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Author Topic: Machining Bevels.  (Read 56389 times)
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ArtF
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« on: December 18, 2010, 08:53:47 PM »

Hi Guys:

 Here we will discuss Ideas for implement ations of Bevel Machining techiques
or possible ways to make Bevel gears under GM..

Ill post some thoughts over the next couple days as to how I intend to proceed.

Art
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ArtF
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« Reply #1 on: December 18, 2010, 09:11:57 PM »

Notes on Bevels:

  Ive implement ed 3 types as a start, Zerol, straight and Helical.

Zerol is like a knuckle gear, its halfway between a straight and a helix. Bevel Helix's are not generated the same as any other type of helix, its a more complex helical proceedur e, but Im pretty sure mathmatic ally its correct. Whiel some specs call for the curvature of radius to be used for the helix tangental circle, Ive used the conedista nce as the radius of the helical cutter, this means less to consider when creating a helical. Ive tried to keep the settings very low, almost everythin g is automatic . For example the helical angle is always 35 degrees.. until someone who knows more than I explains why we need other angles... .then Ill likely add them. Smiley

 
  The facewidth of a bevel is set to a max of 25% of the cone distance. BUT its overridde n at
the width of the GearWidth setting. That is to say if you set a gear width of 10mm, the face will be clipped to 10mm.. thats the face, not the actual gear width. The gear width will be the facewidth times the sine of the pitchcone angle... which is computed from the wheel pinion relations hips.

 Remember, no two bevels will mesh properly unless they are generated for each other.. Always design bevels as a pair..

Shaft angles are allowed from about 1 degree to 90 degrees , so you can have shafts flying about everywher e.

 So the question for the next developme nt period is "How exactly will we make these in the shop".

 This , I think, requires some discussio n. We can do 2.5D, rotary on horizonta l axis, or rotary on the flat ( though Im not convinced this gives us any true advantage on a well working system. )

   There are many ways to do this, each with their own problems in terms of coding it. I dont think any one proceedur e will machine all bevel types or sizes.. but I do think we can solve this one together to make bevels easier than ever before. So give it some thought, and lets break new ground together.


   Im very proud of the helicals Ive seen cut, I think they have worked fantastic ally.. so Im determine d to make bevels work just as well, maybe easier.


  Lets use this topic board for all things bevel. If you have a question on bevels, ask it here. A comment.. same thing. An Idea.. Go for it. Lets see what makes sense and what doesnt.


Thanks
ARt

 

 
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tweakie
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« Reply #2 on: December 19, 2010, 12:01:01 PM »

Art,

Expanding on what I was trying to say in another thread - If the rotary axis is mounted verticall y but tilted and fixed at the angle of the bevel then the tooth being cut would be plane with the table X - Y and the rotary is just indexed to the next tooth and so on in a similar way as a spur gear would be cut one tooth at a time.

Tweakie.


* Presentation1.jpg (16.64 KB, 800x600 - viewed 855 times.)
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ArtF
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« Reply #3 on: December 19, 2010, 03:27:44 PM »

Hi Tweaky:

   This is one method we can consider. I fear that many hobbiests lack the capabilit y to tilt
without a hassle, but I could be wrong.. It woudl require an accurate tilting capabilit y. The cone angle
can easily be reported to the user, and posted in the Gcode. The machingin fo them at that point gets pretty
easy from my perspecti ve.

  Anyone else got feelings on this, is tilting easy enough to consider most can do it, or should I go the
way of tilting the code and find out if this limits the gear too much?

Art
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Greolt
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« Reply #4 on: December 19, 2010, 04:46:32 PM »

In Tweakie's diagram, I am assuming that it is using X and Z axis with A being just an indexer.

Would that mean we can not machine the involute shape of the tooth without an involute shaped cutter.

Or would Y axis come into play as well to achieve the involute with a straight end mill, as it does now with a spur gear.

Greg
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ArtF
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« Reply #5 on: December 19, 2010, 04:55:57 PM »

Greg:

 We should be able to use the shaving technique same as a spur oir helical to get the involutes ..

Involute cutters will be an option..

