# How to layout holes on an 8" diameter plate



## bigblock61 (Dec 6, 2019)

I have a 4" rotary table for my Grizzly mill. I'd like to make an 8" top plate for it so I can have room to bolt down a 4" piece. My thought is to make the top plate with as many holes as I can drill in it and still have the plate remain strong. I'm thinking 3/8" threaded holes 3/4" on centers. I would need the center hole to match up with the center hole on my rotary table and 4 holes roughly 3" around the center hole to bolt the top plate to the rotary table. What is the best way to lay out a pattern of holes 3/8" by 3/4" on centers?

My first thought is to figure out each diameter from 7.5" to 3" staying as close to the 3/4" centers as possible. 7.5" C=(2 x Pi x r) a circumference of 47.1".  A spacing of .785" = 60 holes. 360 \ 60 = 6 so a 3/8 hole every 6 degrees slightly larger than 3/4". Do that for the next smaller diameter and so on. My question is, is my thinking right and is there a better way of doing it?


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## Bob Korves (Dec 6, 2019)

I would probably just lay out a pattern on the plate with 1" spacing in both directions, with a square grid pattern.  The plate for my own RT is blank and only gets holes added to it as needed, the lazy man's approach.  It does put the holes right where you want them for the job at hand, and it also adds time to the projects, but not to making the plate...


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## bigblock61 (Dec 6, 2019)

Bob Korves said:


> I would probably just lay out a pattern on the plate with 1" spacing in both directions, with a square grid pattern.  The plate for my own RT is blank and only gets holes added to it as needed, the lazy man's approach.  It does put the holes right where you want them for the job at hand, and it also adds time to the projects, but not to making the plate...


 
Interesting idea but since the plate I have is already an 8" circle, putting a square pattern would limit my choices for clamping options.


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## bill stupak (Dec 6, 2019)

If you have a mill and dro. www.doov.com/cgi-bin/bolthole.cgi


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## Janderso (Dec 6, 2019)

Big Block,
One of my, to do, projects is adding a plate to the rotary table to be a clamping, sacrificial plate.
I would like to add a fence that is parallel to the milling machine table X feed when the rotary table is set to zero.
In order to accurately accomplish this the plate needs to be installed to the RT quickly and easily.
A smart fellow in our local group has his plate set up with close tolerance pins that slip into the RT's t slots.
Just throwing ideas around.


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## Bob Korves (Dec 6, 2019)

Look at the first four videos in this list:








						oxtoolco
					

The Oxtoolco YouTube channel is an ongoing video journal of a life spent designing and building special tools, instruments, and mechanical devices for the sc...




					www.youtube.com


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## Bob Korves (Dec 6, 2019)

I would also not make the plate round if it is currently square.  The idea being that we do not know what our next project might be, so why cut anything off or drill anything until it needs to be for the current job.  I would just build a good centering arrangement (spud, Morse taper, or whatever) and something like a tight fitting pin (or more) to set the clock position of the plate.  Done right, it could also act as a fixture plate for your mill vise.  Or for the mill table.  Or for a lathe chuck.  The more things you do to the plate up front, the more limited become the future possibilities for using the plate...


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## francist (Dec 6, 2019)

You could lay it out like a spider web wth diameters increasing every 1-1/2” and rays radiating from the centre. When the spacing of holes becomes too loose or too tight for your liking, add or delete a set of rays. Personally I think 3/8” holes on an 8” plate are way too big. I would go 5/16”, no more. It’s for clamping on a 4” RT, not holding up an engine block.

I do tend to agree with Bob though, regardless of where you put your holes there is an equal chance they will be in the wrong place. Been there several times.

-frank


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## homebrewed (Dec 6, 2019)

Some time back I wrote up a little spreadsheet for calculating bolt circles.  It's attached.  DROs would be a handy thing to have for this kind of business, but not absolutely necessary.


