Author Topic: Easy way to make a plastic gear!  (Read 17193 times)

Online John Hill

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Easy way to make a plastic gear!
« on: August 16, 2010, 09:57:56 PM »
Hardly needs any words!


The set up.....



... and the result!




I think this plastic is ABS,  I cut a disc of random size and mounted it in the lathe with spare change wheel mounted on an axle in the tool post.

The lathe was run at about 400rpm and the change gear pressed against it, of course the gear started to turn and I increased the pressure a little.  After about a minute the sound changed somewhat which I assume indicated the plastic was  getting hot and starting to soften,  at that point I increased the pressure and in about 15 seconds or less the teeth were formed.


The blank does not have to be the same size as the gear and as far as I can see no matter what the size of the blank the teeth which are formed will always be shaped in true involute form.  It should be possible to make gears with odd numbers of teeth (according to the diameter of the blank) and of course there is no need for any indexing devices.

BTW, this does not always work and it is easier with bigger diameters.

Now if I used a steel blank with an oxy/acet flame playing on the edge it might get hot enough to form the same way..... :scratch:

« Last Edit: August 16, 2010, 11:34:37 PM by John Hill »
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Offline Stilldrillin

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Re: Easy way to make a plastic gear!
« Reply #1 on: August 17, 2010, 02:06:05 AM »
Hardly needs any words!


John,

Yer right!  :thumbup:

 :clap: :clap: :clap:

David D
David.

Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!

Offline cidrontmg

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Re: Easy way to make a plastic gear!
« Reply #2 on: August 17, 2010, 05:43:03 PM »
Thatīs neat! I guess you still have to calculate (and machine) the plastic gear blank just the same as if you were milling/hobbing it from any other material, for the number of teeth that you want in it. Have you tried the method with small gears and small pitches?
Obviously it wouldnīt work too well with racks, but it just might work also for inside gears... The "tool" gear would need to be small enough to fit inside the blank... Hm.  :scratch:   I might need to experiment a bit about this. Inside gears Iīve never been able to machine satisfactorily  :bang:
You might also be able to do the same with aluminium... The "tool" gear action would be more like that of a knurling wheel, instead of hot forming, though... But with a steel gear, not cast iron...
Food for thought and experiment. Thanks for a very good idea!    :clap:
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Online John Hill

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Re: Easy way to make a plastic gear!
« Reply #3 on: August 17, 2010, 06:08:13 PM »
Yes, you have to calculate the size of the blank if you want a praticular number of teeth but when just testing the process I used random sized blanks.

The bigger blanks are easier than the smaller ones as obviously the error from a random size is less in the bigger blank (due to the error being spread over more teeth).  Something really small like a pinion would require carefule size of the blank but I think it would still work OK,  I have not tried this.

Haha, I never thought of inside gears but now I must make one!

My original idea was not to make plastic gears but to make aluminium ones, I just made the plastic gear as a trial to ensure the teeth would be formed correctly as some people on another board argued it would not.  My idea was to cut the aluminium gears  with a hand ground single point tool and use this process to finish the gear shape.  I have cut about 15 teeth on a 60 tooth aluminium gear at this time and when that is finished I will try this method to finish the gear shape.

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Offline RichardShute

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Re: Easy way to make a plastic gear!
« Reply #4 on: August 21, 2010, 09:52:22 AM »


Now if I used a steel blank with an oxy/acet flame playing on the edge it might get hot enough to form the same way..... :scratch:


I have seen film of gears being made essentially just like that. I think it was from Ford's in the '60's but I may be wrong, it was a long while ago. A red hot blank was rotated with a pair of 'forming gears' one either side. The formers were sprayed with coolant to keep them cool and a finished gear popped out every 15 sec. or so. It was obviously auto fed etc, but well pre CNC.


Richard
« Last Edit: August 21, 2010, 10:11:49 AM by RichardShute »
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Offline Lew_Merrick_PE

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Re: Easy way to make a plastic gear!
« Reply #5 on: August 21, 2010, 11:03:19 AM »
My original idea was not to make plastic gears but to make aluminium ones, I just made the plastic gear as a trial to ensure the teeth would be formed correctly as some people on another board argued it would not.  My idea was to cut the aluminium gears  with a hand ground single point tool and use this process to finish the gear shape.  I have cut about 15 teeth on a 60 tooth aluminium gear at this time and when that is finished I will try this method to finish the gear shape.

