'Slide-Fit" "Press-Fit" "M6 Caps" and "Good Finish

[Sirius]

Do you use these terms on your drawings???. As far as Im concerned its VERY bad practice, but I dont have the experience of working at many companies.
I have worked for a smallish toolmaking company for a few years in design & the 'old-skool' draughy's use some really sloppy techniques (in my opinion) and so far I have not completely followed thier practices in fear of it being frowned upon if I ever went anywhere else!!! . (As in " whose this cowboy?!&quot.
I am talking about a leader line to a counterbored hole and tapped hole on adjacent parts and just typing "M6 Caps" as an abbreviation for M6 Caphead Screws to give all the infornation in two words!!- which is still wrong in my view - as I like to put the dia of the counterbore,depths etc and say "Drill 5.0 & Tap M6 Through" or similar. A lot of "to suit" stuff like this goes on. I think its pure lazyness and so that no problems can come back to them.

Another thing is when assembling bushes and pillars/guideways/ bearings etc, they put "P/FIT" for press fit, "Push-Fit" and "Sl-Fit" for Slide Fit and just put a nominal dimension on the diameter in question on both parts Eg "20 dia P/FIT" rather than putting the limits on the two assembling parts to achieve the required effect. "good finish" "flat" "smooth" and "Close-Fit" etc are other classics!!!!.

Is it me? or does everybody else do this? what do you reckon to it?. *I am not learning the sort of limits needed for things going on like this* - I wouldnt know what limits to put on things to achieve a so called 'Slide-Fit' and I want to know these things.
They rely on the shop floor guys to make it to this and if its ever wrong its difficult to blame anyone, the designer says "well it says thats "Flat" and that part there is a "slide-Fit" there on the dawing", but surely the shop floor guy is quite within his rights to ask how good a "slide fit" is required and to what degree the flatness has to lie within etc?????. It all seems a nonsense to me - they just always make everything as accurate as possible to cover thier backs on the shop floor, and surely this is counter productive??.

Should I forget my principles and follow the flow, and never gain the knowledge, or should I keep quietly batting away for proper limits and forms (although I dont fully understand the correct/feasable proportions to apply). I understand that it would take a fraction more time to apply a proper value, but surely its better when there is a specific value to work to.


If other industries/companies do this sort of thing, then I guess Ill have to change my viewpoint a bit.

I know its a bit nit picking, but its always quietly bugged me since I started in design (got shown the other way (my way)at college).

Cheers,

Sirius.

 

[insideman]

Sirius: I agree with your complaint. I feel yhat a drawing should be able to stand alone, so am lavish with notes, tolerances, special views. More work, but really defines what's wanted and avoids pi**ing contests when something goes sour.

I have never been faulted for too much, but have sometimes had trouble because of not really finishing a drawing.

Let us know what happens. OK?

 

[knobhead]

I think the only bad practice is a failure to communicate what it is that you want.
For example, using ISO tolererance boxes could be seen as 'good practice' but it's not if your shop floor chap gets confused betweent the symbols for concentricity and roundness!
Ambiguity is bad, and so yes, most of your examples are definitly bad practice.

The bottom line is, your drawings should say what you want, and be utterly impossible to misinterpret, and if that means breaking draughting 'rules' then who cares? It's better than bad parts.

But don't guess information. If you don't know what information to put, then find someone who does, even if it's the guy on the shop. The last place you should bluff is on an engineering drawing, because it WILL come back, and the drawing is king. The shop guys know that they cannot be wrong by following the drawing, even if it IS wrong. If they deviate, even correctly, from your drawings, then they are leaving themselves open to critisicim, so it is always your responsibility to make sure they know exactly what they are doing, and also you must be open to critiscim of your work from those same guys. If they don't understand, or arn't sure what you mean, then it is WRONG, no matter how 'right' or 'properly' you've drawin it.

 

How would a subcontractor make your part if you had to send it out, or would replacement parts still fit? Like the others said, everything should be on the drg.
In the UK we have standards for limits and fits for different applications, I suppose it would be similar to where ever you are. You could get and idea from these, together with engineers "bibles" like Machinery's Handbook & Kemps (in UK)

 

[butelja]

In a perfect world, your drawings should use Geometric Dimensioning and Tolerancing (GD&T) as defined by either ASME Y14.5 or the almost equivalent ISO standard. That being said, you will probably need to train your shop people in how to properly read real drawings. Any drawing that you produce should be able to stand alone both witin your company or to a subcontractor outside of your company. That is the technical aspect of the problem.

