# how to figure the off set to drill this hole at an angle



## werowance (Aug 30, 2022)

hi,  so this is not engine related but just a fun project i wanted to do for my grand kids.  a siren whistle.  i did not make the 3d stl files, i just downloaded from thingieverse.

but for this part the holes are drilled around the perimiter at lets say 45deg angles.  and for the life of me i cant figure out how to offset my mill so the center line of the hole is in the middle of the disk.

lets say the part is 1" diameter and .125 thick, the holes are .125 dia and angle of the drill is 45 degrees.  all just made up examples.

when i set my rotary table at 45 degrees would i offset the drill to the left or right by half the drill bit dia?   just trying to figure this out.  if i drill in the same spot i would if it were not at an angle hole then the oposite where the drill breaks thru will be way out of place and likely even closer to the parts edge than the top part.  i need to figure out the math so that the exit of the drill hole is = / oposite as to the top side of the hole.  man this is hard to describe but i hope it my question makes sense.


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## gbritnell (Aug 30, 2022)

By the illustration the holes are offset from the centerline ? Is there a dimension for that!  I would need a little more information.


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## Jasonb (Aug 30, 2022)

central axis of the drill  should enter 1/16" off of the ctr line measured at the top surface of the disc


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## Jasonb (Aug 30, 2022)

If you look at the ctr lines from the side you can see the maths is just trig where if you know the angle and half the thickness of the disk you can calculate the offset


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## werowance (Aug 30, 2022)

but i dont understand the math you are using.  i get you are going half of the .125 drill for the offset but thats assuming a 45 degree angle drill.  what if the hole degree is say 60 degree?

or are you going half of the .125 part thickness? or a combination of the 2?   

thing is,  i plastic 3d printed the part and will insert the drill into the hole and then determine the actual angle of the hole.  which my guess is that its really not at 45 degree's.  and likely not even .125 diameter so im not sure how measured to get that offset.


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## werowance (Aug 30, 2022)

Jasonb  the info in this picture you provided is what im looking for.  but i guess a better way to ask instead of saying i "dont understand the math" would be to say what formula did you use to get what is circled in red below.  im guess it will involve knowing the angle of the hole. the size of the drill and the thickness of the part.  but i just cant figure the formula you would use to get that so it could be used for many different angles or drill sizes etc.


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## Lloyd-ss (Aug 30, 2022)

Werowance,
Forget about your made up example because there is some conflicting info in there that just happens to work out at 45 degrees.
Try this, based on my pencil sketch. It requires 2 different trig calculations.
Here are your knowns: (use the convention of lower case letters for side distances and Capital letters for the angles opposite those sides.)

Angle A. The tilt angle of the rotary table from the flat position. Flat would be 0 deg, vertical is 90 deg. Note that in my sketch, I have shown 60 deg to be where I labeled it, and also where i show it as angle A.

Distance b.  The distance from the surface of the work piece where you want the CL of the drill to pass thru. If the part is .400" thick, and you want it to pass thru the center plane of the part, then b is .200".

You do not know distance a, how far along the face the hole CL is offset.
You do not know distance a1, how far the mill head needs to be horizontally offset from the CL of the part.

Here are the calculations.
First, calculate distance a.  This is the Tangent of angle A times the distance b.  So, tan 60 deg times .200.
1.732 x .200 = .3464  

Now, calculate distance a1. This is the Cosine of angle A times the distance a that you just calculated.
So, Cosine 60 times .3464.
.500 x .3464 = .1732

Let me know if that doesn't make sense yet. Trig can be tricky.


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## bluejets (Aug 31, 2022)

Best to run an end mill bit in first to get a square line for the centre.
Use a rotary table and divide hole centre radius into 7.
Doen't matter where first one goes so long as the rest are equal.....simple.


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## TonyM (Aug 31, 2022)

Jasonb said:


> If you look at the ctr lines from the side you can see the maths is just trig where if you know the angle and half the thickness of the disk you can calculate the offset


No need for maths or trig. Your drawing is just job.  
Draw cross section of disc, draw horizontal and vertical centrelines,  draw centreline of hole at the correct angle through the intersection of those and just let CAD measure the offset.


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## timo_gross (Aug 31, 2022)

Brain twister, maybe I get something wrong here. I think I do not have to offset anything at all, do I?


This disk is angled at some angle and the hole on top will line up for drilling. 
Then you just rotate to the next hole and drill again? (use some drilling tool, that stays straight is the biggest challenge. )




You just need to center between the upper edge and the lower edge on angled axis and the sides at the not tilted axis?

If the Angle becomes 90° the holes will end up tangential.  



If the holes are evenly spaced, you can start one and just work around or not.

Greetings Timo


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## Jasonb (Aug 31, 2022)

This is the same image I posted but with the maths added.

You know the thickness T, you know you want the ctr lines to cross half way through the disk so they cross a T/2 down. You know the angle so the rest is triganometry to find the offset x

Tan of Angle = x / (T/2) 

Tan 45 = x / 0.0625 Look up Tan 45deg on a calculator

1= x / 0.0625

Multiply both sides by 0.0625 to get rid of the fraction on the right side

0.0625 = x

Just change T and angle for whatever setup you have


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## Whittler (Aug 31, 2022)

Hi, good luck with your project, will be interested to see the result.  Here is one I made earlier.  I also had issues drilling the holes, more by trial and error!




