Re: [CAD_CAM_EDM_DRO] Re: Driving a Bridgeport Knee

In my opinion (and experience) there is nothing wrong with cnc'ing the
knee and using a rolled (.003") ballscrew and shimming the bevel gears
to remove backlash. I have one job that I run periodically and I can
hold 0.02mm (.0005") in Z all day long - but as the machine warms up, I
have to compensate for thermal effects. Don't get too concerned about
.003" error over 12" - you're almost never going to make anything that
big on a knee mill anyway. And while it is an error, a zero backlash
ballscrew is exremely repeatable hence you can compensate for it.
Admitedly, I'm not doing mould work where this could be a big factor but
in 2.5D and most small 3D parts I make, it's a non issue. I have
certain dims I have to hold and it takes care, but is not hard. I too
had planned to eliminate the bevel gears, but it turned out to not be
necessary.

Cheers, Peter

cnc_4_me wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
>wrote:
>
>
>>The knee due to its mass and precision of the acme screw is far
>>
>>
>more accurate than the X and Y will ever be. And there is zero
>backlash. The Z mechanism is twice the resolution of X and Y i.e. 1
>rev equals .100 opposed to the .200 per rev of X and Y. A ballscrew
>is not needed. Backlash in the bevel gears can be removed by shimming
>or compensate for it in the software. The major drawback to driving
>the knee is slower acceleration due to the weight and the need for a
>larger servo to power it. The quill wouldnt be any more accurate.
>Probably not as accurate.
>
>
>>
>>Ron
>>
>>
>
>
>The best acme screw spec I can find is .003" per foot and that is
>today's technology. Couple that with bevel gear error and what do
>you have???
>
>On the other hand, most of us have not had a problem with this error.
>
>On the third hand, what if we want to try new things on our CNC like
>making molds, now how accurate do we have to be. If we leave the
>acme screw in and bevel gear, will we limit ourselves later?
>
>On the 4th hand, are we kidding ourselves that we can get ½ thou
>accuracy from our Bridgeport. Probably. My guess, working on a metal
>parts for a couple hours, best ballscrews, good controls, .003" error
>min. But hey,I am an optimist!!!
>
>And round and round it goes. I have been hashing this over in my
>mind for months.
>
>
>Wally
>
>
>
>
>
>
>>Polaraligned <polaraligned@o...> wrote:
>>
>>
>>Well, I don't think they get it very much below .001". You will
>>have backlash to contend with. But the bigger question is how
>>accurate are those gears anyway? People get ground screws over
>>rolled ones- yet settle for sloppy bevel gears? I just like the
>>idea of driving the leadscrew directly and saving the bevel gears
>>for manual operation of the machine. I may even get a ballscrew
>>for the Z axis, but I have not really decided if this is just a
>>
>>
>waste.
>
>
>>As it is all commercial conversions (that I have seen) drive the
>>
>>
>quill
>
>
>>because it is more accurate. To each his own...
>>
>>Scott
>>
>>
>>
>>
>>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" wrote:
>>
>>
>>>Well for what it is worth, the people i talked to about driving
>>>
>>>
>with
>
>
>>>bevel gears said to shim them and take out the backlash.
>>>
>>>Wally
>>>
>>>
>>>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
>>>
>>>
>>>
>>wrote:
>>
>>
>>>>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me"
>>>>
>>>>
>>>wrote:
>>>
>>>
>>>>>I was wondering if this was feasable also, if my machine
>>>>>
>>>>>
>wasn't
>
>
>>>>>buried under a ton of stuff i would investigate further.
>>>>>
>>>>>
>While i
>
>
>>>do
>>>
>>>
>>>>>not have any answers i do like hearing your ideas or anyone
>>>>>
>>>>>
>elses
>
>
>>>on
>>>
>>>
>>>>>the subject. One thing i am affraid of with usaual methode of
>>>>>driving the knee, is breaking the bevel gear with to much
>>>>>horsepower...
>>>>>
>>>>>Wally
>>>>>
>>>>>
>>>>>
>>>>I really am resistant to driving through the bevel gears as they
>>>>are very sloppy. I also like to be able to move all axis by hand
>>>>should the need arise or for the "quickie" machine job. My
>>>>
>>>>
>machine
>
>
>>>>is currently completely apart for a rebuild, and I can see no
>>>>
>>>>
>reason
>
>
>>>>why this won't work. Just can't bring myself to drive through
>>>>the damn bevel gears....
>>>>
>>>>Scott
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
>>>>>
>>>>>
>>>>>
>>wrote:
>>
>>
>>>>>>I am thinking about driving the knee with a pulley just
>>>>>>
>>>>>>
>below
>
>
>>>the
>>>
>>>
>>>>>>bearing on the Z-axis leadscrew. I would machine the
>>>>>>
>>>>>>
>leadscrew
>
>
>>>down
>>>
>>>
>>>>>>to accept the pulley. The motor would be mounted on the
>>>>>>
>>>>>>
>side
>
>
>>>of the
>>>
>>>
>>>>>>knee, and to get the timing belt out I would cut a
>>>>>>
>>>>>>
>rectangular
>
>
>>>slot
>>>
>>>
>>>>>in
>>>>>
>>>>>
>>>>>>the side of the knee.
>>>>>>
>>>>>>Advantages:
>>>>>>Motor is out of the way.
>>>>>>I can still use the handle to move the knee via the bevel
>>>>>>
>>>>>>
>gears.
>
>
>>>>>>(can't leave the handle permanantly attached!)
>>>>>>
>>>>>>Disadvantages:
>>>>>>Lose about 2 inches off the bottom of the knee travel
>>>>>>Changing belts would be a real pain in the you know what.
>>>>>>Permanantly alter the knee with hole (which does not bother
>>>>>>
>>>>>>
>me)
>
>
>>>>>>I think the motor will have no problem with clearances.
>>>>>>Does this sound good? or am I missing something.
>>>>>>
>>>>>>Scott
>>>>>>
>>>>>>
>>
>>
>>
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>OFF Topic POSTS: General Machining
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>http://www.metalworking.com/news_servers.html
>
>http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
>NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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