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## Bob Korves (Dec 6, 2019)

On a fixture plate for a 4" RT, I would not make the threaded holes larger than 1/4"


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## bigblock61 (Dec 6, 2019)

Janderso said:


> Big Block,
> One of my, to do, projects is adding a plate to the rotary table to be a clamping, sacrificial plate.
> I would like to add a fence that is parallel to the milling machine table X feed when the rotary table is set to zero.
> In order to accurately accomplish this the plate needs to be installed to the RT quickly and easily.
> ...



I like that idea and will incorporate it into my plans. thanks



Bob Korves said:


> I would also not make the plate round if it is currently square.  The idea being that we do not know what our next project might be, so why cut anything off or drill anything until it needs to be for the current job.



The plate I have is 1/2" thick and it is already 8" round, that can't be changed. 



Bob Korves said:


> On a fixture plate for a 4" RT, I would not make the threaded holes larger than 1/4"



I settled on 3/8 threaded holes because most of my clamping fixtures use 3/8's hardware and all of my T-nuts use 3/8 as well. Changing the hole size necessitated all new clamping hardware


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## bigblock61 (Dec 6, 2019)

Bob Korves said:


> Look at the first four videos in this list:
> 
> 
> 
> ...



That first video was hard to watch. and hour of setup and 5 minutes of making a hole bigger. I'm pretty sure you wanted me to see his initial setup and the homemade clamps. I like that idea and may put it too use. My thinking is that after the top plate is made I'll make another thinner plate out of aluminum as a sacrificial plate.


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## dirty tools (Dec 6, 2019)

For myself 
I would just drill any holes that I need for the current project .
you can always drill more hole but plugging them and redrilling new holes can be very challenging


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## bigblock61 (Dec 6, 2019)

francist said:


> You could lay it out like a spider web wth diameters increasing every 1-1/2” and rays radiating from the centre. When the spacing of holes becomes too loose or too tight for your liking, add or delete a set of rays. Personally I think 3/8” holes on an 8” plate are way too big. I would go 5/16”, no more. It’s for clamping on a 4” RT, not holding up an engine block.
> 
> I do tend to agree with Bob though, regardless of where you put your holes there is an equal chance they will be in the wrong place. Been there several times.
> 
> ...


 This is sort of the idea with smaller and smaller rings that are offset to keep the spacing somewhere around 3/4" from any other bolt hole. I'll have a 3/8 hole and a 3/8 space in any direction I go on the plate.


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## BtoVin83 (Dec 6, 2019)

R U sure it's a 47 inch circumference?


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## bigblock61 (Dec 7, 2019)

BtoVin83 said:


> R U sure it's a 47 inch circumference?



No its not. I didnt figure out my mistake until last night, It should be 23.55 with 30 holes every 12 degrees.


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## tjb (Dec 7, 2019)

BtoVin83 said:


> R U sure it's a 47 inch circumference?


I think the calculation is off by a factor of 2.  pi x 2 x r = pi x 7.5 = 23.56194.  If I'm reading the OP correctly, sounds like the diameter was plugged into the formula instead of the radius.  2 x 23.56194 = 47.12389.

I have a 12" rotary table and made an indexing plate with toe clamps for it that works very well.  Not nearly the same design, shape or size being discussed here, but all those are variable.  It has a centering hole and four additional mounting holes for alignment on the RT.  It is rectangular with a recess cut in the front and back edges so it can also be clamped in my vise, or mounted directly onto the mill table.  The holes are all drilled and tapped to 1/4-20.  I tried to design it to be as versatile as possible.  Works pretty well, and it's hard to come up with a piece to machine that can't be attached to it somehow.

Regards,
Terry


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## bigblock61 (Dec 7, 2019)

tjb said:


> I think the calculation is off by a factor of 2.  pi x 2 x r = pi x 7.5 = 23.56194.  If I'm reading the OP correctly, sounds like the diameter was plugged into the formula instead of the radius.  2 x 23.56194 = 47.12389.
> 
> I have a 12" rotary table and made an indexing plate with toe clamps for it that works very well.  Not nearly the same design, shape or size being discussed here, but all those are variable.  It has a centering hole and four additional mounting holes for alignment on the RT.  It is rectangular with a recess cut in the front and back edges so it can also be clamped in my vise, or mounted directly onto the mill table.  The holes are all drilled and tapped to 1/4-20.  I tried to design it to be as versatile as possible.  Works pretty well, and it's hard to come up with a piece to machine that can't be attached to it somehow.
> 
> ...



