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Author Topic: Bevel Gear Stock Prep  (Read 15369 times)
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markwoodard41
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« on: March 22, 2011, 03:39:17 PM »

I am a bit confused by my analysis of the various angles shown in the Project screen as compared to the 2d DFX (and research machinist handbook, wikipedia et al to understan d the definitio ns).

For a particula r gear (20T, 12 dia pitch, strt, bevel) the GM project screen shows the face angle to be 58.324, root angle is 69.3434,  pitch cone angle to be 63.4349 and the cone distance is 0.9317" yet the tap file shows the face angle to be 68.5459 and the 4th axis tilt to be 63.43 degrees  (root angle not stated). 

I did some dimention ing of the DFX drawing and I think the face angle as drawn is 68.55, root is 57.53 and the cone distance is 0.93573". 

I know nothing about gears (or machining for that matter) but if I am reading the definitio ns correctly, shouldn't the face angle always be less acute than the root angle?  I suspect that I don't have the root angle dimension ed correctly because I am assuming the drawing doesn't show points to use to measure it.  (seems to me I read that it is an imaginary point someplace between the bottom of the tooth and the face of the tooth, guessing it is related to pitch diameter). 

As I write this, I see that it may be that the only discrepen cy is with the GM project screen showing the face to be 58.324 with the tap file and the DFX consisten t at 68.55. 

I am trying to figure out the angles to use to cut the blank and my head hurts...

Do I cut the blank at 68.55 degrees (face angle) and the opposite side of the blank at 21.45?  I will tip the 4th axis to 63.42 when I get to the point where I can actually cut it.

Mark
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ArtF
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« Reply #1 on: March 23, 2011, 01:21:57 PM »

Mark:

 Im thinking the tap file is correct and the project screen wrong on the face angle.
I'll checkout why. When I cut my tests I used the tap file settings to make my blanks.
   Its noteworth y too, that you cannot use standard machining definitio ns to check the bevels,
just as each company uses various definitio ns, I too do so. My Bevels are made with proper
tooth form to the initiatin g spur gears, variosu systems distort the tooth form to confrom to
differing machining limitatio ns based on the machines used. I decided to start with standard teeth
and allow for stubbing, pitch shift and such to match whatever someone else may use.

  Standard gear bevels though seem to run fine here, use the tap file settings for the blank till
I check it out. A new version will be out soon, so Ill check why the discrepan cy in display before I release. Ive added GT style timing pulleys and a couple other features that I just completed,
so Ill hold back to check this out before I do more.

Art
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markwoodard41
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« Reply #2 on: March 24, 2011, 09:18:46 AM »

Thanks for clearing me up Art.   I am trying to make some time to finish the work to tilt my A axis accuratel y.  (Sine plate fixture on my rotary indexer)  I hope to cut some bevel gears from maple stock before the end of the week.  If all goes well I may figure out how to show the results in the forum.  If not....

Any idea when you will have 4th axis functiona lity for the timing pulley function?  I want to cut some XL pulleys 0.5" thick but given my current cutter inventory, I can't make the 0.5" depth of cut in 2.5D.  4th axis would be the way to go for me.

Mark
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ArtF
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« Reply #3 on: March 24, 2011, 09:50:28 AM »

Mark:

 Hard to say how long that will take. It takes special classes to do it for each gear type. Im condierin g if I can make a generic class to do any tooth form, but thats experimen tal so I cant promise.

Art
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markwoodard41
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« Reply #4 on: March 25, 2011, 11:08:09 AM »

Thanks Art.  I'll take the DFX and create my own 4th axis tool paths until you sort yours out.  Just more complicat ed for me.  :-)
But do-able.

Mark
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bosr
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« Reply #5 on: March 25, 2011, 05:00:27 PM »

Same here, I have 2 One Inch thick pulleys to make.  I wanted to mill both at the same time from a single piece of stock (1:1) so that means a depth of 2 plus inches for me.

4th axis is the only way I can cut gear stock (ie 6 or more inches).

