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How Big

How Big

(OP)
Hi Guys,

I'm designing some basic things but I need some clarification regarding the gap that I should leave. If I have a cutout and then into that cutout I insert another part of the same shape, how big a gap should I use? If you add paint, the part that I'll be inserting will become larger, also, CNC machines aren't exactly perfectly precise so this needs to be taken into consideration also, any helpful hints, ideas or links to sites that talk about this?

Thanks.

RE: How Big

Leave enough. Consider the other factors too.

RE: How Big

I try to leave about the width of my small finger.

RE: How Big

A good rule of thumb is to have the hole be bigger than what goes in it, but not always.

RE: How Big

What type of paint? Coatings can vary from .01"-.05" easily.
What is the tolerance on the CNC parts?
If any of the frame parts are welded, then what is that tolerance?
What happens if you have a gap? Do you worry about water intrusion or not? How much of a gap is too much?

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies to make the best use of these Forums?

RE: How Big

Seems like you need to have some idea of the process capability for your likely manufacturing process. Don't run up to this unnecessarily, within reason leave the largest tolerance you can functionally accept. Now in situations where there isn't a real functional limit, I just use a figure well within likely process capability and taking into account things like allowing them to leave external profile stock etc.

You also need to understand if there are any functional constraints beyond simple fit.

When dimensioing take into account not just the impact of nominal size but also how variance in angle etc. can play into it - use of surface profile potentially simplifies this but not clear you're familiar enough with dimensioning & tolerancing yet to take advantage of that.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: How Big

I have prepared a haiku about fuzzy questions:

Preparing dinner
How much time in the oven
Salad is on fire

All in good fun.bigsmileDetails about your application would help greatly. Even then, we likely won't be able give you a number, or even refer you to a book that contains the number you're looking for. Fear not, a number isn't what you really need. You'll be cruising with new knowledge in a jiffy; lots of experience and smarts on this forum.

RE: How Big

AtomicSheep,

What do you want to accomplish?

The Machinery's Handbook has a section on ANSI limits and fits, which I use extensively. How accurately do you need to locate stuff? Are you trying to seal something?

--
JHG

RE: How Big

While I know you aren't designing a punch for a punch press, the clearance between those features depends on the size of them. It's the same with a keyseat and keyway except that the clearance is a lot more. The point is that there is no one number that will be correct without knowing what the function of the interfacing features is.

Nescius,

That haiku was awesome.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: How Big

When I need a number, I use "42" first.

Microns, thousandths, mm's, hectares......take your pick.

What's the cost of being wrong, anyway?

Proud Member of the Reality-Based Community..

To the Toolmaker, your nice little cartoon drawing of your glass looks cool, but your solid model sucks. Do you want me to fix it, or are you going to take all week to get it back to me so I can get some work done?

RE: How Big

While we're on the topic of clearances, I meant to ask how much I need for the part I'm working on. It's a part that goes over another part.

Thoughts?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: How Big

More than you've got room for.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: How Big

Quote:

What's the cost of being wrong, anyway?
Boy, I wish they had taught that one when I was in school!

RE: How Big

PH - combine them into a single part - problem solved.

Berlanger - there's a nice TED talk on being wrong. The presenter asks some people in the audience what it feels like to be wrong and they answer that it feels unpleasant. Then the presenter says that's what it feels like when you find out; that just being wrong can feel just like being right.

https://www.ted.com/talks/kathryn_schulz_on_being_...

This should have been taught in school.

RE: How Big

Dave,
That's why I love this forum so much. Thank you.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: How Big

Good point, Dave. Ask me again after next Tuesday and I hope to be able to tell you that I am right! surprise

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: How Big

Every day I do something wrong.
Fortunately, since I do thousands of things every day, the chances that I will get one of the really risky things wrong is pretty small.

STF

RE: How Big

(OP)
Hehe a good sense of humor on this forum :).

No, not trying to seal anything, not welding anything, not designing any punch presses and nothing to do with liquids. Just designing both static parts and dynamic parts and would be nice to have something other than a guess to estimate gaps.

Just general design work i.e looking for info that will serve me in a wide variety of design problems.

MadMango - Not sure what type of paint at this stage, most likely automotive. But as I mentioned, not talking about clearances just for paint coatings.

KENAT - sumed it up nicely.

drawoh - Machinery's Handbook - had a quick look at it, have realised that a quick look is not enough :), will look into it some more thanks for the suggestion.

RE: How Big

Quote (AtomicSheep)

Just designing both static parts and dynamic parts and would be nice to have something other than a guess to estimate gaps.
You only give enough to guess with.

You literally just defined every part in the world. They're all either "static or dynamic" so yea... your answers will usually only be as good as the question.

RE: How Big

When I was fabricating and machining I had three different size hammers, and a nice array of files, grinders, and a torch, plus presses to push things. Now that I'm modelling items at nominal size I don't need any of those tools - suddenly everything fits. I thinking of either selling them off or giving them to my grandsons and guiding them towards a life of experience.upsidedown

Diego

RE: How Big

(OP)

Quote (JNieman)

You only give enough to guess with.

You literally just defined every part in the world. They're all either "static or dynamic" so yea... your answers will usually only be as good as the question.

Fair comment, I guess I was asking for pointers on where to read up on tolerances in general. Got a couple of leads so got the answer I was looking for.

RE: How Big

"I guess I was asking for pointers on where to read up on tolerances in general."
Now there is a question folks can seriously respond to.

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