The real problem with changing things will have more to do with interpersonal communication, pride, and "inertia" in doing things the way they've been done in the past. If you are the lone voice in the wilderness, you have an uphill battle ahead of you. The Draftsmen should work for the Engineers, not the other way around when it comes to preparing drawings. Hopefully the higher ranking people in the Engineering department can be convinced of the merits of doing things correctly. If this is not the case, you can either "go with the flow" or seek an alternate place of employment. If you let the sloppy practices become your habits, you will become less employable in the future. Something to keep in mind.

 

[Sirius]

Thanks all!!. I wish I could employ the use of geometrical tolerances,to me its the only system that makes sense of the ambiguity.

I have placed GDT on some drawings in the past where it was getting awfully long-winded trying to note what I want eg. "This face must be perpendicular within 0.02 to datums A,B and C and flat within 0.02/100mm to...." etc. The only trouble is you cant really make these values up, and because I dont have a full understanding of GD&T I tend to stick to easily defined ones when applying them as I understand that it is dangerous for me to use inappropriate tolerances or symbols etc.
When I issue these drawings I always identify the meanings of the symbols clearly for the shop floor guys....so there is no excuse to confuse symbols.

I perhaps put too much on my GA/Detail drawings sometimes in general (centerlines,hidden detail,cross hatching and special views etc), but I find this makes me faster when detailing out all the seperate parts from the GA, as I readily have the info drawn for all the bits and I am sure in my mind that it will all fasten together without screw clashes etc......the other guys leave everything very sparse and you can easily sit there looking at one of thier drawings for hours just trying to decipher whats actually going on!! . Blocks are just on there not fastened down in any way on the ga, but on the details there are tapped holes etc!.
It saves 'design time' cutting these corners I suppose, but it must increase future time on similar jobs and especially the shop floor who just go working 'blind' making each little part seperately and hope it all goes together OK!!!!. Sounds like "Penny wise - Pound Foolish" to me.

It doesnt help when we dont ever get the info required to fulfill the design requirements, eg. A fixture to finish grind a casting. We just get the component drawing or a sample casting and have to make up an idea for a fixture for it. If it was clear that for that operation, it must finish within a certain flatness/parallelism or something, then I could properly assess the flatness/parallelism of the jig baseplate etc.....nobody ever gives this information because they either dont know themselves or leave it ambiguous on purpose to cover thier backs.

Anyway, back to drawing practice. The point made that it doesnt necessarily need to conform 100% to a drawing practice is fair enough. "M6 caps" gets the job done 'in house' etc as the toolmakers have learnt the sizes required to make the parts - but what if it was an outside company that had to make it or an apprentice??, or even if for some specific reason a certain dia/depth of counterbore was required?. "flat" - Flat to what/within what? its a nonsense.

I generally muddle through, somewhere inbetween the 'proper way' and 'sloppy way'.

The aerospace guy (above) says I should ask the guys on the shop floor. They all have the same frame of mind "Well, I just make it to within 0.01mm so it cant be wrong" and this doesnt help me or anyone understand whats going on.
I respect the shop floor guys in a big way, but I always get that type of answer. The fact he may be spending an extra hour per workpiece grinding it perfectly flat etc doesnt matter to him as he's paid his wage at the end of the week and aslong as everybody else does the same, he cant get done for taking too long. He's generaly not interested in the cost to the customer or the company, as long as he doesnt look too slow. You all know the phrases like "I got it within a nats knacker" or "within a tadpoles eyelash".

Perhaps this is just "toolmaking" practice, but I would be glad to hear any toolmaking poeple who use GD&T. I suppose at the end of the day its a compromise between what you can get away with to save time and 'proper' design to fully distinguish a part. For the time being Im going to keep quietly plodding away mid-line, but I am never ever going to put "M6 Caps" or "Good Finish" on anything I do!!!!. Its Bonkers.

