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Author Topic: Bevel Gear Stock Prep  (Read 15508 times)
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ArtF
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« Reply #15 on: April 03, 2011, 02:22:59 PM »

yeah, looks to me like your actually cutting on the back blank..no t on the face. Isnt that the actual fact pointing downwards at the rear?

Art
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markwoodard41
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« Reply #16 on: April 03, 2011, 02:44:28 PM »

I hoped it was something simple. 

I reversed, re-zero'd and cut a couple more teeth.  The stock does indeed angle down now but I must be confused.  I didn't think that face represent ed the face of the tooth. 

Ignore the effort in the lower portion of the pics.  That was the first cut attempt.

Mark


* IMG_0132[1].JPG (789.83 KB, 1600x1200 - viewed 475 times.)

* IMG_0132[2].JPG (789.83 KB, 1600x1200 - viewed 502 times.)
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markwoodard41
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« Reply #17 on: April 03, 2011, 03:10:37 PM »

I may be onto something .  The stock was in the indexer correctly the first time.  It may be that I dont' have the indexer tilted to the correct angle. Let me investiga te and will get back here with results.  I think I may have the indexer tilted the the complemen tary angle to what is required. 
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markwoodard41
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« Reply #18 on: April 03, 2011, 04:57:48 PM »

Nope.  That wasn't it either.  I am out of time and out of blank now.  I'll create a whole new gear and blank dfx with the GM 3.1 and have a go again another day. 

The tap file says "4th axis tlted to 50.19".  I took that to mean that I tilt my indexer to 50.19 degrees.  Should I be tilting the indexer spindle (gear shaft) to 50.19d?  That would require me to tilt my indexer chuck face to 39.81d.  The router Z axis would then be presented to the stock at 50.19d. 

Art, when you said the stock face should appear tilted from horizonta l slightly, that rang a bell.  When I tilted the indexer to 38.81 and presented the stock face to the cutter, the face was tilted down slightly.  That was good and I got optimisti c.  The resulting cut did not pass the cutter all of the way through the blank though.  The machine also did a couple of "air passes" on each side of each tooth.  It made a couple of involute shaping attempts at the extremes of the A rotation and above where there was stock to cut.  This makes me think that my stock isn't right.

Examining the gcode shows that the cutter is not programme d to travel the full 0.5" of the blanks face.  That was as I specified in GM.  So it seems that I have sized the blank incorrect ly but it is extremely close (within a few thou) to the dfx drawing of the involute.

 As I said, another day.  Thanks for the assist here.  I am sure I will get this hashed out. My bet is I have the blank sized wrong.

Is anyone else trying these beveled gears?  How are you making out?

Mark



* IMG_0133[1].JPG (571.41 KB, 1600x1200 - viewed 500 times.)
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ArtF
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« Reply #19 on: April 03, 2011, 05:13:16 PM »

Mark:

 No-one else has tested these.. OR at least mine are the only oes Ive seen. I too get an air pass on the sides occasiona lly depending on the blank, but since Im not real accurate on the blank creation, and mine have been of foam..Ive never been sure if its a math error or process error. ( or just me. ). Ive been waiting for testing to show..

  So it may not be you at all. Ill have to investiga te it.

 I guess Ill have to do this in a CAD program and tilt it to see how that all figures.. its what I had to do in the end to verify the helicals and normal involutes ...bevels just adds a few more variables to the mix.. very hard to simulate in the hea dor on cad..

 Ill schedule myself to look at this next week to figure out what may be wrong.. I foudn when I was very close with the blank, it seemed
to cut fine. The cutter is programme d to cut the face width plus the tool diameter on each side of the face. Ive been tilting to the angle requested in the tap file though, not a converted angle. The program starts with the assumptio n that the gear is 90 degrees to the Z axis when not tilted, so a requested tilt of 45 degrees shoudl show the face almost flat, but tilting down at 3 or 4 degree's..whatev er the addendum angle is..  That woudl be the clue that the face is properly aligned.. Most of mine requested a 35degree tilt or so.. ( close to my maximum ), and I always tilted to that angle..an d my blank then always looked rigth with a slight down tilt on the face.. ( I never attenmpte d to measure it..

