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Author Topic: Machining Bevels.  (Read 56750 times)
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John S
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« Reply #30 on: December 26, 2010, 01:33:03 PM »

Can't see a problem and if it works for you then why not go this way.

At the moment we have no way to cut bevels easily - period, so anything is going to be better and Chucks idea of locking the angle to 45 is very good as if you need a tilting devise for say a rotary table then you only need the one fixed option, far easier to make / modify / setup.

Art,
Can I request that you keep the tapered cutter option on the table for small pitches, they are easy to make in HSS or carbide and very strong.
I have just run the figures thru for a pair of 20 DP bevels and they require a 1/16" max cutter, any DP finer than this will need even finer cutters.
If a tapered cutter was used with a defined angle, your call, then the tip would need to be 1/16" but the body could slope up to 1/4" or whatever in metric.
At 1/2 PA angle it would cut most of the gear on it's 'blocking out' operation .

John S.
« Last Edit: December 26, 2010, 01:43:52 PM by John S » Logged

John S.
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Dan
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« Reply #31 on: December 26, 2010, 02:53:32 PM »

Good idea! If it is doable code-wise and you can do any bevel angle this way - go for it!

Dan
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Dan
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« Reply #32 on: December 26, 2010, 02:55:43 PM »

Can I request that you keep the tapered cutter option on the table for small pitches, they are easy to make in HSS or carbide and very strong.


Hi John,

Would be nice if you could share your experienc e in making these. They are not cheap to buy.

Dan
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Chuck
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« Reply #33 on: December 26, 2010, 03:52:23 PM »

Dan,

The small engraving carbide cutters are about $2US each on ebay.

For larger pitch a strait endmill will probably be best but won't do the gears down to 80DP I want.

look on ebay the taper endmills and engraving bits are sold by taper and tip diameter. ...the stock angles we are likely to use are 20, 30, 45 and 60 degrees included angle.  A 20 (or maybe 22.5) degree pressure angle should work well with the 45 cutters but 30 should do any pressure angel we see on bevels.

If Art gets the bevels working with taper cutters maybe he can go back and work on spur gears with taper endmills. Then the 4 axis spurs can also be cut down to 80DP..... ..Art??

http://cgi.ebay.com/10-PCS-CNC-Router-Carbide-PCB-Engraving-Bits-30-0-5mm-/180604601886?pt=LH_DefaultDomain_0&hash=item2a0cdf861e

Chuck in Wyoming
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ArtF
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« Reply #34 on: December 26, 2010, 04:29:43 PM »

Chuck:


  Other than the Z suface modifcati ons the bevel Gcode source will be very close to the spur gear source. (In fact Im starting from an exact copy and mofiying it to do bevels. So while Im doing that, I will find a tapering hook and see if I can add tool taper to the mix. If it works, Ill port it back to spurs.

Art
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Dan
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« Reply #35 on: December 27, 2010, 02:45:30 AM »

Hi Chuck,

Actually engraving bits never came to mind. When I thought about tapered end mills I was meaning these: http://www.menlo-usa.com/catalog/ProductDetail.aspx?pid=242

Thanks for another good idea. However, they are good for very fine gears, but not for something more substanti al like a 1.5 module gear. The tip radius is too small and they are too weak to take any real cut - compared to the real tapered end mills. But still probably better than using a plain tiny end mill....

Dan
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Chuck
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« Reply #36 on: December 27, 2010, 01:18:15 PM »

Dan,

I will make you a deal,  when Art gets the taper end mills working I will write and post a program to generate Gcode to make any 1, 2 or 4 flute taper mills.  What I have now is code to make strait flutes but can add code to do spirals too.  Strait cut flutes have a much higher cutting load than spirals.

I use a setup on my 4 axis mill with a Dremel grinder and diamond wheel to make carbide taper endmills.  You could use hardening stock and an endmill, or hard drill rod or carbide and a grinding wheel.

The first gears I made this way was back in 1974 with an early Fortran version of the "one tooth hobbing" program I posted in 3rd party programs on this forum.

Chuck
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Dan
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« Reply #37 on: December 27, 2010, 01:38:44 PM »

Very interesti ng, Chuck!!! Would be nice to be able to grind taper end mills from carbide stock.