Thanks for your reading patience,

Sirius. (UK)

 

[Adrian2]

Dear Sirius;

I have had the opprtunity to see tooling drawings from a variety of T&D shops in Canada and they also make a lot of abbreviations. For many small tooling shops it is quite sufficient to put "Press Fit" on a drawing because you are dealing with tool makers and they know what a press fit is. Many tooling shops have standards for dowels and bushings so that often only a letter designation is all you see calling up a standard part. To an outsider this makes tooling drawings all the more difficult to understand.

While I think "M6 Caps" is a little sloppy, if everyone in the company is used to that, then no problem. Tooling drawings are generally used only once to make a specific die and not usually farmed out for others to work with. Sometimes drawings are not used at all. Many toolmakers can and do build entire progressive dies whith little more than sketches on the back of paper napkins and their own experience.

I think what you are dealing with here is peculiar to the tooling industry.

Regards

Adrian

 

[Sirius]

Cheers Adrian. I think youre right. I suppose I just struggle having only been in engineering for about 5 years and in toolmaking for about 3 (Shop Floor - Drawing Office). I guess my head is just made for general engineering.

It often seems madness though, theres some real 'pearlers' crop up doing it the way they do, I can tell you!.

The trouble is you can rely too much on 'experience' and whats in peoples heads - if there is a lack of fully trained personel who can fill thier shoes when they retire or if they go under a bus, there is major trouble ahead. "Give that to Dave, he knows about that sort of thing, you dont need to put 'x' and 'x' on the drawings". Dave leaves. "ERMM. What happens with this then?, Dave used to do that. How do we set that up to machine!? - does that piece need to be flat?". etc.

A lot of work is indeed 'one offs', but saying that, 80% of the work is look-alikes and altered drawings from one thing into another, if its done comprehensively the first time, it will be comprehensive faster and every time its used as a blueprint for something else.

In my area of England, a lot of poeple just arent interested in engineering/toolmaking as a career path as its just not economically viable unless you get your foot in a big company. Toolmaking knowledge relied upon could quite easily get lost in a few generations at this rate.


Anyway, if thats the way it is, then so be it - It doesnt really rile me or anything, its just often frustrating at the inane-ness of it all. Im just going to keep plodding on, somewhere down the middle and thank you all for your input as it has been both interesting and informative - I am better off for it!.


Cheers,

Sirius.

 

[smokehouse]

Hello,
I have been in plastic injection moldmaking since 1970. I've built molds for a long time and now design them. There is "movement" on every mold --vertical, horizontal and sometimes angular. The drawings are complex as it is and if there were dimensions called out for every type of fit in a mold, the designs would be far more cluttered than they are.

Every ejector pin must be a slip fit, they vary from the manufacturer (DME). It would be almost impossible to put a 4 place dimension on the holes, because the pins DO vary a little. The leader pins have a certain slip fit.
Horizontal slides must be able to move freely after being heated to +500 degrees. Slides can be complex and to assign tolerances on all the mating/moving parts would mean an E-size drawing on a 6 inch slide.

I call out a C'bore like this: DRILL & C'BORE FOR 3/8-16 SHCS (TYP) (ex)...Even the apprentice knows what that means.
A tapped hole is: 1/4-20 TAP X 5/8 DEEP (ex)

After a job is designed, I sit down with the lead moldmaker and the shop floor boss and we go over every sheet. When he starts the job, there are no misunderstandings....

The top guys have built molds for 25 years or so --they know what every fit and application is..

When we send things out (ex. jig grinding) then I will call out every hole and with a +&- tolerance.

To have everything toleranced is a real good idea but time is of the essence and doing it would not make the job go one bit faster....Anyone that has worked on the floor knows what a 4 place dimension means..

I'de pay more attention to the guys that have been actually doing the work for a long time--why? Because they know what works..

I will agree with the outsourcing scenario though--- IF you are getting parts made outside your shop then you will have to tolerance dimension everything..

 

[Sirius]

Yeah, I see where youre coming from. We use three/four decimal places for 'tieing up' accurate stuff, and I admit that I tie stuff up to three places and put 'g' on the surface to indicate grinding sometimes.

