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Author Topic: CNC Gear Cutting  (Read 8169 times)

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Re: CNC Gear Cutting
« Reply #10 on: November 12, 2008, 06:30:48 PM »
Bill,

I already had that set but did go to the shop and try again.  I played with the settings a little and re-read the posts on this subject but no luck.  Not sure what's different but I may have to double check my version and make sure I have the most recent version as I noticed that some posts sugested it was a new or fixed feature.

Best Regards.

Carl
Re: CNC Gear Cutting
« Reply #11 on: January 06, 2009, 10:41:19 PM »
mach 3 will round pulses up. I also had the same problem in my head. here is how I figured it out.
first I set my 4th axis to 1 pulse in motor tuning then I used the "cut spline or gear" wizard and selected gear then set number of teeth to 32  and posted the g code, zero the 4th axis and run now you can see how mach 3 handles fractions of pulses. 11.25 rounds to 11 and 22.5 rounds to 23 and so on. But if you use this wizard it will continually correct it self and not add the rounded increments by use of G90 in the code it produces. 
« Last Edit: January 06, 2009, 10:43:05 PM by sparkness »

Offline RICH

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Re: CNC Gear Cutting
« Reply #12 on: January 07, 2009, 12:08:26 AM »
Just remember that 1 sec is equal to .000004848" in an inch and originaly you were talking
around 2.52 MINUTES  of concern which is .0007" in an inch.
Machine and enjoy.  ;)
RICH
« Last Edit: January 07, 2009, 06:55:18 AM by RICH »

Offline simpson36

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Re: CNC Gear Cutting
« Reply #13 on: January 07, 2009, 08:45:10 AM »
Interestring thread. I just thought I'd add my 2 cents.

I"m new to home grown retrofitting and manually programmed CNC, but I have many years experience as an engineer and have done a lot of machining for prototypes of all sorts. My comment is that I have to agree with the folks who opined that this is really an academic discussion as the minute error at issue here is insignificant for almost any real world application.

The 'accuracy' of a gear from a 'timing' perspective is related only to the number of teeth. Any inaccuracy in individual tooth spacing would only effect that specific tiny portion of the trace and be completely gone by the time the gear moved to the next tooth. i.e. 32 teeth is 32 teeth and that is how many teeth will pass per revolution, no matter if a tooth or two are slightly off of their true theoretical azimuth or slightly over or under thickness.

Manufactured gears have a tolerance. Many are broached in a single pass. Like bearings, there are many different accuracies available with corresponding cost.  Gear tooth contour accuracy effects mostly the amount of sound (harmonics) produced and wear characteristics. No cut gears are really "accurate". The only applications that I know of requiring super accurate gears call for hardened and ground teeth and are for super precision, super smooth (quiet) or very high rpm service, none of which would apply to a clock, I would imagine.

Even in the most accurately produced gears, ultimately the 'final machining' is done by the gear mesh itself during break-in.

So, as has already been stated, use absolute angular positioning to eliminate a tolerance stack, rely on the accuracy of your setup (which from where I'm sitting looks more than adequate), machine and enjoy!







Re: CNC Gear Cutting
« Reply #14 on: January 07, 2009, 08:57:34 AM »
Simpson36,
    VERY GOOD explanation.
More like $2 worth than 2 cents.
Thanks,
RC
Re: CNC Gear Cutting
« Reply #15 on: January 07, 2009, 10:02:37 AM »
My concern was not with machine accuracy, but with program accuracy, and I needed to prove it to myself. I wanted to make sure that if I cut a gear that I do not increment and accumulate on a rounded number. the spline and gear wizard creates code that adds 1 rounded increment to the correct absolute value each move. Therefor every tooth cut will have the same nano error instead of progressively increasing to the last tooth cut. This is correct way to calculate each move. Program accuracy and machine accuracies are factors in the final finished parts accuracy. So now I know what mach3 is doing this gives me insight as to how to write good code.

Offline simpson36

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Re: CNC Gear Cutting
« Reply #16 on: January 07, 2009, 11:38:33 AM »
Interesting . .

Out of curiosity, I just did a few tests changing the increment value accuracy and the steps per degree setting to see the relationship.

My findings are that Mach sets the A axis to the nearest physical step, and that is what shows in the DRO. The nearest step is a physical necessity and showing that value in the DRO seems logical to me. However, internally it would seem that Mach is maintining the full accuracy of whatever increment is specified in the G-code, and uses that for calculations.

If that finding is correct, then Mach will automatically utilize the maximum accuracy your machine is capable of, provided you input enough decimal places in your increment.

To test this yourself, simply use a wizard to generate a 17 tooth gear. Then set the axis steps at 200 and then at 10,000. note the difference in the incremental DRO readings at each step setting. Yet in both cases, it ends at 360 without being commanded to do so, indicating that there is no accumulated error inside Mach (given the accuracy of the increment).