How to accuratly plot bolt circle radius?

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
Reply to
JR North
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Caliper the distance between inside to inside edges, and outside to outside edges, and average the two.
Reply to
Richard J Kinch
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.
Harold
Reply to
Harold and Susan Vordos
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.
Reply to
Don Foreman
The open center is asymmetrical. JR Dweller in the cellar
D> >
Reply to
JR North
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
Karl
Reply to
Karl Townsend
Firstly, put the screws in and then tighten up a lock nut to remove the wobble. Then do your thing with the micrometer/calipers.
If alignment is that critical I'd make everything a bit loose and then adjust with a signal for max/min whatever. D
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Reply to
Dave Garnett
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.
Reply to
Pete Snell
That does complicate matters!
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.
Reply to
Don Foreman
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.
Harold
Reply to
Harold and Susan Vordos
snip--
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.
Harold
Reply to
Harold and Susan Vordos
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.
Pete
Reply to
Pete Snell
You *are* a stubborn old cuss, H. ;~)
m
Reply to
michael
(snip)
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...
Reply to
Lew Hartswick
Thanks, Don, my cad is up to the task. Already spent a considerable amount of time modeling in the various components for mods in cad. I will go with the drill blank scheme; should provide the necessary accuracy. JR Dweller in the cellar
D> >
Reply to
JR North
They may start with nominal dimensions somewhere, but many or most dimensions become whatever they must for things to fit and function.
Reply to
Don Foreman
Chuckle!
I'm old. I've earned a right to be. Besides, I can do it without all the flash.
H
Reply to
Harold and Susan Vordos

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