Art
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ArtF
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« Reply #20 on: April 03, 2011, 05:16:20 PM »

Mark:

 Just so Im not confusing you with the angles.. I put a digital angle meter on top of my 4th axis, ( so zero degrees is what a normal 4th axis looks like, and I tilt it to the required degrees'. SO dont consider the 4th axis when perpendic ular to the Z to be at 90 degree's.. I consider that zero. I then tilt the face upwards by the requested angle.. The fact should then look correct..

Art
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markwoodard41
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« Reply #21 on: April 03, 2011, 07:05:49 PM »

Ok.  I am on the same page.  I also have a machined surface on the top of my index.  It is parallel with the chuck's axis of rotation. 

When I put my inclinome ter on that surface, it should read the angle that you need to axis tilted to for your code to work. 

I had been using the surface of the chuck to check my angle of tilt.  I acutally only use the inclinome ter to double check my sine plate apparatus

My machine has a movable gantry and I needed all of the clearance avalable to clear the indexer.  I was going to use a sine plate to mount the indexer to but the sine plate uses up 2 inches (and I didn't have one so I would have to hit ebay).  I milled two slots with a 0.375 ball end mill exactly 5" apart and exactly the same depth.  I turned one into a hinge with some ground rod and affixed the ends directly to my table.  I attached the center to the bottom of the back of the indexer (a machined surface).  The upper rod is attached to to indexer back.  I use it and some gauge blocks to set the angle.  So in essence , I was setting the face of the chuck to the recommend ed angle.  Now i see that I need to set it to the complemen t of your recommend ed angle so that the shaft is at your required angle.

Now that that is sorted, I just need sott figure out why I don't get a cut completel y through the face.

Mark
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ArtF
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« Reply #22 on: April 03, 2011, 08:27:49 PM »

Mark:

   It should be zeroed at the back of the tooth with 1/2 tool hanging over the edge.. That assumes the rear bevel is cut.. I rarely do.. If it isnt cut, then youd need to start with the tool at the start of tooth.. a bit more to calculate ..

 Id try one with whatever cut you get, then drop the tool and rotate the indexer to clear it all off, we'll go from there to see whats happening ..

Art
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markwoodard41
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« Reply #23 on: April 03, 2011, 08:52:37 PM »

I zero the cutter at the point where the arrow is in the pic below. 

I did some more thinking about this.  The code calls for movement in the X direction from -.123 (cutter dia) to +.2193 for Root Clearance (total move is .4653).  For both shaving operation s the cutter moves in X direction from -.1215 (not my cutter dia) to +.5481 (total .6696). 

I would think that it needs to move 0.5 + 2x cutter diameter.  At least 0.746 minimum.  The clearance doesn't do that nor does the shaving operation

Mark


* Bevel Gear DFX.png (31.07 KB, 1152x748 - viewed 496 times.)
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ArtF
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« Reply #24 on: April 03, 2011, 09:16:28 PM »

Hi Mike:

  I did find that I coded from ( assuming a .25 radius tool for an example.)

-.5 to facewidth + .25

  Ive modified that to -.5 to face + .5 for next release.

 Now the shaving and clearance may not show that..onl y a X motion at zero degrees will
show that sweep length. As the indexer rotates, the X dimension shrinks and the Y increases .
 So you may not see an intuitive length in the Gcode that way. Its because the face surface
as the index rotates, is moving the face away from your zero position. .so the shaving actually moves forward as the rotation and Y axis moves .. the result is a very nonintuit ive face length
for the root and shaving..

   Its all a very complex thing to imagine codewise. . The slightest thing being off can affect results to varying degree's. I foudn for example that the final 2-3 shaving passes may hit nothing but air.. ( due to very slight tilt error or very small rotationa l zeroing.. ( or some bug on my part I havent seen yet. Smiley )

Art


Art
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markwoodard41
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« Reply #25 on: April 03, 2011, 09:24:48 PM »

I thought about the rotationa l aspects of the cut but the X axis can only move in a line.  To get from the edge of a 0.5" part to the other edge it must move at least 0.5".  Sure the length of the cut along the rotationa l axis in combinati on with the Y moves result in a longer cut in the material .  But to get to the other side don't you still have to move the cutter to the other edge?

