Trouble with 4th axis

[Stephen Fornelius]

Hang on a sec - my stock is 1" and my gear is something like .78.  But what I see is that the plunge we're talking about is on the rotary table side of the stock and not the "free hanging end".  If it was plunging in air, then the problem with the G0 plunge would be irrelevant wouldn't it?  The depth of cut would be the only issue and that seems to work fine for me.

I'll set up the program again a couple of different ways and see if I can get the cut to start in air.  Maybe I have to reevaluate the "handedness" of the rotary table.  As it is now, I have it pointed to the right, which I'm assuming means x++.  Maybe I've got that wrong?

Steve

[philbur]

Its why the requirement to zero is to zero on the front edge, the system then assumes from there outward is empty space, and the backside is the same dimensions. The plunge I think is only used in 2d work as the 4th axis cutting was never supposed
to actually cut anywhere except from side to side at the correct depth..

Here's a piece of 4th axis rooting code for my 4 tooth helical.

G0      X 4.0000  Y 0.0000 Z 5.0000  A32.4114
          X 4.0000  Y 0.0000 Z -1.0000  A32.4114

GO is 2,800 mm/min!!!!

The thing is that for some of the low tooth count, small module worms I have in mind I think it is going to be useful to leave the gear teeth attached to the parent stock (shaft) on both flanks of the gear.

I have been zeroing GT on the x centre of the blank?

Phil :)

[Stephen Fornelius]

Testing again - the program wants to start at the center of the workpiece, even if they x-axis is set to zero at the right end of the workpiece.  If I set the right end to .7 (in my case), then the tool goes to the left into the workpiece and traverses completely to -.7 and then returns to .7. 

Maybe this problem is just that the 4th axis doesn't index as expected?

Steve

[philbur]

Hi Steve. Do you now that you can set the part offset in GT, you pick where you want the x,y,z zero in the gcode processing window/tab?

Phil

[ArtF]

Hi Guys:

  I do take that back. Zeroing in GT is on the center of the balnk.  As in this code on mine for a spur gear..

G0        X 3.2500  Y 0.0000 Z 0.1000  A0.0000
          X 3.2500  Y 0.0000 Z -0.3000  A0.0000

  So this is assuming zero center, x++ is away from the chuck in my case. The blank is 5mm thick, so a move of X3.25 puts the 1mm tools center .75  mm from the front of the blank , the motion is then a G0 to the front position, G0 down ( because after all no material is there) and then a G1 sweep through the blank.
The code is assuming the blank has no material on the back side either, so we're clear by .25mm at a minimum. If you wish more, you just set the "Depth" from 5mm to 8mm, the system will now move 4mm to the front before going down to cut depth..

  No attempt is made to plunge because 4th axis was designed to never cut into material on the way down, only to position itself in front ( or back) of the blank.

  (Sorry for any confusion here, I rarely cut 4th axis, have coded two programs for it, and each is different and has changed over time. This means I sometimes have to check to see exactly what I did. ) However, now that Ive checked..:-) The system assumes center in middle of blank, doesn't plunge in 4th axis as it assumes it never cuts on the way down), cut side to side and also assumes no material is on the other side of the blank. Use "Depth" to override the actual blank thickness you have.

  While it would be handy to have a plunge for when you may want to leave an edge on either side or something,  the code doesn't necessarily rise to prepare for next cut , it may assume all is clear because material is only in the blank ... in this case 2.5mm on either side of center..and simply rotate to the next tooth without rising up to clearance level.

  When GM is euthanized in the near future Ill be rewriting the Gcode screens in GT for added options and Ill add something for this ..

Art


 


Art

[Stephen Fornelius]

Art and Phil,

This current feature is almost exactly what I wanted to do anyway.  Visualize a workpiece in the chuck pointed to the right.  On the right end, the material has been cut down past where the gullet of the gear would be (call it the right collar).  Then the width of the gear is 0.11" (or sprocket in my case) is at full diameter, then on the left side of the gear portion (left collar) is another cut down below the gullet (probably double or more the width I want) to allow for parting off.  Then the stub goes into the chuck at far left end.

So  I could actually set my zero at approximately the right end, then the program would traverse left and right through the sprocket portion and "plunge" in the collar area.  Should be no problem that way!  I'll try it today on my test piece.

Steve

[Stephen Fornelius]

Did another test and it's truly scary.  I divided the program into two portions - the roothing portion and the toothing portion.

