How do I check whether this fixture is overconstrained?

EmmaBell0637 · 22 May 2026, 06:20 UTC

Reply to discussion
EM
EmmaBell0637
I'm reviewing a nest for our planned placing drilled components in an assembly fixture with Universal Robots UR5e and a milled support bracket in a precision fixture review area. Its multiple surface contacts look convincing in CAD, yet the bench samples seat differently. A stronger clamp has been suggested; I'd like to check for conflicting location constraints before accepting the tooling.

14 replies

RA
RachelBrooks0870
Replying to EmmaBell0637

@EmmaBell0637 Which contacts establish the drawing datums? Mark those separately from supports and clamps, then compare that map with the samples that rock.

10 points
EM
EmmaBell0637
Replying to RachelBrooks0870

The holes and the outside wall both locate sideways. The sketch calls all of them locators. It doesn't explain which takes priority.

10 points
RA
RachelBrooks0870
Replying to EmmaBell0637

Show the fixture designer where those constraints overlap, alongside the permitted hole-to-wall relationship. A relieved wall contact is one candidate for their review.

10 points
TH
TheoAllen0323
Replying to RachelBrooks0870

That proposal depends on the required datum scheme. Without establishing it, relieving the wall might remove the reference needed by the operation.

13 points
RA
RachelBrooks0870
Replying to TheoAllen0323

I got ahead of the drawing requirement there. Establish the necessary datum scheme before deciding which secondary contact can be changed to accommodate permitted variation.

15 points
EM
EmmaBell0637
Replying to RachelBrooks0870

I'll have the designer mark that explicitly. I'm not modifying the nest from a nominal CAD view and a hunch.

5 points
IS
IsabelBell0648
Replying to RachelBrooks0870

@RachelBrooks0870 I'm lost on support versus locator. Aren't both just touching the part?

9 points
RA
RachelBrooks0870
Replying to IsabelBell0648

@IsabelBell0648 Both touch it, but a locator establishes position. A support carries load without imposing a conflicting position. The design needs to make those roles work together.

12 points
AA
AaronCarter0958
Replying to RachelBrooks0870

In my separate fixture job, contacts labelled as supports turned out to constrain the same direction. Drawing the tolerances exposed what their labels had obscured.

7 points
EM
EmmaBell0637
Replying to AaronCarter0958

Did you measure the awkward samples first? Our instinct has been to put them aside, which might be throwing away the useful ones.

10 points
AA
AaronCarter0958
Replying to EmmaBell0637

@EmmaBell0637 Keep them identified and compare them with the drawing. A sample fitting badly in your nest doesn't, by itself, make the sample wrong.

19 points
EM
EmmaBell0637
Replying to AaronCarter0958

Our inspection sheets show the awkward samples are within drawing limits. Their hole-to-wall dimensions differ within that allowance. So rejecting them would need another reason.

15 points
EM
EmmaBell0637
Replying to EmmaBell0637

I'm holding the tooling decision. We still haven't established that the nest can locate acceptable parts consistently, and a stronger clamp doesn't resolve that missing basis.

4 points
RA
RachelBrooks0870
Replying to EmmaBell0637

@EmmaBell0637 The missing basis is clear: consistent location of acceptable parts hasn't been established. That explains why the tooling decision remains open.

2 points

Add to the discussion

Welcome to Application Robot

Everyone can read the forum. Sign in or create an account to start a discussion, reply, or upload photos.

Forgot your password?

By creating an account, you agree to our Terms and Conditions and community guidelines. Read our Privacy Policy for how your information is handled.