# Standards of accuracy



## KC2IXE (Mar 24, 2009)

OK, anyone who has spent any significant time around a machine shop, we all know that working to TOO tight a tolerance just slows you down, but working to too LOOSE a tolerance, and all you have is scrap (we are not talking class of fit here, but tolerance)

Under "Normal" conditions in your shop, assuming tolerance is NOT called out on the drawing (say prototype work) what kind of tolerances do you try to hold?

In addition, lets say you're tramming in a mill vise, How close is "good enough". 

What got me to thinking about this was I was tramming in my vise today, and I don't OWN a .0001 indicator. I do have 3-4 .001 "Last word" indicators that are graduated in .001, but it's reasonable to split that a LOT finer, particularly for change. I usually clamp a 6" parallel in the vise, and tram till I basically can't see any change - let's call it "less than 1/2 pointer width", figuring that the mill probably has more play than that.

Thoughts?


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## gadget_lover (Mar 24, 2009)

I think that somewhere in Machinerys handbook there is a spot where they say what the assumed tolerance is when not called out.

I forget what it was. 

I'm not 100% sure it was in there.

I'd like to hear the answers too.

Daniel


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## 65535 (Mar 25, 2009)

I believe it's somewhere around .005 for the tolerance.


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## gadget_lover (Mar 25, 2009)

65535 said:


> I believe it's somewhere around .005 for the tolerance.



Yep, that is what I was thinking too.

Daniel


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## LEDobsession (Mar 25, 2009)

I've heard of this "assumed" tolerance. I cant remember off the top of my head what it is so Ill have to check it out.


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## KC2IXE (Mar 25, 2009)

OK, I know what the hadbook says (and it totally depends on how the part is demensioned).

What I'm talking about was more like - how accurately do you tram your vise? (It'll make a difference in your tolerances) - How accurate to you you worry about fixtures and jigs?

Plus, it's something to talk about. Like I said, it's easy when the spec calls for it, but when your just knocking something out for yourself....


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## Rothrandir (Mar 25, 2009)

My general rule is to aim for dead nuts, and stop when you get frustrated that you're not there yet.


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## will (Mar 25, 2009)

I think that tolerances tend to be in the +.005 or -.005. range most of the time. 

You really have to look at the project itself. If you have 2 parts that should fit inside one another the the smaller is going to be -.005, the larger +.005.


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## wquiles (Mar 25, 2009)

Rothrandir said:


> My general rule is to aim for dead nuts, and stop when you get frustrated that you're not there yet.





That was good. I can definitely relate to that 

Will


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## 65535 (Mar 25, 2009)

For things like tramming a vice I would aim for .0001 or less. Chances are the piece to be clamped will be off by more than .005 so you are just compounding the problem of wrong dimensions.


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## StrikerDown (Mar 25, 2009)

Rothrandir said:


> My general rule is to aim for dead nuts, and stop when you get frustrated that you're not there yet.



You are the first person I have heard of whose aim is to have dead nuts, Perhaps an anvil and bigger hammer would help reduce the frustration!


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## Torque1st (Mar 25, 2009)

When you are working with metal parts just heat or cool them until they measure right!


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## mdocod (Mar 25, 2009)

In my very limited experience thus far with machining, there are almost always unforeseen obstacles that throw out the accuracy more than anticipated. I've learned through experience already a number of ways to correct for those issues, and am in the process of changing procedures and tooling to hold tighter tolerances in production while at the same time improving productivity. I don't have any need to hold a thousands in any of my designs, but what amazes me is how much inaccuracy can crop up in strange places. My suggestion is to get your setup/layout of work-holding as accurate as reasonably possible with whatever gear you have available so that it's easier to identify where other sources of inaccuracy are coming from.


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## precisionworks (Mar 25, 2009)

> how accurately do you tram your vise?


As long as the TIR (total indicated runout) is .001" or less, your vise should be fine for most jobs.



> I don't OWN a .0001 indicator


They aren't really useful for general shop work. You see them primarily in grinding shops, or in the inspection lab. For 99.9% of mill & lathe work, a .0005" indicator is the standard. The best one I've ever used is the Interapid 312B-1, which has a large dial (1.5"), very large range (.060" total), & .0005" graduations. 

Not the least expensive dial test on the market, but certainly one of the toughest. Built like a Swiss watch, perhaps because it's made in Switzerland

The built in stem extension (right side) makes the Interapid at least 10X faster to use than anything else - in a drill chuck, collet, or Indicol holder. Other makers have knocked off this design, but it's worth the money to get the real deal.


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## modamag (Mar 27, 2009)

I normally use one of these to hold my indicator and sweep across the flat portion of my vise to check if it's within 0.001" (R2") once my flycutting or facemill starts cutting in one direction.

Tramming within 0.001" is good enough for home hobbiest use.

As for design tolerance it's depends on what your need is.
Press fit? Snug fit? Loose fit?

For most flashlight apps it's no more than 0.005" standard, with class 2 threads.

Take a look here for some basic on tolerances.
http://www.engineersedge.com/tolerance_chart.htm

Also highly recommend you read ASME Y14.5.1M.


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## precisionworks (Mar 27, 2009)

> Take a look here for some basic on tolerances.
> http://www.engineersedge.com/tolerance_chart.htm


+1

Tons of great information there, I use it frequently.


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