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Torx head profile of a surface questions
3

Torx head profile of a surface questions

Torx head profile of a surface questions

(OP)
I'm trying to establish if there is a need to establish a datum C for a torx head screw, and how it could be done properly with GDT. My only understanding til this point of leaving out locking down rotation is on purely cylindrical revolved geometry.

I just encountered a torx print that did not have datum C and allow rotational freedom in Z. Maybe this is correct, maybe it is dealers choice, and that is why I'm asking.

In my attempt to lock down rotation, I attempted to use a radius surface for the datum feature, but when I looked up the standard it shows the radius as a basic dimension then the datum is attached the profile of a surface FCF....both 2009, 2018 standard pics are below....they are in different chapters so I put them below.

If I followed suit into the standard and created datum C from the profile of a surface callout it would conflict with the desired profile callout listed on my print. On the otherhand if datum C is listed as is on my print and I measured this with a CMM and a cad model the profile of a surface could get all messed up depending on the radius.

If I just forgot about datum C and hence rotation and made the radius basic, the problem seems to go away.

I'm confused. Thank for having a look. 2009 Fig. 4-29 & 2018 Fig. 7-30

*edit...the prints I've come across so far with torx like heads use min max diameter as basic dims for the drive head, so I followed suit here.
*edit...the datum surface is not much in terms of degrees....I know that is a problem for cmms,but is there something written about this for the standards?







RE: Torx head profile of a surface questions

Unless something other than the recess requires it "C" serves no function.

Also, all dimensions describing a surface controlled by profile need to either be basic or, in the special case of a single nominally flat surface, not exist.

RE: Torx head profile of a surface questions

It would only make sense to lock the rotation of the Torx about the axis if it had to be at some specific orientation relative to something else. Then that something else would become the tertiary datum reference. Not useful for a simple screw. As it was pointed out, all radii defining the true profile of the Torx shall be basic if it is to be controlled by profile. BTW, if it is a standard Torx, there is ISO 10664 that can guide you on the dimensions and allowed variations.

RE: Torx head profile of a surface questions

(OP)
Thanks for the replies, I figured something was wrong with the toleranced surface, but I overlooked the referenced radius as well. I saw a print with profile that included the small lobe referenced so that was incorrectly put together. Appreciate the ISO reference as well, I'll look that up.

RE: Torx head profile of a surface questions

Are you actually having screws made to spec from custom tooling? Seems like an awful waste of effort for something so standardized. Almost certainly better to spec compliance with an existing standard, e.g. SAE AS8538.

RE: Torx head profile of a surface questions

(OP)
I saw two prints recently from customers with a profile of a surface callout, both if I remember with a reference dim on a lobe. I'd tend to agree with what you said, I do prints anytime I can yo learn both the mechanics of using diff cad drawing tools and improving my gdt. I'm also doing a practice part on a cmm to align this more effectively.

RE: Torx head profile of a surface questions

Looks like a pretty large Torx.
The largest standard one is T100, and on it the diameter specified as 1.50 in your drawing is .871.

RE: Torx head profile of a surface questions

Yeah - a 2 inch diameter screw head. How likely is that?

"(a) A zero is not used before the decimal point for values less than 1 in."

If it's per Y14.5, then this is a metric dimensioned drawing of the recess to accept s T5 tool.

RE: Torx head profile of a surface questions

If it's metric a leading zero is missing for the profile tolerance.

RE: Torx head profile of a surface questions

Oh, no, a drawing that isn't compliant. Glad you are there to point out the tiny flaws when ignoring the clues to the actual size and making big errors.

RE: Torx head profile of a surface questions

And...
2.0+-0.1 should be 2+-0.1. 1.50 basic should be 1.5.

RE: Torx head profile of a surface questions

3D,
Good to have you here to complain about it.

RE: Torx head profile of a surface questions

I always figured that these rules about leading and trailing zeros were intended solely so nitpickers could have something to complain about when they didn't understand the engineering intent of the drawing. That way they could feel they were making a contribution and not feel left out. If either approach was clearly better it should be the only approach.

Interesting that it's important to nitpick now when it didn't occur as a problem earlier.

