Typical Grinding Tolerances

Evening All, Im looking into some high precision stuff for work, and Im wondering what sort of tolerances I can expect from a ground part.
Briefly I need a hemispherical cup mounted on a bar. I can make the seat on a mill using the 45 degree head and a boring bar method, and I can see that you could then finish grind it using a similar method on a grinder, but I dont know enough about grinding processes to know if wheel breakdown will effect the result. The process will need a wheel of a certain diameter, to generate the hemisphere correctly. Ideally it would be nice to hold the part to better than 0.001mm, but I suspect thats a tall order?
Any experience of the sort of tolerances that are 'routinely' (i.e.not super expensive) held in grinding would be useful. I know my 540 is marked in 0.0001" (0.0025mm), but I dont know if thats just a dial thing, not an achievable thing IYSWIM
Dave
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wrote:

I have a pal that makes hemispheres on a Bridgeport ...he doesn't grind them
being optical work, he works in microns
and his work is closer than 1 -3 microns on most
its not how close your going to get the your figures ...its how you are going to measure them.they have to be measured in all planes...he has the equipment .......and it ain't a dti.
all the best.markj
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Hi Mark, Is that Alwyn at Oak engineering? Ive seen the PM thread with some pics, and I think we'll be contacting him shortly. I work for a spin out from Loughborough Universitys Mechanical Engineering - the Optical Research group, so I hope they have access to the required measurement equipment. I dont think I'll be making these ultimately, but I might have to 'knock up' a prototype or 2, and provide some 'practical grounding' to the ideas. The seats will need to be finished in situ on the end of a 1 meter bar, as we need an overall length to be precise.
Dave
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wrote:

Forgot to mention that the seats will need to be hardened, hence asking about grinding, as the Heat Treat needs to be finished before attaching the seats to a CF tube.
Dave
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wrote:

Hi Dave..Yes it is Alwyn
Hmmm you're not giving much away with your answer ..
Is this a forming tool ...a die-casting tool....a mold ?
I've never seen him get the things hardened before ...
He does them in cast iron ...aluminium....bronze and brass
They don't have to be hardened ...because abrasive petals are glued onto them.
The measuring equipment (sphereometer) is available from a company in Ruthin Denbighshire..........the equipment is used in conjunction with lots of maths ...meaning the results it displays have to be put into an equation to get the radius etc
http://www.optical-tools.co.uk/sphereometer.php
One could measure with a dti ...but you would have to have the dti's pivot point on the exact centre of the sphere ...and like, how are we going to know where that is ..........when in practise we don't even know if we have created the sphere of dimensions we are after.
BTW...........after the spheres are finished ..........the optical companies go on to grind them ....sometimes.
He sends all his other work out for heat treatment/ hardening ..there's nothing in house .
If you want his company telephone number ..it is
01745 571447
Secretary is there.........8am - 12 noon...........so phone only gets answered then...if it gets answered after this time you're lucky..........as all employees there, are very busy...making lots of noise ..and not in the office..
all the best.markj
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sorry, its not a secret, we need to make a (quite special) ball bar, I started a thread over on the homeshop bbs, http://tinyurl.com/79g4xk I forget that just cos we all frequent the same internet places doesnt mean we read everything... Its a pair of ball seats in a carbon fibre tube. They will need to be glued (bonded) in, and need to be hardened. To hold the length tolerance required I think that they will need to be finished once they are in situ, as even the curing of the expoy could move the seat to much.
Dave
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dave sanderson wrote:

How about making these seats out of wear resistant **plastics** :-)) No kidding, but if you coat the seat with one of the products from diamant (they do make the Moglice too), put in the balls, adjust to length and let cure, you'll get what you want. They do have products that do not shrink. An other product they say that it shrinks shrinks 1 µm with a thickness of 10 mm. I'd contact them, they are very helpful!
<http://moglice.de/>
Nick
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dave sanderson wrote:

