Typical Grinding Tolerances

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

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

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

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

You might even be able to do it on the 540 if the diameter required is 4-6" or so :-)

Mark Rand RTFM

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

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

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

sorry, its not a secret, we need to make a (quite special) ball bar, I started a thread over on the homeshop bbs,

Dave

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

Nick

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

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

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

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

Nick

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

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

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