Art
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Jeff
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« Reply #6 on: December 19, 2010, 05:49:58 PM »

Hi Tweaky:

   This is one method we can consider. I fear that many hobbiests lack the capabilit y to tilt
without a hassle, but I could be wrong.. It woudl require an accurate tilting capabilit y. The cone angle
can easily be reported to the user, and posted in the Gcode. The machingin fo them at that point gets pretty
easy from my perspecti ve.

  Anyone else got feelings on this, is tilting easy enough to consider most can do it, or should I go the
way of tilting the code and find out if this limits the gear too much?

Art



 If you can cut the gears with out tilting that would be the way to go for me. I don't have a forth axis capable tilting (with out major additions) - Just my 2 cents.

Jeff
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ArtF
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« Reply #7 on: December 19, 2010, 06:02:02 PM »

Jeff:

  Even Im reticent about tilting my axis.. not that it cant be done,
but cone angles can vary quite a bit so there'd be no standard to the
tilt amount, so the only way to quick-setup such a system is to have a good variable tilt mechanism of some sort made up.. easy to do I supose.. but Im suspectin g most hobbiests would rather cut without the tilt.. assuming its possible. A lot of us arent machinist s but hackers who know from experienc e the fewer setup's we need to worry about the better the end result..

Art
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John S
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« Reply #8 on: December 19, 2010, 06:31:34 PM »

To machine with an involute cutter would require the cutter to cut on the side of the gear in which case you can use a normal 4th axis and slew the axis on the bed in relation to the X axis.

Problem I can see here is that the interfere nce with an involute cutter will not allow the true profile.
Normally when using an involute cutter to cut bevels you do a centre pass, second pass with the cutter raised and dividing head rotated in the ++ then a last pass with the cutter lowered and the head rotated in the --

Even then you get a section of the tooth that has to be filed to get a proper fit.



One work round may be to still cut on the side and use a slitting saw instead of an end mill to plane the shape on?
Advantage s are saws are cheap and strong given they can go down far thinner than end mills.

I think for end mills then work on top and tilt the table. Anyone serious about bevels will have to have something very similar already.

John S.
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John S.
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ArtF
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« Reply #9 on: December 19, 2010, 09:40:51 PM »

John:

   I "Think" I understan d what you mean. Am I right in assuming the tooth must be filed because the involute is actually wrong? While you can design an involute cutter to match the profile at the start of outside of the bevel, since the bevel tooth shrinks as it goes to center, the correct involute is actually the involute of an ever shrinking base circle?

   I was thinking this was the problem with using a inv. cutter.. there is no way to dynamical ly match the involute curve..

   Provided the end mill can fit in the small end of the root, can you see any reason I cant do the same technique as a helical, only with simultane ous Z motion during the cut? To my mind if we control the facewidth as a function of the end mill max diamter we want, we should be able to do most bevels, perhaps with facewidth restricti ons due to bit size...

   I was thinking if all Z cutting passes started closest to lowest root, and cut upwards and inwards to gear radius, we shouldnt have any real plunge problems. Im suspectin g the tough problem is an algorithm to see the tooth
profile as an angled primitive for the clearance s during cutting..



Art



 Just want to be sure Im understan ding..

ASrt
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Dan
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« Reply #10 on: December 20, 2010, 03:51:19 AM »

   Provided the end mill can fit in the small end of the root, can you see any reason I cant do the same technique as a helical, only with simultane ous Z motion during the cut?

If possible, this would be best. The biggest problem I can see is with helical bevels, where the end mill would have to be very small to fit in the tooth root close to the centre.

  To my mind if we control the facewidth as a function of the end mill max diamter we want, we should be able to do most bevels, perhaps with facewidth restricti ons due to bit size...

Exactly! The smaller the face width the bigger the end mill that can be used, but in the same time the tooth is weaker. May be give the user the control over it...?

   I was thinking if all Z cutting passes started closest to lowest root, and cut upwards and inwards to gear radius, we shouldnt have any real plunge problems.

Yes, mentioned it in some other thread. If we choose the normal 4th axis mounting, then this is the way to go. You can go full feed this way.

