Author Topic: helical gear with cnc  (Read 1746 times)

Offline michelko

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helical gear with cnc
« on: January 16, 2019, 09:08:36 AM »
Hi,
after some Research and heavy thinking i was able to adapt the cfellows helical gear tool into my cnc machine.
Beeing a Little lazy i didn┤t want to build the fixture. At first i tryed a 45 ░ helical gear.
I mounted the cnc dividing head under 45 ░ Degree on the table and used a m0,9 involute gear cutter.
After some calculations a made a quick and dirty short program.
at this setup the mill has to move three axes simultanious (x, y a)

Here is a video from machining:

And here is a video testing the gears on my lathe.

came out realy nice.

Michael





« Last Edit: January 16, 2019, 09:11:49 AM by michelko »

Offline Admiral_dk

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Re: helical gear with cnc
« Reply #1 on: January 16, 2019, 11:45:22 AM »
That looks nice and easy, but I kind of expect that it took quite some calculations and verification, before these videos  :thinking:
Or did you have a CAM program that took care of that for you ?

Best wishes

Per

Offline kvom

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Re: helical gear with cnc
« Reply #2 on: January 16, 2019, 12:13:24 PM »
The good news is that each "valley" is machined with a single g-code move, so you just have to determine the cutter and material positions at start and end.  The control synchronizes getting all the axes so that they reach the end at the same time.

It looks as if the axes here are x, z, and a.

My fourth axis is very heavy and setting it at a 45 degree angle would be a hassle.  I'm thinking that a button type cutter would work with the gear blank horizontal in X and tool on top.  Then Z and Y would be constant and I'd just move X and C together.

Offline michelko

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Re: helical gear with cnc
« Reply #3 on: January 16, 2019, 01:20:33 PM »
Hi,
the quick and dirty g-Code looks something like this:

g91 g1 x-15 y-15 a -190,98 f30  [syncronized cutting move]
g91 g1x15y15a190,98 f60 [retract]
g91 g0 a-36 (next tooth)

Then copy and paste these lines 9 times and you have the program for an 10 tooth gear

Michael

Offline Admiral_dk

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Re: helical gear with cnc
« Reply #4 on: January 16, 2019, 09:42:30 PM »
Thank you Michael

OK this is relative simple - though I have to admit that I haven't used the G91 instruction yet ....

One question from one who haven't made gears yet. If you wanted a 12 tooth gear here, would it just be a-30 in the last line and maybe adjust the depth of cut ?

Offline michelko

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Re: helical gear with cnc
« Reply #5 on: January 17, 2019, 03:57:41 PM »
Hi yes the last line is the tooth angle (360 /tooth).
In this case The absolut coordinates are not relevant.
Just touch of and set the machining depth.
Because of the aluminium i used just one pass, steel requires maybee more smaler passes.

Michael

Offline kvom

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Re: helical gear with cnc
« Reply #6 on: January 17, 2019, 06:59:52 PM »
I may be blind, but why/where is the Y axis move in the video?  The spindle must move downward as the cut proceeds.  That's Z on my mill.

Offline michelko

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Re: helical gear with cnc
« Reply #7 on: January 17, 2019, 07:57:21 PM »
 :facepalm:
Offcourse you are right. My fault

It moves x,z and a :ThumbsUp:
Y is setup at the beginning as cutting depth and then left steady

Michael

Offline kuhncw

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Re: helical gear with cnc
« Reply #8 on: January 19, 2019, 09:19:15 PM »
Michael,

Thanks for posting your code.

"g91 g1 x-15 y-15 a -190,98 f30  [syncronized cutting move]
g91 g1x15y15a190,98 f60 [retract]
g91 g0 a-36 (next tooth)"

What is the function of the 98, following a-190 and a190 ?

Regards,

Chuck

Offline kvom

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Re: helical gear with cnc
« Reply #9 on: January 21, 2019, 06:23:00 AM »
In Europe they use comma instead of period as a decimal point.

Offline michelko

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Re: helical gear with cnc
« Reply #10 on: January 21, 2019, 08:46:53 AM »
You are Right. for Linuxcnc it must be something like this

g91 g1 x-15.00 y-15.00 a -189.9 f30

michael

Offline kuhncw

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Re: helical gear with cnc
« Reply #11 on: January 21, 2019, 03:49:48 PM »
Thanks for setting me straight.  I keep forgetting about , vs .