Dan
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ArtF
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« Reply #38 on: January 08, 2011, 02:46:05 PM »

Hi Guys:

  Just a status report..

 Ive learned quite a bit about the math of making a bevel with involute shaving, but still havent managed to complete the algorithm s to cut them. I can see trouble though..

   When we create a helical or spur on the 4th axis, we do it by tangental shaving. This means we take a point on the involute curve of the tooth, and rotate it till its tangent to the Z axis ( the tool). Since its tangent to the tool we're OK to make a pass which will shave only points above that involute tangent point..
( Hard to visualize I know..).

   Our problem with bevels is that tilted gears do not change tangent at the same rate as a spur.They rotate their tangent at a ratio of 180 - 2*PitchCone angle / 180 . This means some bevels never hit a tangent point
during their rotation, others have to be rotated to a involute angle times the ratio above.

   What this all means is the amount of involute angle for a tooth is limited by the above ratio ,
so as the gear increases its pitchcone angle, the amount of rotation of the toolpath necessary
 to get to a shaving point increases . When it hits 90 degrees your ususally hosed.

 The best way to picture this is to picture a 90 degree bevel pitchcone gear.
 This is a gear laying totally flat. It has no angle. Its teeth go straight in to center. Obviously,
there is no rotationa l position that will make those teeth go tangent to the Z axis. The involute tangents
 point the same direction in any rotationa l angle of the gear.   The other extreme
is the 0 degree bevel, ( a normal spur gear ), where we'd have no trouble as the tangent rotationa l angle is equal to the
involute angle for the point in question. Since the involute angle on  a gear never goes as high as 90, the rotation that
has to be done is always less than 90 degrees. But in a 20 degree pitchcone angle, for example, you need to rotate the gear almost 180 degrees. ( cutting the gear on its back face).

   The numbers tell me that 90 degree pitchcone angles are impossibl e, while 0 degrees are easily done. Basically
all bevels fall within the two extremes. Whether a bevel is possible or impossibl e then, falls to a certain amount
of variables that affect the end max involute angle.. Tooth count is important as the lower the tooth count the
higher the involute angles are, thus limiting the amount of pitchcone that can be cut. The higher the tooth count the
more easily it CAN be cut, but the larger the gear. ( A limitatio n for most of us based on rotary table size. )

 John Stevenson, who is annoying correct almost always, was also correct in his assumptio n that tilting to the pitchcone
angle ( thus making the tooth pitchline flat at the top of the table ), would make the calculati ons easier. In fact tilting to
an arbitrary degree such as 45 degree's, makes the calculati ons near impossibl e. By tilting to a straight pitchline Ive managed to
create several routines that check the numbers for me that prove out the above statement s on limitatio n of tangental angles
based on toothcoun t , gear diameter, and pitchcone angle.


  Sooo.. Im starting a third iteration of the code. ( Not unusual for me to go through several iteration s on complex algorithm s as each
try teaches a valuable lesson in getting your mind in the right frame of 3d referance . Smiley Im still convinced its possible, but with limitatio ns
as to what gears can and cannot be cut. Ill need to implement some calculato r to auto figure out if we can cut this on a 4th axis or not.
If nothing else it explains to me why no-one else does this in the context of a 4 axis mill. ( That 5th axis would certainly come in handy here. Smiley


Just a status update as to why Ive been pretty quiet.

Art
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John S
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« Reply #39 on: January 08, 2011, 07:09:41 PM »

Art, Gert says annoying yes, always correct no....... ......... .

John S.
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John S.
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« Reply #40 on: January 09, 2011, 04:08:04 AM »

Hi Art,

Looks like you have been doing an awful lot of work on this over the holidays when you should have been enjoying the break.

Totally impractic al I know but wouldn't it be nice to have a rotary mounted on a rotary ?    Grin Grin

Tweakie.


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ArtF
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« Reply #41 on: January 09, 2011, 09:40:37 AM »

Ive thought o the same thing.. Smiley I kinda doubt it'd fit on my table. Smiley

 Also, the math would be horrendou s I suspect. lol

Art
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Chuck
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« Reply #42 on: January 10, 2011, 09:33:10 PM »

Art,

Thanks for all your hard work...