This is 'understood' that the plate has to be accurate and to get three decimal places (metric) it has to be ground and a good surface grinding machine should give you a precisely flat face etc (or good enough for the general job). But, what happens if it is not good enough afterwards. I cant hold the toolmaker to blame because he is following the drawing, and the drawing hasnt got this information on....meanwhile I can get out of it in the usual company way bay saying 'Well, the toolmakers should know.......' etc. (Although I dont).

As for cluttering up the drawing, a small goemetric tolerance box say 6mm high and 15mm wide attatched to the dimension line is hardly clutter - especially if applied only to the crucial parts. I'm not saying put special tolerances or lenghthy notes on *every* piece, just the absolute critical parts. I think you may be over exagerating the 'clutter' and on todays CAD systems (if you got a decent one) would let you add a full GD&T frame in about 10 seconds or less.

You say if the work was for outside, you would put full tolerances on everything. Fairy Nuff. But the point is that you (having worked in the industry 30 years) know all the limits and fits and sensible values to attach in terms of form eg Parallelism etc and for someone who has just started off and only knows 'P/FIT' and 'Good Finish' how am I supposed to learn this stuff. No body has the time to show you and its deemed (like you view it) unneccessary and a waste of time anyway.

If it was properly specified on things and people didnt use terminology like 'within a tadpoles eyelash' then I would have no doubt picked it all up by now. Its just "Dumbing down" and if I ever had to work elsewhere they would laugh in my face when I sheepishly go and ask "what does n6 finish mean" or something and they see me putting 'To Suit Screw' on my drawings.

The example of hole descriptions you gave is in my opinion OK for the tapped hole, but why type 'Drill & C'BORE FOR 3/8-16 SHCS' when you can (in the same amount of time) put eg: 6.5 Dia thro', C/Bore 11.0 Dia x 7 DP?......Perhaps on the drawing board days (where the mentality of most toolmaking design stems from) then it would take longer to neatly hand print in pencil all this information, but CAD has wiped out the need to apply short cuts such as this and shotcuts like drawing one piecce and saying 'one off opp.hand' - you could just mirror it iun seconds and save possible confusion etc etc etc.

I suppose, if some half way house is met then everybody would be happy. You say you go through the job before it goes out - this is good and when you detail work up in this 'toolmaking manner' its a good idea to spend some time explaining the functions and in and outs of a job. I agree thats a good compromise.

Our firm never has the time to tell anybody what it is, what its for, etc and most often than not because were screwed tight for delivery dates the managers have say 8 seperate poeple working on different parts (even mating parts) and nobody is up to speed on what everyone else is doing.
The drawing of a block may say "50 Sl-Fit'" - the guy has made a mistake and its gone under to 49.9 and carried on with the rest of the features on the block and its also slightly tapering - no problem usually, you just make the matching piece to compensate. However, the plate has just finished being made by somebody else and its a fraction oversize and has a slight taper on it the opposite way so one of the parts (after deciding the most fastest to remake) is scrapped off.

If in the above example, the plate and the block were given proper sizes like 50.00 +0.00 -0.03 and on the plate: 50.02 +0.02 -0.00 and tied up for parallelism then this may never have happened. The 'liscence' to make whatever the heck you want is removed and it is clear to anyone what values have to be met and this may predetermine the setup required to produce the feature (say grind, instead of finish mill).

Dont get me wrong, Im not trying to fall out with you or anything it just that I want to make it clear where Im coming from, and then someone can fully justify the reasons for doing it the way 'toolmaking' folks do....I dont really buy into the 'clutter' and 'lots of extra time' arguments much yet. Perhaps if time was spent as you do, going through the job and if fewer people worked on the same job on the shop then a lot of 'difficulties' would not arise.

Its good poeple have different views on stuff isnt it? It would be boring if we were all the same!!!!.

Thanks again for replying,

Sirius.

 

[smokehouse]

Sirius,
You have a different scenario at your shop than I do. One moldmaker oversees the job, there is nothing made on his mold that is out of his control.

If I had the serarate components of a mold made in different shops, then I would have the tolerances and surface finishes called out on all fits.

RE: the bolt dimensions..
3/8-16 shcs tells the toolmaker all he needs to know about installing this hole and C'bore..He knows the clearance hole will be .406 dia. For me to put more than this would be redundant.

