Re: [CAD_CAM_EDM_DRO] ...but how much precision????

Hi bill

It probally is not good to equate backlash with precision. Leadscrew
backlash can create problems on a CNC machine that will cause it to
loose precision but it does not directly affect precision. Now before
too many react to that statement let me try to clarify the statement.

Imagine you have the worlds most perfect slide assembly driven by a
brand new lead screw with a lets say 0.030 backlash. If after taking
up backlash you still can position to with in 0.001" then you have a
pretty accurate system. Now lets say that lead screw has worn for a
year, and the nut is adjusted for 0.001" backlash but now you are only
able to postion accurately, due to leadscrew wear, to with in 0.010".
Which is the most precise system?

In the past I regularly worked on CNC systems that where expected to
position to within a couple of microns 24 hours a day 7 days a week.
Yes this required a great deal of maintenance, but when the tolerance on
your parts is 2 or 3 microns you do what you have to do. Even then,
in a temperature and humidity controlled environment, continous
metrology was required on the product produced.

As far as your nut adjustments go, they are probally to tight.
Generally if they are to tight to operate by hand they are going to have
problems when driven hard by a CNC system.

But agian it is a question of what you are doing with the machine. It
is possible to have huge amounts of backlash in a system and still
repeat on parts to within a few microns. You won;t do this unless the
rest of the machine is up to snuff but putting to much stress on
leadscrew backlash is really focusing on only a small part of the
problem of precision.

Bill Kichman wrote:

>Thanks for sharing your experiences guys...what I was basically after, is,
>how precise must my machine's backlash be, to attain a reasonable amount of
>precision, all other things being equal, without having to resort to
>ballscrew conversion. And additionally, how much precision is typically
>merited in machining work As I understand it now, most professional cnc
>machines are lucky to attain 0.0005 to .0015 accuracy. Sound correct?
>I spent a part of last evening adjusting the acme screws on the old 1945 Van
>Norman mill that I will be converting to cnc, and using a dial indicator,
>was able to reduce the backlash on all 3 axes to within (1) thou. The
>handwheels are a bit tighter than I would want for manual use now, but I
>don't intend to do a LOT of hand turning :-) This machine is in great
>shape, they don't make things like they used to.
>
>
>Bill Kichman
>103 Old Furnace Road
>Cornwall, PA 17016-0643
>tel/fax 717/270-0714
>----- Original Message -----
>From: "Les Watts" <leswatts@...>
>To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>Sent: Friday, May 09, 2003 12:16 PM
>Subject: Re: [CAD_CAM_EDM_DRO] ...but how much precision????
>
>
>
>
>>I have also measured and mapped my gantry machine.
>>
>>I used 12", 6", and 3" precision micrometer test bars in various
>>combinations
>>to measure over the entire 5'x5'x1.5' work envelope. I also have a huge
>>class
>>N precision square. After correction I was within 0.001 positioning
>>everywhere
>>at precisely 25 degrees c. Cutting error is of course poorer.
>>
>>And if the shop temperature rises just from 25c to 30c the whole thing
>>
>>
>grows
>
>
>>about
>>5 thousandths. Expansion from ballscrew heating in hard use results in
>>
>>
>even
>
>
>>more error.
>>
>>I think position error goes as to about the inverse square of cost. It is
>>
>>
>a
>
>
>>big
>>deal and big $$$ to do this.
>>
>>Les
>>Leslie Watts
>>L M Watts Furniture
>>Tiger, Georgia USA
>>http://www.alltel.net/~leswatts/wattsfurniturewp.html
>>engineering page:
>>http://www.alltel.net/~leswatts/shop.html
>>Surplus cnc for sale:
>>http://www.alltel.net/~leswatts/forsale.html
>>----- Original Message -----
>>From: "Bill Kichman" <billkichman@...>
>>To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>>Sent: Thursday, May 08, 2003 10:39 PM
>>Subject: [CAD_CAM_EDM_DRO] ...but how much precision????
>>
>>
>>
>>
>>>Has anybody truly measured the degree or precision you are getting from
>>>
>>>
>>your installation? Is 1 thousandth attainable from home cnc?
>>
>>
>>>Bill K
>>>
>>>
>>>
>>
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>OFF Topic POSTS: General Machining
>If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
>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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