Need Help!- Gear Cutting

[Martin Long]

I need to cut a gear on a X4+, however the gear is a larger diameter than any axis travel, approx 300mm.
My thoughts are to lay the 4 axis flat on the table facing up in Z direction, using a 2-3mm endmill to cut as many teeth as I can before running out of travel, (or even one at a time) return the tool to start, index the 4 axis to an amount equal to the number of teeth previouly cut, and start the cycle once again, do this until the gear is complete.
Any thoughts on an easyer method, and how difficult would this be to programe using BobCAD V21?
When anwering please be aware I'm new to this game!!!

Martin

[HMB3000]

depending how tight the gear needs to be it may or may not work..but it is worth a try....

[HuFlungDung]

You may not actually need the 4th axis. It may take up valuable height that you need.

Instead, you can index the part via accurately spaced dowel holes, going through the gear face and into a 'subplate' of your own making. Let's say you drill, bore and ream 4 holes at 90 degrees on one circle centered on the gear bore. Then, you can mill either 180 degrees or 90 degrees worth at one setup. However, you must pay careful attention to the tooth number, and whether it is divisible by 2 or 4, so that when you restart the program, the tool is in the correct place. If not, then you would need to reprogram after indexing. Or, perhaps drilling a different number of dowel holes would be in order, if there were a number that divided evenly into the tooth number of the gear.

To keep the setup simple to operate, I would include a centering boss on the subplate to keep the gear on center when it is rotated on the dowels. That is one less thing you need to dial in, plus you do not want to alter the original alignment of the dowel pattern.

[Martin Long]

The gears are be for a clock, so a little too much backlash should not be a problem. Any thoughts on how to write the G code programe?

Martin

[HuFlungDung]

Can you draw the gear tooth profile in Bobcad? Seems to me that you used to be able to draw one tooth, then copy and rotate to make the whole gear. It may make sense to copy only two teeth, do all the trimming, then do the final copy and rotate.

Once you've got a trimmed profile, then its just a simple profile to mill so far as Bobcad is concerned. Used to be able to do profiles with cut chain in Bobcad Maybe had to do an offset first, and retrim that. Sometimes offsetting will show flaws in the original trim, or lack thereof.

[sansbury]

The free CAD program that eMachineShop gives away has a gear profile wizard that seems decent, and you can export to DXF. Might be worth a try to see if it gets you part of the way there. Personally I feel like the fewer manual indexing steps, the fewer chances to do something stupid.

[smurph]

BobCAD has a gear function. In V22, it will do the whole profile. I remember the gear function in V21, but I can't remember if it did the full profile or not.

Steve

[Martin Long]

I can draw a gear with V21, the attached photo is of a gear I cut last night.
My problem is not knowing where to start to generate the G code using X, Y and 4 axis, if I have the g codes I can lay the blank flat on the 4 axis which would index around inconjunction with X and Y.

Martin

[Kiwi]

You may be able to cut the gear in two halves.
Drill two holes, both in line, alone the X axis.
Bore the centre hole and machine first half of gear.
Reposition the material. Line up using pins in the two holes and a straight edge.
Centre from the centre hole and machine second half.
See attached pictures.
If the gear has a even number of teeth you could use the first half code for second half, just rotate the material.

[moldmker]

Martin,

Sounds like your wanting to try programming simultaneous 4th axis and say X axis motions.

Can you use 4th axis conversion procedure as outlined in the v21 manual (Section 87) and use the pitch diam. as the cylinder diam.?

I've never done this, but it sounds interesting. For ease of programming, I think you would want to keep the Y axis at your gear centerline and cut with the X and A axes.(or vice versa).

Can you loop your program steps or set-up a subprogram? Then you would only program 1 tooth in incremental and loop as needed.

I think you would want to convert the path of the cutter centerline and not the tooth profile. Not sure if path would need interpolated into straight line segments.

There may even be a script possibility that compensates for the ever-changing cutter distance from A axis center as I think this constantly alters the angular conversion.

Sorry to answer your question with more questions, also I should confess that I haven't programmed a 4th axis with Bobcad. I'm just aware of it's presence.

moldmker

[Martin Long]

Moldmker,
That is the sort of thing I was thinking about, although I may need to learn a bit more about BobCAM before I can tackle it.
All the info I have seen relates to the 4 axis being used parrallel to the X not at 90% to it.
Maybe there is a simple proceedure, I was hoping someone has some experience in this area who might like to share it????

Martin

[Kiwi]

One method would be to cut one tooth, rotate (Z Axis) material the angle of one tooth, then repeat.

Re #9.....Sorry, didn't read your post correctly.
Also suggest you consider the same system but divide the gear into four parts if these would fit your table.