I think tilting the 4th axis has its advantage s and this is the most logical thing to do, but it has its difficult ies. For me, I would prefer not to have to do this. Besides, Gearotic doesn't follow common logic in how it cuts gears Wink

Dan
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John S
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« Reply #11 on: December 20, 2010, 04:48:05 AM »

Art,
I think Dan has done all the replies necessary, I see no reason why an end mill can't be used, in fact because the DP changes as the tooth approache s the centre it's probably the only way to generate a tooth that requires no finishing .

Just to make it clear take a sample gear and say this gear is 10 DP measured on the outer diameter, pitch circle etc.
This gear has a face width of say 5/8" and because it's a genuine bevel it tapers on pitch and root to the cone point which is zero.

So at the inner face diameter this gear has a DP of probably 12 DP.

This is the problem milling, do you use a 10 DP cutter or a 12 DP ?

Your method will allow the profile to change. Using an involute cutter on the side it won't, it's back to milling.
Using a slitting saw probably won't work either as the the saw will interfere as it passes thu.

In the case of spur and helicals its on the cards ? to allow Gearotic to generate it's own involute cutter shaped like an end mill / D bit to allow very small profiles to be done that can't be done with conventio n end mills because of risk of tool breakage.

This feature won't be possible with bevels because of the differing DP / MOD.
One way round this may be to allow the user to define a tapered cutter similar to an engraving tool where the user specifies an included angle and tip width. The advantage s of this are a stronger tool for a given size.
These are quite easy to make in HSS or even carbide given a couple of simple jigs.

Now back to mounting and cutting the gear.

If we have 45 degree bevels then cutting from the flat or horizonta l is exactly the same as regards axis travels.
The Z axis is probably the most important as it will have to raise from the inner DP to the outer DP.

The problem I can see is the differing angles.



Take this 45 tooth wheel and the 10 tooth pinion, using a 4th axis the 10 tooth pinion will be easy as the cone angle is quite low. However the 45 tooth wheel is a different matter in that the cone angle is nearly vertical and an end mill will not be cutting perpendic ular to the pitch line, in fact it will be hard put to form the tooth depth.

I know not everyone will have access to a tilting dividing head but there are simple ways of doing the same, this is one



My take on this is that anyone wanting to do bevels will either be geared up or be prepared to gear up so if the president is set that the blank HAS to be on the pitch angle it will make the maths a lot easier.

John S.
« Last Edit: December 20, 2010, 04:55:57 AM by John S » Logged

John S.
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ArtF
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« Reply #12 on: December 20, 2010, 09:16:37 AM »

John:

   Yes, I think we're starting on the same page. ( Isnt that a nice picture of a bevel :-) ), the pinion in your case is faily easy I think, while the wheel is hard.  IF we dont tilt, then the wheel IS an example ( I think ) where machining would be very difficult . ( Though I suspect that I may be able to compute the milling bits vertical interfera nce in steps to even do that provided wheel.. but if you add a helix to it , it becomes near impossibl e to do the wheel.. )


 I suspect some experimen ts are in order.. For lower ratio's it all seems possibel without tilting, its the higher ratio's such as you show that worry me without tilting..

Art


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Dan
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« Reply #13 on: December 20, 2010, 01:57:38 PM »

Art,

If we want to stay away from the need of tilting the 4th axis, I think we are back to the option where we would have a selection between: horizonta l 4th axis or vertical rotary table. Given you can do any bevel up to 45 degrees cone angle (still not sure 45 degrees helical bevels are doable when face width gets large - tried to visualize it with Gearotic, but just can't see it), the two variation s of the 4th axis mounting should cover whole the range.

Dan
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Maxmachine
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« Reply #14 on: December 20, 2010, 03:09:36 PM »


If I may venture an opinion here (many of you are far superior to me in knowledge of gear making) the tilt/not tilt issue seems to be not very significa nt compared to  the ease of machining derived from the use of it.  It is easy to build a tilting table, and it is even easier to use a sine bar to get the proper angle to the required accuracy. Many low cost tilting tables are out there, some even come with worm drives to allow close adjustmen t.

Mark
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Engraver for 35 years, retired with shop. Avid tool collector, too many interests to be really good at any! Machine shop, Fabricati on shop, Wood shop. Always intereste d in meeting like minded individua ls.
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