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Gerald_D
Registered
Username: Gerald_d
Post Number: 101
Registered: 11-2005
| | Posted on Thursday, June 01, 2006 - 07:24 am: | 
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I have no experience of the old cable-driven ShopBots and wonder if they gave the same chatters as the rack-and-pinion (R+P) ones?
In theory, a R+P drive is as smooth as a ballscrew and cable drive. (Chain over small sprockets is inherently rough.) However, a R+P drive is only smooth while the flanks of the gear teeth do all the work - the tips of the teeth must never contact the root valley between the teeth. If the flanks wear down, then the tips have a rubbing action in the roots.
The quickest and cheapest thing one can do to improve the chatters is to grease the racks. Next thing is to fit new pinions. Next thing is to turn the racks around (if you work a lot more at the one end of the table). Also consider replacing the rails for new, and having your new pinions hardened. It's a pain to grease those racks (we use a stiff brush dipped in the pot), but it does give us a significant improvement in cut quality, plus we know that the gears will take longer to wear out.
I don't think it is good practice to force gear teeth together to eliminate backlash - this force causes excessive wear. When the chatters really bother me one day, I plan to wedge the motors slightly away from the racks and run a test..... |
Gerald_D
Registered
Username: Gerald_d
Post Number: 103
Registered: 11-2005
| | Posted on Thursday, June 01, 2006 - 11:36 am: |
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Here is a technical article by Douglas Wright (Department of Mechanical and Materials Engineering, The University of Western Australia) on gears, and it contains the following mesmerising graphic:
The red dot is the contact point. The two mating surfaces mostly roll relative to each other, they have little slip. This is a proper functioning of gear teeth.
Imagine what happens if those teeth are forced together..........lots of grinding at each other. And multiple "red dots" moving in crossing paths.
Imagine further if the tips were driving in the roots.... The effective diameter (the driving diameter) would increase/decrease/increase/decrease.... |
Gerald_D
Registered
Username: Gerald_d
Post Number: 104
Registered: 11-2005
| | Posted on Thursday, June 01, 2006 - 12:16 pm: |
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This is the right way of spring-loading a gear to take out backlash:
but it is very expensive and normally not used for pinions with small numbers of teeth.
You might get excited to look at this page, till you see the sizes...and prices....
We could put in two racks side by side and spring-load them apart......... |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 9
Registered: 12-2005
| | Posted on Friday, June 02, 2006 - 01:38 am: |
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Here's a pic of a double pinion. Both are driven by one motor. Adjusting it tight would eliminate backlash, and you get twice the teeth. |
Mike Richards
Registered
Username: Richards
Post Number: 6
Registered: 05-2006
| | Posted on Friday, June 02, 2006 - 04:26 am: |
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Dirk,
I really like that design, but I don't see any adjustment to 'tighten' one of the spur gears, nor do I see any kind of 'idler/tensioner' roller.
Are you familiar with that particular configuration? Are the spur gears on eccentrics? |
Gerald_D
Registered
Username: Gerald_d
Post Number: 106
Registered: 11-2005
| | Posted on Friday, June 02, 2006 - 05:26 am: |
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Good stuff Dirk! Did you space the pinions so that there is a half-pitch difference? (ie. so that while one pinion's tooth is at the top, the other has a "root" at the top)
(You could set up anti-backlash by having the the one pinion spring-loaded towards/away from the other....) |
Gerald_D
Registered
Username: Gerald_d
Post Number: 107
Registered: 11-2005
| | Posted on Friday, June 02, 2006 - 07:00 am: |
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Dirk, thinking of it further, during the commute to work this morning, you need to do some tensioning/adjusting between the two pinions to make sure that the belt tooth spacing does not fight the rack tooth spacing. Driven with a timing belt, a second pinion can only help to share the load if it is very precisely positioned - otherwise it adds to the load but takes out backlash. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 10
Registered: 12-2005
| | Posted on Friday, June 02, 2006 - 01:58 pm: |
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Gerald, Mike
This is someone else's machine, not mine. I saved the pic but not the link. I would think the distance between pinion shafts is fixed. You would tension belt by adjusting the motor. You could then adjust one of the pinion gears to take out any slop. You would probably have to use something a little more exotic on that pinion than a simple set screw burying into the shaft. They make pinions that lock to the shaft using an eccentric and the position accurately set. Assuming timing belts offer no backlash, only one pinion will be doing the work for a particular direction. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 11
Registered: 12-2005
| | Posted on Friday, June 02, 2006 - 02:47 pm: |
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I found the link of the site I found the pics.
http://www.dallur.com/
Dirk |
Gerald_D
Registered
Username: Gerald_d
Post Number: 109
Registered: 11-2005
| | Posted on Friday, June 02, 2006 - 02:51 pm: |
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Hey, that Dudley Dix mentioned in that article sure gets around! We used to cut stuff for him when he lived about 5 miles from here. |
Jarl Stefansson
Unregistered
| | Posted on Monday, June 12, 2006 - 11:34 am: |
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Hi Everyone,
The picture is from a machine I am building with my friend.
There is are a few things that are missing from that picture.
1. We replaced the plastic pulleys with aluminium ones
2. Each of the pulleys can be adjusted on it's axle (dual stop screws).
3. There is a tensioner which acts on the belt between the small pulley and one of the large ones.
I don't have any good pictures of the arrangement but I will try to post some this week.
Here are some links to pics/videos:
Pic
Enlarged Pic
Video |
General