Plastic gears--wrong curves

[iter]

I am cutting involute gears from polycarbonate sheet. I start with a DXF, import it into PartWizard, and generate inside and outside toolpaths. The curves look fine in PW. I use a 1/16 straight bit.

The cut quality is good. The cut path is not quite involute. I notice each tooth has a bulge on the right side (red arrows) and what looks like baclash in the dedendum (yellow arrows--the tool cuts clockwise).

My first reaction is that this part is too small for the Bot's resolution (the small gear is just under 2" diameter). Then I notice that all teeth have almost identical defects. I wonder if this is an artifact of PW's .sbp generation. I make gears using both "Shopbot inch" and "Shopbot inch arc" processors, with almost identical results.

I wonder how I can find where the problem occurs and what can I do to make more accurate gears.

Ari.


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[zeykr]

I wonder if defect will move to other side of gear if you reverse cut direction.

[jerry_stanek]

can you post the dxf file that you made the cut from?

[cutitout]

Recycling CDs -- "polycarbonate sheet" I will remember that one

[gene]

What program are you using to do the designing in? I have been wanting to do some gears . Any help is appreciated. Thanks Gene

[cutitout]

Ken has a point -- I think the bit is pulling itself into the cut. you may have to make several cuts with offsets then a finish cut to clean up.

[dana_swift]

My suspicion is that it has to do with the direction of the bit. What does the SB3 program show in the preview? If it does not show the flaw, the problem is in the cut direction issues. If it shows the asymmetry the problem is in PW presuming the DXF is correct.

One of the problems is expressing an involute to PW and then to SB. Neither program supports that curve form.

I wrote my own involute generator that outputs SBP files directly to save one level of interpolation. I have never cut gears with less than a 0.5" pitch so I don't know if it would have the same problem or not. I wanted control over all the parameters such as pressure angle, axle-axle distance, etc.

Hope the preview idea provides the clue you need-

D

[khalid]

I think you have to check the backlash in your Router..

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[harold_weber]

There is a Tolerance parameter in the profile toolpath option in Part Wizard. What is it set to and have you tried reducing it?

One way to check if it is a machine backlash problem might be to use the new Version 3.6.5 software and look at the simulated cut.

[garyb]

As its the same on each tooth and in the same positin for the full 360° tells me its in the DXF drawing. Appears one tooth was drawn then radial copied therefore duplicating to ea tooth.

too consistant for backlash