What tolerances can I expect from a local machine shop?

[metalman8357]

Hi all,

I'm having simple fixtures made for some material testing. They are essentially plates with through holes to insert precision dowel pins. The center to center spacing for these holes must be accurate, and my design allows for no more than a +/-0.001" tolerance. My question is, is it safe to assume that a normal machine shop can hit these this tolerance no problem? Also, what type of tolerance can I typically expect for the overall width of the plate? (both faces will be end milled). From what I remember working at my uncle's machine shop as a kid, it wasn't uncommon for us to hit +/-0.0005. I just want to make sure that I'm not making some absurd request.

Thanks,
M

 

[hydtools]

Depends.
Did you ask the machine shop that is making your fixtures if they could deliver your tolerances before you gave them the job?

Ted

 

[metalman8357]

No I havent given the job to anyone yet. I'm just wondering what is typical for a machine shop. Everything will be done on a CNC.

 

[KENAT]

Within reason almost any tolerance is achievable - it's just the cost to do it that gets problematic.

There have been threads about 'typical tolerances' before, maybe you can find them with a search.

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[FACS]

Make your prints, and put on the tolerances that you require.
Then send them out for quotes. The machine shop will make your decision, and answer your question.
If you don't really require tight tolerance, then don't put them on the print; ask for what you need.

If you really need the tolerances that you are asking about; then it really does not matter what to expect.

Charlie

http://www.facsco.com

 

[mfgenggear]

Metalman

all of the advice given is very good & are very good suggestions.

These are my thoughts. for your FYI only.

The fixture tolerance should be approximately 1/3 max of the tolerance of the production parts. rough rule of thumb.
for very tight tolerance. .001" x .333 = .00033 max, it's more complicated than this but this works.
+/-.001 is a very reasonable tolerance, but may be not adequate enough.

if these test are one time only, then the tools can be made pre hardened tool steel. available from Mcmaster carr. p-20 tool steel, apply your dim's &
get your quotes & ask for the cost saving suggestions from the manufacture. I would use a shop that has grinding abilities. but not necessary for your tolerancing.

if this a test that is to be rerun for production, have instructions on your print tool surfaces to be heat treated to 60 min HRc then ground.
depending the steel used. (if it's heat treatable)
If a more accurate requirement are needed. the holes & plate surfaces will require to have stock left for finish grinding.
this will prevent the tool from wearing out prematurely & have a very accurate tolerance.

As I seen the other senior members point out on other post, you only have to apply your requirements on the print.
and the manufacture has figure it out. but I like to advise what is involved.

send out three quotes & see who is of more help, as well as cost sensitive.

good luck

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[John2025]

I'd suggest that if(when) you send out an realistically toleranced dwg, you also talk to them to make sure they understand that the tolerances are "for real" and you're going to reject them if they are out of spec. It's not unusual for machine shops to "interpret" what they think you really need. Most have seen so many absurd requirements over the years that they get cynical.

 

[mfgenggear]

yes I agree with john, for production tooling however is normally held at a very close tolerance.
the cost is base on the Time & Material,"tolerance, Labor, cost of machining, cost of material, heat treating & process required",

HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[dvd]

Specify the machining and inspection temperature if you are serious about achieving the tolerance. Material (usually) expands with temperature increase and if there is a difference in the temperature of the plate in the shop, the inspection department, and your end-usage site there will be problems.

 

[ajack1]

That is an interesting idea dvd, does anyone actually do it, I have certainly never seen it done?

I have always thought all dimensions should be at 20 degree C. If you start specifying a different temperature that would add a huge cost to the part as all machines and inspection equipment I am aware of are calibrated at 20 degrees.

Personally I would go down the route of changing the part to be functional at whatever temperature is required but inspected and machined at 20 degrees not insist that all production machines and inspection equipment are calibrated at a one off temperature.

 

[mfgenggear]

It may not be machined at 20 deg C but it better be inspected at 20 deg C . depending at where in the world it's being machined, usually unless it's ultra close tolerance normal room temp is OK for machining.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[MikeHalloran]

I heard somewhere that NASCAR specifies the cubic inch displacement of an engine is to be measured at a particular ambient temperature, within a maximum of three hours of shutting the engine off. ... and that engine builders routinely size their product to shrink to the proper size at 2:45 or so.


Mike Halloran
Pembroke Pines, FL, USA

 

[dvd]

I wouldn't call it ultra-precision machining. If a mild steel plate arrives off the truck at 90 F and goes to the machining center and has holes bored while at 80 F, say 24 inches apart, then the plate gets inspected at 68 F, there is an (80 - 68) = 12 F differential. Using 6.5 E-06 in/in-F for expansion coefficient yields: 24 inch x 6.5 E-06 in/in-F x 12 F = 0.0018 in.

My point was not to use an oddball temperature for machining, but to machine and inspect at the same temperature. Depending on the distances involved, a few degrees of temperature difference between machining and inspection can result in out-of-tolerance results.

 

[mfgenggear]

dvd

agreed, just that who ever is machining must be smart enough to make the correct adjustments.
I know many areas in the world requires air conditioning & or heat depending if it's winter or summer.
I live in southern California which has Mediterranean weather, which is 70 deg F most of the year.
I forget other parts of the world are not the same.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[KENAT]

dvd, if you reference an appropriate drawing standard that will define at what temperature dimensions apply.

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[dvd]

The point to me is that the "local machine shop" can mean any number of things. I have seen "local machine shops" that convert millimeter dimensions to inch by multiplying by .040 in/mm: works okay for short dimensions, but fails miserably on longer ones. If one is going to the guys down the road, it is probably a good idea to establish their level of sophistication. Thus the topic of the post here, I suppose.

 

[KENAT]

Couple of the threads on 'typical tolerances' and the like.

thread281-280471
thread1103-287585

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[mfgenggear]

DVD

I am in agreement with you
It depends the the level quality required & what is required by your customers which now is a more complicated answer.
certification to ISO & so on.

as a supplier it should be audited to make sure the machine shop is qualified to make the parts. yes
With the correct quality procedures Inspection & manufacturing equipment, that traces back to the bureau of standards ect ect. or depending where it is situated in the world.

Kenat

It it not part of being certified to AS9100 or ISO 9100?
and as for U.O.S. I prefer a tolerancing block for untoleranced dimensions.
but I believe it is better to use ASME/ANSI and directly dimension all attributes.







Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[BillPSU]

No one has commented on it depends on the type of work you are doing. I once worked at a plant called McNally in Pittburg, Kansas. They machined a lot of cast iron and steel but it was rather large. They made tire molds with an outer steam jacket and an inner tread ring which were press fit together. It happens the press fit diameter was about 130" in diameter. There was a vertical boring lathe in the shop with a 16' diameter table and could swing 21' diameter part. Maximum RPM on the machine was 4. Holding +/-.001 on these large parts/machines is not possible and impractical.

What tolerance can and should be held 'It depends'

 

[KENAT]

mfgenggear my point was ASME Y14.5M-1994 1.4k "Unless otherwise specified all dimensions are applicable at 20°C.

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