# How To Do Lay Out For 8 Holes



## Kroll (Sep 20, 2015)

Guys I need to try and figure out how to lay out 8-8/32 holes to match the SB mill gear.The other 1/16 thick plate mounts to the gear to keep the timing belt from sliding down has broke into and the old holes in the plate is wore out and very loose even when the screws are tight.I can figure out how to cut it into a circle but I am at a lost how to lay out for 8 holes that needs to match the gear.I have painted the plate so that I can see and try to do a good layout.
For me this is not an easy project cause I am not good at laying out holes(any kind of math) to match and I know that it will be hard for me to comprehend the best and easies answer but I am going to try.If you know of any websites that explains or and links to Youtubes,etc please let me know


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## Billh50 (Sep 20, 2015)

After making the I.D. and O.D. use the old ring and a transfer punch.


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## RJSakowski (Sep 20, 2015)

First, determine your bolt circle.  If you have a digital caliper, use the inside measuring jaws to span one of the holes.  Set the scale to "o" and then measure the distance between two directly opposite holes.  Measure several combinations and average to get the best reading.  Once you have determined the bolt circle diameter, mount your part to be machined and find its center.  If you set your machine coordinates at o.o, then the four holes at N, S, E, & W are 0, +r; 0,-r; -r, 0; and +r, 0.  The remaining four holes are at a 45 degree angle so the coordinates are +.7071r, +.7071r; +.7071r, -.7071r; -.7071r, -.7071r; &-.7071r,+.7071r.  

This procedure can be done on any mill but is most easily done with a DRO equipped mill.  Some DRO's have a bolt circle routine which automated this process.  Another way to make the ring would be to use a rotary table which would simplify the process.

If you will be using a drill press to drill the holes and making the ring on the lathe, scribe the correct bolt circle diameter on the ring using a lathe tool.  A pair of dividers can then be used to divide the circle into eight equal segments.  Center punch and drill.  Not as accurate as the previous methods but get you close enough.  

Bob


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## Terrywerm (Sep 20, 2015)

One thing that I have done in the past (before I had a mill and DRO) and works well is to draw out the part in a CAD program, allowing it to space the 8 holes equally around the ring. Print out the drawing, lay it over and fasten it to the part, then punch the eight holes with either a prick punch or a center punch, then drill your holes. In your case using your mill is obviously out of the question. Do you have a drill press?


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## Billh50 (Sep 20, 2015)

I have made parts like that with just a sabre saw, belt sander, drill press, sanding drum and transfer punches. I laid the old part on my material and traced the I.D. and O.D. Then after cutting closely with saber saw. I dykemed the inside and outside edges of the old part and clamped it to the newly cut piece. Using the belt sander I sanded the o.d. down til I just touched the dykem. Thus making the outside close to a true round. Did the same thing using a drum sander in a drill press. Then transfered the holes and drilled.
This works well if one does not have a cad program or a dro.


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## Bill Gruby (Sep 20, 2015)

The OD and ID are easily found, just measure them from the old ring. The Bolt Circle is a tad different. You know the holes are 8-32. Use the gear, not the ring to determine the bolt Circle diameter. Put an 8-32 bolt in two of the holes 180 degrees apart or directly across from each other. Measure from outside to outside on these two bolts. Subtract the diameter of one bolt and you have the Bolt Circle Diameter. Old school but extremely accurate. The bolt locations are as stated in another post, an even 45 degrees apart on the Bolt Circle. It can be done faster than reading this post.

"Billy G"


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## brino (Sep 20, 2015)

My standard way is to measure two opposite bolts. Measure the outside-to-outside and then inside-to-inside and average them.
-brino


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## Bill Gruby (Sep 20, 2015)

All you need is outside to outside then subtract one bolt diameter. Why complicate it?? Try it your way by averaging then try mine. I'll bet the farm both answers are the same. You could even do it inside to inside and add one bolt diameter and the answer will still be the same.  You are correct "brino" but it's a little more math. Thank you for adding that.

