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  #1  
Old Wed 25 May 2011, 03:46
southernduckie
Just call me: Andrew (Duckie)
 
new south wales
Australia
hobbed or ground rack?

Hi,

Don't know if i'm missing something but is the rack ground or hobbed?
Apart from the price difference is ground rack required? motors would run smoother on ground rack i think and accuacy would be improved not sure it this would affect cut quality.

Thanks in advance. i hope
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  #2  
Old Wed 25 May 2011, 05:23
Kobus_Joubert
Just call me: Kobus #6
 
Dalview
South Africa
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Not hobbed...all ground I think
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  #3  
Old Wed 25 May 2011, 06:06
Surfcnc
Just call me: Ross #74
 
Queensland
Australia
Gear ratio would be the biggest determinant of cut quality.
That is why the Oriental Motors steppers with the gearbox built in or the 4 to 1 belt reductions are so popular.
Both these options improve the resolution many times over trying to improve the fitment of the pinion to the rack itself.

TEA have the rack here in Australia, up to a 3m length for the X axis.

Regards
Ross
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  #4  
Old Wed 25 May 2011, 10:46
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I don't think that anyone has used ground rack. Our machines are definitely with hobbed rack and hobbed pinion gears.
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  #5  
Old Wed 25 May 2011, 11:21
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hobbing is a process whereby a metal cutter "shaves" away cuttings.....similar to milling.

Grinding uses stone to get a finer/smoother surface after hobbing is complete.

Hobbed gears have a slightly rough surface that soon wears smooth.
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  #6  
Old Thu 26 May 2011, 03:16
southernduckie
Just call me: Andrew (Duckie)
 
new south wales
Australia
Thankyou

Thankyou, think i can put this one to bed and save a heap of $ to hobbed rack it is. it's questions like this that keep me up at night
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  #7  
Old Thu 26 May 2011, 03:33
Kobus_Joubert
Just call me: Kobus #6
 
Dalview
South Africa
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Shows how much I know about engineering..
All I can do well is FOLLOW THE PLAN..
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  #8  
Old Thu 26 May 2011, 07:24
Surfcnc
Just call me: Ross #74
 
Queensland
Australia
Kobus - I read the wiki on Hobbing at http://en.wikipedia.org/wiki/Hobbing after your comment and was still not able to figure out exactly how the rack was made.
You have plenty of company

Regards
Ross
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  #9  
Old Thu 26 May 2011, 08:12
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I don't think that a rack, which is a gear with infinitely large diameter, is "hobbed" at all - I think that is plainly "milled"

To see/understand hobbing, you need to watch some youtubes. The first one I found had poor video quality but it showed the basics: http://www.youtube.com/watch?v=F_qflpT6__o On the left is the (spiral) cutter, on the right is the gear being cut. the cutter and gear are speed controlled so that the cutter doesn't just strip off all the teeth. The spiral cutter is held at an angle to the gear, equal to the angle of the spiral, so that the gear's teeth end up straight. The cutter moves across the face of the gear at a fixed distance from the gear's center. Maybe skip to the end of the video to see the bigger picture when the camera zooms out. There must be many better videos available.
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  #10  
Old Thu 26 May 2011, 08:21
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
http://www.youtube.com/watch?v=7j9OUR3mKK0 Hobbing cutter left, gear being produced on right. I skipped a lot of the video but I think it was a first pass going up and a full depth pass going down.

Edit: another pass back up
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  #11  
Old Thu 26 May 2011, 09:23
Kobus_Joubert
Just call me: Kobus #6
 
Dalview
South Africa
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I know the basics of hobbing and have done it on a small pinion...that is why I thought our Module 1 racks were Milled and not hobbed ..or whatever..
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  #12  
Old Thu 26 May 2011, 09:36
KenC
Just call me: Ken
 
Klang
Malaysia
The important part is, IT WORKS!!!
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