I hope the 4th is bumped in priority.
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John S
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« Reply #6 on: March 25, 2011, 06:50:47 PM »

You can use the 4th axis for timing pulleys if you go to spur gears and select the timing gear you want from the library.
They are only modified spur gears which can be cut with a 4th axis.
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John S.
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BobL
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« Reply #7 on: March 25, 2011, 08:43:07 PM »

Hi Guys;

 I hope I'm wrong John, and forgive me if I am, but I don't think 4th axis cut is possible with timing gears at this point, only 2D for now.... I know the topic was discuss earlier and I don't think a solution exist yet for 4th axis cut with timing gears ... Art didn't mention it to me anyway... I give it a try and will post my finding tomorrow.


Cheers
Bob Sad
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John S
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« Reply #8 on: March 26, 2011, 04:36:11 AM »

Heretic.

Oh ye of little faith.... ......... ......... ......... ...... Smiley
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John S.
Nottingha m, England
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« Reply #9 on: March 26, 2011, 08:26:49 AM »

lol
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Bob
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« Reply #10 on: March 26, 2011, 09:33:55 PM »

Hi:

 Johns right. If you select a timing pulley type to generate the gear, you cannot cut it 4th axis, BUT , if the gear is a normal gear that you fudeged to be the right size of a timing pulley ( roughly..Smiley ) , then it will cut in 4th axis. You cannot though, fudge a GT type gear.. but the xl series you can get close to with using tooth stubbing, and width etc.. to get close.. in that case the 4th  axis should work..

  The problem comes in because in reality the timing pulleys are not generated mathmatic ally as a curve type .. ( involute or epicycloi dic..) they are a series of spec'ed curves with no real mathmatic al relations hip to each other.... so the 4th axis has no parametri c basis to derive its tangental motions from.

   This is why the 4th axis is limited to involute tooth type or bevels.. Im thinking of ways to do general DXF point data tangental s to make a generaliz ed algorithm to do 4th axis on any gear.. but that may take awhile...

Thx
Art
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John S
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« Reply #11 on: March 27, 2011, 05:51:27 AM »

Never worked with  GT pulleys and never studied them so can't comment on their tooth form BUT if you switch to HTD or HDT depending on who's making them they can be cut by just using a ball end milling cutter and indexing round one tooth at a time with the cutter at the top.

The HTD 3mm pitch series needs a 1.82mm ball nose end mill and the closest is 5/64" or 2mm at a push.
The HTD 5mm pitch series needs a 3.12mm ball nose end mill and the closest is 1/8"
The HTD 8mm pitch series needs a 5.14mm ball nose end mill and the closest is 5mm
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John S.
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« Reply #12 on: March 27, 2011, 07:19:41 AM »

John:

 Interesti ng.. I guess that tells me what I need to know for that series then.. the GT's are actually quite complex curves, not round. ( The specs I have for them even contain a few "mistakes" to make curving difficult . )

Art
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markwoodard41
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« Reply #13 on: April 03, 2011, 01:46:48 PM »

I have finally finished my tilting A axis and created stock for a beveled gear using the DFX file parameter s.  In a previous post I discussed inconsist encies between the DFX and the tap file but I now think it was just my lack of understan ding.  The DFX has the same face angle as the tap file.  I believe that I cut the face angle to correct spec.  The angles on the stock seem to match up with what I see on the GM project screen.  (appearanc es can be deceiving).  I set the A axis to the proscribe d angle in the tap file and zeroed the cutter at the rear, top of the blank. 

After that, the fun began.  I cut a a tooth and didn't like what I saw so I stopped the operation .  The tool doesn't cut all the way through face of the blank.  I have included two photos showing the postion of the cutter at 0,0.0 and a closeup of the blank/tooth. 

I am afraid I have the blank cut wrong but am at a loss to identify what is incorrect . I notice that the face of the blank is not in the x/y plane.  Should it be?  Any observati ons or assistanc e would be much appreciat ed.

Mark



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ArtF
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« Reply #14 on: April 03, 2011, 02:21:26 PM »

Mike:

  Looks to me like the blank is backwards on the indexer?

When placed for cutting, the fact angle shoudl be almost flat. In fact when the tool is placed to the back of the blank, the blank shoudl always tilt down by the addendum angle.. usually pretty low.. so the face will look almost flat, but tilted down a few degrees.

It looks ( hard to say ) like if you reversed the blank at the same tilt angle, that blank woudl be near flat.. slightly tilting downwards?

Art
 
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