BIG Dro Problem

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

Moderators: Harold_V, GlennW

Mr Ron
Posts: 1827
Joined: Tue Dec 22, 2009 12:36 pm
Location: Vancleave, Mississippi

BIG Dro Problem

Post by Mr Ron » Sun Aug 05, 2018 5:53 pm

I finally finished installing the Dro Pro on my 6x26 knee mill. The DRO works perfectly, but there is a problem with the mill itself. When I set coordinates on the X and Y axis, and when I lock the table, the readings on the display change with about .004 on the X axis and .010 on the Y axis. I checked the gibs on the mill and they are tight. If I push on the table, the DRO readings on the display jump around. What can be the problem? The mill hasn't gotten a whole lot of use, so I don't think it is worn out.
Mr.Ron from South Mississippi

User avatar
Bill Shields
Posts: 5508
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: BIG Dro Problem

Post by Bill Shields » Sun Aug 05, 2018 7:31 pm

Oofta...this is generally indicative of worn ways or ways that are not properly fit (for one reason or another).

Just because gibs are tight does not always mean that there is good contact across the entire length of the surface...either the gib or the way itself.

First you need to see where the 'non-contact' is coming from - the gib or the way itself.
Too many things going on to bother listing them.

John Evans
Posts: 2031
Joined: Tue Jan 20, 2009 9:33 pm
Location: Phoenix ,AZ

Re: BIG Dro Problem

Post by John Evans » Sun Aug 05, 2018 8:52 pm

Pull the gibs and check for polished high spots or a chip lodged in it. Also make sure the gib does not have a bow in it, that would make it feel tighter than it is.
www.chaski.com

Mr Ron
Posts: 1827
Joined: Tue Dec 22, 2009 12:36 pm
Location: Vancleave, Mississippi

Re: BIG Dro Problem

Post by Mr Ron » Mon Aug 06, 2018 11:01 am

Thanks; I will try that.
Mr.Ron from South Mississippi

jmanatee
Posts: 28
Joined: Mon Mar 09, 2015 12:17 pm

Re: BIG Dro Problem

Post by jmanatee » Mon Aug 06, 2018 5:57 pm

Try to snug the lock before setting table final position and then tighten the lock slowly the rest of the way

Mr Ron
Posts: 1827
Joined: Tue Dec 22, 2009 12:36 pm
Location: Vancleave, Mississippi

Re: BIG Dro Problem

Post by Mr Ron » Mon Aug 06, 2018 8:06 pm

jmanatee wrote:
Mon Aug 06, 2018 5:57 pm
Try to snug the lock before setting table final position and then tighten the lock slowly the rest of the way
I tried that, but when I tightened down the lock, the reading changed. Is this a common problem with other mill owners with small 6x26 mills? I think this is a pretty good mill; I bought it new and it doesn't have a lot of hours on it. It has never been abused. It was made in Taiwan, not China in 1986. The bearings are SKF, FAG or NSK and the ways are hand scraped
Mr.Ron from South Mississippi

johnfreese
Posts: 181
Joined: Thu Jan 21, 2016 2:10 am

Re: BIG Dro Problem

Post by johnfreese » Mon Aug 06, 2018 8:44 pm

I have a relatively new Bridgeport clone. I see a change of a couple tenths when I lock the Y axis. I don't see it often on the X. There must be a little clearance in the slide ways in order for them to move without binding. When you tighten the clamps you are pushing the gib against the sliding member which means zero clearance on one side of the member. The looser the gibs the more change in the DRO you can expect when you tighten the clamps. The problem exists on all mills with slide ways. The larger and higher quality the machine the less shift you should expect.

Mr Ron
Posts: 1827
Joined: Tue Dec 22, 2009 12:36 pm
Location: Vancleave, Mississippi

Re: BIG Dro Problem

Post by Mr Ron » Mon Aug 06, 2018 11:36 pm

johnfreese wrote:
Mon Aug 06, 2018 8:44 pm
I have a relatively new Bridgeport clone. I see a change of a couple tenths when I lock the Y axis. I don't see it often on the X. There must be a little clearance in the slide ways in order for them to move without binding. When you tighten the clamps you are pushing the gib against the sliding member which means zero clearance on one side of the member. The looser the gibs the more change in the DRO you can expect when you tighten the clamps. The problem exists on all mills with slide ways. The larger and higher quality the machine the less shift you should expect.
No one told me the problem exists on all mills; bummer. Then how do guys get around this problem? Maybe Harold has some ideas to fix this problem.
Mr.Ron from South Mississippi