I ran the rooting portion with no problems.  Made nice little shallow cuts at .05 depth on one cut and .1 cut on the next using a .185 round nose bit.  Not as deep as I wanted but spaced OK.

Ran the toothing portion of the program.  It starts, indexes and then immediately drops the z axis to .3 inches down, cuts across the material and then does nothing but cut air for the rest of that tooth.  Repeats on the next tooth with that .3 deep cut.  After going one way around, it does the same cutting pattern in reverse - one deep cut and then it moves up and cuts air the rest of the time.  Perhaps my bit is too big and leaves nothing for it to tangentially cut?  If the bit were smaller, that first cut would snap it off.  I may try again tomorrow with a .125 end mill and see if it leaves something for the tangential cuts.

Attached are pics of the result of today's experiment.  Ignore the off center hole (different experiment).

Steve

[ArtF]

That does look like too large a bit to do it.. Ill be doing some changes to the rooting for tomorrow.
Ill see if I can make it trigger a stop if it senses the bit is too large..

Art

[Stephen Fornelius]

I notice in the program that when I play with the bit size, a little green bar appears in the simulation.  I was assuming that was telling me that the bit would only go down to that point.

Steve

[ArtF]

Steve:

  Not sure what Green line we're speaking of..

Here what the path should look like . ( Press the left arror perspective to this view..)

Art

[ArtF]

Steve:

  Take note in that photo of the trouble in making a sprocket in 4th axis. The tangental shaving can work only when the tangent angle is above PI. So only the top of the tooth is tangentally shaved. The rest is up to the rooting, so youll get many stepping artifacts o a sprocket cut with a flute. Sprocket shaped flycutter is probably the only way to get a really good sprocket profile..

Art

[Stephen Fornelius]

Art,

I see what you mean about the rooting doing most of the profiling and the toothing just barely doing much of anything on a sprocket.  My math is weak, so I'm not sure what you mean about pi, but I'll take your word for it.  I've already made a sprocket fly cutting bit, but I'll have to work out how to set it up and use it without beating the blank to death.  Again, it seems like it wants to run through the part way too fast, but I only played with it cutting air once.  I'll let you know how it works after I try it a couple of times. 

Steve

[philbur]

Its going to be a very long cutting edge with a fly cutter! You will need a very rigid set-up. I haven't read the whole thread but I would have thought that 2D was the way to go?

Phil :)

[ArtF]

lol, Im with Phil, a sprocket in 2.5D is easy and very accurate, hard to do in 4th axis.

When I speak of a tangential angle and PI, I'm really saying that for many types of profiles, there is a point at which you cannot tangentially shave below. Its because if you rotate that point to the correct spot for shaving, the tool will
shave off points above that point which are not supposed to be cut. Involutes can be cut from base circle upwards, sprockets from about 70% upwards to the top. Some pulleys can be shaved a bit, some not at all. The real rule would
be , "If a points normal goes below zero degrees or above PI, it cannot be shaved by tangential shaving."

  This is why GM never allowed anything other than spurs...involutes are the only shape that truly allows shaving.
GT tries though a bit of AI to do any shape, and will attempt to do the best it can based on the tool shape I store in memory. Its not based on formulas but on coordinates of a stores tool shape. As a result, "Results will vary"

  Its all a matter of what the math allows more than anything else. Ill willing to implement anything, if its
possible,many things just aren't, even though they sound like every reasonable wishes. :-)

Art

[Stephen Fornelius]

I'm a great believer in learning from those who develop ideas.  Mucho respect, and I'm going to try both the fly cutting and the 2.5D cutting. 

A couple of ideas I'm playing with if I want to keep the integral collar idea.  1)  hold the workpiece in a vice with a v-block, 2) mount it in my 4-jaw on my rotary table (paid for it - got to use it!). 

The prep work will be to lathe the collars leaving .11 for the sprocket, then use a long enough end mill (.75 maybe), so I can do a slow cut on the sprocket portion.  Once the sprocket it cut, then part off the piece on the lathe (or hacksaw) and face the ends.  Drill and tap for a set screw.

I'm going to try the 4th axis with something smaller so I can use a typical epicycloid fly cutter.  Thanks Art for making the profiles on the drawings.  Helps a lot and I also check them against an existing wheel with the proper tooth profile.  Just more things to learn to do!

Steve