Dovetail myth regarding pin dimensions.

[Parlo]

I'd just like to mention that the dovetail might NOT be critically dimensioned by dowels packed in the socket,
if the those surfaces (big-end and slant side) aren't those that determine the position of the fill item; you don't
care about the depth (and big-end face) of the socket if the dovetail mates with contact of the outer/upper face of
the socket and the sidewall.

Part of the idea was to allow the first part to be made to a loose tolerance. This first part would then be measured over the pins to give a dimension to make the mating part. This will only work with a specific diameter pin to keep the dimension the same for both the male and female parts.

 

[Bruce Billett]

Thank you, but O fear that makes no sense?

b

 

[Parlo]

Thank you, but O fear that makes no sense?

b

Sorry I could not explain clearer.
Perhaps knowing that there are quite a few calculations to arrive at two different dimensions to get a mating pair if no regard is given to the pin diameter. Usually it is suggested that a pin roughly half the dovetail deep is used with two separate sets of calculations, one for each part with both needing to be accurately machined.

 

[Bruce Billett]

So if I have two blocks I wish to cut dovetails into, one male one female. I register one and cut the female noting start and stop positions of the cuts for the actual dovetail. Remove the first block and replace setting the same registering it the same as the first. Allowing for the clearance offsets, This will not give me the required parts?
Yes I understand this is a very simplistic way to look at this issue, but I am a simplistic person.
I might see the need for using the pins etc. if you are trying to match an existing part that has seem some wear and the angle is now off by 1/2 deg. or something like that, but if I can determine the angle, depth of cut , and width of the throat, why doesn't that work?
Remember, simple guy!
And thank you for trying to answer this one.
b

 

[Parlo]

So if I have two blocks I wish to cut dovetails into, one male one female. I register one and cut the female noting start and stop positions of the cuts for the actual dovetail. Remove the first block and replace setting the same registering it the same as the first. Allowing for the clearance offsets, This will not give me the required parts?
Yes I understand this is a very simplistic way to look at this issue, but I am a simplistic person.
I might see the need for using the pins etc. if you are trying to match an existing part that has seem some wear and the angle is now off by 1/2 deg. or something like that, but if I can determine the angle, depth of cut , and width of the throat, why doesn't that work?
Remember, simple guy!
And thank you for trying to answer this one.
b

Great question, let's see if I can answer helpfully.
Take the first block and cut the female dovetail as you describe. Let's say that the Y axis was moved +/- 10mm either side of the centreline.
This will make a female dovetail with the two internal sharp corners 20mm bigger than the cutter diameter measured over its external sharp corners.
Are you assuming that the Y offset for the male part will be +/- ( 10mm + the cutter diameter )?

Please follow this link to see the difference when using arbitary size pins compared to the calculated size pins.

DOVETAILS.pdf

drive.google.com

 

[Bruce Billett]

Great question, let's see if I can answer helpfully.
Take the first block and cut the female dovetail as you describe. Let's say that the Y axis was moved +/- 10mm either side of the centreline.
This will make a female dovetail with the two internal sharp corners 20mm bigger than the cutter diameter measured over its external sharp corners.
Are you assuming that the Y offset for the male part will be +/- ( 10mm + the cutter diameter )?

Please follow this link to see the difference when using arbitary size pins compared to the calculated size pins.

DOVETAILS.pdf

drive.google.com

If I follow what you are saying, then yes. One would also have to take into account some sort of clearance. Lets say .01 mm or some such.
Your drawing would seem to by default include some sort of ANTI-clearance, and the parts would never slide together?

 

[Parlo]

If I follow what you are saying, then yes. One would also have to take into account some sort of clearance. Lets say .01 mm or some such.
Your drawing would seem to by default include some sort of ANTI-clearance, and the parts would never slide together?

It would seem obvious that using the 10mm + cutter diameter would give a size for size fit. Then taking a bit more off would give you the fit you need.

Unfortunately this is not the case. The external corner of the cutter will cut the internal corner of the female dovetail. However the corner on the cutter does not cut the mating external corner on the male part. There needs to be a further calculation that includes the depth of the dovetail to arrive at the correct offset.

Here is a sketch to show how this works.

 

[Bruce Billett]

I see what you mean but I don't think you understand what I said.
cut the first cut as you show. The mating cut would use the first cut CL offset y the cutter DIA. Plus some clearance, say 0.5 mm. Giving you a total clearance of 1mm. Not another 10 mm, that would give you a total of 20 mm clearance. I suspect that would be a bit much no matter how big the parts are?
The operative part here is using the cutters CL as your reference, not the CL of the part.
How one gets that measurement is a different matter, I suspect knowing the size of the cutter, depth of the cut and the width of the throat, that should just fall out on it's own?
Do I make any sense?

 

[Parlo]

The drawing used the centre of the part as the Y datum.
All movements on the Y axis are to the cutter centre from the part centre. ( 10mm either side of the part center ). This keeps the dovetail in the centre of the part.
The mating cut example moved the Y axis by the original Y offset (10mm ) + the cutter diameter. It shows a 5.8mm error if this formula is used.

 

[Bruce Billett]

Well this is true, but that is not what I said, Least not what I thought I said.
My intention was to cut the first part.
Noting the location of the "cutting" Y axis. Not the part center, but that can certainly be done just adds to the mess.
Then one would simply take the "cutting" Y axis add the cutting Dia. plus/ minus (inside or outside cutting) what ever you wanted for clearance, and there is the CL for the next cut on the new part.
Now granted. for making a new part to fit an existing dovetail on a machine you don't want to put in mill just to the tool CL Yea you measure from the CL of the part. etc.
I still do not understand, and the reason for the initial question, Why the pins?
Depth of cut give you everything you need for one side? The other side falls out when you know the width of the throat? Is this not correct?