Hi, just thought I would post a pic of a 4 toothed M2.5 worm I just completed in low carbon steel with an 18mm OD. It was just a test piece. I should have watched the CAM module tutorial before I cut it, then I would have understood rooting and toothing and why the tooth depth isn't correct. If all else fails read the ...... manual.
Pity the rooting wasnt done, but I have to say "Wow" anyway. That looks really good.
I see on the side profiel where the rooting failed to clear the sides fully, but its still
a very nice looking worm...
Hi Art, here's my second attempt. As you can see something went wrong with the roughing stage and the cutter crossed the involute line. As you point out in the tutorial, it is very difficult to see what is happening with the tool paths when dealing with helical gears. Any ideas how to catch this before the cutting stage would be appreciated.
Is it possible to show only an instantaneous tool path cross section only, rather than the complete path across the whole width of the gear, then it would be as easy to read as a straight spur gear.
Phil :)
PS: looks like maybe a 0.5 mm thick gear gives a clearer view of the tool path!!
Last edited by philbur on Fri Apr 18, 2014 9:58 am, edited 1 time in total.
Do me a favour and drop a copy of your project file here so I can try this one. That just looks wrong....and Im not so sure its your fault.
That first try looked like a proper cut of the fine path generator. This one looks as if the rough path is junk. Im not sure why..
Drop the project file on this thread and Ill run some tests to see why the screwup..
Im happy the path shows the end shape you got...but Im not happy with that shape. :)
>>gets less as the number of teeth increase but I don't think the error is ever zero
It doesnt. I think I found the error. Its in the tooth profile stored for tool shape. I use that shape to actually plan the
cut and Im wrong on the helicals. The tooth shape IS correct, from a transverse point of view, but not for the
machining planning in 4th axis. The problem is the shape needs to be identical to an equivalent non helical shape.
I will fix this and put out a fix. Thanks for pointing this out, a the tooth count increases youd notice it less and less
but its always there as an error. Once I fix the machining tooth shape, I think it should be OK..
The problem is that to do toolpaths, and in order to allow 4th axis epicycloidals and such, I changed the way I do things. I now take
the tooth shape which is shown on DXF's and such for tool grinding..and I wrote a routine that traces it with a tool. The theory was that this would
work for multiple tooth shapes and was self correcting in that it wont allow a bad cut as nothing can exceed the shape of the tooth shape used
to make the tooth. The problem with that, is that helical's in particular have their pitch diameter increased by the secant of the helical angle.
This means the tooth shape was never correct as shown, the shape shown was what youd see looking at a helical directly sideways( tranversely),
but the shape you NEED was the shape looking at the gear at helical angle ( perpendicular to gear normal direction).
So..what Ive done is change the tooth tool shape, ( not the gear..its the same..). Ive done this by changeing the tooth count used to make
the tools shape. The equivalent spur tooth shape is given by a spur gear with more teeth. ( Toothcount of equivalent spur is the helical tooth count
divided by the cosine of the helical angle cubed. ). This fixes your rooting trouble..BUT Ive noticed an inconsistancy in the output, the model doesnt
match the toolpath. This tells me somethign is wrong. ( The beauty of math is that it should match when looked at from any angle. ). Something
has gotten ugly in there. Im suspecting I have inadvertantly not made the chordal helical tooth arc correctly, so it "may be" your teeth will be
slightly too thick. Im investigating, but Id be interested in whatever you find in testing. This is one of those complex situations where I know either
the model OR the toolpath is slightly incorrect, but Im unsure exactly which.. I suspect its the toolpath being slightly thinner than required, but
Ill make sure before I change any more. A second issue with this is that a helical really isnt a worm mathmatically. Helicals are speced to be 12
teeth or larger mathmatically, though this is often ignored, so the fewer teeth you have the toth equivalency count formula's blend off a bit.
Undercut in a helical is supposed to be verboten, though I do allow it in GT and GM. SO a 4 tooth is pushing it ..in math terms, personally I like
seeing it though.
:)
The new version with this fix is now online.. and I continue to investigate.,..
Art
As Bob said... The numbers only rarely match up, Im terrible at numbering and I turn out fixes or adjustments too frequenctly.
Whenever a release is very stable I update the top one, and then always update the bottom one.. :)
Hi Art, I've just been looking at the gcode generated for an involute helical gear using an involute saw (I assume this is a B&S type involute cutter). It looks like a series of slots cut by passing the cutter across the gear blank with the blank angled at the helix angle. If this is the case then I do not think that it will generate the correct form. The error is easiest to visualise if you consider a large helix angle and a very wide gear blank. As the cutter passes across the gear blank it will have reduce depth of contact either side of the centre point of the blank. If the blank is wide enough the cutter will completely lose contact at the two edges of the blank. To work correctly I think maybe the cutter needs to be angled at the helix angle and the gear blank rotated as the cutter passes across the blank.