RE: Clutter..
A fully dimensioned 2D drawing of the core detail (for ex.) will be quite complex and filled with dimensions.
To add anything that the toolmaker knows already will be ignored anyways.
For example he knows that core pins from DME are .001 oversized ... He knows that ejector pins are usually .0005 to .001 undersized.
He knows that after heat treat the holes may need some more "attention"..
Nothing that I add can help this.

Again, I point out that all the components of a mold we make are made in-house so we have the latitude to "make" things fit. (excluding pins)

 

[gbent]

Sirius
It sounds as if you have a big task ahead of you. One thing our shop used as a standard is errors of form. The form error of anything is limited to 1/2 of the locational tolerance. This helps eliminate some drawing notes. for example, if the location of a surface is +/- .001 in., the surface must be flat within .001. If the location of a bored hole is +/- .001 in., the maximium out of round for the bore would be .001. If different requirements were needed, they would be noted seperately.
Producing a drawing that lists every requirement for the part, without telling the machinist how to make the part is difficult. If the work is done in-house, a set of shop standards could take care of many drawing details. The shop standards could be referenced on the drawing. If some overflow work is sent to vendors, keep your vendor list small, and also provide them with a copy of the shop standards.

 

[cranekiran]

Dear all,

It has been very interesting to read all your views.I think every thing concerned with this question has been answered in depth.I want to make only one comment:-

My thinking is that tolerances are provided in the first place because you give some range to human error.The more tolerance the more range you give to the operator to work in a sloppy manner.Of course,Assembly references are important and are definetly the reason for tolerances in the first place but the former reason is a very important human factor too.Drawings in any part of the world would not give you total information about the part or Assembly;You just have to be knowledgable or learn the job as I call it the first time you come across a problem.Later when you are more familiar you don't have to even see the same drawing again.
Designers on the otherhand are not so lucky since they have to face new challenges all the time; that's the precise reason you have standards to work on.My suggestion is stick to one standard i.e if it's I.S stick with that throughout your lifetime unless the company or World opinion(other newer standards) wants you to change your designing style.

 

[smokehouse]

Sirius,
In rereading this thread I noticed a statement you made:

"In my area of England, a lot of poeple just arent interested in engineering/toolmaking as a career path as its just not economically viable unless you get your foot in a big company. Toolmaking knowledge relied upon could quite easily get lost in a few generations at this rate."

That seems to be the mentality here too.. We are losing toolmakers to retirement and few kids want to learn this trade..

It takes longer for a man to become a class A moldmaker than it does to become a doctor!

At some time down the road I can only hope the "worm" will turn.

I want to see the day when toolmaker is a desired and respected job again, with a respectable wage also.

We make molds for some big well known companies and are losing their business to the Chinese. With all the math skills required and tools to be bought, the average moldmaker in this area gets about 18.50/hr. It might take 10-15 years to get there besides -- no wonder the kids don't want any part of it.

 

[Stoker]

Sirius,

Another issue which may affect your company is ISO. If your company has ISO status or is considering it, these notations may not be popular with the auditors. In general,they will object to any fabrication specifications which are open to interpretation - no matter how seemingly insignificant the range of interpretations may be. Also,if these drawings are to be sent out to subcontractors who are ISO certified you may constantly get requests for more specific notations. At the very least, it would be a good idea to generate a document which outlines exactly how your personnel are to interpret terms like "press fit". This will help when training new employees and ensures that everyone is interpreting these terms in the same way.

 

WillP58, youre right. I didnt even start off with Toolmaking in mind, but after a heck of a lot of training and college and hard work in 'general mechanical'engineering(I am not the brightest button in the box when it comes to maths!), the only half decent job going was in toolmaking....I can see exactly why younger folks steer well clear (I am only 24 though) and after full NVCQ's and HNC I was only on minimum wage till about 12 months ago because without travelling really far and loosing the extra money in petrol/car costs, the jobs just arent here.
Generaly, engineering has taken a bit of a nosedive of late hasnt it??. Just picking up again slightly........We too are loosing out to China/Japan in some areas.

Stoker, We have ISO9001/2 but its very sketchyly followed sometimes and sort of seen as an "Evil Necessity" to get work. Bare minimum is stuck to (if that), but youre right about a fully documented in-house book about things - it could be a darn good idea because there is nothing of that sort at the moment.....thanks.

Thanks all,

Sirius.