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..

---------------------------------
http://my-woodcarving.blogspot.com/

[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

[khalid]

Gary.. you are indigenous
.. However, if the gears are made with gear software, its hardly a chance of wrong output or it may be a bug in that software...

-------------------------------------------------
http://my-woodcarving.blogspot.com/

[iter]

Thank you guys for jumping in o this one. I am grateful for all your feedback.

Ken--the defect moves as you say. The tool cuts the inside of the large gear counterclockwise. The artifact is on the trailing edge of every tooth regardless of cut direction.

Eric--recycling is a major trend I notice in fashion and politics. Re bit pulling into the cut: CD material is .047" thick. I wonder how many passes I can meaningfully have throgh it.

Gene--I use Inkscape to draw gear outlines, then QCad to draw everything else, then PartWizard to generate toolpaths.

Dana--In my DXF, the gear outline is a series of short, straight segments. This translates well into PW's view of the world (and into M2 .sbp commands). Thank you for the preview idea. The path looks smooth in preview. Thank you for clearing my suspicion of PW error.

Khalid--I am not clear on how to check backlash in my router, or how to reduce it. I use a spindle, if it make a difference.

Harold--the tolerance on my toolpaths is .001

Gary--the artifact indeed repeats through 360°. Dana's idea of examining the preview tells me that my DXF is correct, the toolpath is correct, and ShopBot controller's interpretation of the toolpath is correct. I am leaning towards a mechanical explanation.

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[garyb]

Check the node location in the dxf & see if there is one located at the problematic point.
Post a section of your dxf file might help.

[iter]

Gary--thank you for your suggestion. The first file (red on black) is a bitmap export of the DXF. I see no problem with it, furthermore I see no problem with the .art file or the .sbp as it appears in preview mode. Your theory is plausible but I see no evidence that is consistent with it.

Ari.



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[jerry_stanek]

When I look at the bit map it looks like there is the same bulge as on your cut.

[dana_swift]

What are the parameters for your gears?

Pitch, pressure angle, backlash, etc?

I would love to generate a dxf for them and go cut some myself and see how it compares.

D

[iter]

Jerry--you may be looking at a pixelization artifact.

Ari

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[joe]

Dana my friend,

I'm not sure that would prove very much. All of these machines are different with more or less tollerance, backlash, and wear. Each one put together, usually for the first time, by a novice. They can be fine tuned but even then each piece of equipment has it's limits.

If anyone could make these work, I know you'd be the man.

Joe
www.normansignco.com (http://www.normansignco.com)

[iter]

Dana--I use 20° and .22" pitch. These numbers aren't particularly important to me. I want the smallest gears I can make out of this material on the Bot.

You can also try my DXF a couple of posts above.

Ari.

[dana_swift]

Ari- how many teeth on the outside and inside spurs?

Joe- it may not prove anything, but I would like to see if it gives a good result. Whatever the result I will post photos, and my dxfs.

D

[iter]

24, 45, 64

Ari.

[gene]

Is the toolpath saved as shopbot arc inches ? Could that be something to check ?

[dana_swift]

Result:

I cut only the inner spur gear, using a CD/R material. I did not get the gear centered well enough, so the other side of the gear is "missing teeth".. woops. Oh well, it did what I was looking for.

The CD reflective material flaked off, so the edges look rough when in fact they are not.

Gear Parameters:

24 teeth
0.220 pitch
65% tooth height
20 degree pressure angle
5% backlash by design

Cut parameters:

0.060" end mill, single flute up spiral
1"/s feed rate
Porter cable router,
45 seconds to cut the axle hole and 6 mounting holes.
20 seconds to cut the outline.

Hold down:
Super-77 to HDPE spoilboard for small parts (documented elsewhere)
HDPE was held to the base using my vacuum system (documented elsewhere)

Here is the cut preview from SB3:

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Here is a photo of the resulting part:

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Here is the DXF I imported to Aspire to generate the tool paths:

The source DXF

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My results:

I measured the ID of the mounting holes and axle:
Design: 0.250" Actual 0.250-0.252" measured about 20 places.