Pick your poison, all three ways are below. Let's use a bolt dia of .250

Your way "brino" ---- Outside is 6.250,  Inside is 5.750, Average is 6.000. ( 6.250+5.750=12.000/2= 6.000)

My way ---- Outside is 6.250. Minus .250 = 6.000 (6.250-.250=6.000)

Third way Inside is 5.750. Plus .250 = 6.000 (5.750+.250=6)

All three ways are equal at 6.000 Bolt Circle Diameter. There are many was to skin the proverbial cat. I like simple ones.

"Billy G


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## British Steel (Sep 20, 2015)

Quick way - turn the ring, fit transfer screws[1] in the pulley threads, clamp ring in correct centred position, tap over each transfer screw in turn - voila, eight hole locations prick-punched.

Slow way - measure over pairs of bolts, work out bolt-circle radius,
centre-punch centre of plate and set dividers to radius, scribe hole circle,
straight-edge and scriber, intersecting centre mark and hole circle, scribe first pair of hole locations; centrepunch 'em,
set dividers to 1-1/2 the radius, scribe arcs from the first pair which intersect at 90-degrees from the first pair - lay out scribed line through intersections and centre; centrepunch where the line intersects the bolt circle radius,
same again and again, splitting the 90-degree angles.

For threes and sixes, leave the dividers set to the radius and "walk" them around the circle, giving a mark every 60 degrees (pick alternate marks for three),

For twelve, mark out for six, then set the dividers slightly wider, bisect the 60-degree angles by laying lines from the centre to where the pairs of arcs intersect.

Old Skool, but it works as a last resort 

Dave H. (the other one)

[1] Transfer screws:
take a bunch of high-tensile bolts and saw the heads off,
off to the bench grinder with remains (studs) and a cordless drill,
spin studs with drill and put a nice sharp point on 'em.
grind a pair of flats at the pointy end to grip while screwing in or out.

Screw in until just the point's proud of the surface (by 20-40 thou"), tapping the plate will put a nice neat punch=mark in the plate for each transfer-screw point.

This works really well of chuck-to-backplate drillings, lost covers for gearboxes etc. too!


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## Bill Gruby (Sep 20, 2015)

Yup, that's another way.  Most hobby machinists don't have transfer screws though.

 "Billy G"


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## British Steel (Sep 20, 2015)

Transfer screws are so useful, so easy to make! I mostly work in Metric, so it's not a long job to make ALL the sizes, any specials (my lathe's all BSF and BA threads) get made as and when needed, stored in old 35mm film cans with labels. My Grandad showed me how to use them when I was about 4, sat on the end of his workbench (he was a toolmaker, then spent WW2 calibrating and test-firing field guns, died of his war wounds without going to fight - went stone deaf, didn't hear the bus coming...)

It occurs to me that Kroll could initially skip the turning / cutting the ring and use transfer screws on the raw plate, then use his nice new punch marks to find the centre for machining / marking and cutting before drilling the bolt circle, after drilling he'd then have some useful holes to locate the raw material on e.g. a faceplate to trepan the ring from the sheet (assuming he has a lathe).

Dave H. (the other one)


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## Bill Gruby (Sep 20, 2015)

Thanks for sharing that story.

 Transfer screw sets are available and well worth the cost. If you only need them once you will be glad you have them.

 "Billy G"

http://www.globalindustrial.com/g/m...-up-tools/transfer-screws/transfer-screw-sets


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## Kroll (Sep 20, 2015)

Guys thanks for all the great ideals which I do understand the directions given.I was hesitant to ask this question cause I thought the answer would just be to complicated for me to understand.I do think that I will order me couple of those screw transfer sets.I already need one for finishing my back plate and now a size for this project.Guys thanks again for all the ideals


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## Bill Gruby (Sep 20, 2015)

If you ever find an answer to a question is too complicated for you just say so and we will pull it apart till you do understand it. That is why we are the Forum we are. No one is left behind. It's an equal playing field here.

 "Billy G"


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## Billh50 (Sep 20, 2015)

We will also give a few different ways to do things depending upon skills and equipment. Some may be easy with minimal tools and some may be real easy with some machinery. You get to chose the way you feel more comfortable with.


