GearHeads Corner

Hi all,
I'm really new at CNC and gears, so please be kind! I used Gearotic to generate a wheel, 32P, 124T, now translating that to metric, I get the module as 0.793 something, the wheel diameter works out to be just over 100mm, if I set the Use Involute Cutter in the Output Manager and tick 4th axis GCode with a 1mm end mill, I get a cut that makes sense to me, a round wheel with 124 straight cuts, I guess I would need a special cutter that is the same as the tooth profile? However, my understanding is that I can cut this profile with a normal end mill? If I do not choose the Use Involute Cutter in the Output Manger, the machine start cutting the gear, but then moves the Y and A axis and eventually ends up to the left of the A axis just cutting air.... My setup is as follow,
   
    X+

Y+ <> Y-

&nbsp; A facing the gantry

&nbsp; &nbsp; X-

With this setup, I guess the Axis Parallel to Rotary, the Y needs to be ticked and the Chuck faces Axis box in Output manager need to ticked? Sorry for all the questions, but I need to understand this.
Thanks for the help!
&nbsp;

Hi:

&nbsp; &nbsp; Yes, as you found when you check involute cutter youll get code that cuts with a cutter shaped like a tooth. You may get
&nbsp; &nbsp; more than one pass per tooth depending on the perpass depth till it gets to final pass.
&nbsp; &nbsp;
&nbsp; &nbsp; Now, without the involute cutter checked the system will do tangental shaving using a straight fluted end mill. In order
&nbsp; &nbsp; to tangentallly shave it needs to move laterally during the rotation so the axis must be set up properly.
&nbsp; &nbsp;
&nbsp; &nbsp; In your case you would NOT check the axis faces -- checkbox, thats to indicate the axis faces the negative side of the axis,
&nbsp; &nbsp; yours seems (from the diagram) to face the X++ axis. ( again if I understand your drawing..). The " Chuck faces Axis "
&nbsp; &nbsp; box is really " Chuck faces Axis --" , it means the chuck is facing the negative direction. Doesnt sound like it should be
&nbsp; &nbsp; checked. If this box is wrong, the axis will rotate in the wrong direction during the shave.
&nbsp; &nbsp;
&nbsp; &nbsp; Now, the important thing here is the zeroing. The system is calculating an angular offset combined with a Y motion (in your case)
&nbsp; &nbsp; to find the point where the target point is tangental to the bit, so the bit moves laterally in the Y axis while the blank is rotated.
&nbsp; &nbsp; If the zero is off, even a small amount, the final shaving steps could cut air, this isnt a problem generally, ignore it. Let it cut
&nbsp; &nbsp; until its on the third tooth, if the teeth looked gouged out, stop..something is wrong. Probably the blank is rotating in the wrong
&nbsp; &nbsp; direction. Its not uncommon for a cnc system to have the A axis rotating backwards by default. Its usually un-noticed for anything other
&nbsp; &nbsp; than cutting gears in this way. If the tooth looks like its getting chewed away, but your pretty cure everything else is correct
&nbsp; &nbsp; try reversing the direction of your A axis rotation in the cnc program. ( If its mach3, toggle the DIR bit for the A axis ).
&nbsp; &nbsp;
&nbsp; &nbsp; If you look down the X-- axis towards your chuck, it should rotate CCW when jogged A++.. (Im assuming from your diagram the A axis
&nbsp; &nbsp; chuck faces down the X++ direction...)
&nbsp; &nbsp;
&nbsp; &nbsp;
&nbsp; &nbsp; If you check all this you should find it properly cutting your gear. It is VERY important that you enter the proper tool diameter,
&nbsp; &nbsp; an actual measurement. Just being off by .1mm may be the difference between seeing every shaving step touch the gear or the final
&nbsp; &nbsp; 2 or 3 moves totally missing the gear. This wont actually invalidate the gear, its quite common to miss a couple shaving steps,
&nbsp; &nbsp; the gear will still be fine. The more accurate your tool diam is, and the more accurate youe blank dianter is and your zeroing
&nbsp; &nbsp; is, the better the gear will be, and if your deadly accurate the tool will cut on every pass, but if it doesnt, dont worry, even mine
&nbsp; &nbsp; often has 2-4 steps of shaving air because Im slightly off on one measurement or the other. This typically wont affect your gear badly
&nbsp; &nbsp; its just the tip of the involute may be slightly rounded or a bit too sharp. The final shaves are only to sharpen the tip of the involute.
&nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; The number of shaves will be the number you enter in "Evolute Segmentation" , typically set to 8 to tell the system to shave each
&nbsp; &nbsp; &nbsp; tooth 8 times&nbsp; so the involute curve will be made of 8 individual segments. You can raise this if youd like tighter curvature.
&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; Feel free to ask if any of this confuses, its a hard thing to fully explain. :)
&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; Art
&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp;
&nbsp; &nbsp; &nbsp;
&nbsp; &nbsp;
&nbsp; &nbsp;