User avatar
NP317
Posts: 1915
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon

Re: BIG Dro Problem

Post by NP317 » Tue Aug 07, 2018 12:18 am

My big new South Bend milling machine registers a DRO reading change of 0.0002" when I lock the X or Y slides.
I accept this as normal for any machine with oiled surfaces and gibs for sliding ways.
I figure the accuracy of the mill cut is within this dimensional change.
If I require high accuracy, I leave some drag on the locks.
~RN

Harold_V
Posts: 17688
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: BIG Dro Problem

Post by Harold_V » Tue Aug 07, 2018 1:55 am

Mr Ron wrote:
Mon Aug 06, 2018 11:36 pm
Maybe Harold has some ideas to fix this problem.
Unfortunately, I do not, and I'm not convinced there's a solution. You just have to come to terms with the idea that a milling machine isn't a very high precision machine, and they aren't (please---those who may read these words-----lets not start a dead-end debate on this subject. If you've ever operated a jig borer or precision grinding equipment, you already understand what I mean. If you haven't, no amount of talking will help you understand the differences).

As has been stated, a slide must have some clearance in order to be able to move. Machines like a Bridgeport (I realize yours isn't one) ARE NOT SCRAPED---contrary to popular opinion (although they may show flaking, which has nothing to do with scraping). They are machine finished, and have a reasonably high degree of precision, but they are not scraped to improve that condition. They also have the potential to move with time, as stresses relax. Considering those conditions, and assuming that any given machine is reasonably good at the outset, they have a respectable amount of clearance, which allows them to move without excessive effort. That can manifest itself as slop when locks are tightened, and generally does on mills, where pressure is applied to the gib to narrow its clearance, creating the required lock.

On some machines, the lock isn't related to the gib, and slides can be locked without losing position. A CNC relies on the drive motor to keep the slide on location, which it appears to do quite nicely.

I am not attempting to judge any given machine. Lesser expensive machines may not have a high degree of precision when they are made. In order for them to operate with reasonable effort, they may have excessive clearance. The more clearance a machine has, the more movement you can expect when tightening locks that bear on the gib. As an example, I've always known that my BP loses at least a half thou when locks are applied. I deal with that by not using them unless necessary (it isn't always necessary to lock a slide), or by setting them lightly, making movements without moving the lock, so the error remains somewhat constant. As a machine wears, however, that becomes impossible.

If a machine has excessive wear, you'll find that the screw and table/saddle assemblies move easily when in the center of their travel, and get tight, even too tight to move if the gib is snugged up with either of those slides at an extreme end. Under that condition, the gib must be operated in a somewhat backed off condition, otherwise you'll limit the travel of the given slide. Assuming you have backed off the gib on either of those slides, you may be able to detect movement when you apply side pressure to the table, as if you're trying to rotate it either clockwise or counterclockwise. If tightening the gib limits or eliminates that movement, then you know that the mill has excessive wear, and that the gib must be operated loosely in order to not lose full travel. At that point, the solution is to rescrape (or scrape, for the first time) the machine. Without that, it won't get better, only worse.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Mr Ron
Posts: 1827
Joined: Tue Dec 22, 2009 12:36 pm
Location: Vancleave, Mississippi

Re: BIG Dro Problem

Post by Mr Ron » Tue Aug 07, 2018 9:40 am

Thank you Harold. You explanation makes a lot of sense. I'll try to do the best I can. When drilling a pattern of holes, I can leave the table unlocked. I will also check the gibs. They have not been addressed since the mill was new, 30 years ago.
Mr.Ron from South Mississippi

RSG
Posts: 1086
Joined: Fri Feb 04, 2011 9:59 am
Location: Ontario, Canada

Re: BIG Dro Problem

Post by RSG » Tue Aug 07, 2018 1:11 pm

Mr. Ron,

I have the same problem with my smaller knee mill although not as bad as what you are saying. It will jump a few tenths so what I have learned to do is to set it while the machine is running and stop a few tenths before landing on the number so when you tighten the lock it lands on the correct number.
Vision is not seeing things as they are, but as they will be.

Post Reply