You nailed my mistake. I plugged in the diameter instead of the radius. Oops! I do like that design and if the plate wasnt already round I would go with a square piece. It would be so much easier to lay out a hole pattern. I'm trying to do this with stuff I have on hand so I am ending up limiting myself.


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## tjb (Dec 7, 2019)

bigblock61 said:


> You nailed my mistake. I plugged in the diameter instead of the radius. Oops! I do like that design and if the plate wasnt already round I would go with a square piece. It would be so much easier to lay out a hole pattern. I'm trying to do this with stuff I have on hand so I am ending up limiting myself.


Yes, I get that you're trying to use what you have on hand.  (My indexing plate is its size and shape for precisely the same reason.)

I haven't thought this through completely, but I wonder if the simplest approach might be to carefully locate and mill your large center hole, then mount and center it on your rotary table.  From there, you should be able to mill the additional holes wherever you want - including the ones to mount to the RT..  My kneejerk would be the outer set would be approximately equivalent to the numbers on a clock face - 12/1/2/3/etc.  After you've done the outer set, you can fairly easily move to the adjacent set and decide if you want to line them up straight or stagger them (maybe 12:30/1:30/2:30/etc.).  I think I would be inclined to chuck up a spotting bit first and just mark ALL the holes before cutting anything.  That way, you can adjust if it doesn't look right.  Totally different size/shape/pattern but that's essentially how I laid out my indexing plate.

Regards,
Terry


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## tjb (Dec 7, 2019)

Quick PS:  Obviously, you'll need to start somewhere.  I can't visualize any way to mount an 8" piece of round stock on a 4" rotary table without first drilling a set of four holes that will line up with the t-slots.  Maybe use a compass to lay out a circle (at 3" according to your original post?); then mill those holes first so you can at least mount it to the RT.  Then proceed according to whatever pattern suits your objective.

I would suspect your FIRST measurement with a compass should be at @8" to make sure you're at or near centered.  Then you're more likely to be where you want to be on the 3" circle.

Regards,
Terry


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## bigblock61 (Dec 7, 2019)

tjb said:


> Quick PS:  Obviously, you'll need to start somewhere.  I can't visualize any way to mount an 8" piece of round stock on a 4" rotary table without first drilling a set of four holes that will line up with the t-slots.  Maybe use a compass to lay out a circle (at 3" according to your original post?); then mill those holes first so you can at least mount it to the RT.  Then proceed according to whatever pattern suits your objective.
> 
> I would suspect your FIRST measurement with a compass should be at @8" to make sure you're at or near centered.  Then you're more likely to be where you want to be on the 3" circle.
> 
> ...



Thanks, I was already thinking that I would mount the plate to the table and then use it to drill the holes. My thought was to find the center of the plate set the dro to 0,0. Then drill and countersink the mounting holes for the rotary table 1.5" in the X Y directions. I have a plan in my head. I just need to work out the fine details before I drill the first hole.


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## Bob Korves (Dec 7, 2019)

First make something to locate the plate radially with the RT center to get the plate centered,  and only then begin with locating it in clock position.  The hole in the center of the RT is your friend...


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## Bob Korves (Dec 7, 2019)

After being constrained radially, the plate can then be held down to the RT with flat socket head screws and t-nuts.  Also have at least one locating pin to locate the plate accurately and repeatably in clock position.


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## tjb (Dec 7, 2019)

bigblock61 said:


> Thanks, I was already thinking that I would mount the plate to the table and then use it to drill the holes. My thought was to find the center of the plate set the dro to 0,0. Then drill and countersink the mounting holes for the rotary table 1.5" in the X Y directions. I have a plan in my head. I just need to work out the fine details before I drill the first hole.


That's exactly the way I would approach it.  Just be careful to make sure you're as close to center as possible before you drill anything.

Ditto, Bob.