RE: Torx head profile of a surface questions

3D,
You are obviously confused because you are the one who brought up the "zero is not used" rule.
I pointed out the clues to an inch drawing.

RE: Torx head profile of a surface questions

I didn't nitpick the drawing. You did. Still, it's good to have rules to allow easy confusion to happen rather than not caring about leading zeros trailing zeros and instead looking at the title block to see what the units are.

Maybe they are scaling a metric recess by 25.4 times when a more typical and less costly choice would be a spline.

Keeping track of zeros sure gives checkers something to do when they don't have any other useful thing to add.

RE: Torx head profile of a surface questions

(OP)
That's my bad on including the leading zeros, sorry about that, I do like that rule. This is a personal drawing for an oversized 3d printed torx head I'm using to probe on the cmm and pick up the larger torx radius to orient the part with leeway as to user error upon placement orientation in a collet and without the operator having to manual find surfaces. The size of the print is to make the 2mm probe tip have similar room that a .5mm tip would inside something like a size 10 torx. I can barely see a .5mm tip let alone the shaft so I'm using a larger probe to practice on.

RE: Torx head profile of a surface questions

3D, I didn't nitpick on the drawing. As usual, you fail to see the context, just like you failed when you were nitpicking on the plural verbiage of the standard in the simultaneous requirement definition.

Edit: by the way,

Quote (3D)


Yeah - a 2 inch diameter screw head. How likely is that?

Not less likely than an approximately M1 sized thread with a T5 Torx.

RE: Torx head profile of a surface questions

sendithard, it's all good. I figured it's an inch drawing because there are more clues to it than to a metric drawing, that's why I was intrigued by the size of the Torx. Thanks for the clarification.

RE: Torx head profile of a surface questions

Quote (3D)

I always figured that these rules about leading and trailing zeros were intended solely so nitpickers could have something to complain about when they didn't understand the engineering intent of the drawing. That way they could feel they were making a contribution and not feel left out. If either approach was clearly better it should be the only approach.

"something to complain about..."

"didn't understand the engineering intent of the drawing..."

"could feel they were making a contribution and not feel left out"

3D, that turned out to be a precise description of most of your input in this thread.

RE: Torx head profile of a surface questions

sendithard,

If the goal was to also encompass the radius specified C as part of the all-around profile tolerance, in addition to the fact that it needs to be basic the result is that you have created a self-referencing specification. This means that the datum feature C is controlled by a profile tolerance which also references C in |A|B|C|. Now since C only constrains [w] rotation I don't believe you'll get the normal effects of a self referencing tolerance zone as when translational DOF are being constrained (ie: half the tolerance zone is available) however there may be some unintended consequences - at the very least you've created a potential headache for the people inspecting it unless this is something they are familiar with.

In this case as others have noted you could just remove C as it really adds nothing as you probably don't care about orientation of the torx feature to any other feature on this bolt (note - all DOF do not always have to be fully constrained in every case, as in this case [w] rotation can be left free as orientation/clocking of the torx to the thread doesn't matter), however its important to be mindful of this self referencing as its easy to specify a datum feature and then wrap it in an all-around/all-over profile tolerance that specifies the same datum feature. There are situations where its potentially desirable however in my opinion it should be generally avoided unless necessary.

RE: Torx head profile of a surface questions

Quote (Burunduk)

if it is a standard Torx, there is ISO 10664

If it is a standard Torx(tm) then it does not follow ISO 10664, it follows the Camcar Textron specification; they are not identical. E.g. for the size 6 internal hexalobe feature, according to a document I have from 1992 (has it changed?) for the Torx(tm) internal feature it specifies an outside lobe-lobe width of .0680 to .0700 (inches) while ISO 10664 specifies 1,695 to 1,709 (millimeters, conversion: .0667 to .0673 inches).

