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
--
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Doubt yourself, and the real world will eat you alive
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JR North writes:

The screws rock slightly in
the threads, so I can't just mike the screw shafts and extrapolate.

Caliper the distance between inside to inside edges, and outside to outside
edges, and average the two.

"JR North" wrote in message
...
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


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

On Tue, 29 Mar 2005 19:59:42 -0800, JR North
wrote:

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 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.

The open center is asymmetrical.
JR
Dweller in the cellar

Don Foreman wrote:
On Tue, 29 Mar 2005 19:59:42 -0800, JR North
wrote:


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 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.



--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
If you're not the lead dog, the view never changes
Doubt yourself, and the real world will eat you alive
The world doesn't revolve around you, it revolves around me
No skeletons in the closet; just decomposing corpses
--------------------------------------------------------------
Dependence is Vulnerability:
--------------------------------------------------------------
"Open the Pod Bay Doors please, Hal"
"I'm sorry, Dave, I'm afraid I can't do that.."

The screws rock slightly in
the threads, so I can't just mike the screw shafts and extrapolate.

Caliper the distance between inside to inside edges, and outside to
outside
edges, and average the two.

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

"JR North" wrote in message
...
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
--

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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Harold and Susan Vordos wrote:


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.


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.



--
__
Pete Snell
Royal Military College
Kingston Ontario


The reasonable man adapts himself to the world;
the unreasonable one persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man.
- George Bernard Shaw

On Wed, 30 Mar 2005 00:21:46 -0800, JR North
wrote:

The open center is asymmetrical.

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:
http://users.goldengate.net/~dforeman/cone_adaptor/

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.

"Pete Snell" wrote in message
...
Harold and Susan Vordos wrote:


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.


Don't you mean subtract the diameter of the blanks Harold?

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
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.

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

"Dave Garnett" wrote in message
...
snip--

Firstly, put the screws in and then tighten up a lock nut to remove the
wobble. Then do your thing with the micrometer/calipers.

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

Harold and Susan Vordos wrote:

"Pete Snell" wrote in message
...

Harold and Susan Vordos wrote:


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.


Don't you mean subtract the diameter of the blanks Harold?

snip
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?

snip
opposite holes, and measure accross the pins/screws/screwheads. The
measurement shown on the caliper is the bolt hole circle.


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.

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

--
__
Pete Snell
Royal Military College
Kingston Ontario


The reasonable man adapts himself to the world;
the unreasonable one persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man.
- George Bernard Shaw

Harold and Susan Vordos wrote:
"Pete Snell" wrote in message
...

Harold and Susan Vordos wrote:




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




You *are* a stubborn old cuss, H. ;~)

m

Karl Townsend wrote:


(snip)
Also consider that designers nearly always use a sort of even number.

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...

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

Don Foreman wrote:
On Wed, 30 Mar 2005 00:21:46 -0800, JR North
wrote:


The open center is asymmetrical.


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:
http://users.goldengate.net/~dforeman/cone_adaptor/

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.


--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
If you're not the lead dog, the view never changes
Doubt yourself, and the real world will eat you alive
The world doesn't revolve around you, it revolves around me
No skeletons in the closet; just decomposing corpses
--------------------------------------------------------------
Dependence is Vulnerability:
--------------------------------------------------------------
"Open the Pod Bay Doors please, Hal"
"I'm sorry, Dave, I'm afraid I can't do that.."

On Wed, 30 Mar 2005 11:00:40 GMT, "Karl Townsend"
remove .NOT to reply wrote:



The screws rock slightly in
the threads, so I can't just mike the screw shafts and extrapolate.

Caliper the distance between inside to inside edges, and outside to
outside
edges, and average the two.

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

They may start with nominal dimensions somewhere, but many or most
dimensions become whatever they must for things to fit and function.

"michael" wrote in message
...
Harold and Susan Vordos wrote:
"Pete Snell" wrote in message
...

Harold and Susan Vordos wrote:




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




You *are* a stubborn old cuss, H. ;~)

m

Chuckle!

I'm old. I've earned a right to be. Besides, I can do it without all the
flash. g

H