Teeth looked great also..

D

[khalid]

Dana.. The holes not ROUND!!!! or my eyes has some faults? i find some non-symmetry in teeth too... i think its my eyes or reflection of material...

-----------------------------
http://my-woodcarving.blogspot.com/

[myxpykalix]

khalid i think it is the angle of the picture that is throwing you off. It is not at 90 degrees to the part and what you are seeing is the top half of the circle to the bottom of material and that causes the circles to look out of round.
I think that is your problem with he other parts because its clear material it is distorting your view of it.

[dana_swift]

I took close shots of a hole and a couple of teeth. The angle is now 90 to the part, and you can clearly see the metal foil on one side of the CD has flaked off, but the plastic cut nicely.

Here is the hole close up:

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Here is the teeth close up:

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The holes are round with variations I indicated. I suppose someone could photoshop the tooth photo and check the symmetry better.

Hope that resolves the issue well enough-

D

[garyb]

"Hope that resolves the issue well enough"

Well Dana that doesn't help Ari's plight if you generated a different dxf file, however I did look at Ari's dxf including the node location and it did look ok.

Using Ari's dxf I toolpathed and cut a 2.5" gear which cut fine, still waiting on the photo which I emailed from my phone.....its in the black hole of cyberspace at present, may have to resend for the 3rd time.

Ari you might post your sbp code, maybe Dana will test cut it.
I just have a hard time believing this is mechanical due to the consistancy of it being on the same side and location of the tooth through the full 360° in both climb an conventional.

Incidentally I toolpathed in ArtCam outputting to a custom arc inch post processor in g-code.

[dana_swift]

Hi Gary- the issue I hoped to resolve in my last post was that the holes were round and teeth symmetrical, in reference to Kahlid's observation of my photo.

I accept it as a given that the DXF Ari posted has symmetrical teeth, so the problem was in tool-pathing, or a material issue. I asked the question about the preview to remove the tool-path issue. So the question I had was regarding the material properties. That is why I did my test cut.

I used a CD/R and the original appears to have been a stamped CD, the flaking problem showed up in my material as the metal layer on the surface instead of embedded. If my material had exhibited the same problem as Ari encountered the problem would be due to the material, and more would have been learned useful to the forum.

This is the smallest gear I have ever cut, and wanted to know if there was some problem I might need to address, so this thread was of particular interest.

Sorry about not being specific about what issue I thought I had resolved.

Ari- sorry my experiment yielded nothing of real value to your case. I posted my cutting info in case there was any significant difference between our methods that might account for the difference in the result.




D

[garyb]

Gotcha Dana, I missed what you were refering to as resolved

[srwtlc]

Let's see this 1/16" bit. How long, shank diameter, how sharp, how fast are you pushing it?

[iter]

Scott--I use an Amana Tool 45190, 2-flute straight bit, 1/16" diameter, 3/16 length, 1/4" shank. Different speeds from 3,000RPM to 15,000 yield similar results. I hesitate to push it beyond 0.75"/s for fear of breaking it.

Dana--I am grateful to you for posing your results. I know now that what I want to do is doable. This is a very important result for me. Also thank you for posting about S-77. Old news for many, but news for me. This is by far the easiest mounting method for this application and I intend to use it. The photo I post is what happens tonight when I spray the label side of a CDR. When I come to pull the CD off my spoilboard, it lifts the foil clear off the disk. An interesting way to get clear CD blanks and an interesting visual effect on the spoilboard.

One problem I may have (and one that's not come up here) is slop in the Bot's gears. The machine I use lives at http://www.sawdustshop.com and has many users. With the controller on, I can move the gantry a noticeable amount back and forth by hand, perhaps as much as 1/20" or more. From what I know about this Bot's operation, I expect that I'm the first person to look at this slop after its assembly 3 years ago.

I notice that the stepper motors mount on adjustable plates. I wonder what is the correct procedure for adjusting their position.

Ari.


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[khalid]

Guys..tell us where you are going to use these gears?

