Calculating Bolt Hole Radius....?

[YankeeMetallic]

I frequently get chucks and backing plates from a customer in the Glass Blowing business. It seems Glass Blowers often do whatever it takes to get parts put together just to keep the machines running. There are so many bizarre compilations of components, chucks, and backing plates in that business. The Glass Blowing industry prefers to use metric increments in it's parts and accessories but that is not always the case. Each Blower also prefers their own combination of bizarre chucks, Drill Chucks, on face plates, back plates etc. for the repetative parts they specialize in.

My dilemma: HOW TO DETERMINE THE BOLT HOLE RADIUS FROM EXISTING BOLT HOLES?
I make parts to refurbish Glass Blowing Lathes (Typically Litton models). When I have to make a new back plate for instance, I have to match the 3, 4, and sometimes 5 bolt hole pattern already drilled in the chuck / plate. Since the chucks all have thru holes, I cannot use a compass to find the centerline and determine the bolt-hole radius to make a new backplate etc. I have a method that I use where I install screws/ hole pin gages into the threaded holes, and try to create triangle dimensions by measuring the outside of each pin, subtracting half the width of the pin to find center.
This cannot be the optimal method as there are so many variations that affect precision: metric screws, uncommon screw pitches, worn out screws that tilt in the holes when being measured, undersized or varying screws peak to peak. Often times I do not have the ability to contact the user / owner of the parts since I get the parted-out work from my customer. Sometimes I get the original screws so I can determine by pitch if they are metric or Imperial, which helps in determining the closest common radius, other imes I don't get the screws that were originally used to combine the chuck to the back plate or spindle nose. I often resort to screwing taps into the tapped holes to figure out the pitch thread and therefore the screw size.
I have also used a Co-Ax indicator to determine the center of the hole when inserting Hole pin gages. But that only gives me the point of a triangle/ pentagon so I can make a math calculation from there.
Many glass blowers require tight tolerances. In my attempts at figuring out the bolt hole radius, my measurements fall between common metric and Imperial numbers so I don't know which system was used.
Those of you who have experience with glass blowing chucks will know where I am coming from.
I know there is an easy way, but many of my old timer machinist friends that could answer this have passed on in the last few years. I have searched the internet, this and other machinist sites but cannot find the answer.
I have a DRO with 'Bolt Hole pattern' to drill the new holes, but I cannot figure out how to use it in reverse, and it is not in the Accu Rite manual.
Obviously I have no formal machinist training, but I scratch out a living with decent machines and information from books / friends.
Thank you in advance.

[Black_Moons]

Do you really need to know the bolt hole center distance?
If you clamp the two parts togethor, you can use transfer punchs through the existing bolt holes to mark the exact locations on the rear plate, and then just drill visualy (the transfer punchs will leave a dent that will guide your flexable drill bit, or give your center drill a very precise spot for visual alignment)

Also, if the center has a hole, you can messure from the center hole to the bolt hole, and just add half the diamiter of each hole to your messurement.

To accurately determin the size of a hole, Try telescopic and half ball diamiter messurement tools, your caliper directly is inaccurate for inside messurements.

[Doozer]

In an odd pattern, like 5 or whatever, measure from one mount bolt to the outside of the backing plate, the nearest way. Next measure from that same bolt across the diameter, way over to the other side (the farthest way). Subtract your first measurement from the second, and you have the bolt circle.
I think I explained that OK.

--Doozer

[oldtiffie]

Centre-distances and pitch circles

I frequently get chucks and backing plates from a customer in the Glass Blowing business. It seems Glass Blowers often do whatever it takes to get parts put together just to keep the machines running. There are so many bizarre compilations of components, chucks, and backing plates in that business. The Glass Blowing industry prefers to use metric increments in it's parts and accessories but that is not always the case. Each Blower also prefers their own combination of bizarre chucks, Drill Chucks, on face plates, back plates etc. for the repetative parts they specialize in.

