About two years ago, ARC used to sell a product described as 'Precision Bench Block', as shown in the picture below:
It is an imperial size 3" in diameter, with nine oversized holes ranging from 1/8" to 5/8". The block is hardened and ground steel, ground vee groove to accommodate round stock.
If you have a look at the top view of the drawing below, two of the holes are located in the vee groove, but they are not center to the vee. One of the holes is 6.6mm in diameter, and the other is 3.7mm in diameter (yes I know I am stating metric measurements here.. because that is how they are made). Anyone know the exact reason why their positioning is the way it is?
There is no fault in the original drawing/specification for this product. This is exactly how it has been specified by an American importer, many moons ago.
Some people refer to this block as a jewellers block, others refer to it as a block for gunsmithing. For engineering, I would suggest that it is a great support when drifting out pins and drilling small parts, but I am unable to figure out why the positioning of the holes are the way they are in the vee groove.
The offset 6.6 mm (1/4" clearance) might be derived from gunsmithing practice. Specifically removal of the link pin from 911 style barrels, as fitted to the Colt M1911 handgun and clones, which I believe is slightly off centre when the barrel is laid in the Vee groove.
How many did you get returned as 'crap Chinese shoddy drilling' ?
If not something for the gun nuts I was gong to suggest an original drawing made in imperial fractions that got converted by some kid on a holiday placement 'cos it wasn't important enough to waste a proper draughtsman on it.
The original patent (US1442415A) was taken out Arthur Hermone Starrett in 1921. The patent drawing doesn't show the holes being offset, and no claim about them is made in the wording. It's all rather vague:
These holes may be spaced about the top of the block in any manner, but are preferably arranged in the symmetrical manner disclosed in Fig. 1. Intersecting certain of the holes is a diametrical centering groove 5, preferably V-shaped as indicated in Fig. 3.
and
The intersection of the groove 5 with the three sizes of holes makes it possible to set or remove a pin from a finished cylindrical object without danger of having its slip or dent, while the pre dominant flat top of the block will support any flat object.
My 50th Anniversary Starrett Catalogue of 1930 says: 'The V in the center is a feature needing no explanation.'
I usually suggest patent evasion for odd features like this – someone claiming their Block isn't a copy of the Starrett because of the differences. In this case, if I read it right, the Patent expired in 1940.
No mention gun-smithing or jewellery either, it's a general purpose engineering tool for driving pins in and out of round and flat work.
Mine doesn't get a lot of use, but it's handy when it does. Made by Soba, the holes are centred in the 'V'…
After reading Clive Fosters explanation and seeing the drawing on SpeedyBuilders link, it is making more sense as to why the maker we were buying from makes them the way he does…. especially considering that he predominantly makes for the American market.
Bazyl: I had 6 pieces returned from a batch of 300pcs. based on the comment you made. However, it would be wrong to presume that the product was made incorrectly. I raised the question via this thread to get some explanation, which I now have. The maker we purchased from has been making this block in this way for a specific U.S. dealer for a very long time. The only reason we stopped buying was because there was no certainty on delivery. Our demand is low, and we have to wait to fit in with a high volume purchase… which only the U.S. could give.
Plasma: Thank you for the photograph of the block you have from STARRETT. Looking at the photo, it gives an impression that the vee groove on the left is narrow (with larger hole), and wide on the right (with smaller hole)?…similar to SpeedyBuilders link?.
S.O.D.: Thank you for the original patent details. After reading the various comments, I would suggest that this product has been modified/ or developed futher perhaps? to meet the demands of the U.S. importers market.
Clive is right on the money. I have seen this type of block (with offset holes in the V) used to remove the link pin on Colt style 911 pistols. This I hasten to add was a gunsmith friend of mine, who operates in the USA.
I believe that Brownells sell the offset hole block in the US for this very purpose.
Plasma: Thank you for the photograph of the block you have from STARRETT. Looking at the photo, it gives an impression that the vee groove on the left is narrow (with larger hole), and wide on the right (with smaller hole)?…similar to SpeedyBuilders link?.
Interesting to see that the Gunsmiths version is plastic
MichaelG.
I suppose plastic is more than good enough for knocking out a few little pins, I thought they were 3D printed until I saw the ejector pin marks.
One thing puzzles me, how can anything with clearance holes be classed as 'precision'? And precisely what bit of the 'precision bench block' is the 'precise' bit? that'll be two things that puzzle me!
One thing puzzles me, how can anything with clearance holes be classed as 'precision'? And precisely what bit of the 'precision bench block' is the 'precise' bit? that'll be two things that puzzle me!
Ian
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Jason is correct [many a true word is spoken in jest]
Precision is really about repeatability … So if the clearance holes and the vees are well made, and correctly positioned, and the material is suitably robust; the Starrett item [for example] can be regarded as ’precision’ … because it can be relied-upon to serve the same purpose every time it is used.
Arc’s version … likewise, if the holes in the vee were deliberately placed off-centre.
But if their positioning is not understood by the user … who then rotates the block, unwittingly, between jobs … precision is lost.
Gunsmiths use the plastic ones so they do not mar the finished surfaces on the gun parts. You can buy them (in America of course) with all kinds of specialised niches and slots and holes for specific tasks on certain common guns.
Precision is a subjective/relative term. My mate the builder regards anything within a quarter of an inch as precision. Within a couple of millimetres is spot-on in his book. Gunsmithing can be a bit like clock making in that a precision result is reached sometimes by careful hand fitting rather than high-precision measurement and machining. So, no the holes in the pictured block may not be sized or located to the nearest tenth of a micron. No focused beam microscopes required.
Everything is relative, Hoppers mate's 1/4" may be over 50ft which is 5thou per foot which is not bad if all you have got is a 3ft spirit level.
Still not sure how conversion from imperial to metric would affect the angular positions of the holes as Bazyle mentions, drawing suggests that is how they have been dimensioned on the drawing.