Mark


* Bevel Gear DFX2.png (27.75 KB, 1152x648 - viewed 471 times.)
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ArtF
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« Reply #26 on: April 03, 2011, 10:17:59 PM »

Mark:

 Thats what I mean about being non-intuitive..

 Look at a bevel from the top as its in the tilted indexer. The Top tooth is straigh on the X axis.. so yes, that lien is say..1" long..

 But look at that same tooth when the bevel rotates by 5 degrees. The tooth now is no longer straight on the X axis, its tilted inwards on the Y axis.. The length of the face is still 1", biut to traverse it, the X axis will move .75" and the Y will move .25".. still a 1" move.. but very hard on the head in Gcode..  In act to grasp it properly, you ned to have one in your hand thats cut..as I do..

  The further off Y=0 you are, the less the X is moved and the more the Y moves at the same time to encompass the length of the now tilted tooth.. Imagine the tooth tilts the entire way to 90 dgerees..( cant happen of course..) the X woudl move zero.. The Y would move 1".. The 1" face would still be cut..  Basically the X shrinks by the cos of the tilt angle as the Y increases by a similar amount.

Art


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markwoodard41
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« Reply #27 on: April 04, 2011, 10:41:49 AM »

Art,  you are absolutel y correct.  It is not intuitive

Thanks very much for your attention to this little problem.  I am amazed at the type of support you are providing .  I'll try to help get to the bottom of this dilema by suppling data and observati ons.  This is a great learning experienc e for me.

So assuming the code and your logic is sound, then perhaps the problem is elsewhere so I examined the stock and the dimension s of the finished gear.  I see that the face of the finished gear tooth is approxima tely 0.430".  I say approx because it is tough to measure.  To project that face completel y through the blank, the finished tooth face would need to be approx 0.650" long. 

I specified a face of 0.5".  The blank's face as built is 0.5" measured radially. 

I went back to the GM project screen specs.  I see that the Gear Thickness is specified as 0.4737".  I notice that the DFX blank shows the thickness of the blank to be 0.600".  There is no thickness specfied in the Tap file.  (you recall the discrepen cy I noted between face angle noted in GM project screen vs. Tap file where you said to use the tap file.)  So I measured the thickness of the gear at the point where I completed the cut in the X direction .  If the blank were 0.500" thick, the cut would make it completel y through the blank.  Allowing for 1/2 radius of my cutter, that makes it close to the 0.4737 spec. 

I appolgize for the bad photograp y.  In the first pic you can see where I used a pencil to highlight the uncut portion of my blank.  You can't make out the caliper but you can make out what I am masureing .  The 2nd pic shows that had the blank been cut to the thickness specified in the GM Project Screen, the gear tooth would have been completel y carved out.  I hope this helps.

Mark


* Bevel Gear GM spec.png (22.3 KB, 1152x648 - viewed 416 times.)

* bevel gear dfx3.png (21.53 KB, 1152x648 - viewed 466 times.)

* IMG_0136[1].JPG (570.58 KB, 1600x1200 - viewed 464 times.)

* IMG_0137[1].JPG (565.6 KB, 1600x1200 - viewed 483 times.)
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ArtF
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« Reply #28 on: April 04, 2011, 11:00:38 AM »

HI Mike:

  Yes, the math on bevels really can hurt the head.. very hard to wrap around..a nd I soon forget as well.

 The thickness displayed on the project screen is really the thickness of the entie extruded gear on the screen. Its probably correct.
The DXF however, doesnt have the ends cut off right I dont think.. Ill investiga te why the discrepan cy..
  I think your correct though.. the dxf is wrong, the project screen is correct..

Thx
Art
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markwoodard41
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« Reply #29 on: April 04, 2011, 11:54:53 AM »

Art,  when I get time, I'll modify the DFX to accomodat e the correct thickness, recut a new blank and make another attempt using the original gcode.  I will also take pains to set 0,0,0 accuratel y to see if I can eliminate/reduce the "air cuts".  I may take the next step and cut its brother to check the fit.   

I'll let you know how I make out.  It may not be for a few days though.

Thanks again,
Mark
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