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Machining Tolerances

Benjgordon

Plastic
Joined
Dec 11, 2016
Location
somerset
Hi all,

I have been given a job with tolerances of +/- 0.007mm and I am wondering if it would be achievable to do on a lathe.
I have managed to achieve it before but needs to be machined over a batch quantity.
We also have people opening and closing a big shutter door letting all the warm air out, how much would this affect the tolerances?

Even worse job has +/- 0.006mm

any suggestions would be great.
 
You will have to provide more info before anyone can give you an answer. How long are the features you need to hold to that size? Brand of machine? Workholding set up? Material? Diameter? Print?
 
Hi all,

I have been given a job with tolerances of +/- 0.007mm and I am wondering if it would be achievable to do on a lathe.
I have managed to achieve it before but needs to be machined over a batch quantity.
We also have people opening and closing a big shutter door letting all the warm air out, how much would this affect the tolerances?

Even worse job has +/- 0.006mm

any suggestions would be great.

With a good machine, and if you can keep it 20C in your shop, wear gloves when handling measuring tools, let the part equalize to 20C after machining, sure.
 
A lot will depend on the size of the part. Temperature does not matter as much on 3 or 4 mm parts. If the part is 200 or 300mm you better have your ducks in a row.
So as the first response indicated more info is needed.
 
Hi all,

I have been given a job with tolerances of +/- 0.007mm and I am wondering if it would be achievable to do on a lathe.
I have managed to achieve it before but needs to be machined over a batch quantity.
We also have people opening and closing a big shutter door letting all the warm air out, how much would this affect the tolerances?

Even worse job has +/- 0.006mm

any suggestions would be great.
.
.
very rigid stiff part shape needed as well as short rigid stiff tooling. common to run slow as vibration at higher feeds and speeds can cause vibration problems. i would normally
1) leave 0.1 to 0.12mm
2) take cut to leave 0.025 to 0.075 mm (measure real accurate)
3) take final cut
.
many materials you need to take a minimum to max final cut. too low and tool does not bite in, only rubs. then increase depth and then cuts too much. each material acts different. i have had better results final cut not too much or too little
 
that's a tolerance of over half a thou... cakewalk, blindfolded!

a tenth or two on tricky superalloys, now we're talkin.

Tell us more about the part and machines and tooling and you'll get some proper answers back.
 
that's a tolerance of over half a thou... cakewalk, blindfolded!

a tenth or two on tricky superalloys, now we're talkin.

Tell us more about the part and machines and tooling and you'll get some proper answers back.

Check your math .006mm is a little over 2 tenths.
 
I've done long, thin parts (.750 x 8 inch) between centers in hard materials and held that tolerance, well .0002 overall not +/-. but everything matters and it's not easy. Even when you get it figured out you will junk every 10th part or so. That is operating beyond the edge of capability of my lathes.

It's all about shape and material of the part and the condition and capability of the lathe. Half thou isn't terrible but it get exponentially harder below that.
 
Not difficult at all, really....... Just remember that EVERYTHING matters.....10,000,000 variables play into that. You just have to design a process that eliminates or reduces the effect to nil of as many of those as possible, and then control the ones you can't.

Measuring is another issue..... how are you trying to measure this and is it an OD, ID or length? more info required.
 
Achievable, even on our old Mori Seiki lathe, which still has good repeatability. We once had a blueprint and there was a measure of dia 12mm +- 0.00mm. We didn't think it was a mistake (even if it was) but decided to make parts that were as close to the 12mm diameter as possible. After 5 or more break-in pieces, the dimension stayed fairly constant and within +-0.005mm range as long as the lathe kept running.
 
i have done holes with boring bars to +/-.0001" tolerances. my company calls it selective grinding but it is really done with boring bars. they measure bearing actual size as bearings often vary .0002" then we bore to actual size of bearing that is going to be used.
.
as hard as i try to make a .00005" tolerance i am often .0001" off target size. note picture of .0001" bore gage wrapped in rag so heat of my hand does not effect it.
.
note ring gage with known size compensated error amount (usually under .0001") kept in cnc so at same temperature and kept in same orientation as indicating bore gage.
.
holding tolerance only works with rigid parts and short rigid tools. if tool or part is vibrating no way can anybody do tight tolerances
 

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Why even bother starting a thread with a post that provides so little info?
The answer to your question is that with the info provided it's impossible to know.
 
you would need rigid part shapes. flimsy parts are often impossible to hold tight tolerances or require extra care machining
.
Swiss machining lathes do long skinny parts as they slide out of the chuck ? so often you need the proper machine too to make some parts
 
Thank you for all the replies, I am new to this website and my first forum I've joined.
Now I know what I need to post, I shall give a more accurate post next time.
Problem is with our workshop we have shutters opening all day so the temperature of the room fluctuates like crazy.
I've achieved it, only just!
 
Nice job young man! Now you know it CAN be done. Half the battle in this line of work is overcoming the mental obstacles. Once you've done it - the next time is way easier.
 
If its a batch even on a good holding manual lathe you would have to stop leave a consistent finishing cut mic it then apply the finishing cut,A batch if you ran them off to your dial marks would be surprised if you got a 100%pass rate.A grinding op would be my choice on a manual lathe
 
optimal tolerance

How to calculate the optimal tolerance. No CAM software development company can answer to this question until now.
I have the rethink and the algorithm that allows to identify the optimal tolerance and other mathematical models that can be programmed in CAM or post processor software in order to improve the high speed machining quality of freeform surface and increased the useful life of the CNC machine elements . Essid Mohamed . Teacher researcher . ISET SFAX and UGPMM. ENIS. Tunisia.
https://www.linkedin.com/in/essid-mohamed-567865177/
Mohamed ESSID | Ecole Nationale d'Ingenieurs de Sfax, Sfax | ENIS | Mechanical Engineering Department
 








 
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