Regards,
Terry


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## Tim9 (Dec 7, 2019)

I would just center the RT then mount the plate to the 4” RT. Use the RT to then drill your pattern holes. I too wouldn’t go overboard on drilling the holes just yet. Drill enough for the current project and maybe a few more which would expand capabilities. You can always drill more at a later date.


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## extropic (Dec 7, 2019)

homebrewed said:


> Some time back I wrote up a little spreadsheet for calculating bolt circles.  It's attached.  DROs would be a handy thing to have for this kind of business, but not absolutely necessary.



I downloaded the spreadsheet and was playing with it.
Thank you for posting it.
I have a question. When I enter a "First Hole Angular Offset" value, the table changes appropriately but the "Sanity Check" chart does not.
I tried the "Calculate Now" button but no change. Can the chart be made to agree with the table data or is this an Excel anomaly?


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## Illinoyance (Dec 7, 2019)

Make your layout on paper.  Once you have a satisfactory layout all you need is the radius of the bolt circle and the degree spacing between holes.  Dial in the degrees with the RT and the radius with either the X or the Y feed.   Spot drill the holes with the plate on the RT.  Remove the plate, clamp it down on parallels and drill & tap the spotted locationc.

For quick alignment of plate to RT I used fixture keys.  3 are all that are needed.  Pins can substitute for key.   Flat head SHCS and T nuts were used to hold the table in place.


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## bigblock61 (Dec 7, 2019)

Illinoyance said:


> Make your layout on paper.  Once you have a satisfactory layout all you need is the radius of the bolt circle and the degree spacing between holes.  Dial in the degrees with the RT and the radius with either the X or the Y feed.   Spot drill the holes with the plate on the RT.  Remove the plate, clamp it down on parallels and drill & tap the spotted locationc.
> 
> For quick alignment of plate to RT I used fixture keys.  3 are all that are needed.  Pins can substitute for key.   Flat head SHCS and T nuts were used to hold the table in place.



You and I think alike. I'm sitting here with autocad drawing it up now and adjusting as needed.


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## BtoVin83 (Dec 7, 2019)

IMHO the holes are for clamping and don't need to be super accurate. In a lathe or with dividers layout your BC. Use a center head to layout 0 and 180 if the pattern is even numbered. Use your dividers to layout the bolt hole, you should be able to get the chord length from Machinery's Handbook


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## Illinoyance (Dec 8, 2019)

If you adr doing the layout in CAD you might consider dialing in holes with X and Y coordinates.


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## homebrewed (Dec 8, 2019)

I checked my spreadsheet and discovered an error in the formulas I used to calculate the coordinates .  I apologize for that.  I've attached the revised version and also will revise the version I put on the Resources forum.

BTW, although the spreadsheet gives the coordinates for 12 bolts, of course you'd only use the first 4 for a 4-bolt circle, 8 for 8 bolts etc.


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## extropic (Dec 8, 2019)

homebrewed said:


> I checked my spreadsheet and discovered an error in the formulas I used to calculate the coordinates .  I apologize for that.  I've attached the revised version and also will revise the version I put on the Resources forum.
> 
> BTW, although the spreadsheet gives the coordinates for 12 bolts, of course you'd only use the first 4 for a 4-bolt circle, 8 for 8 bolts etc.



The chart seems to work now. Thank you for fixing that.
I was hoping you would explain the fix so that I (we) could learn something about Excel.
I've tried to compare the two versions, cell by cell, but can't see a change.
The fix is obviously 'over my head', so I would appreciate it if you would provide a clue where to look.
Thanks,


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## homebrewed (Dec 8, 2019)

Sure.  The difference is in the table calculations,  columns C & D, lines 11 through 22.  I incorrectly added the angle offset -- I'd already done that in the angle calculations in column B so the calculated coordinates were incorrect.

If you click on one of the cells in the table you will see the formula pop up in the edit box that's near the top of the window.  Dollar signs in front of the cell ID (like $C$4) tell Excel not to update that particular ID when you copy and paste it into another cell. Cell C4 is the radius of the bolt circle so that stays the same for all the calculations.  The angle changes so you DO want that ID in the formula to change.


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