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Gear center distance question

Gear center distance question

Gear center distance question

(OP)
I have a print from a customer when he only has the straight line distance from one hole (a datum) to the other holes

I know he doesn't care about the gear position per se, just the center distance so that the gear meshes correctly.

I have a CAD model so I can get the XY distance from hole to hole, but can you use TP with just one dimension?

If not what would be the best way to GD&T it without over constraining it?

Thanks,
Robi
 

RE: Gear center distance question

I'd say you are missing 2 dimensions.

You either need the angles for the 2 holes, or the 'vertical' or 'horizontal' dimensions from B.

It doesn't have any note about MBD or other digital definition to allow measuring off the CAD model/drawing.  In fact it explicitly says 'DO NOT SCALE DRAWING'.

I'd go back to customer and get the missing dimensions, or worst case put in your best guess and tell him that's what you'll make it to unless told otherwise.

Also, are you sure you wanted to post something 'confidential'.

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RE: Gear center distance question

(OP)
Hi Kenat

The problem with just adding another basic dimension is it would lock down the position radially as well and my customer does not want that. He's ok with the radial position moving, but  the center distance of the gears needs to be controlled.

As far as the drawing goes I changed the dims and put it on my drawing sheet, so I'm not giving anything away.

Robi

 

RE: Gear center distance question

As currently dimensioned the holes could be outside of the outline of the part, or interfere with the other features (holes?) is that OK?

Adding an angle for each hole, or equivalent vertical or horizontal dims, is not over constraining it, it's adequately constraining it.

Unless I'm missing something.

Perhaps do either you or your customer not understand position tolerance?

I notice you don't have dia symbol which may be correct.

Do you actually want unequal bilateral tolerance?

You'd still need the extra dimension but you'd also add FCF to control movement in the other direction.

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RE: Gear center distance question

I am with Kenat in saying that additional basic angle dimension or two 'vertical', 'horizontal' basic dimensions are needed for defining TP of each holes precisely, otherwise holes can be anywhere, theoretically even outside the part.

If your customer is interested more in controlling distance between datum B and the holes rather than their angular relationship with B, I would suggest method shown on fig. 7-29 in Y14.5-2009 std. or fig. 5-42 in 1994 version.

RE: Gear center distance question

Robi,

Assuming [B] has nothing to do with the axis to axis distance between the functional gear centers... attached is sketch of how I guess the refinement would be detailed if my functional assumptions are correct.

The lower position tolerances to [A|D] control the center distances exclusively with cylindrical zones perpendicular to [A].

I would sanitize and possibly alter a customer's drawing before posting it to the internet.

Paul  

RE: Gear center distance question

The specification of datums "C" & "A" are a little unconventional.
I would agree with Paul's modifications if his assumptions are correct, it is the same assumption I made looking at it.
The standard states "the location of each feature is given by basic dimensions, or such" that establishes the true position, the ideal location, you let it float with the applied tolerances and if those gears float you will want them to float together
Frank

RE: Gear center distance question

Robi, take a look at section 5.8 of asme Y14.5m-1994 where it talkes about bidirectional positional tolerancing of features.

I think this might be what you're asking about, especially 5.9.2 Polar Coordinate Method, & figure 5-42 especially.

I've attached a VERY crude sketch (I don't have much time), hopefully enough to make you go look at the standard.  http://files.engineering.com/download.aspx?folder=9fa6279c-b2f5-46a5-a0fa-5300d4563bf4&file=polar_coord_bilateral.tif

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RE: Gear center distance question

(OP)
Hi guys,

Thanks for the input on this.  I actually know that the customer has not specified this correctly.  My question was how to keep the center distance controlled tighter than the radial location. Which the customer did not even have on his drawing.

Kenat's last post I think is the way to go on this.  I don't actually have the standard and all the books I have on GD&T don't mention bidirectional positional tolerancing. So I wasn't aware of this.

I have asked my QA manager to buy the standard. Aftermarket books seem to leave things out and if you don't know that you don't know something......

fsincox, the callout for datum C is for a locating pin on the mating piece which is flush to datum A.  I interpreted that to be the center of the notch since they give  a (non-basic) dimension to datum B from the center of that feature.  is this correct?

They have agreed to let me redo their print and I want to do it correctly.

Thanks,
Robi

PS the drawing was cleaned up and put on my title block. If this is considered inappropriate I will edit it.  

PPS how do you edit posts? I couldn't find an edit button or a FAQ on how to edit a post.
 

RE: Gear center distance question

To edit you redflag and ask management to make a change, however if in fact there is no confidential info on there it may not be an issue.

I think the issue of polar bi directional tolerancing has come up before, maybe you can find it.  This is the better sketch I did at that time http://files.engineering.com/download.aspx?folder=c93dd132-e5eb-41c8-b9f3-9077d109649e&file=5-42-APPROXIMATION.tif

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RE: Gear center distance question

Robi,
I mean the leader for "A" should terminate with a dot, when the intent is the surface normal to the view, and the box should not be attached directly to the leaderline without the "flag".
Frank

RE: Gear center distance question

(OP)
Thanks for the info on editing.  I made an even more generic print and will probably replace it just because.