Hi Art,
Thanks so much for the explanation, it does make sense (I think) attached are some photos to explain my setup
I will definitely try this over the weekend and let you know if it worked for me, as far as the zeroing is concerned, I take it I place the bit on top of the blank at 0 deg on the A-axis and then move it just past the blank on the X axis in negative and then zero my X-Y-Z-A settings?
Thanks again for the detailed explanation!

Hi :

&nbsp; On most gears you zero to the blank just ahead of it, ( side furthest away from chcuck of the&nbsp; A axis). The Gcode will tell you at the top where to zero. Bevels zero the the chuck side because the blank is beveled, but most gears zero to the side away from chuck. ( If you have it wrong youll know because it wont cut anything. :) )

&nbsp; If you find it seems wrong, just reverse the A axis direction. But, yes, when you zero, zero everything. Then cycle start, it should start by cutting into the gear at 0 degrees to start root formation of that tooth.. It will become
clear as you watch it cut what its doing.. :)

Thx
Art

Come on weekend!! Thanks Art!!

Hi Art,
I tried this again, but there is a lot of time spend just cutting air, I tried reversing the A axis, but that did not help either, I also had to set the "Axis Parallel to Rotary" as Y, I'm a bit confused with this setting, let me do a drawing again

&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;X+
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;^
Y+ &nbsp; < &nbsp;Gantry &nbsp;> &nbsp;Y-
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Spindle

&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Blank
&nbsp; &nbsp; &nbsp; &nbsp; <A- A+>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; X-
The gear was cut, took about 2 hours, meshing seems to be ok, but the teeth depth seems to be a lot, changing that Final Depth does not seem to make a difference. I'll try to upload a video to illustrate the air cutting...

Attached is the G code if that would be of any help?

What air are you cutting? Is it as though the gear were much thicker than it is?

It seems to be cutting too deep and too wide, could this be a problem of unit conversion somewhere in user interface entry field?

--Justin

Hi Justin,
It cuts the 1st tooth, then shaves the tooth, then move to the Y- way past the blank and then starts cutting air, once that is done, it move back on Y+ to the blank, cuts the 2th tooth, shaves and then moves back in Y- and starts cutting air again. I'll try and compress a video I made and upload it

Is the tool comp/tool radius in inches or mm?
.5 inches + 7mm + .5 inches = 32.4mm. Is that about how far it is moving?
It should&nbsp; be moving:
.5mm + 7mm + .5mm = 8mm.

Justin

Hi,
I've uploaded the video to http://youtu.be/QaW5jkDZGoY , maybe that would help to illustrate the air cutting&nbsp; ???
As far as I know, everything was set to mm, in the screenshot of the output manager the diameter of the tool was set to 1mm
Thanks for the help so far!!

It looks like it is trying to cut with the side of a tool that is an inch wide, not a mm wide.

In the Output Manager, in the NC Parameters box, in the Tool Diameter field, try dividing that number by 25.4 (mm/in).

There is another, proper place where this should be corrected, but it slips my mind (I'm only out of bed for a "business trip").

Thanks Justin,
Will try the settings!

Hi,
Strange but my settings are set to metric, but I will try the INCH to MM over the weekend for the tool diameter

I take back everything I've been saying.
Your posted g-code and the video do not appear to be doing the same thing. I don't think this is a Gearotic issue.

When the root of the gear gap is cut, the X and A axes seem zeroed, but when the tapers are cut, the X and A axes seem to not be zeroed. The "left taper" and the "right taper" are both off to one side, for both the X and A axes. What are you using? Mach3? LinuxCNC? Is the G-Code interpreter reading a different ASCII character definition? Does the code your NC program shows on the screen match the code Gearotic puts out **EXACTLY**?

Very weird.

--Justin.