[erik_f]

Ari,
After 3 years and many users there is a good chance your pinion gears are no good. Someone previously did suggest you may have some backlash in your router. I suppose when we say router we can mean two things, the actual router that is contained with in the machine...or the entire cnc router machine. I think this person was talking about the entire machine. Check all axis for slop and adjust it out. Also grease those gears! There is no specific method to adjust the drive gears. You don't want them so tight they bind so tighter is not always better. You can find the manual on this site for your machine, I would read it. I usually start by trying to leave about the thickness of a piece of paper between the gears and then make small adjustments from there.

[dana_swift]

Ari- I cut from the foil side of the CD/R using an up-spiral which is why the foil flaked so badly. Your HDPE looks almost exactly like mine!

The S-77 was on the clear side when I did the cut. I cleaned off the residue from both sides using a rag with Naphtha. Naphtha is a very weak solvent and does not attack the polycarbonate enough to matter. The S-77 comes right off.

Normally I would have kept the result to myself, since it was not "presentation quality". I learned from it, and thought it worth sharing in spite of the ragged appearance.

I have down-spiral 0.063 bits, but they are much more expensive, so I used the cheap bit in case I broke one. Down-spiral may have done a better job on the foil, but that remains for some future experiment.

Backlash is quite likely the only problem in getting the results you want, my result numbers show very little backlash in my BT-32, also runout is very small. You can tighten up each of the stepper mounts in case they have "backed away" from best engagement. Some steppers are mounted with a spring load to maintain a good pinion fit. I don't know what your system has.

My 1/8 shank bit is held in a collet I learned about from Brady Watson's column. I went to the source he suggested, and have had great results. The 1/8 adapter is held in a 1/2" PC collet. I ran at 12000 rpm.

Ari- good luck! I am curious what these gears are for? If its artwork, you are off to a great start!

D

[iter]

Khalid--I have all these old CDs lying around, I'm trying to come up with /some/ use for them. I am toying with the idea of making a grandfather clock out of them. I like the irony of using a pure-digital medium in a pure-mechanical system.

Dana--My spoilboard is not so much HDPE as MDF with melamine covering. I like your HDPE idea and plan to acquire a piece for this project. Thank you again for sharing your results, they are very important to me. I don't mind presentation quality so much as I am glad to know it is doable at all.

Thank you for pointing me to Brady Watson's post about collets. I wonder where you buy your smaller bits from. The bit I use has a 1/4" shank and fits in a regular collet, but I want to use smaller bits if I can get the machine's backlash under control. The gears on this machine have no springs. There's just a plate that the motor sits on, and 4 machine screws that hold that plate to the gantry.

I am not clear on the meaning of "runout" in this context.

Ari.

[hh_woodworking]

Ari, I use this company for small bits and collets : http://www.precisebits.com/gateways/ColletsNutsHome.htm fast and good to work with
Ed

[dana_swift]

Ari- I get little bits from precisebits also. Sometimes on ebay too.

When I measure runout it is the accumulated "wobble" in the tip of the bit. Measure it with a dial indicator on the shaft of a bit just above the cutting area. Turn the router by hand and note how many thousandths of change there is as the bit turns.

Runout can be from the bit being off-center, being crooked, bearing troubles, etc. Ideally it should be 0.000 all the way around, it almost never is exactly. I don't check it very often, but I can any time I suspect it is a problem.

Using shorter bits deeper in the collet helps minimize runout.

That definition may not be precise..

D

[iter]

I'm getting somewhere. I adjust 3 motors to take as much slop out as I can. I also adjust the lower bearings on the YZ car--one is loose enough to turn over by hand, and the whole car wobbles up and down Y.

I run the toolpath clockwise (climb) and then run the same toolpath counter-clockwise. The result is not perfect, ut it's a big improvement.

Dana--I wonder what bits and settings you use to route HDPE. I want to cut grooves into my spoilboard to position CD blanks.

Ari.
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[dana_swift]

Ari- looking VERY good! Are the spiral holes for some function?