Comparing to the mating tolerances for Torx(tm) for the T-6 screwdriver the outside lobe-lobe dimension could vary from .0650 to .0670 inches so at the extremes of both features it has a ~.0003" interference with an ISO 10664 hexalobe. Will that situation ever occur? Subject for the "statistics" thread? :)

RE: Torx head profile of a surface questions

BiPolarMoment,
1,695 to 1,709 mm in ISO 10664 is the size variation for the GO gage, not the size limits for the actual feature.
The NOT GO gage is 1.778-1.785 mm.
Note how the minimum size of the NOT GO (1.778) matches the maximum value you see in your document (.0700 inches).

RE: Torx head profile of a surface questions

chez - it's correct that the self-reference to C doesn't cause the same problem as when there's use as a location tolerance that loses a portion of the tolerance zone along that part segment to the other side of the datum simulator. Here the worst effect is that it doesn't really help define this particular feature.

Edit: In thinking a bit more on this - since this was an RFS specification - if there was a portion of the feature around the remainder of the perimeter that was slightly out of tolerance, this RFS tertiary reference would prevent shifting that tolerance zone in a way that the datum feature surface remained in the tolerance zone and allowed the other portion to be shifted in; this would reject a usable feature as being unacceptable. Less a problem for the inspector and a big problem for the factory. An MMB reference would preserve the ability to shift it.

Depending on the CMM software there's the potential to scan a few thousand surface points and rotate them to verify they fall into the tolerance zone, so the benefit of a tertiary reference is if the CMM software cannot do that rotation - I would have chosen the smaller radius feature for that purpose, but the CMM inspector can chose their own alignment to minimize the rotation requirement.

In production this inspection most often be done with a digital microscope or Go/No Go gages.

There might also be a use for the tertiary if there was some other feature that had to be clocked relative to the recess and the desire for establishing a simultaneous requirement. One area lacking in the Y14.5 standard is an example of clocking screw threads, though for this recess I can't come up with a need for clocking - it repeats at 60degree intervals which is sufficient.

RE: Torx head profile of a surface questions

@B... Woops, I skipped the part where I was looking at a gauge definition somehow--the tolerance certainly should have clued me in but since it was in metric it didn't throw up any red flags (+/-.0035, no problem!)

RE: Torx head profile of a surface questions

Quote (3DDave, 16 May 22 17:35)

since this was an RFS specification - if there was a portion of the feature around the remainder of the perimeter that was slightly out of tolerance, this RFS tertiary reference would prevent shifting that tolerance zone in a way that the datum feature surface remained in the tolerance zone and allowed the other portion to be shifted in; this would reject a usable feature as being unacceptable. Less a problem for the inspector and a big problem for the factory. An MMB reference would preserve the ability to shift it.

I figured there could be some unintended consequences - I agree this could be one of them.

Quote (3DDave, 16 May 22 17:35)

so the benefit of a tertiary reference is if the CMM software cannot do that rotation

I hadn't thought of that, that would be an interesting reason to add a self referencing datum feature however it would align the tolerance zone as you mentioned. Of course as you know the MMB solution would likely aggravate that issue.

RE: Torx head profile of a surface questions

(OP)
Chez,

Appreciate the comment. I discussed this a little in the original post as I knew something was wrong...but I didn't see it like you so well said as 'self-referencing'. I just saw it as a problem to have a datum C referenced surface that had a tolerance applied to it. My ambition to to lock this part down in rotation on the cmm overwhelmed my knowledge that it didn't need it in real life. The juxtaposition is that I am probing what I called datum C to clock that part in accurately so the profile can then be appropriately measured against A|B. I learn a lot from you all and I appreciate that you all help educate people with much less knowledge. I'm embarrassed I used a leading zero, I knew better, glad I got smacked for that one.

RE: Torx head profile of a surface questions

The only embarrassment for a leading or trailing zero should be among those who demand it as a stand-in for specifying the units for a drawing. Specifying the units is already required to be clearly put on a drawing under "IDENTIFICATION OF LINEAR UNITS." So controlling leading and trailing zeros serves no purpose. This is particularly ridiculous since there is a big section laying out how numbers are all considered to have an infinite number of zeros past the last significant digit beyond the decimal point.

The main reason for padding the numbers is just to allow for, on US inch drawings, the practice of putting default tolerances in the title block based on the number of digits - which should be deprecated by now - and isn't mentioned in Y14.5 recent versions; not sure it ever was.

