the promised undercut.

Greetings Timo

I believe production commercial and precision gears would likely be cut with a Hob instead of a profile cutter. The Hob gear cutter is in the shape of a rack and generates the tooth profile as the hob is synchronized to rotate with the pitch of the rotating gear and would likely begin undercutting in that type of gear cutting process once a minimum number of teeth have been reached. Check out "gear hobbing" on you tube to see the process. It's pretty impressive.

 

[timo_gross]

The gears are for a model traction engine I am trying to built.
They are not super critical dimensionally or otherwise.

Regards Nikhil

Maybe you give the small one a try. Curious what comes out of it.
A regular 10 tooth gear will only be 12 mm outside diameter and 7.5 mm root diameter, so a geared shaft will be easier to make.
Another option is to go down with the module to the next smaller size, increasing tooth count and keeping center distance same. (provided one is having all the gear cutters, or is willing to make them).

 

[TSutrina]

This is the gear modal 1mm 10 and 6 teeth 20 deg pressure angle and pitch circle. Created by part design work bench on freeCAD21. This is a good design. As you can see this is an undercut gear.
The only problem I have seen with the Gear work bench is the clearance at the root of the gear that may not have the recommended radiuses and specifically for ring gears. This clearance is missing from the above gears profiles.
The gear cutting set of cutters list the range of number of teeth that it can cut. YouTube has a handful of videos on DIY gear cutting with some not using a purchase cutter from a set.
There two methods that can cut undercut gears and independent of the number of teeth. The first is a rack multiple trapezoidal tooth cutter. This method may not achieve sufficient smooth teeth of an undercut gear without making at least two passes. This is a bobbing tool.

This is the tool for the trapezoidal cutter to use with a rotating table. http://www.helicron.net/workshop/gearcutting/gear_cutter/
The second method uses a slitter to cut gears and requires multiple passes to achieve a involute shape. This give insight into setting up the second pass for the first method.
And their are DIY youtube for hobbing a gear with an stepper motor driven rotating head.

 

[Jasonb]

Several of the CAD programs that have gear generators don't really do them with the correct form so you won't see the under cut.

Timo, using different DP gears was actually done on full size traction engines, the 2" scale Fowler that I made replicates by using 12, 10 and 8DP gears with the 12DP being on the smaller dia crank and 2nd shaft.

 

[timo_gross]

I believe production commercial and precision gears would likely be cut with a Hob instead of a profile cutter. The Hob gear cutter is in the shape of a rack and generates the tooth profile as the hob is synchronized to rotate with the pitch of the rotating gear and would likely begin undercutting in that type of gear cutting process once a minimum number of teeth have been reached. Check out "gear hobbing" on you tube to see the process. It's pretty impressive.

Yes, most gears are hobbed. There are hobs for final dimension and others have slightly different tooth to give a grinding allowance.
The symbol on the hob suggests an undercut. From the side you can see the profile of the "rack".
"basically a glorified tap"?

 

[TSutrina]

The CAD designed gears are not being cut with a CNC program that would require a very very accurate gear. The freeCAD program allows you to increase that points along the involute. http://www.helicron.net/workshop/gearcutting/gear_cutter/ Is it good enough? However DIY gear are cut with a few methods that do not depend on the shape created by a CAD program. and bob fixture for mill

 

[timo_gross]

Youtube channels I checked out:
LSCAD: (very good approachable DIY gear content, using commercial tooling) https://www.youtube.com/@prof-rieg
AndysMachines: All sorts of gear cutting methods. (e.g. mentioned method, using a slitting saw, hobbing and more see post from TSutrina)
Evolvent design: making gears on "commercial machines" with explanations https://www.youtube.com/@EvolventDesign
Robertt4522: gear making videos and diy rack hob https://www.youtube.com/@robertt-cs8fe
Toms rabbit hole: Must have seen. https://www.youtube.com/@thomasstover6272
clickSpring: some videos how to make gear cutters
BaxEdm: Maybe bit off toppic, he "brute forces gear cutting with wire EDM machine" https://www.youtube.com/@baxedm9806


The hob appears to be very expensive at first, but keeping in mind that one hob can do all tooth counts. A 120USD hob from (you know where) replaces a complete set of disc cutters. Only to be surpassed by the 10USD carbide slitting saw method, tedious but unbeatable flexible.

 

[timo_gross]

... As you can see this is an undercut gear....

Hello, no I do not get it. Will it make a real undercut?

For the screenshots from the Freecad program I could "pull" the tooth gap out radially. Undercut means, I thought, that it would not be possible to make a disc cutter with the right profile.
I might try it and post the results here. But not today.

Greetings Timo

.p.s. after another while reading, I think I got it all wrong. The word "undercut" means that the tooth root is skinnier than the tooth at the pitch diameter.

 

[Peter Twissell]

Am I missing something?
I don't see how it would be possible to generate an undercut in a single pass with a rotating cutter.

If the tips of the cutter were shaped to form the undercut, they would remove material from the flanks of the gear, leaving no undercut and an incorrect form.

I can see how the slitting saw method could do it, but not a full form cutter.

I have access to a Drummond gear shaper which generates the form in a series of linear cuts, incrementally rotating the cutter and blank.

 

[Jasonb]

peter as I said you can't undercut with a typical involute type cutter.

I can be done with the rack form cutter though they are not really single point but do act like a gear planer with more than one tooth cutting per pass which will create an undercut on the cuts either side of the one that is on ctr line. Ideally you still need to shift the cutter up and then down to get a better profile on small gears adjusting the indexing of the gear to match so not a single pass

 

[timo_gross]

Am I missing something?
I don't see how it would be possible to generate an undercut in a single pass with a rotating cutter.

If the tips of the cutter were shaped to form the undercut, they would remove material from the flanks of the gear, leaving no undercut and an incorrect form.

I can see how the slitting saw method could do it, but not a full form cutter.

I have access to a Drummond gear shaper which generates the form in a series of linear cuts, incrementally rotating the cutter and blank.

Yes and no.

As I think (from what I was reading recently), undercut means that the tooth at the root diameter is thinner than at the pitch circle. (dashed line). But when the Tool can pe "pulled" out to the side without interference it should be possible to cut the gap, or not?
A four tooth gear exaggerates the undercut, but still the tooth gap has no wider part that would interfere. (maybe I am missing something, hoping for someone with more knowledge to solve the riddle)

Greetings Timo

p.s. the slitting saw can do it definitely, even if I have a misconception.