Reference Dimensions

[kunfuzed]

What is your general take on reference dimensions, specifically on aerospace drawings?  Do you ever use them for inspection, or just ignore them?

 

We deal with mostly Sikorsky prints from the 50's through mid 70's, and until recently I've thought of reference dimensions as a mere clarification, because typically they are present elsewhere on the print, or defined by other dimensions.  Recently though we were working on a fairly complicated fitting, with dimensions to angled intersections, planes, points, yada yada... and for stupid political/bureaucratic company bs reasons, we couldn't properly check this thing on a cmm (kind of a dept feud).  So we were instructed to check this thing on the surface plate.  Anyways a couple of the guys inspecting this were doing some sketchy inspections from points to intersections and what not (since company policy is not to check ref dims), and I got to thinking there just had to be a better way.  While I think CMM technology may have been around in 1975, I still think a lot of stuff was done on a plate, and a lot of these reference dimensions were on the drawing specifically for that purpose.  Giving a linear dimension and an angle for instance as ref, to check a plane defined by coordinate point off of  other datums.

 

Any take on this?  And while I'm at it, I would really like to hear of a good book/reference material for old school surface plate inspection, since that's how we seem to be forced to check things most of the time...

[crazy^millman]

Get a Master3DGage and be done with it. Allows all the flexibility of checking on the plate like a standard Height Gage, but the power and reporting of a CMM.

 

No Reference Dimensions are just that for reference. Good to have them in those other areas to see how it applies to a certain area, but I have had drawing with upwards of 2000 dimensions on them and sometimes 50% of those were Reference Dimensions. Someone told me I had to check them all. I would clock out and go find me another job. Cannot check it on the CMM is just stupid arrogant pride from someone The inspection department should always and I mean always work hand and hand with Manufacturing and Vice Versa. A good inspection department is the checks and balances in a good company not the hindrance it seems to be at your place. I see it time and time again where an inspection department is holding up production.

 

Go to the Master3DGage web site I have a white paper there that outlines things and maybe someone will sit down and see the lost money what is happening right now is costing your company and see the ROI would be very quick.

[kunfuzed]

Damn I wish!  Has been brought up before... lot's of dogma going on around here.

 

We even have $200K+ worth of retrofitted CMM and software (that can program to solid models) that's just been sitting for over six months or so!  Not fully sure why, somone said the Mittitoyo rep that was supposed to train had personal issues come up, but come on, there's gotta be someone else!  Plus the CMM operator is a scared of it in my oppinion.  He seems to just like using the other one that he has to "manually program".  Which is why we had to check it, would have taken him to long based on his estimate...

[kunfuzed]

6 people, and five days of inspection for 250 dims... insane.

[Del.]

A reference dimension is mostly a repeat of a dimension or a reference from other tolerance dimensions or values. They are not to be part of an inspection.

[DODGERFAN]

It should be perfectly legal to throw rocks at people like that.

[Guyinthedesert]

6 people, and five days of inspection for 250 dims... insane.

But, how many trips to Jersey Mikes did that include?

[BenK]

A reference dimension is essentially meaningless. Its purpose is more to tell you to look for the real value if you ask me. A lot of the time the reference dimension can be rounded and doesn't show the tolerance of the feature.

 

I agree that there is too much of the "us vs them" mentality between departments. My philosophy has always been that we are working for the same company our goals should be the same as well.

[Guest]

A "Reference" dimension is just that. It is there for.... wait for it.... REFERENCE. :

[kunfuzed]

Lol, yeah yeah I get it... Was just curious if the intent some times was to aid surface plate inspection. :-P

[Müřlıń®]

I have worked with hundreds of aircraft prints over the years quite a few Sikorsky.  del is right...in Most of my findings a reference dim is just a duplicate call out of the same dimension either in another area on the same page or on another page in a group of prints.

 

Every once in a while it is a redundant dimension and  be rounded off.....like there will be linear dims and then an angular one that will be be called a ref because the linear is what you need to go by.

[BenK]

Lol, yeah yeah I get it... Was just curious if the intent some times was to aid surface plate inspection. :-P

 

The person applying the dimensions most of the time doesn't even know what a surface plate is much less how to use it. I went to school for drafting and design as well, and one of the biggest things I was told is not to call out a manufacturing process. That includes an inspection process, that is up to manufacturing and quality to determine. 

[Newbeeee™]

one of the biggest things I was told is not to call out a manufacturing process. That includes an inspection process, that is up to manufacturing and quality to determine. 

Long one - I'm sat having my tea and writing this...

 

I'd say yes and no to this.

If you're a big company, then the production engineers are the ones who earn their money. Think the car game - look at the swept complex shapes the 'designers' model - and then the clever boys (production engineers) have to turn the dreams into affordable manufacture.

 

If you're in a medium size company (100 ish people) I'd say no.

Engineering should set the overall standard while quality maintain that standard.

I've seen time again (example) where an Inspector rejects a part for a paint finish, where when assembled, the face in question isn't even visible to the outside world ie doesn't matter.

Or Alocrom is a bit streaky on the inside of a sidewall - not seen and no problem whatsoever.

I've seen SPC literally kill a job, as (expensive) parts were scrapped for being out of (SPC) tolerance band, but within drawing tolerance.

I've had 3x different customer requests (from QA) to supply 100% dimensioned checked report (marked up dims on a print adjacent to the print dim) for parts we're going to make. Not a FAIR, but all parts in every batch, now and the future. My std reply is added cost of course and we'll have to engrave the parts to serial number them so you can match the part to the dims. Each time it's 'we don't want to part mark' and I've said as the parts are treated and painted, we won't know which part goes with which dimension sheet. Each time this request has been dropped, and I never got a real reason why they wanted it in the first place.

I've also had a couple of times, asked new customers whether their dimensions are prior or post plating. And the buyers have given me drawing office contacts to phone, who have said 'errr, ummm, good question...'

 

Drawings are only a means of communication. Where I used to work (small Black Box company), we had our own manufacturing standards and used to quote on the drawing things like 'Drill 4mm' or 'Ream 5mm'.

Our standards would show that Drilling process meant a hole tolerance of -.0/+.1mm and Ream Process would be -0/+0.01 (size dependant of course).

If a hole was just dimensioned as 15.0 (for example) then the standard +/- drawing border dims would apply (so you could drill it or mill it).

We'd also sometimes specify circular lay (O Ring Seals) if required.

And sometimes (Nickel Plate finished parts which were visible to customers) we'd specify something like 'Bead Blast using 60/80 media at 50PSI with a Venturi Feed Gun to remove all marks prior to plating'.

At this company, all Production Engineering did was organise departmental factory moves and wired plugs etc at workstations.

And when I 1st started there, the Chief Inspector who was responsible for gauge control, would in-house calibrate his sets of Bowers bore mics (his pride and joy) every 12 months using the ring gauge in each box. One day I was in Inspection discussing a problem with an Inspector and I noticed the Chief Inspector was calibrating the Bowers. In all innocence I asked 'when does the ring get calibrated', and he replied 'I'm doing it now at the same time as the bore mic!!!'

Looking back, the whole company was run by the Engineering dept.