HDPE is a very soft material, similar to Teflon. It cuts with just about anything, being so soft it is very tolerant of cut speeds variations. I tend to cut initial clearances around 4"/s and cleanup around 2"/s. My max cut depth is the current bit diameter, that is only for chip clearance.

Its moderately pricey, I buy it by the 8'x4' sheet in all kinds of thicknesses from a local plastics supplier. Some suppliers will sell you drops, which would probably be good enough for your needs.

D

[iter]

Dana--Thank you for your definition of runout and notes on HDPE.

I don't post here very often, and it feels great to still get the great feedback that you guys are giving me. I feel privileged to learn from your experience.

The holes serve a spirograhic function.

Ari.


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[tporter1001]

Inkscape has a gear making function. From the toolbar select Effects...then Render...then Gear...select the tooth count....export as DXF.

[dana_swift]

Terry thanks for the info on Inkscape, I will check it out! I have wondered why graphics and CAD programs don't just have gear-generators.

D

[dana_swift]

Terry- I cant find the gear generator in Inkscape.. I'm lost at "toolbar Effects". Where is that? Nothing I hover over says "Effects". There is an "Affects" which has nothing related to Render under it.

D

[iter]

I use Inkscape 0.46 to generate my gears--see my post from Tuesday. Here's a screenshot of the menu Terry talks about.

Ari.


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[dana_swift]

I downloaded 0.47, it has a MENU (not a toolbar) labeled "Extensions", which has the Render gear option..

finally found it! Looks like a great program- love the price!

Thanks

D

[joe]

Ari,

So, have we determined the problem Ari was having to be with the software or slop in the hardware?

[hespj]

Have you guys looked at this and similar?

http://woodgears.ca/gear_cutting/template.html

"Gear template generator"

Look slike this one prints or outputs HPGL, but you might be able to convert, or you might fond one that outputs dxf if you search.

John

[bill.young]

Hey John,

FYI there's an hpgl converter in the ShopBot Control software [FC]. Must admit I've never had the need to use it, though.

Bill

[hespj]

Thanks Bill. I downloaded the default gear but the SB conversion didn't work (or maybe the HPGL file is incorrect). I might have done something wrong.

Anyway, I thought I'd use the print option to print a pdf, which I then opened in Rhino. Worked beautifully with one vector per tooth.

[Ryan Patterson]

When I first saw this post I thought it would a good exercise to create a program to generate a gear. So I did some learning about involute curves. Here is a link that explains an involute curve and show how to draft a gear in a Cad application http://www.cartertools.com/involute.html.
After a couple days of learning about gears this is what I have developed. This program is not complete and will draw the profile of one tooth based on Pitch, number of teeth, and pressure angle. It will then save the profile to a dxf file. You will need to use a cad program to array the tooth profile. In the next few days if there is interest I will complete the gear.
Here is the download link. http://www.cabinetpartspro.com/GearMaker/GearMaker_Setup.exe

If you have any questions you can email me at info@cabinetpartspro.com (mailto:info@cabinetpartspro.com) .

[wberminio]

Nice job,Ryan

Thanks

[k4mg]

Thanks a lot, I am making gears!. I get an error when I open the generated .dxf in V-Carve Pro. I can click YES and continue and no problems. I have attached the error.

Great work, looking forward to the final version.
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Barry

[Ryan Patterson]

Barry,
I am not sure about the warning message you are getting, it should not cause any issues.
I have received a few emails asking how to complete the gear. I did a small video that shows how to complete the gear in Aspire.
http://www.cabinetpartspro.com/GearMaker/GearAspire.mp4

[k4mg]

Ryan,

Thanks for the video, however I am not having any problems cutting gears with your software, I just wanted to tell you about the error message (I too develop software, see http://www.k4mg.com/FreeSoftware/index.htm).

Looking forward to seeing your final version, thanks for the program!

Barry

[dana_swift]

Ryan & Barry its good to see other software developers on here.

I do C, C++, C# predominantly. Basic only when I have to.

D

[iter]

Dana--I am a software engineer by trade. I write my own software to generate .sbp files for these: http://www.lib.aero/~ari/mosaics

Ari.


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