Shop Floor Talk  

Go Back   Shop Floor Talk > Welding and Metalworking Forums > Machining

SFT Search:   
Reply
 
Thread Tools Display Modes
  #1  
Old 02-03-2021, 06:03 PM
camdigger's Avatar
camdigger camdigger is offline
Roving Reporter
 
Join Date: Aug 2008
Location: Central Oilberta
Posts: 3,945
Default Free hobbing

I was asked to reproduce a plastic worm wheel the other day.

The easiest way I can think of would be to generate a worm to match and cut flutes to make a free hob. The plastic used for the original seems to be an injection mouldable nylon or PE. Fairly hard as plastics go. Do plastics lend themselves to free hobbing?
https://youtu.be/vghcnXPki5s

The teeth are quite small, roughly 0.090" pitch. The whole gear is only 2.100" OD. The gear thickness is 0.500".
__________________
Design to 0.001", measure to 1/32", cut with an axe, grind to fit..

Last edited by camdigger; 02-03-2021 at 06:09 PM.
Reply With Quote
  #2  
Old 02-03-2021, 07:43 PM
Ironman's Avatar
Ironman Ironman is offline
Iron Modification Investigator
 
Join Date: Feb 2008
Location: Warburg, Alberta, Canada
Posts: 15,381
Default

Quote:
Originally Posted by camdigger View Post
I was asked to reproduce a plastic worm wheel the other day.

The easiest way I can think of would be to generate a worm to match and cut flutes to make a free hob. The plastic used for the original seems to be an injection mouldable nylon or PE. Fairly hard as plastics go. Do plastics lend themselves to free hobbing?
https://youtu.be/vghcnXPki5s

The teeth are quite small, roughly 0.090" pitch. The whole gear is only 2.100" OD. The gear thickness is 0.500".
This is the easiest way to do it, but I would have a washer and nut on top so the gear does not wobble around like in the video. And he never showed the fact that you have to center the gear so the lathe center line bisects the gear width center line that you are cutting.
__________________
Gerry
You got freedom of speech, if you don't say too much.
Aaron Neville.

The virtue is always a cover for the sin. That's the key to understanding the modern left. Whatever they're accusing you of doing, they are doing themselves but more enthusiastically. And that's definitely the story of Justin Trudeau. Tucker Carlson
Reply With Quote
  #3  
Old 02-04-2021, 09:24 AM
Lew Hartswick Lew Hartswick is offline
Elite Member
 
Join Date: May 2008
Location: Albuquerque NM
Posts: 4,186
Default

Tht example wasn't centered very well :-) A simple heavy weight on the top of the wheel would have helped a lot and a captivated restraint would have been even better. :-) But the idea is there anyway.
...lew...
Reply With Quote
  #4  
Old 02-05-2021, 02:38 AM
clive's Avatar
clive clive is offline
 
Join Date: May 2007
Location: Western Australia
Posts: 2,052
Default

The diameter would need to be devisable by the thread pitch but it’s a keeper.
Already thinking of a use.
__________________
It's So Easy
Reply With Quote
  #5  
Old 02-05-2021, 03:14 PM
camdigger's Avatar
camdigger camdigger is offline
Roving Reporter
 
Join Date: Aug 2008
Location: Central Oilberta
Posts: 3,945
Default

Quote:
Originally Posted by clive View Post
The diameter would need to be devisable by the thread pitch but it’s a keeper.
Already thinking of a use.
I am hoping that since I am copying an existing part, most of the design questions would be answered with some careful measuring.

I have heard and read about freehobbing before. I understand from that research that an Acme tap will give something very Close to an involute shaped tooth. I have seen and read about many examples done in metals, especially Brass, Bronze, and Aluminum. I am still a bit curious how it will work with plastic because it is so stringy.
__________________
Design to 0.001", measure to 1/32", cut with an axe, grind to fit..
Reply With Quote
  #6  
Old 02-05-2021, 09:25 PM
LKeithR's Avatar
LKeithR LKeithR is offline
Hey...wait...is there a prize?
 
Join Date: Aug 2011
Location: Langley, B.C.
Posts: 5,411
Default

The basic idea in that video is sound but the execution is really poor. As others have said, a better fit on the pin and something to keep the blank from flopping around would have produced a much more accurate result. Notice when he takes the gear off the pin that the teeth are cut way off centre because the blank was tipped up.

Quote:
Originally Posted by camdigger View Post
...I am still a bit curious how it will work with plastic because it is so stringy...
Not all plastics are stringy; some of the harder delrin materials will break a pretty good chip. Also consider that if the gear you're producing is fairly narrow like the one in the video you're going to have an interrupted cut as the hob moves across the blank--each tooth is only in the material for a short distance...
__________________
Keith

Measure twice and cut once...or...wait, was that the other way around?
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Web Search:

All times are GMT -5. The time now is 06:06 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.