I always see people laughing at mechanical drawings that have really precise dimensions.
I wanted to ask how detailed should my drawings be when I asked for quotes.
I don't want to get expensive quotes back because I carry the drawing to the 42nd decimal place. However I need to make sure that my parts will work.
I have a cylinder that is 12mm OD x 6.5mm ID x 2mm long.
I have a push rod that is 6mm that rides inside the spacer. It cannot be any bigger than 6mm.
I'll be using 1mm diameter o-rings. So I'm guessing that I need to have the o-ring groove to be ~.50mm. That will give me .25mm of compression.
Is .25mm enough clearance to provide a sliding fit on the push rod?
What should I ask for on my tolerances? Is asking for a 6.25mm hole to much?
All of these measurements are approximate. I'm only looking to keep about 300psi from blowing past the o-ring.
However I'd prefer the parts to be as precise as possible. But I don't need the parts to break the bank.
**Corrected the ID on the cylinder. Changed from 6.25 to 6.5MM**
Last edited by Deviant; 03-25-2007 at 10:13 PM.
It's not of a cylinder, it's more of a seal cap/spacer.
Cylinder just seemed to be the proper term. Maybe a super duper o-ring washer would be a better description. *grins*
It will more than likely be incorporated into another part.
At this point it's just an example. Trying to see how I should ask for the part on my drawings. I.e. 6.00mm +0.00/-0.15
Okay, that makes sense.
I think in inches and because I was up at 4:30 am this morning to catch a plane it is way too late for me to convert to centimeters except for this conversion; 0.15mm is around 0.006" so that is not a tight tolerance. 0.0006" a tightish tolerance.
You need to do a bit of searching for information on O-ring fits. The clearances for O-rings, the amount of compression of an O-ring and the clearance between a piston rod and the hole it passes through or between a piston and a cylinder are more or less independent of the size of the cross-section of the O-ring or the O.D. or I.D. of the O-ring. Notice that I say "more or less" I am being a bit sloppy for the sake of simplicity.
The clearance for a typical O-ring seal whether it is between a piston rod and its hole or a piston and its bore needs to be kept fairly small otherwise the O-ring will simply compress and extrude out through the gap.
You mention 300 psi and 'blowing' which suggests you are dealing with a gas. So I will make the side comment that 300 psi is high for gases...I do hope you are being careful.
But to get back to O-ring clearances; for a 1mm, i.e. 0.04" cross section O-ring you would need to keep your radial clearance below about 0.004". But the O-ring clearance is not the only thing to consider. When you are specifying tolerances for sliding fits you need to keep in mind that the hole always needs to be bigger than the shaft. To keep things simple assume that you decide that 0.001" is the minimum clearance acceptable between shaft and hole; this means that the variance between the minimum clearance and the maximum clearance is 0.003". But this variance is divided between two parts so each part can vary by 0.0015".
So you need to specify your sizes so that the clearance never gets below 0.001" or above 0.004" with the 0.0015" applied to each part. So you specify for the shaft a diameter of 0.236" +0.0000", -0.0015" and for the hole a diameter of 0.237" +0.0015", -0.0000". If my arithmetic is correct and the lack of sleep and too much wine has not created too much confusion this should give you a situation where the largest acceptable shaft in the smallest acceptable hole giving a clearance of 0.001" and the smallest acceptable shaft in the largest acceptable hole giving a clearance of 0.004".
And I hope I have pointed you in a direction so you can more or less figure things out from these types of principles.
These websites give a rough idea on the gland design and clearance for pressure.
As for being safe. No worries there. Except the seal blows and jams the assembly. That will make for quite an adventure to remove.
O-Ring Gland Design.
O-ring Clearance Guide.
Thanks for the follow up. That gives me a little more information to file away.
I think if you read those references you will see there is not much chance of your O-ring blowing at 300 psi; 3000 psi might be a bit dicey. Normal 90Durometer O-rings used at recommended clearances to handle up to about 1500 psi (I think); when you want to go above that it is necessary to use back-up rings and then you are good to at least 3500 psi (again I think).
Mind you this is all hydraulic pressure; the idea of using O-rings on 3500 psi gas is a bit scary.
Do you think it would be more cost effective to quote a part with undersized holes then hand ream them myself?
I'm fairly handy with tools, do you think I'd be able to attain the results I need? What type of accuracy is normally seen from hand reamers?
If I was going to hand ream them. What sized would I be looking for.
I'm reaming a hole to 6mm. What size would I want to start with?
Assuming that reaming is the way to get the best accuracy. Would you agree that I need to keep the design where the holes are even diminsions?
6, 8, 10mm?
Then I'd need to have the shafts undersized? Which would be the exterior of the parts and thereby easier to machine? Also, I'm trying to keep all the parts with through holes so there are no blind holes that require reaming.
Thanks for all the help.
I want to make sure the parts are as easy on the machinist as on my pocket book.
The Parker handbook
Has a wealth of good information on it. I was an engineer for 18 years, and my advice is that if you're not working closely with the shop who is creating the parts, you can't be too clear.
I would suggest applying whatever reasonable tolerances that the guide books suggest, but include geometric tolerancing on the drawing as well.
I've seen flanged cylinders with bearing bores in each end return from the machine shop with the bearing bores .060 out of concentric, and the flange bolt holes on one end 5 degrees misaligned from the other because there was nothing explicit on my drawing stating how much misalignment was tolerable.
What seems common sense to you might not to the person running the lathe.
As far as the teeny-tiny parts you are making, tight tolerances will be required. An o-ring will only squish as a percentage of its size.
When it comes to O'ring's, get the design manual from Parker or whomever is supplying the O'rings for the project and follow their recommendations TO THE LETTER for tolerances. Period.
When/if the thing doesn't work, someone will be blamed. If you use the wrong bore size and/or finish tolerances as well as the wrong gland tolerances, you rest assured that leakage WILL be a problem. At that point, the designer is at fault.
Keep in mind that there are also different tolerances depending on whether it is a static seal or a dynamic seal. Sometimes, even the material necessitatates different fits and/or tolerances.
WHen it comes to high pressure sealing, do NOT think that SWAG's will work. Should your machine shop say " bull, you don't need that level of accuracy" find one that will meet the specs called out by/for the seal by the seal vendor.
Rest assured that if it doesn't work, the guy who machined it to some lame "close enough spec", will NOT be willing to pay what it will cost to fix or redo the project entirely. IF he doesn't meet your drawing spec, you have legs to stand on - if the specs suck, you're screwed.
Re: hole sizing - what's the hole being used for??? If it is for a O'ring seal, FIND THE SEAL FIRST, then size the hole accordingly. You can NOT always find a seal for the hole size your create. As far as hole size for anything else, it all depends what sort of accuracy you're needing in the fastened joint.
Example: to hand a muffler, you can use a 8mm bolt in a 10mm hole and cover the difference with a washer. IF you're trying to make a precision joint in a con rod cap, the cap needs to precisely fit over the bolt with minimal slop and, in some case, with a snug, almost press fit.
Precise bolt fits can be achieved with a drill and ream fit/finish - a knurled shank bolt will accept more slop than a ground shank bolt. Not knowing EXACLY what you're trying to do limits the explicitness of a bolted joint design recommendation that can be made.