Chuck

Offline jadge

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Re: helical gear with cnc
« Reply #12 on: January 22, 2019, 11:34:50 AM »
I'd be interested to know how the OP got on with feedrates. Any sort of X, Y or Z move is obviously in distance per time, and a pure rotary movement is in degrees per time. As far as I can see a combined linear and rotary movement should be in distance per time, accounting for the longer path due to rotation.

When I cut helical paths, to machine a worm, I had all sorts of issues with the feedrate. My rotary table just ran flat out irrespective of the specified feedrate. This was with a Tormach customised version of Mach3. I wasted a huge amount of time trying different things. In the end I came to the conclusion that this particular feature in Mach3 plain didn't work. I ended up using inverse time feedrates aka G93. For those not familar with G93 the time (in minutes) to do the cut is the inverse of the number. So if the code is G93 0.2 then the cut will take 5 minutes. Oddly, this worked absolutely fine in Mach3. There was a small time error,  few seconds, but it was consistent and fixed.

Andrew

Offline kvom

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Re: helical gear with cnc
« Reply #13 on: January 22, 2019, 01:38:56 PM »
A CNC control is responsible for ensuring that coordinated axes move to arrive at the end point at the same time and in a linear fashion.  I had read that Mach3 had a problem doing this with a rotary axis unless using inverse time, but I have verified that my PathPilot control does it correctly.  I'm not sure how to calculate feed rates, but I'd probably treat a round cutter like a saw.

Offline jadge

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Re: helical gear with cnc
« Reply #14 on: January 22, 2019, 04:01:56 PM »
Good to know that PathPilot works properly with 4-axis moves. I've upgraded, but haven't tried it with continuous 4-axis moves, just indexing which works fine. In my hiccups with Mach3 Tormach effectively stated that the 4-axis moves, and diameter/radius compensation boxes (whatever they are), didn't work.

I'm not sure I understand the comment about feedrates? For a helical cut I simply unwrap the helix mathematically to get the total length of cut and then apply normal calculations as for a simple linear cut.

Andrew

Offline michelko

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Re: helical gear with cnc
« Reply #15 on: January 22, 2019, 05:07:02 PM »
I am using linuxcnc and it worked right out of the box on the simultanious moves.

Offline kvom

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Re: helical gear with cnc
« Reply #16 on: January 22, 2019, 11:31:30 PM »
For a 45 degree helix the length of cut is the hypotenuse of the triangle or 1/cos(45) * linear length.  The rotation is the wrapped length of the opposite side or (1/sin(45))*L*360/(pi*D).

Feed rate for the linear cut depends on the material and the form of the cutter.  It seems to me that circular gear cutter cuts more like a saw than an endmill for initial calculations.

PathPilot is linuxcnc under the covers.


Offline jadge

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Re: helical gear with cnc
« Reply #17 on: January 23, 2019, 07:47:55 AM »
It seems to me that circular gear cutter cuts more like a saw than an endmill for initial calculations.

Correct. To calculate the spindle speed simply take the OD of the involute cutter and use that to calculate in the same way as for an endmill. It works the same for side & face cutters and slitting saws. Feedrate is simply cutter rpm times number of teeth times the chip load per tooth. Both spindle speed and chip load depend upon the material, but I don't take the cutter form into account.

Andrew

Offline jadge

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Re: helical gear with cnc
« Reply #18 on: January 23, 2019, 07:51:36 AM »
I am using linuxcnc and it worked right out of the box on the simultanious moves.

Did you measure the actual feedrate? I only twigged I wasn't getting the correct feedrate when I cut my first worm in steel rather than plastic. After roughing out with 6mm and 4mm carbide endmills the final cut was with a tapered HSS endmill at a significantly reduced spindle speed and feedrate. I got chatter and poor finish which is what made me look into the motion in more depth.

Andrew

Offline michelko

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Re: helical gear with cnc
« Reply #19 on: January 24, 2019, 08:22:11 PM »
I didn't measure it but linuxcnc shows the programed feedrate in the display.

Michael