Regards

Paul


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## Badhippie (Aug 31, 2022)

Question for everyone wouldn’t it be easier to scribe a line from the center of the part to the centerline of the holes. Around the diameter to connect all the holes??? The part is already on the rotary table so you can use it for the degrees,minutes,seconds. For the spacing between the holes. And since you scribed a line around the diameter of the part for the center line. Turn the table to whatever degree you need for hole placement center punch hole then put your rotary table on a 45 and drill or use a end mill to get hole started.


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## Jasonb (Aug 31, 2022)

TonyM said:


> No need for maths or trig. Your drawing is just job.
> Draw cross section of disc, draw horizontal and vertical centrelines,  draw centreline of hole at the correct angle through the intersection of those and just let CAD measure the offset.


That's only OK for those that have CAD and know how to use it.

I can also do trig using the calculator on my phone while down in the workshop or out on site, saves having to come back to the computer to draw it out in CAD


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## werowance (Aug 31, 2022)

Lloyd and Jasonb, thank you both for that.  i had since i posted that found a web site talking about how to calculate this for installing plumbing pipe and drilling the holes in the floor, walls etc for it.  same math and i was struggling with it reading those sites.  but with your answers and formulas i do understand now.  complex in my opinion but to me thats just math and im terrible at it.  but as long as i have the formula i can do it.  i REALLY APPRECIATE IT.  THANK YOU!


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## werowance (Aug 31, 2022)

Whittler,  that looks like one of the pattent drawings i saw (didnt have dimensions or anything) so i found a similar one on thingieverse and printed it out to get the dimensions from (slightly different than what yours and the pattent drawing looks like).   by chance do you have drawings and dimensions for yours?  im thinking that looks more to the period of what i was trying to recreate than the one from thingieverse.  i wouldnt mind paying for a copy of the prints if you have any.   yours looks great.


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## werowance (Aug 31, 2022)

Whittler said:


> Hi, good luck with your project, will be interested to see the result.  Here is one I made earlier.  I also had issues drilling the holes, more by trial and error!
> 
> View attachment 139611
> 
> ...


  sorry for the double post but didnt get the quote in the one above

Whittler, that looks like one of the pattent drawings i saw (didnt have dimensions or anything) so i found a similar one on thingieverse and printed it out to get the dimensions from (slightly different than what yours and the pattent drawing looks like). by chance do you have drawings and dimensions for yours? im thinking that looks more to the period of what i was trying to recreate than the one from thingieverse. i wouldnt mind paying for a copy of the prints if you have any. yours looks great.


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## timo_gross (Aug 31, 2022)

"telling bone" can be used as calculator?   "nothing works"!


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## Whittler (Aug 31, 2022)

Hi Werowance, I know the old patent sketches you mention, I came across them too.   Sorry, I don’t have a decent drawing, it was more or less ‘make it up as I go along’!   I would have like to achieve more of trumpet effect, but think the original sirens were ‘spun’ and not turned out of solid.  Good luck in getting yours to work, if I can find any of my notes I will let you know.  Paul


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## timo_gross (Sep 2, 2022)

werowance said:


> hi,  so this is not engine related but just a fun project i wanted to do for my grand kids.  a siren whistle.  i did not make the 3d stl files, i just downloaded from thingieverse.


Hello werowance: 

is it this one you refer to? 









						Siren Whistle by BbrnKyle
					

This is my version of the popular siren whistle. Makes the wife angry... Measured at 92Db




					www.thingiverse.com
				




I got it to work after getting rid of the "elephantfoot", so better print it with a raft.  
The thing works sort of, need quite a bit air. (do not pass out if you use it too much)
I wonder if there is the need of making this many different plates with holes in them. 

Greetings Timo


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## werowance (Sep 6, 2022)

timo_gross said:


> Hello werowance:
> 
> is it this one you refer to?
> 
> ...


not that one but i did look at it.  its this one which is very similar.  i figured i would use a roller bearing on the spinny part and make it out of aluminum.  the body from brass and the horn part - i have been practicing metal spinning with surprisingly decent results lately so was going to try and spin the horn from brass.  that is if i can ever get a form printed out.  for metal spinning i have been experimenting with 3d printing the form to spin metal around it and that works surprisingly well.  easier than turning a piece of wood down for it.









						Amazing Siren Whistle (fun, loud, and fast to print) by Shmoee
					

Update 7 and 8: 11/16/2021 and 11/17/2021 These are still my recommendations: Siren_Whistle_Mouthpiece v50.stl Siren_Whistle_Horn_Alternate_shallower_indented_cone.stl Rotor_Modification_4p6.stl For some additional experimenting fun with other good parts, try out these to pick a favorite...




					www.thingiverse.com


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## timo_gross (Sep 7, 2022)

werowance said:


> not that one but i did look at it.  its this one which is very similar.  i figured i would use a roller bearing on the spinny part and make it out of aluminum.  the body from brass and the horn part - i have been practicing metal spinning with surprisingly decent results lately so was going to try and spin the horn from brass.  that is if i can ever get a form printed out.  for metal spinning i have been experimenting with 3d printing the form to spin metal around it and that works surprisingly well.  easier than turning a piece of wood down for it.
> 
> 
> 
> ...


Hello, 

yes the two designs are similar, the one you were refering to is a "remix" Shmoee seems to have done a lot of experimenting and variations. 
Hope it works out fine. Are you making it as a lungth training device or to use it on a steam engine?

Greetings Timo


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## werowance (Sep 7, 2022)

just for fun.  if it works decent then gifts for my grand kids.  i had one when i was little and loved to annoy my parents with it when i was little.  wanted to pass that joy along to my grand kids and their parents.    im sure my Daughter will appreciate it as much as my parents did lol.  assuming it works as good as the one i had when i was little.


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