In reverse engineering my radar scanner for mods, I am in the process of machining an adapter, and I have run into a conundrum. I need to very accurately plot a bolt circle radius consisting of 4 evenly spaced 3mm screws, about 1/2" radius. The screws rock slightly in the threads, so I can't just mike the screw shafts and extrapolate.There is an open center, so I can't easily locate x0y0. The hole centers must be plotted very accurately for alignment of the vaveguide. Help? JR Dweller in the cellar
If you have drill blanks, it's easy to install them (snug fit and vertical) and then mic across them. Subtract half the diameter of the blanks for your true bolt circle.
If you can't easily locate x0y0 in an open center , you're in a world of hurt. Consult Lautard's "Machinist's Bedside Reader" for how to do this quickly and easily. I don't recall which volume, others may help here.
This is my method. Sence you've got two pairs, do all four measurements. Also consider that designers nearly always use a sort of even number. In this case, I'd look at rounding your result to the nearest 0.1 mm or 0.25mm
Don't you mean subtract the diameter of the blanks Harold? If you have a digital caliper or mike this is easy. Measure the diameter of the pin/screw/screwhead, zero the caliper or mike, insert pin/screw into opposite holes, and measure accross the pins/screws/screwheads. The measurement shown on the caliper is the bolt hole circle.
I would select a drillbit whose smooth shank fits snugly into the threaded holes. I guess I'd need two drills that size. I would then measure the distances between the various holes by measuring between drillbits and adding one drillbit diameter. I would then use a caliper to measure the hole diameter and the distances from the hole centers to the nearest edge of the hole.
I would then take this data to the computer and construct the situation geometrically in AutoCAD using arcs and such. The result would be as precise as the measurements.
I would then let AutoCAD ordinate-dimension the hole locations relative to the center of the hole, which I'm assuming is circular.
Then to the mill. If I already had a hole in a mating flange, I'd find the center of the hole using Lautard's seeking technique, which basically is to use a centerfinder to find opposite edges and split the difference. Doing this alternately in both axes a couple of times will find the center to within about as close as you trust a centerfinder, certainly within a couple of thou. I'd then zero the DRO at the center of the hole, crank in the ordinate dimensions for the various holes, and drill them.
This is how I made the mating flange for the blower:
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If you would like to email me a set of such measurements, I would be glad to set it up in AutoCAD and email you a dimensioned drawing.
No, half the diameter of each blank should be subtracted in order to determine the bolt circle. My rational is that each hole may vary enough that he could end up with two different diameters of drill blanks. That's not at all uncommon, even though each hole should be identical. Half the diameter of the two blanks would be true center, yes?
If you have
But would include the amount of error that was half the difference between two different diameters. That's why I suggested subtracting half the diameter of the two drill blanks.
I don't own anything electronic in the way of measuring tools (not even a DRO, which I refuse to use), so I was not familiar with the feature you spoke of. It's certainly a nice convenience, assuming you're working with the same diameter blanks or other devices.
That's a poor way to determine holt circles due to the very common condition of bolt heads not being concentric with the pitch diameter of the bolts or screws in question. It's far more reliable to work from the minor diameter of the tapped hole.
Which is not the meaning I took from your original statement, which I now see you meant to be 'half the diameter of each blank, added together'. This would work as well.
Ok, I should qualify. IF the bolt holes are all the same size, and IF they are on the same circle, and IF the pins or screws you are measuring across are the same diameter, then the the bolt hole circle diameter will be the distance measured across the outside of opposite holes, minus the diameter of the pin or bolt.
Dont you believe that!! I did the location bit for the mounting holes on a DeWalt router and there isn't any measuring system in the world that would make the dimensions come out nice and neet. Did it in polar and rectangular coordinates, in metric and inches and it still is a BUGGER. :-) ...lew...
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