My QA manager said the standard will be here today.  I will go over those sections you mentioned to make sure I understand them correctly.

It's kinda funny, we had a 12 week GD&T class for our machinists that ended just a few weeks ago and he didn't mention bi-directional tolerancing at all.

Thanks
Robi

 

RE: Gear center distance question

If it was 12 weeks full time then you were ripped off.

If it was one hour a week for 12 weeks, then it's perhaps understandable.  It is only a couple of paragraphs and 2 figures.

The most difficult thing with bidirectional polar can just be getting it to format correctly with most CAD systems!

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RE: Gear center distance question

(OP)

It was 3 hours each Monday, from the local community college. that probably explains it.

I use Inventor 2011, I havn't tried it yet. I hope it will let me format it right!

Robi
 

RE: Gear center distance question

Use a composite frame.  If you have a center-to-center distance and one of the centers is a datum (i.e. datum "B"), then you can control the distance with a tight tolerance position relative to "B", and then a larger tolerance applied to a full set of datums.

RE: Gear center distance question

Robi, Kenat,
Unless the specified bi-directional tolerance references one of the gear centers as the datum feature that establishes X0, Y0 coordinate reference... the bi-directional detailing will only orient the planar tolerance zone to [A|B|C] and if each has a tolerance to [A|B|C] (no matter whether cylindrical or slab-like) their deviation would be compared to the DRF [A|B|C]... not directly to each other.

Thetick,
If one of the gear centers was declared as the datum... a composite control on the other would be confusing if not erroneous. Composite controls are designed to apply to patterns and if one of the 2X pattern was declared the datum feature... maybe you can see the dilemma. It could work if there were leader lines pointing to each of the separate size callouts of the (assumed gear bearing counter bores) ending in a composite FCF  (making them a 2X pattern by dimensional detailing) that would refer to [A|B|C] in the upper segment and just [A] in the lower. The tolerance for the allowable deviation in the center distance would have to be split between both diameters since there would be two cylindrical tolerance zones, one at each gear center perpendicular to [A], that would be controlling the axes of the 2X pattern... This control method would be in my opinion "overkill and/or convoluted."  You would still have to tolerance the other bores coaxial with the gear centers.

I do agree with you though that one of the gear centers should be established as a datum feature to tolerance the axis to axis distance to the other... I suggested that also.

Paul    
 

RE: Gear center distance question

If not oposte, then a pair of  position callouts: one with only primary (or add seondary) with tight tolerance and seond for angular position with looser tolerance.

RE: Gear center distance question

Kenat, Understood.
Maybe I am wrong in assuming that B is not a gear center but rather an assembly locator.
Paul

RE: Gear center distance question

(OP)
Hi everyone,

It seems the discussion has continued without me...

I have created a simplified sketch showing the pertinent info and how it is currently called out by my customer.

Datum B is indeed a gear center. there are three gears in the train. The center distance is what needs to be controlled to +/-.0005 inches. The angular position is acceptable at +/- .005 inches (aprox 2 degs)

The original question is how do use GD&T to accomplish this?

My customer says he is correct but is willing to see what I can come up with.  He says having a TP callout with one basic controls the center distance and the hole can float within the title block tolerance of +/-.005, this would give me an elongated tolerance zone.

I hope this clarifies what I am looking for.

Thanks,
Robi

ps I wonder why the standard does not address gear centers?



 

RE: Gear center distance question

TheTick,

It's good but there wold need to be the note "SEP REQT" under the refinement tolerances otherwise the pesky simultaneous requirements rule would restrict the rotation of the features within the diameter .001 as well.

Paul

RE: Gear center distance question

Quote:

Paul, why are you saying not to use Position and instead use +- tols?
I didn't say that.

Quote:

This seems like an ideal candidate for polar coordinate bidirectional position tolerance or am I missing something?
Your suggestion would work fine! TheTicks would as well!  Your suggestion is subject to the sim reqt rules just like The Tick's but it is marginally insensitive to it because of the slab rather than cylindrical tolerance requirements... but when sep reqt notes are added to TheTick's his becomes an advantage because the zones are free to rotate and not locked at the included angle between the centers.
There are many ways to do it but the best mirrors the functional constraints and liberties... We don't know them! We know only what Robi tells us! Most of us recognize what is missing in the DRF construction, form and orientation controls of the datum features, basics for the features in question, etc... but those details do seem to be important to the discussion here so... there is nothing wrong with the option I suggested last and one doesn't need I depth knowledge of GD&T to understand it.

Paul   

 

RE: Gear center distance question

I say it is alway's function that drives preferences not rules that are made up by designer's or inspector's or machinest's, those that want everything checked with an attribute gage, etc.

If you nail the datum constraints exactly the way the degrees-of-freedom are removed in function (assembly or otherwise) and apply tolerances according to the function liberties and constraints required for the item to perform as predicted... that is the goal.

Paul   

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