Assembly tooling and fixture drawings

[thegardener6]

Hello,

I have a general question about drawing arrangement for assembly tooling and fixture drawings. This may have been covered in previous post, but I wasn't able to find it.

I have seen test/assembly fixture drawings done a couple of different ways. Some drawings have all required information on a single drawing of the finished tooling and sometimes they a collection of separate drawings done for each assembly level to complete the tool or fixture.

My question is what is preferred practice?

For example, lets say I am creating a drawing of some sort of fixture -- basically a block with 2 pins to hold something during assembly and test.

If you have have a single drawing of the finished tooling assembly, wouldn't that open up the door for questions such as what size hole is required to accept the pins? I don't think that it would be very clear to dimension a force fit, so would it be appropriate to annotate the fit class? Also, what is the preffered way to callout the pins so that whoever produces the tooling doesn't attempt to machine cylinders instead of machining the holes, buy some pins and press them in. If a positional tolerance at MMC (or LMC) is required then would the diameter of the pins need to be dimensioned to give a mating envelope?

If you have a separte drawing for the block, wouldn't a projected tolerance zone be appropriate for the holes that accept the pins? I have typically seen holes with no drill point used, so would a counterbore symbol be preffered for the hole callout? I have shown them in section before, but is there a more efficient way?

The assembly drawing of the block and pins couldn't have many dimensional requirements brought from the block drawing because of the nature of assembly and extra cost because the block has already be inspected. You can't inspect the position of the holes because they now have pins in them. Can the pins be held to the positional tolerance of the holes if a projected tolerance zone is used?

I just wanted to see what everyone else does or if there is a standard that I am missing. Please point me to thread if this has been covered before

Thanks

 

[KENAT]

ASME Y14.24-1999 is the standard you are missing.

Fundamentally, pretty much all your approaches are permissable to that standard. You have to pick which is best for your specific situation.

While "multi detail drawings" (section 3.2.2) are generally discouraged tooling is one application where they often make sense. Multi detail is where multiple parts are defined on a single drawing. Since tooling fixtures are typically one offs or maybe low volume and usually in house etc. the documentation requirements are often relaxed somewhat.

This can then over lap with "inseperable assemblies" (section 4.2) which your dowel pins are a common example. In this case you have a parts list to help identify the components even though some or all the parts (except off the shelf items like dowel pins). One difference is that the dimensioning of the individual pieces is done with the parts shown in the assembled condition.

The fit of the pins is a good question. We've debated that here, what we currently tend to do is say how much interference is required e.g. .0002 to .0008 might be typical for stainless pins into aluminum. The dowel (and it's tolerance) is defined by it's vendor part number and description etc. So the drawing doesn't say what size hole to make, the people making it work it out based on the pin. This isn't ideal as the fit can't really be inspected on the finished item and it's delegating to manufacturing but it's what we end up with. We apply the position FCF to a reference dimension diameter of the pin.

One might spec a minimum pullout load which might be able to be tested, though this may not be perfect. I used to spec the hole size, even though it couldnt' be inspected.

You can do a detail assembly which then effectively combines the multi detail drawing and the assy drawing on a single drawing.

Yours is actually a fairly large question, I hope I've helped not hindered. If in the US I strongly recomend taking a look at 14.24, it will show you a number of options and remember, types of drawing can be combined on a single drawing if appropriate.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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What is Engineering anyway: faq1088-1484

 

[drawoh]

thegardener6,

I don't know about standards. Here is how my reasoning works.

You are making one each of your tools and fixtures. Maybe two or three units maximum. This means that your design time is at a premium. The person fabricating and building the tools and fixtures probably will be experienced and skilled. You do not have to specify everything for them. If you generate a general arrangement drawing showing critical dimensions, the worker will figure everything out. They will use materials available in the shop. Being knowledgable and skilled, their ideas and opinions will be valuable to you. You will benefit if you treat them with respect.

In any kind of production, manufacturing wants to use low cost, unskilled workers. Manufacturing must order materials, and they would like to automate the process as much as possible. Parts coming in from vendors must be inspected, and you cannot reliably anticipate who the vendors are going to be. You need to do very complete, detailed documentation.

There is a continuum here. Who is going to make your tools?