One of the tricks I have used over the years when making a gear train with homemade tooling is finish the gears with .004/.001 excess in my over-pins measurement.  I then set up a dummy of the gear train such that I can "drive" each gear the last couple of thou to center distance, fill my dummy gearbox with fine lapping compound, and "run them in" the last couple of thou.  I start the gears being driven (usually with whatever electric motor was lying about my shop) at center-distances that allow it to run and then tighten each of the "adjustments" a bit as the motion smooths out -- this having been done in the days when 600 grit Clover lapping compound sold for $0.75/pint.  The AGMA frowns on such a practice, but it works better than you might think.

Offline jim

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Re: Easy way to make a plastic gear!
« Reply #6 on: August 21, 2010, 12:37:46 PM »
would delrin work?
if i'd thought it through, i'd have never tried it

Online John Hill

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Re: Easy way to make a plastic gear!
« Reply #7 on: August 22, 2010, 04:50:23 PM »
would delrin work?

Someone on another forum saw my post and tried delrin (acatel, I think it is the same stuff) but he was unable to get enough friction in the early stages to heat and soften the blank.  He played a gas flame on the acatel but it was not a success.  Mind you, not all mine were successes either and it must have been incredible luck that the first one I tried came out so well! :med:

I would not give up on delrin.
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Online John Hill

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Re: Easy way to make a plastic gear!
« Reply #8 on: August 22, 2010, 04:58:49 PM »
Further to my original idea, I make a 60 tooth aluminium gear with a hand ground single point cutter and as would be expected the tooth shape is not perfect.

I ran that gear against a steel one for about an hour with as much pressure as I felt my lathe was comfortable with and at the end of it the aluminium gear appeared unchanged! :scratch:
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Offline cidrontmg

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Re: Easy way to make a plastic gear!
« Reply #9 on: August 22, 2010, 05:44:56 PM »
Further to my original idea, I make a 60 tooth aluminium gear with a hand ground single point cutter and as would be expected the tooth shape is not perfect.

I ran that gear against a steel one for about an hour with as much pressure as I felt my lathe was comfortable with and at the end of it the aluminium gear appeared unchanged! :scratch:

Hm. And if you would use something like valve grinding carborundum paste, it probably would embed itself to the aluminium gear, and just grind/polish the steel gear. Rats.
Then, a plastic gear would be even softer than the ali gear, so running those together would grind the ali...  But the ali gear might also wear in the wrong places, destroying the gear profile... Rats.   :scratch:
Olli
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Online John Hill

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Re: Easy way to make a plastic gear!
« Reply #10 on: August 22, 2010, 06:00:12 PM »

Then, a plastic gear would be even softer than the ali gear, so running those together would grind the ali...  But the ali gear might also wear in the wrong places, destroying the gear profile... Rats.   :scratch:

No, I still think you would get the correct shape so now I need to make a wide plastic gear to match my ali one and try some grinding. :coffee:
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Offline cidrontmg

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Re: Easy way to make a plastic gear!
« Reply #11 on: August 22, 2010, 07:18:20 PM »

Then, a plastic gear would be even softer than the ali gear, so running those together would grind the ali...  But the ali gear might also wear in the wrong places, destroying the gear profile... Rats.   :scratch:

No, I still think you would get the correct shape so now I need to make a wide plastic gear to match my ali one and try some grinding. :coffee:

Well, I certainly hope youīre right, and no better way to prove yes or no than trying. What Iīm afraid of is, the plastic gear will be flexible, so it would tend to shy away exactly where it would need to grind the most material away (the highest pressure points). But a theory  :smart:  is worth nothing if practice proves it wrong  :med:
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Offline picclock

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Re: Easy way to make a plastic gear!
« Reply #12 on: September 11, 2010, 10:43:06 AM »
Good idea.

This method would lend itself to making an Annulus (the one with the gear teeth pointing inward) in a planetary gear system. Possibly by heating the material and forming gear with a hot air gun.

Now if only I could figure out how to do it in steel .. .

picclock
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Online John Hill

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Re: Easy way to make a plastic gear!
« Reply #13 on: September 11, 2010, 05:30:05 PM »
Good idea.

This method would lend itself to making an Annulus (the one with the gear teeth pointing inward) in a planetary gear system. Possibly by heating the material and forming gear with a hot air gun.

Now if only I could figure out how to do it in steel .. .

picclock

I have not had a chance to get back this but I do think that if an Oxy/Acet flame was played on the edge of the gear blank as it rotates in the lathe it may be possible to heat it up enough to form against a steel gear.  If the steel gear showed signs of getting red hot it could be cooled by running coolant over it or even running in a cooland trough.  Just my ideas and I daresay hardly original.
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