My dilemma: HOW TO DETERMINE THE BOLT HOLE RADIUS FROM EXISTING BOLT HOLES?
I make parts to refurbish Glass Blowing Lathes (Typically Litton models). When I have to make a new back plate for instance, I have to match the 3, 4, and sometimes 5 bolt hole pattern already drilled in the chuck / plate. Since the chucks all have thru holes, I cannot use a compass to find the centerline and determine the bolt-hole radius to make a new backplate etc. I have a method that I use where I install screws/ hole pin gages into the threaded holes, and try to create triangle dimensions by measuring the outside of each pin, subtracting half the width of the pin to find center.
This cannot be the optimal method as there are so many variations that affect precision: metric screws, uncommon screw pitches, worn out screws that tilt in the holes when being measured, undersized or varying screws peak to peak. Often times I do not have the ability to contact the user / owner of the parts since I get the parted-out work from my customer. Sometimes I get the original screws so I can determine by pitch if they are metric or Imperial, which helps in determining the closest common radius, other imes I don't get the screws that were originally used to combine the chuck to the back plate or spindle nose. I often resort to screwing taps into the tapped holes to figure out the pitch thread and therefore the screw size.
I have also used a Co-Ax indicator to determine the center of the hole when inserting Hole pin gages. But that only gives me the point of a triangle/ pentagon so I can make a math calculation from there.
Many glass blowers require tight tolerances. In my attempts at figuring out the bolt hole radius, my measurements fall between common metric and Imperial numbers so I don't know which system was used.
Those of you who have experience with glass blowing chucks will know where I am coming from.
I know there is an easy way, but many of my old timer machinist friends that could answer this have passed on in the last few years. I have searched the internet, this and other machinist sites but cannot find the answer.
I have a DRO with 'Bolt Hole pattern' to drill the new holes, but I cannot figure out how to use it in reverse, and it is not in the Accu Rite manual.
Obviously I have no formal machinist training, but I scratch out a living with decent machines and information from books / friends.
Thank you in advance.

This should help.

Measure the bolt-hole centre-distance and use the "polygon" to sort out the rest. It works well. No need to try to measure the centres - use the left (or right) side of both holes as they are the same distance apart as the hole centres are.

[YankeeMetallic]

Thank you Moon and Doozer so far.
I guess I should add the following...
I just experienced a backplate where someone drilled three holes they thought were concentric to center, but were not. Then they center punched through the holes into a "Spindle Nose": ( a second backplate of sorts) and tapped and drilled the holes. I had to make a new backplate to fit another spindle nose. I tried measuring the inside of the screw to a gage block parallel to the outside, subtracting the gage block and the screw radius. I took for granted the bolt hole pattern was consistent and made a bolt hole pattern that eventually only lined up one hole. I remeasured the existing holes, which is how I concluded some 'hack' just 'guestimated' and drilled what looked like a concentric pattern, but was not. That cost me time & material. Live and Learn.
I have a very nice metrology collection for measuring hole sizes, but pin gages are the most efficient for small holes. I do not have a metric pin gage set.
Also, glass blowers use a system of a chuck, attached to a backplate, which attaches to a spindle nose. Many of the holes, like in the chuck, are 'blind holes' where hole transfers are not easily done.
Transfers are also not possible if I have to make a back plate that has to attach to multiple size spindle noses. That is when I definitely must have a bolt hole radius to apply to each part.
I have even tried tranferring the bolt hole to sticky backed paper, then using a compass to determine the bolt pattern. That system is not always accurate or possible though.
Keep 'em coming though....
Duke

[LES A W HARRIS]

Calculation of BHC from chord.

Radius = half of the chordal measurment/sin of half the included angle.



ie:
there are 5 holes. the chords average 1.113".

the angle at the center is 72°; half this is 36°.

half the chord is 0.5565"/sin 36°;

0.5565/0.5878 = R = 0.9468".



See also MHB circular segments.


Cheers,

[oldtiffie]

Triangulate.

Duke.

Another method is to use the measured hole centre-distances and a centre-punch and a set of good dividers.

A triangle is "fixed" and "rigid". It is pretty easy to do and quite accurate if care is taken.

If you like I will post a sketch later in the day.

[LES A W HARRIS]

Search Old posts

From old post, same things.
http://bbs.homeshopmachinist.net/showthread.php?t=30851


Cheers,

[YankeeMetallic]

BINGO! I think I've got it now.
Sketches always help, putting equations to practical use.
The links were also handy.
Thank you for the replies!

Duke

[Black_Moons]

For blind holes, they make bolts that basicly have a pointy head, you screw a couple in, place parts togethor, hit with a blunt hammer, done. (you could make your own in the lathe)

What does cot mean on the page http://www.cnccookbook.com/MTLayout.htm for dovetails?

[LES A W HARRIS]

cotangent

What does cot mean on the page http://www.cnccookbook.com/MTLayout.htm for dovetails?

cot = Co-Tangent of the angle = the tangent function of the angle then reciprocated.
ie:

30°, tan = 0.5774, cot 30° = 1.7321

Cheers,

[Paul Alciatore]

I don't know if it is a standard or best method, but I usually measure from hole to hole using calipers. I do both inside and outside dimensions for each pair of holes and average them for the center to center distance.