JHG

 

[thegardener6]

Thanks for the replies. This came about because I was making a drawing for a fixture like the one I described. It isn't very complex at all but I started thinking about all the ways to layout the drawing and I was curious to see how others dealt with this. I was more in the thinking and discussion mode.

I prefer the inseparable assembly approach. It clearly depicts the requirements except for fit, which can be accomplished by a simple note. However, I can foresee some instances in which the fit is really critical, so the holes for the pins might be a required inspection point. Then I would create a separte drawing for the block and assembly.

Kenat,

You aren't a hinderance at all, I appriciate the discussion. I did combine a lot of questions into 1 thread. I do have y14.24, but like you said there are a few ways to accomplish this.

Drawoh,

Eventually the fixture will be procured by our sister company from a supplier of their choice, so the drawing needs to be usable by anyone. The quantity will be close to what you said -- maybe 3-5, one for each assembly station.

I know that skilled manufacturing personel will be able to figure it out. I meant no disrespect with my questions, I just want to be as clear as possible. When my drawings are clear, everyone's life is a little easier. Some of the drawings that I have seen wouldn't even callout a pin. They would just show the dimensional characteristics. To me that means that the entire part needs to be machined. That wouldn't be that bad if there was a big enough block just laying around and tool wear and machine time wasn't a concern. But that usually is not the case even for the low quantity that we are talking about.

 

[ewh]

...so the holes for the pins might be a required inspection point.

You may be able to do this if it is a thru hole. Otherwise, you will have to require a separate inspection of the hole before the pin is pressed in.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

thegardener6, so essentially while it is only a tooling item it will get made "externally" at least to you. So yest you need to be a bit more thorough on the documentation.

However, unless the pull out or what ever on the pins is really critical I wouldn't worry too much about it. I've given a couple of suggestions above for how to deal with it. They may not support inspection in at least one case but maybe that's not much of an issue for tooling. I'd lean toward inspeparable assembly (with parts list that can give the pin details) and specify the desired fit. Unless the pull out is really critical in which case I might specify that but then you're expecting an external machine shop that won't even be in contact with you to make a bunch of calculations etc. As I said you can put a postion FCF to the inserted pins with a reference dim for the pin diameter, plus this way you don't have to worry about projected tolerances etc.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[thegardener6]

ewh,

That's an interesting approach, hadn't thought of that. The hole could be inspected for size prior to inserting the pin.

This could be accomplished by the work order/purchase order and through routers and the like. How would you go about specifying the hole on the drawing though? Would you let the required height of the pin dictate the depth of the hole and just specify diameter requirements? It would need to be in a note that inspection of the hole is required prior to inserting the pins, right? Then, this information could be forwarded to production (internal or external) to set up the appropriate routing.

The inseparable assembly drawing approach still works and is completely un-ambiuous. Also, you get out of the way for purch./mfg/quality so they can do what they need to do as drawoh suggested.

What I am used to seeing is that manufacturing/assembly level is controlled by part numbers. It seems to work most of the time, but you sometimes end up with drawings for no real reason other than process control -- much like the approach of creating a machining drawing for the block and a separate inseparable assembly drawing of the block and pins.

 

[KENAT]

If I recall correctly, the individual pieces on an inseparable assy or multi detail drawing are still meant to have their own part numbers (often using "-" numbers) per 14.24 but you may want to check. However, out company documentation system and CAD system conspire to make this difficult so we have 'soft' part numbering for the individual parts, essentially relying on the item number on the assy to differentiate. Not ideal but for tooling or inseparable assy's seems to work just about OK.

Realistically though, you don't care what the hole diameter is, you care that the pins don't fall out/shift over time. For this really pull out load is probably the thing to spec. To avoid the shop doing complex calcs though you could spec the required fit/interference, e.g .0002-.0008 or something like that, then put a note that the pins shall resist a pull out load of XXXXlb or something, essentially a proof test. Trouble is a functional test like this might damage the pins, if some kind of chuck or something has to 'grab them' with enough force to generate a lot of friction, unless they are in through holes and can be pushed from the non functional side for the proof test. Also, depending how you phrase it you might have conflicting dual requirements, especially if you spec the required fit wrong - so double check your calcs;-).