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## Wreck™Wreck (Sep 20, 2015)

If we assume a 1" radius circle and the first hole is located at the 12:00 position (we will call this  zero degrees) or 12 oclock as on an analog clock for instance, and the center of the circle is at X 0.000 and Y 0.000 the first hole position in the X axis will be the sine of the angle multiplied by the radius, in this case 0.000" as you are already at X 0.000, so you now need to determine your Y axis position which will be the cosine of the angle  multiplied by the radius, in this case the Cos of zero degrees is 1, multiplied by a 1"radius circle  is 1". The next hole is rotated 45 degrees at the same radius therefore, sin 45 times 1" = .707" in the X axis and cos 45  multiplied by 1" = .707" in the Y axis, this is where hole number 2 is located.

Hole number 3 is located 90 degrees from hole number 1.  sin of 90 Degrees = 1 multiplied by a 1" radius circle is of course 1,  the X axis position is 1" and the Y axis position will be zero as the cosine of 90 is zero.

Hole number four will be at 135 degrees from zero, this hole  has exactly the same positions as hole number 2 with the Y axis position (as is common in small machine shop practice, this will be a negative number) .707" in X and -.707" in Y and so on around the circle.
In my experience I have never known of any Government or industry rule making body that determines which side of the 0.000 by 0.000 position is positive or negative so roll your own.
Good luck


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## jocat54 (Sep 21, 2015)

Hope this isn't a real dumb question (senior moment?)
I understand how to measure a bolt circle for even number holes--with opposing bolts, but how do you measure say bolt circle with only 3 holes?


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## Randy_m (Sep 21, 2015)

With a bolt circle calculator. Look on line, enter a couple known measurements and it will give you your x and y coordinates. Or use a d.r.o. its kinda like cheating. I have too because i suck at math.


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## markknx (Sep 22, 2015)

Bill Gruby said:


> If you ever find an answer to a question is too complicated for you just say so and we will pull it apart till you do understand it. That is why we are the Forum we are. No one is left behind. It's an equal playing field here.
> 
> "Billy G"



I agree Bill!


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## markknx (Sep 22, 2015)

Kroll It sounds as though you have got many fine ways to lay out the holes so I will leave that part alone.
But I have a question on one thing you said. You said the ring was worn, and even with the bolts tight it was still loose. Are you saying the bolts do not squeeze the ring tight to the gear? If so is the new material thicker? Could you be missing washers? Not sure how the ring could wear in a way to make it to thin to bolt up tight. Seems the bolts are bottoming in the holes. Is this right? Am I missing some thing?
Ok so that was more than one question.
Mark


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## Bill Gruby (Sep 22, 2015)

Jocat;

 No dumb questions are ever asked, they all make sense. Without looking your question up, this is how I would find the Bolt Circle Diameter.

 Assuming the 3 bolts are of equal spacing they form an Equalateral Triangle. Find the center of the Triangle and the distance from the center to one intersection is the radius of the bolt circle. From there the rest is simple math.

 "Billy G"


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## Bill C. (Sep 22, 2015)

Billh50 said:


> After making the I.D. and O.D. use the old ring and a transfer punch.



I would make the holes, at least one hole, one size bigger just in case they didn't match up like they should.  I hope you get your mill back into operation soon.


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## markknx (Sep 22, 2015)

For any odd number I would guess finding the center of the bolt circle is the first step. Then it is only a matter of center of the bolt circle to the center of any hole in the bolt circle times two. There may be faster more accurate ways as I am pretty new to machinist type work but for metal working/steel fab this works. (you know like when 1/32 is tight enough.)


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## Bill Gruby (Sep 22, 2015)

Here is what we know;

1) The bolts are equally spaced and there are three of them.

2) They create an Equilateral Triangle on the Bolt Circle.

3) Equilateral Triangle -- Three like Angles.

Now ------  bisect each of the three angles. Where these three Bisects intersect is the center of that Equilateral Triangle. You with me so far? From this new found center to any one of the Triangle corners is the Radius of the Bolt Circle and from here you are home free. All you need is the radius.

And some of you thought that after High School you would  not need Geometry ever again.

"Billy G"


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## jocat54 (Sep 22, 2015)

Bill Gruby said:


> Here is what we know;
> 
> 1) The bolts are equally spaced and there are three of them.
> 
> ...




Thank you Bill, now very easy to understand


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