For the gears with teeth near 90 degrees you always have 3D ball milling to fall back on.  Much easier to calculate too but take longer to machine with lots of fussy little passes.

I tried some graphic 3D modeling to try to visualize the problem with gear on 45 rotated A axis and while I know I can do it, my first attempt failed.  A bit like your first 2 efforts at 4 axis tool path generatio n.

But you are totally correct on the limitatio ns of 4 axis bevels near 90!  Tapered endmills will get you farther but even they have a limit.

Solving this with trigonome try may not be the best solution. Even if it looks like the most direct path.

Chuck
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ArtF
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« Reply #43 on: January 10, 2011, 11:19:38 PM »

Chuck:

 Actually, Ive managed to reduce the trig to a few simple equations . I think this next try will work. It isnt bad once I got past a few bad assumptio ns. I suspect the limitatio n will mean as the bevel angle falls towards zero the involute will degrade to a gleason gear type of pressure angle side. At least the math seems to indicate it will. Thats not necessari ly a bad thing, and low degree bevels that have a pressure angled side will likely roll fine on the pinions conjugate s.. at least thats what Im hoping. This is one of those ones that only testing will tell. Ive made it so the hooks are in place for me to more easily add the zerol and helical aspects once the straight flutes prove out one way or the other. Only testing will tell me more. As soon as Ive cut one wheel and one pinion that mesh, Ill release a version with that code and begin to add the helicals while more testing gets done. I dont dare release a code version till I see physicall y what the results are.. Testing is a pain due to blanking requireme nts, but Im using styrofoam, so its much less painfull when I screw up. Smiley   ( pink styro insulatio n makes an easy blank that shows the path well.. )

  I figure a few more days and Ill know how it will all work, the tangent angles I can now compute with some certainty, its all a matter of IF its physicall y possible now.. Smiley

   Your right that we could fall back to ball milling, but I dont like that solution at all, way too slow...an d nasty to clean up..

Art


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Archie
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« Reply #44 on: May 23, 2011, 06:35:16 PM »

Art et al,

I am new to this forum, but am intereste d in gear making. If this discussio n has continued in a different topic, please let me know.

I have a long-lead-time (almost a year) project restoring a 99-year-old Lucas horizonta l boring mill -- I hope to have it running in time for its 100th birthday. One aspect of this is replaceme nt of a pair of 25-tooth miter gears, 5" pitch diameter, with a 5 diametral pitch at the big end of the teeth -- straight teeth. A young student volunteer ed to cut these gears using 3-axis contourin g (ball end mill) and is working on generatin g the tool paths. I was referred to this forum and find it to be very interesti ng. At risk of being out of touch with the current state of the bevel gear cutting project, I would like to "join the party".

I am impressed after an initial study of your approach, that cutting gears with 4-axis contourin g, cutting with the side of an end mill tangent to the involute is a good approach -- far better than using a ball end mill. I also am keen on the idea of using a tapered end mill, noting that this has already been discussed . The one thing I can add is the idea of using a ball end tapered end mill, which would allow cutting a full-radius root in one setup. I would not minimize the gain of rigidity (allowing greater chip loads) gained with a tapered end mill -- when it comes to cutting real gears in steel, the time saved would be a major gain.

 I would like to help address the stated problem of accessing all possible pitch cone varieties . I think I understan d it right that you are rolling the gear to accomplis h keeping the cutter edge tangent to the involute. I am glad to hear that this works well with spur & helical gears because I have thought that it might be a good approach. I see how it can become a problem with the size of the involute varying along the length of the tooth of a bevel gear. I do not think I have a clear answer to this problem, but I am intereste d in being a part of seeking one if that is appropria te. I have been thinking about a set tool paths implied by the antique machines developed for bevel gear planing -- the "nut" of this is that all the lines along the pitch cone (as well as the addendum & dedendum cones) all intersect at the same point. It seems to me that a set of tool paths based on a single common point and the involute form at the "big end" of the tooth might be a starting point for generatin g the 4-axis tool paths.

Pardon me if this does not make sense -- it definitel y is a "half-baked" concept.

Archie
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Archie
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