RE: Torx head profile of a surface questions

(OP)
I'd like to ask one more question on this torx profile topic, Thanks in advance.

Referencing my original image....I've learned to forget about datum C and I've learned that all profile of a surface surfaces need basic dimensions.

Could you then just make the torx profile of a surface .02|B| in my case...no need to even involve Datum A? I know you can still go A|B if you so choose, but the profile doesn't need A b/c B is a cylinder and takes care of 2 trans & 2 rot.

Thanks for you thoughts.

RE: Torx head profile of a surface questions

sendithard,
If you do that you won't be controlling the (perpendicular) orientation of the torx feature relative to datum A, only the the (parallel) orientation to datum axis B. It may be enough in terms of degrees of freedom, however doing it that way or not depends on what is functionally important for you. By the way, the datum features need also to be qualified by controlling the form of the primary and in your case (and generally often) the orientation of the secondary relative to the primary.

RE: Torx head profile of a surface questions

Typically one would want to make it relative to the pitch diameter of the thread. Before the thread is made a cylinder of material will be used to located and orient the part for forming the head and forming the recess. After the head is formed the thread will be rolled onto that cylinder.

Using the pitch diameter ensures that the mating driver would remain in the center and directly aligned with the axis of rotation as the screw was installed, a very important characteristic for automated assembly to avoid damaging the tooling.

The outer form of the head isn't so important and neither is the top surface, particularly as references. The underside will also be formed in relation to what will become the pitch diameter after the thread rolling process; it plays no part in relation to the driving recess.

If this is for training - just measure both ways for the experience.

RE: Torx head profile of a surface questions

I also agree about the importance of the pitch diameter and it's use as a datum feature.
Here is what I consider a reasonable example of a screw drawing. In this case they first related the unthreaded portion of the shank to the pitch diameter of the thread, then used it as a datum reference for other controls, related to the head.

RE: Torx head profile of a surface questions

Burunduk,
Which is the standard you have the embeded picture from?

RE: Torx head profile of a surface questions

It appears to be from ASME-B18.2.1 but I do not know which revision (if that matters to the question).

RE: Torx head profile of a surface questions

Quote (sendithard, 23 May 22 03:15)

Could you then just make the torx profile of a surface .02|B| in my case...no need to even involve Datum A? I know you can still go A|B if you so choose, but the profile doesn't need A b/c B is a cylinder and takes care of 2 trans & 2 rot.

So several have commented about the use of the pitch diameter which was going to be my first point, ie: make sure your datum feature selection reflects function, so I won't add too much there. However you seem to be asking about DOF constraint and order of precedence so I'll add some notes there. I'll use the convention you have in your drawing to make it simple (A is the top of the head, B is the head diameter).

The order of precedence in which datum features are referenced in your Datum Reference Frame (DRF) is important, and lower precedence datum features cannot constrain Degrees Of Freedom (DOF) that have already been constrained by higher precedence datum features. So A in |A|B| is considered primary and is the highest precedence and constrains [z] translation and [u,v] rotation, and B is secondary precedence and only constrains [x,y] translation. Even though B has the ability to also constrain [u,v] rotation, it doesn't because A has already taken care of those DOF. Now if we switch it to |B|A| then B now constrains [x,y] translation as well as [u,v] rotation, and A only constrains [z] translation. In this latter case of |B|A|, A is no longer as important to this profile tolerance as constraint of [z] adds nothing to this particular tolerance.*

Ultimately as noted above it should come down to part function as to how datum features and their precedence are selected - namely, if we're to continue with your convention of A and B even though we know the pitch diameter is the better choice: A and B should be functional/mating surfaces, and the precedence reflects how the part functions/assembles either A->B in |A|B| or B->A in |B|A|.

If you have access to it I would take a look at the difference between Y14.5-2018 fig 7-19(b) and 7-20(b) (Y14.5-2009 fig 4-20(b)/4-21(b)).

*Not to confuse things, but there are situations where you might want this - for example if you wanted simultaneous requirements to apply with another tolerance.

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