Thread: Should CI Straight edges have Frosting?

Originally Posted by shedhappens

At 800mm longer than the plate I reckon you might have some strife getting it flat Simon.
Or do you have a devious plan?

cheers, shed

Hi there,

Was thinking the same thing. No I don't have a devious plan....yet. Actually, at nearly 1200mm diagonal to diagonal, it's 300mm short each end. It may still be worth a look at. You can spot in an item longer than the plate by successive spotting at each end but yes I agree that would be pushng the limit. Like I said, worth a look.

Cheers,

Simon

Originally Posted by simonl

Hi there,

Was thinking the same thing. No I don't have a devious plan....yet. Actually, at nearly 1200mm diagonal to diagonal, it's 300mm short each end. It may still be worth a look at. You can spot in an item longer than the plate by successive spotting at each end but yes I agree that would be pushng the limit. Like I said, worth a look.

Cheers,

Simon

I've got an 1800mm B&S camelback SE that needs checking. Unfortunately I only have a 1200 x 900 plate so max of 1500 across the diagonals.

Given I have no immediate need to use the damn thing, I haven't checked it and won't until it *becomes* necessary. Or I'll take a toad trip to Rockhampton and visit RC.....

PDW

Yea, that seems fair enough. I might be in a pickle with this one. Thing is, my Y ways on the knee are about 800mm long, similar length on the coloumn itself and the table is 1200mm long. How do people scrape these things in without such a long SE, not to mention the dovetails? I can't even look at the dovetails yet and even PDW dovetail straight edges come in at 630mm which is also too short.

Simon

On longer surfaces you can use an external reference to get flat. eg a precision level, autocollimator, tight wire, telescope, etc. I've never done a long dovetail before, but I don't think it will matter too much. You may finish up with a way with a curve in it, but if you don't have all the gear to measure a way that length, then you certainly won't have a way to measure how much you've screwed it up :P

Seriously though, this is where people like Phil (Machtool RIP) earns his keep. With experience, or a LOT of thinking before doing, you can often work around these sorts of issues by looking at the factory scraping. The ends of ways are normally not worn much, sometimes not at all, and the key is to preserve that reference. So if you scrape straight down both ends (firstly working out how you are going to determine whether you really are going straight in the down), you will preserve that original geometry. It can then be a function of connecting the dots. Without seeing the exact problem you'll face it's hard to say a precise solution, but that should give you one example. It's what I like most about this type of thing, there are a whole heap of surfaces that need to reference properly with respect to each other and it becomes a little like a Rubik's Cube

Originally Posted by simonl

Hi there,

Was thinking the same thing. No I don't have a devious plan....yet. Actually, at nearly 1200mm diagonal to diagonal, it's 300mm short each end. It may still be worth a look at. You can spot in an item longer than the plate by successive spotting at each end but yes I agree that would be pushng the limit. Like I said, worth a look.

Cheers,

Simon

l have no idea what l am doing here, l have only scraped gaskets and scraped up after the dog so feel free to straighten out the BS.
Simon and l were having a PM about sagging and bowing of the straight edge when not evenly supported, bit hard to explain some things with words so I made a pic of a potential method of handling the SE, we also need to consider plate deflection as this would be a fair weight if simons new sydney harbour bridge was just ponked on the plate. I think I have a spring for the job, simon could you weigh of the SE ?
The idea of the spring is that it should be easier to adjust and share the weight more accurately between the plate and the lifting point.
If anyone can workout the best placement of the lifting straps to reduce sag/bending it would be appreciated, yes l know, buckleys chance but we might blue it with the straps in different pozzies to see the difference.
What is the most appropriate weight to apply to the plate, anyone?
Both the lightweight welding bench and surface plate stand have wheels so they can be wheeled in and out from underneath as required.
All criticisms and suggestions welcome

cheers, shed

I can pretty confidently say you won't need to be concerned about surface plate deflection of a granite surface plate simply by hanging a straight edge off one edge! Maybe if you were designing stuff for CERN, but not for what we do. You can go 1/4 to 1/3 over the edge and still get reasonably reliable spotting from what I hear. My 3' SE just fits diagonally on my plate, but that's the only place it reliably fits. It won't spot as well elsewhere, as it's down in the tolerance of the plate itself. The numbers we're talking here are pretty weeny. Take my plate, it's accurate to overall flatness of 1.6 um. Even if you could only achieve ten times worse, that's still only 16 um over the entire length of the SE. That's not much, so I think it's important to keep these numbers in perspective. The heat from a decent fart on a SE that length will warp it more!

I use a bungy as a load balancer on my Biax and it helps to take some of the weight. I can't see any reason your design shouldn't work just fine. You're going to get sick of hoisting rather quickly however; there are a LOT of cycles in scraping a straight edge to any real precision.

Originally Posted by PDW

I've got an 1800mm B&S camelback SE that needs checking. Unfortunately I only have a 1200 x 900 plate so max of 1500 across the diagonals.

Given I have no immediate need to use the damn thing, I haven't checked it and won't until it *becomes* necessary. Or I'll take a toad trip to Rockhampton and visit RC.....

PDW

I have a 1000mm x 2000mm plate and the exact same 6ft brown and sharpe straight edge as you, not that I'm close but feel free to pop in any time if you need to borrow the plate.

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