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Bridgeport clone quill / knee conversion

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David Colwill20/11/2010 13:03:02
774 forum posts
40 photos
I have started CNCing my King Rich bridgeport. X and Y are fine but I am hovering between the quill versus knee issue. I had originally planned to do the knee by fitting a ballscrew and nut with a timing pulley mounted directly on the ballscrew and having the stepper motor underneath the table, the whole lot to be counterweighted by two gas rams. I was told that this arrangement had problems due to inertia  and that working with small drills and cutters was a problem. I then started to think about doing the quill but the issue there seems to be that you can't get the drive close enough and mounting motors etc seems problematical. Has anyone done any of this? Any thoughts?
Weldsol20/11/2010 14:39:09
74 forum posts
Could you not drive the auto feed spindle ?? then you could select wether to use manual or CNC operation
KWIL20/11/2010 15:50:37
3548 forum posts
70 photos
There was an article in Digital Machinist some time ago about powering the Quill, but the problem of their approach was that it was via the downfeed handle input shaft with all the backlash that that involved with a rack operated Quill. The same will apply to the auto feed because that also goes via the rack on the rear of the Quill. The only way around this is a direct input to the quill itself which has been done by connecting the "stop" hole on the front face of the quill to a suitable leadscrew arrangement, there have been some commercial units which do this I believe.
Steph02/12/2010 06:50:30
9 forum posts
Hi, I too am in the process of converting a bridgport (clone) to CNC. With regards to the quill having some kind of drive attached, most conversions seem to favour a direct connection to the front of the quill- shaft, in manual mode this has a direct attachment to the adjustable depth indcator.  A similar setup is used on XYZ machine tools Protrack systems. A handwheel is also attached to he motor shaft to provide manual feed.
BTW are you using steppers or servos?
Regards Steve
David Colwill05/12/2010 22:45:06
774 forum posts
40 photos
Hi Steve,
I have been looking around and I agree most commercial systems use the front of the quill. I am starting to head towards the option of motorising the knee, partly because of rigidity and partly because I like the idea of having a high speed head on the other end of the ram for occasional routing jobs. I have used steppers which seem Ok for my purposes although I will have to use gas rams to lighten the knee if I go down that route.  Let me know how you are getting along and good luck!
David Senior11/06/2011 06:32:15
18 forum posts
5 photos
It may be a bit late to add to this thread, but I motorised the knee of my Myford VMF mill (article published in MEW issue 137). I had problems with the weight of the knee assembly, and the conversion had it's limitations. To solve this, I added gas springs, which was very successful. I submitted 2 follow up articles to the editor but unfortunately they have never been published.
I was very pleased with the overall conversion, and it allowed me to dabble with some ideas for products to fit to my classic mini. Things moved on from there when I decided to try and sell those products, such that my company bought a 'proper' cnc mill. Now the VMF gets little or no use, and seems incredibly slow compared to the scary speed of a modern machine!
Let me know if any more details would be useful.
GLENN BUNT12/06/2011 08:38:44
10 forum posts
34 photos


I have recently completed a CNC conversion of a Tom senior Light vertical and I decided to use the knee as the Z axis. I chose Arc Euro Nema 34 stepper 6.5 N.m motors wired in parallel configuration mounted alongside the knee and connected to the hand wheel arbour via timing pulleys and belt. I chose a 25mm dia ballscrew for the axis and used the ends of the old leadscrew to retain the straight bevel gear mechanism. I fitted two gas struts from to even out the load on the motor. This works fine, in recent tests the axis was repeatable within .0005” and as an additional benefit the machine can still be used manually. It won’t compete with a dedicated CNC machine for speed but it was very satisfying to see 90+KG of machine rapidly traversing up and down via CNC control. I hope to submit an article later this year on the conversion.

David Clark 112/06/2011 09:22:10
3357 forum posts
112 photos
10 articles
Hi Glenn
I am interested in the article.
Might be able to use the X and Y on my light vertical.
I would not bother with the Z.
Use it like a Protrack mill, I did a lot of useful work on one of them with just the X and Y and you could still use it as a manual mill.
It opens possibilities.
regards David
Jim Nolan12/06/2011 16:25:51
77 forum posts


You might want to Google Bridgeport knee CNC conversion or something like. In the last month I have stumbled across a pretty comprehensive guide to the knee conversion. There was a load of photographs and detail on how it was done. Don’t know where it was might have been CNC zone but I cannot remember.



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