Tony, are you calling accuracy to 1/1000 inch (0.02mm) on a 3 inch V-block "a silk purse"?
As far a the 45 degree chamfer goes it needs to be accurate to within 1°.
The problem with V-blocks that have that "bridge style" design is that the screw threads take up valuable lateral space. This means that the central V-becomes too small. For this reason I would need to go for a slightly larger V-block than 1 3/8inch (35mm) – maybe more like 2 inch (50mm)…
Btw, I was slightly tempted by this design:
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**LINK**
Even though its U shaped clamp is taking up a lot of space above the part, it allows you to 'bury' the sides of the U-shaped clamp a little using the stepped sides, plus I like the way that you can clamp stuff with parallel sidesfrom a 45 degree angle.
Apart from being silly money (GBP105) the spec looks promising.
"Stepped groove construction. z 90° V groove are central, parallel and square with all surface within .0003”. z Harden steel ground. z Two clamps only order No.: 420-5491. z Sold in pair"
To be fair, it's a fraction larger than I really need, as well as (if we believe them) it is more accurately ground than I need, but it's a clever design and at least it is claiming a genuine 90°. The 45° screw doesn't look like it can cope with huge forces but it would probably be strong enough for my purposes.
I can't really justify the cost, but I'm tempted… Any thoughts?
J
PS Although I do like the idea of being able to clamp stuff from 45°, I slightly worry that the screw will be weakening one side U of the clamp, particularly the way they have failed to build in a collar around the 45° screw so as to reinforce it in the same way as the central screw has been done.
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