Machining accuracy

[spider80]

Hello,
I recently had a tight toleranced dwg where I had called out a +/- 0.00025 on distance between two holes and the same tolerance on the height of the block.
Now the machines I know (like typical bridgeport and other conventional mills) can do tolerances like +/- 0.0005 or lesser, however am not sure about the precision below half a thousand.
I have a company who claims they can do this, but am not sure if they are going to do it exactly.
I have this question
Are there machines that can do +/- quarter of a thousand ? (exactly without rounding off to .0003 or .0002?)
If so can you give me some names/websites where I can read more about them?
Thanks

 

[alexit]

Sorry I am metric, this is ±0.006 mm right? Most surface grinder can do this flat, and good machinist can surface grind to height in this tolerance if he has correct measurement equipment to know when to stop.

Hole position is more difficult, good accurate VMC (Bostomatic BD12) can position to 1/3 this but may tool doesn't start exactly right. Look also at sink EDM or jig bore (Moore tool still makes jig borer IIRC.)

How many parts you make? Dedicated multi drill head can do this but costs too much unless you are making autos.

 

[MikeHalloran]

These guys can make the part or sell you the tools:

http://www.mooretool.com/

Mike Halloran
Pembroke Pines, FL, USA

 

[EnglishMuffin]

Any good Jig Boring machine can do this. Just search for Jig Boring, Jig Borers etc.

 

[Tofflemire]

Hi,

Most CNC machines have a tolerance of +/- .003. Is this part you making going to be for huge production runs or a one of? I would try not to use that kind of tolerance on huge production runs. If this was a mold to make other parts but this is still hard to achieve little lone to measure with your average hand held calibers, maybe you should request a CofC (Certificate of Compliance) with this part.

Tofflemire

 

[NickE]

Jig grinding (is that what you guys mean by jig borer?) can easily achive those sorts of tolerances. our die details are commonly drawn around .0001" and need to be with in .0002" of nominal. (roughly)

 

[MikeHalloran]

A traditional jig borer resembles a Bridgeport, but is sturdier, and position of the spindle relative to the table is set with gage blocks and indicators rather than calibrated screws. The spindle rotates and translates a single point boring tool to make or enlarge a hole. { The jig borer operator appeared to be on the hourly payroll, but always drove a really nice car and golfed, with the Chairman. It may have been a law. )

A traditional jig grinder is similar, but the spindle orbits and translates a high speed grinding spindle to enlarge an existing hole. This was also commonly done with an attachment to the jig borer.

The jig borer has been made obsolete by super- precise CNC mills, which can run autonomously. I get the impression that jig grinders are still operated manually. I don't know if they still use gage blocks, or super- precise DROs.



Mike Halloran
Pembroke Pines, FL, USA

 

[NickE]

mike- our grind shop uses both gage blocks and really precise DRO's. Of course our shop still runs theirs manually, I have a feeling that newwer ones have gotten microprocessor control and with feedback loops are even more precise than teh old school ones.

(Although really precise/accurate old school equipment is awesome. We had an XRD machine (Scintag?) in college that had a tube powersupply and a chart recorder. The staffperson that ran the X-ray labs said it was better than the newer Siemens machine by far.) Teh goniometer gears were springloaded I think to reduce lash.

 

[ajack1]

The height is no problem; provided it is flat, not contoured, just look at the limits on slip gauges.

The distance between the holes is more problematic, you do not mention the distance, if it is say .500” okay if it is 20 feet you will have your work cut out.

You stand next to no chance in doing it on a Bridgeport or any other similar turret mill, get your self a nice “tenths” clock bore a hole undo the locks move away in two axis go back to zero do the locks up, is it within 0.0002”? No I thought not.

Even CNC or jig borers that have had a “hard life” will struggle with this. To have any degree of certainty I would say you need someone who has at the least a temperature controlled shop if not an environmentally controlled shop.

Ask to see the calibration records for the machines and there inspection equipment, if they don’t have them the chances are you will get some paper weights to put on your desk.

 

[GregLocock]

Devil's Advocate: if the coefficient of thermal expansion of the work and the machine are matched then temperature control is less important?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

The machines are not made of a single material.

... and ...

At micron tolerances, beyond the temperature controlled room, you have to do anal stuff like let the spindle run for a couple of hours so its temperature distribution, and dimensions, will stabilize.



Mike Halloran
Pembroke Pines, FL, USA

 

[icrman]

I've never had to do that with the new CNC stuff. But in the old days we used indicators and rods called standards. That was our form of digital read out.
And yeah has to be temp stablized. So why is it so important to have that particular tolerance? It must be a gage? Or for micro electronics?

 

[unclesyd]

What is the material?

What is the size of the part to be machined?

What size are the holes and precision required on same, assuming C/L dimensions?

 

[truck]

As I'm sure you already know, the more zeroes in the number the more expensive it is (exponentially so)!

The real question to ask is: Why do I think I need that tight tolerance? One which dimensions?

Truck