One advantage of specifying required fit for small volume stuff is the machine shop can gauge the individual pins and ream the holes to suit. This is good if they have limited press facilities or there are concerns of deforming parts at the high end of the interference etc.

I wonder if you're over thinking this a little though for low volume tooling, or maybe I've bee under thinking it for a while now.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[drawoh]

thegardener6,

Your pin is an external diameter, extending up from whatever surface it is inserted in. You care about the size, location and ruggedness of the diameter. You do not care how it gets there. Maybe, your toolmaker wants to machine down a thick piece of material. Pin or machined feature, GD&T will let you specify geometry.

Imagine that I prepare drawings of an assembly consisting of two plates screwed together, with a couple of cylindrical features sticking up. I apply accurate diameters and positional tolerances to the cylinders, using ASME Y14.5M-1994. I apply no dimensions to the screws holding the plates togther. Maybe I won't even show the sizes of the screws! In this scenario, I am showing the toolmaker what matters to me.

How much direction do your toolmakers need? How much control over the details of your tool do you need?

In production, you must deliver complete, accurate parts lists so that MRP/ERP, your warehouse and your assembly kits can be organized. You are not in production yet, by the sound of it.

JHG

 

[EdMcCabe]

I dimension the pins as follows: (note that this is for 2 round pins with 2 holes, as opposed to diamond pins and/or 1 hole and 1 edge for location)

2x dia. PPPP+/-.0001 pins |TP|dia.0016 M | A | with A as the flat mating surface. The PPPP size = a commonly available pin size, such as .1250, which are commercially available with the +/-.0001 tolerance.

the mating holes are 2x dia {PPPP+.0037} +.0008/-.0019
|TP|dia 0 M | A |

additional callouts for features related to the pin fit (i.e.: Type II error):
|TP|dia .0032 L | A | BL | CL |

Note that this example is for jig boring. If the parts were being made using N/C boring, I would increase the .0016 tolerance for the pin locations to .0032, and if the parts were being made by drilling and reaming, I would increase it to .0041.

I have used these formulae for years without problems, hope they work for you as well.

 

[EdMcCabe]

forgot to add, in the additional callouts, datums B and C are the pins themselves, sorry.

 

[NomLaser]

I disagree with drawoh. Your tooling should have the same level of detail as your final assembly prints. The reason is that someone in the future may turn around and decide to outsource this assembly and tooling. Now you have left them with incomplete drawings under the assumption that it will always be made internally.

In the past I have created tooling parts and called which tooling (with visual illustrations) to use on the assembly drawings.

 

[drawoh]

NomLaser,

I assumed that the first set of tooling could be subcontracted. Your drawings show the critical dimensions that must be achieved by the tool. They allow the toolmaker to select the non-critical hardware, allowing them to use stuff that is lying around the shop. This may be important if you are in a blind panic rush for the tooling.

The drawings have to show the requirements for a functional tool. Anything that meets those requirements ought to work. Anything more than the basic requirements is probably a non-value-added cost.

The manufactured product requires much more detail in the documentation because of manufacturing organization, and support for service. If your service manual claims that an M6x1x25 hex socket head cap screw will work, an M6x1x25 hex socket head cap screw has to work.

JHG

 

[ewh]

I agree with drawoh... unless you are creating a product line of the tooling, or require that all parts of the tooling be interchangable between assemblies, you don't really need to specify more than what is necessary to accurately produce your part.
Our tooling is stamped with the p/n, manufacturer, date manufactured and serial number, and are stand-alone assemblies.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

I too side with the 'minimal documentation' school of thought for the tooling. Just what this level is may vary depending on situation and in some cases might be almost a complete drawing pack.

However, I can't agree that this should be the norm.

I'm also hesitant about referenceing tooling on drawings. To me the Assy drawing should have the requirement, how you achieve that is almost immaterial in most cases.

Tieing manufacturing (be it your own or another company)to some tooling you came up with for the prototype or whatever might be a bad idea, maybe they come up with something better or automated or whatever. However, to meet the non verifiable drawing requirement they have to use your tooling.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[NomLaser]

I can agree with you guys to a point. Most of my tooling design has been in custom generator manufacturing so it was build specific. My reply was based upon that experience and can see where different industries will be different.