# How do you measure the depth & Width of V cut?



## EmilioG (Apr 2, 2017)

I'm working on my finger plates and need a way to accurately measure the size of the V shaped slot.
One will be .535" wide x .290" deep.  I need to mill the V's to within a few thou on one FP design.
The other isn't so critical.

I watched a video by That Lazy Machinist using a depth mic with a
gage pin in a shop made V block he was milling. Not sure if his technique and he doesn't go in to details
of that measurement, so if anyone knows how to do this, please share.  Thanks


----------



## Tony Wells (Apr 2, 2017)

Pin and depth mic are probably the most straightforward means, but that assumes the angle is cut accurately. A little trig and you can get some good numbers if the angle is correct, or you can get an accurate measurement on it and use the actual rather than the design angle.


----------



## Charles Spencer (Apr 2, 2017)

Math is not my strong suit but it seems to me that the Pythagorean theorem should provide the answer.  The depth of 0.290 provides one side of a right triangle.  Half the width of 0.535 (0.2675) provides the other.

0.290 squared =  0.0841
0.2675 squared= 0.07155625
------------------------------------
sum                        0.15565625

square root of above:  0.39453295....

say, 0.395


----------



## Technical Ted (Apr 2, 2017)

If you search the internet (I like Google) you may be lucky and find a website with a calculator of sorts that may allow you to just plug in the numbers. It would probably be worth a try to see if such a thing exists. Otherwise, math is your friend.

Good luck,
Ted


----------



## Wreck™Wreck (Apr 2, 2017)

EmilioG said:


> I'm working on my finger plates and need a way to accurately measure the size of the V shaped slot.
> One will be .535" wide x .290" deep.  I need to mill the V's to within a few thou on one FP design.
> The other isn't so critical.
> 
> ...




Without an angle this will be difficult to calculate, .535 X .290 will encompass a nearly infinite number of angles. Using a specific angle makes the math far easier.


----------



## Tony Wells (Apr 2, 2017)

There is another presumption to consider. That is that the angle is actually symmetrical and the centerline (bisector) is perpendicular to the reference plane used to measure.


----------



## Charles Spencer (Apr 3, 2017)

Wreck™Wreck said:


> .535 X .290 will encompass a nearly infinite number of angles.



Not really.  Bisecting the angle yields two equal right triangles.  The angle of the right triangle can be calculated here:

http://www.csgnetwork.com/righttricalc.html

and doubled to get the actual angle.

Tony is right though.  Nothing in the information above indicates that the base of the angle is parallel to the surface.


----------



## EmilioG (Apr 3, 2017)

The V groove for one FP design will be 90°, with the above posted WxD.   As long as it's 90 degrees, it could be a bit wider/deeper.
I have a small Starrett V block that has the dimensions of .535 x .290 that I can use as reference.  As I make shallow cuts, I can measure each side of the V with a depth mic.

For the 2nd V groove?  From what I can measure, each side of the V should be .250" L
with a centerline depth of .20" x .390" w (on my second) V grooves FP.

I have a Starrett blade type. So, if I can somehow reference each side accurately, i.e. put the edge of the depth mic on each side
the same way?  What size gage pin should I use for this V groove dimension?


----------



## Cobra (Apr 3, 2017)

You can use a known pin for straight grooves or a ball for curves.
It is easiest if the pin or ball is a little smaller than the current groove.
I have attached a sketch for the calculation assuming that the groove is a sharp bottomed 45 degree cut.


----------



## Cobra (Apr 3, 2017)

Sorry, there should have been a plus sign in the formula not an equals.
New sketch attached.


----------



## EmilioG (Apr 3, 2017)

How would you accurately measure the above diagram with a depth mic, the middle of the pin? You would have to hit the pin dead center with a depth mic.
Getting repeat measurements is my concern here.


----------



## chips&more (Apr 3, 2017)

A standard depth micrometer has a flat end about 1/8" round (my sets) on the end of the measuring rod. It’s not hard at all to use in this application. Being dead center is not a concern…Dave


----------



## Cobra (Apr 3, 2017)

chips&more said:


> A standard depth micrometer has a flat end about 1/8" round (my sets) on the end of the measuring rod. It’s not hard at all to use in this application. Being dead center is not a concern…Dave



Mine is similar.  Have not had a problem repeating measurements. (Other than my normal ham-handed beginner variability!)


----------



## EmilioG (Apr 3, 2017)

I bought a V groove Melin cobalt cutter in 1/2". This is the DP-1 version for V grooves.  This will make the work a bit easier. I can get a good starting cut on center and drill the hole 
	

		
			
		

		
	





	

		
			
		

		
	
 for the bushing at the same time.  For the .390" wide x .20" d, I just have to bring the cutter down to that depth, .20 to get my .390 V.
The larger V on the 2nd FP will have to be finished on an angle.


----------



## Wreck™Wreck (Apr 3, 2017)

Charles Spencer said:


> Not really.  Bisecting the angle yields two equal right triangles.  The angle of the right triangle can be calculated here:
> 
> http://www.csgnetwork.com/righttricalc.html
> 
> ...


Exactly, no included angle specified nor an angle from a reference plane, for all we know the proposed block looks like so.


----------



## EmilioG (Apr 3, 2017)

It's a 90° V groove, not what is shown in your nice diagram.  I'm making two finger plates.  One of the V grooves is  .390" not .290.
I typed in the wrong number.  It is .390" W  x .20 d  Sorry about that.

The other V groove is .535" W . 90 degrees.  I guess you would have to see the drawing.
I worked out how I am going to machine this.


----------