If there is an even number of holes, I measure all the opposite pairs and average the average readings for each pair. And, yes they usually do not agree from pair to pair.

If there is an odd number of holes I use equations like dickeybird posted. Again, I measure a number of pairs of holes for each distance and in the equations and average either the distances or the calculated diameters.

I use calipers because in most cases - in all the cases that I have ever measured - there is really no need for higher precision. You WILL get at least several thousanths difference between different pairs of holes. You will have to use pins or other devices if the holes are small or badly worn on the edges. Just keep the pins parallel and averaging the inside/outside measurements will take care of any difference between the pin size and hole size.

[oldtiffie]

Set-out sketches

Once again - back to the OP:

I frequently get chucks and backing plates from a customer in the Glass Blowing business. It seems Glass Blowers often do whatever it takes to get parts put together just to keep the machines running. There are so many bizarre compilations of components, chucks, and backing plates in that business. The Glass Blowing industry prefers to use metric increments in it's parts and accessories but that is not always the case. Each Blower also prefers their own combination of bizarre chucks, Drill Chucks, on face plates, back plates etc. for the repetative parts they specialize in.

My dilemma: HOW TO DETERMINE THE BOLT HOLE RADIUS FROM EXISTING BOLT HOLES?
I make parts to refurbish Glass Blowing Lathes (Typically Litton models). When I have to make a new back plate for instance, I have to match the 3, 4, and sometimes 5 bolt hole pattern already drilled in the chuck / plate. Since the chucks all have thru holes, I cannot use a compass to find the centerline and determine the bolt-hole radius to make a new backplate etc. I have a method that I use where I install screws/ hole pin gages into the threaded holes, and try to create triangle dimensions by measuring the outside of each pin, subtracting half the width of the pin to find center.
This cannot be the optimal method as there are so many variations that affect precision: metric screws, uncommon screw pitches, worn out screws that tilt in the holes when being measured, undersized or varying screws peak to peak. Often times I do not have the ability to contact the user / owner of the parts since I get the parted-out work from my customer. Sometimes I get the original screws so I can determine by pitch if they are metric or Imperial, which helps in determining the closest common radius, other imes I don't get the screws that were originally used to combine the chuck to the back plate or spindle nose. I often resort to screwing taps into the tapped holes to figure out the pitch thread and therefore the screw size.
I have also used a Co-Ax indicator to determine the center of the hole when inserting Hole pin gages. But that only gives me the point of a triangle/ pentagon so I can make a math calculation from there.
Many glass blowers require tight tolerances. In my attempts at figuring out the bolt hole radius, my measurements fall between common metric and Imperial numbers so I don't know which system was used.
Those of you who have experience with glass blowing chucks will know where I am coming from.
I know there is an easy way, but many of my old timer machinist friends that could answer this have passed on in the last few years. I have searched the internet, this and other machinist sites but cannot find the answer.
I have a DRO with 'Bolt Hole pattern' to drill the new holes, but I cannot figure out how to use it in reverse, and it is not in the Accu Rite manual.
Obviously I have no formal machinist training, but I scratch out a living with decent machines and information from books / friends.
Thank you in advance.

Duke,

as promised, I have completed the sketches for hole spacings, centre distances and lay/set-outs for equi-sized and equi-spaced holes - Sketch/sheet 1 - as well as the general case - Sketch/sheet 2 - where the holes may or may not be the same size or equally-spaced and may be on or off the centres or pitch circles.

I hope it clarifies things somewhat and is of help:

[Carld]

Hey Tiff, after some manipulation of the size of the page of formulas I was able to get it on one page in Word and will store it and make some printed copies to use in the shop. I don't think I have ever seen a simpler set of formula to find the bolt circle dia. or sides of the circle.

Thank you very much.

[Lew Hartswick]

[quote]In my attempts at figuring out the bolt hole radius, my measurements fall between common metric and Imperial numbers so I don't know which system was used. [endquote]

Don't be supprised if they come out NEITHER. :-)
I did this for the base mounting of a DeWalt router and using every
measurement (collet to shoulder screws all the way around every
combination) and converting to metric or english in rectangular or
polar coordinates, and nothing could be made to come out even.
:-) So I just used the numbers as they came out and it was perfect.

Sure wish I could have ask the ME or draftsman that layed it out
what he intended. :-)
...lew...