I'm envisaging something like the ball-and-magnetic-stick construction toys?
In which case, do you really need a hemisphere? In fact, would it work? You couldn't fit two rods on one ball, except maybe in a straight line.
A ring, possibly centered at the 45 degree tangent, would do. Final grinding to size could be done with a sacrificial sphere (good ball bearings are accurate to better than 0.001 mm) and diamond goop, with a rubber wheel turning the ball while the rod turns slowly.
However, it might be a better idea to use a three-pronged version, where the prongs from two rods could intertwine - that way you could get two rods on one ball at bigger angles.
-- Peter Fairbrother
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Yes, a bit like the magnetic sitck toys, but bigger and more expensive ;) The balls are 50mm dia, and the seats are as small as we can get away with, as that way there is less visual obstruction of the ball from multiple angles. Its an optical system. Making the seats small means that several can sit on one ball at once. ultimately we need to calibrate a measurement volume (bit like a CMM) of 40 meters long, and (from memory) 10m meters in x section. Thats not to hard, the hard part comes when you learn we need to calibrate that volume to under 40 microns (0.04mm!). There is (needless to say) some doubt as to whether that is even possible...
We are considering several seat forms, the hemisphere is nice as there is a large contact area, so the hertzian deformations are small. with a flat plane and 3 interposing balls forming a seat you get (IIRC) ~0.003mm deformation, which uses a huge amount of the tolerance before you start. a 3 pad hemisphere might be the best compromise, as you can have a large 'fitted' seat, but with only 3 points of contact.
Dave
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dave sanderson wrote:

Hahaha! You will put 10 sticks in a line and hope to get that accuracy? Don't forget to drill a hole into the stick, because changing pressure (whether) will change the length.
Let me see ... Rule of thump: 100°C difference on 100 mm make a expansion of 0,1 mm in steel. Glass filled resin (I assume carbon is about the same ballpark) has about triple that expansion rate. So 10m with 1°C change will give you 0,3 mm expansion. Or 0,1°C will give 0,03mm. Don't tell me that you know the temperature over the full length to 0,1°C.
Nick
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The cool thing about the correct CF layup is that it can be made to have a -ve coefficient of thermal expansion (CTE), and with proper matching to the metal bits the rod can be made to have overall zero CTE over a reasonable temperature range.
Dave
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wrote:

Looks like ... according to nick ..youre going to be talking to clock makers ...and building it like a pendulim with bi-metal.
all the best.markj
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dave sanderson wrote:

Ah! I saw that carbon has a negative CTE! Didn't know that, so sorry!
Did you calculate the mixture? Looks like it requires a lot of resin to compensate the carbon.
Nick
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On Mon, 19 Jan 2009 11:11:06 -0800 (PST), dave sanderson

Mount fixture _on_the_head_ of the grinder, not on the table. Fixture to have horizontal, motor driven, shaft exactly on centre line of grinder spindle. Thrust (taper roller) bearings with spring preload to control axial location. Axial micrometer adjustment to adjust depth of cut. Vertical swivel (taper roller or centres to control location) to generate sphere.
Dress wheel to diameter smaller than hemisphere diameter. Adjust axial location of fixture to put on cut. When re-dressing wheel (dresser can remain mounted on grinder table) in-feed part by the amount dressed off to restore cut.
Only significant issues are 1) The wheel will be cutting on its corners and will break down quickly (so use a radius dresser to dress slightly smaller than hemisphere radius). 2) Surface grinder spindles can have a good half a thou of end play. May need to use a universal head designed to control spindle end play. 3)Air gauging might be the only reasonable way to test the results.
You might even be able to do it on the 540 if the diameter required is 4-6" or so :-)
Mark Rand RTFM
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So a fixture a bit like the radius dresser unit that attaches to the wheelhead, but you put the thing to be ground where the dresser would go, on a spinny shaft. I can see how that works :) Unfortunatly the radius is 25mm, so I guess a T&C grinder with a small wheel is in order.
Dave
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