Re: backlash compensation
Posted by
Marcus & Eva
on 2001-06-06 08:22:08 UTC
Hi All:
I've been reading the debate on the value (or not) of backlash
compensation on machine tools.
My "Real World" experience: (I tried this on my Defiance yesterday)
If I turn off the backlash comp several bad things happen:
1) circles that I mill are not as round as they were with comp on.
2) holes that are jig bored are not as accurately on location as they were
with comp on.
Admittedly, I am talking about a machine where I set the comp values in X at
0.0003" and in Y at 0.0002" (in other words the machine is pretty tight for
backlash).
However, all theorizing aside, the machine performs measurably better with
backlash compensation.
For what it's worth, here's what I believe:
- I think that invoking backlash compensation to overcome significant
mechanical deficiencies in the machine is a losing proposition in milling
operations because of the wide variety of forces you are dealing with.
- However, if the machine is mechanically sound, and you are working to
sub-thou tolerances, you will notice an improvement with comp on. I assume
that you are compensating for factors such as inertial stretch of the screws
and mounts at direction changes much more than for sloppiness in the system.
To give you an idea of the required tightness for this to become noticeable,
this is the test for backlash that I performed:
- Angle plate bolted to table, tenths indicator bolted to head casting.
- Indicator tip on angle plate.
- push table hard and release. note indicator reading.
- push table hard the other way and release. note reading.
- call the difference the backlash.
My machine is 0.0002 in X and 0.0001 in Y
BUT: I can deflect the whole works a good 0.002" while I am pushing on the
end of the table.
That is the stretch, and that's what I compensate for empirically (I
think!!) when I set the comp.
In support of this theory; when I have a heavy block on the machine, I can't
work as accurately as I can on a lightweight block unless I reset the comp.
The effect becomes more noticeable at high feedrates.
Cheers
Marcus
-----Original Message-----
From: Smoke <gordonr@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com <CAD_CAM_EDM_DRO@yahoogroups.com>
Date: Tuesday, June 05, 2001 7:22 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Climb Milling (again)
I've been reading the debate on the value (or not) of backlash
compensation on machine tools.
My "Real World" experience: (I tried this on my Defiance yesterday)
If I turn off the backlash comp several bad things happen:
1) circles that I mill are not as round as they were with comp on.
2) holes that are jig bored are not as accurately on location as they were
with comp on.
Admittedly, I am talking about a machine where I set the comp values in X at
0.0003" and in Y at 0.0002" (in other words the machine is pretty tight for
backlash).
However, all theorizing aside, the machine performs measurably better with
backlash compensation.
For what it's worth, here's what I believe:
- I think that invoking backlash compensation to overcome significant
mechanical deficiencies in the machine is a losing proposition in milling
operations because of the wide variety of forces you are dealing with.
- However, if the machine is mechanically sound, and you are working to
sub-thou tolerances, you will notice an improvement with comp on. I assume
that you are compensating for factors such as inertial stretch of the screws
and mounts at direction changes much more than for sloppiness in the system.
To give you an idea of the required tightness for this to become noticeable,
this is the test for backlash that I performed:
- Angle plate bolted to table, tenths indicator bolted to head casting.
- Indicator tip on angle plate.
- push table hard and release. note indicator reading.
- push table hard the other way and release. note reading.
- call the difference the backlash.
My machine is 0.0002 in X and 0.0001 in Y
BUT: I can deflect the whole works a good 0.002" while I am pushing on the
end of the table.
That is the stretch, and that's what I compensate for empirically (I
think!!) when I set the comp.
In support of this theory; when I have a heavy block on the machine, I can't
work as accurately as I can on a lightweight block unless I reset the comp.
The effect becomes more noticeable at high feedrates.
Cheers
Marcus
-----Original Message-----
From: Smoke <gordonr@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com <CAD_CAM_EDM_DRO@yahoogroups.com>
Date: Tuesday, June 05, 2001 7:22 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Climb Milling (again)
>I've been looking at some of these responses for "software compensation for
>backlash".
>Real HOGWASH IMO. If the machine "jumps" due to excessive load under
>backlash, ho the H is the software going to know how much "jump" to
>compensate for? Furthermore, how's the software going to know how much
>backlash you have in your machine...or how much extra backlash to compenste
>for due to wear or poor machine adjustment?
>
>Smoke