Dovetail myth regarding pin dimensions.

twhite said:


Cutting oil is my blood.
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Excellent link, thankyou.
It proves my simple theory of using a rod diameter of 1/3rd of the dovetail depth & also shows how hard it can be.

For the female - I entered S1 as 10 and a rod diameter of 3 which gave a theoretical sharp corner distance of 18.196.
For the male - I entered S2 as 16 ( S1 + 2 rollers ) rod diameter again as 3 with a height H of 9 ( 3x rod dia ). and gave the same distance of 18.196.
Proving that only a rod diameter of 1/3 the depth gives the same dimension over the correct size rollers for both parts.

It also shows this formulae if you prefer...

dovetail formula1.jpg
 
Well boys and girls, Just thought I would write and ask if you knew where I could get something like a 26 deg. dovetail bit? Or what to do if one can not find such a creature?
Tried the pins, didn't work, was pretty sure that was just me. Tried my way, Better but still....? What? Hey, wait. Those angles are NOT the same! My bit is 30 deg. ( I assume standard ) My tool holder is 26 Deg. ? Well that ain't right! No wonder nothing fits. With that said... My only real option is to toss this Chinese crap and buy American?
Thanks for any thoughts. Must be a metric 30 Deg.?
 
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WobblyHand said:
Most twist drills are 118 or 135 degrees at the tip. How about a picture or two so we can see what you are talking about?
Here's a page for some dovetail cutters showing the angles. https://ktoolinc.com/ktool/30-15-dovetail-cutters
Here's a link to Harvey Tools, they have quite a few sizes to choose from. https://www.harveytool.com/products/dovetail-cutters
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I'm sorry I should have recapped. I am trying to make a female dovetail to match my quick change tool post. The issue was trying to match a dovetail and the complicated way to determine where to cut the part so they matched the existing dovetail. Matching pins sized to the existing dovetail. and all the math involved. Just wanted to know why when it seemed to me that knowing the angle, depth, and width of the throat one could simply cut the dovetail to match. Silly me, I assumed that the world used a 30 deg. angle for most all dovetails. Never guessing that the Chinese were using the Wentworth angle scale where it is NOT 30 deg. but 26 deg.? I found this out buy going through all the bother of the pins Etc. only to not even come close. Thinking this is me not truly understanding what all that was about, I tried my way. It was a much closer fit, but something wasn't right? I looked down the dovetail as it SORT of fit and noticed that the two facing surfaces were not even close to parallel? What? Only thing I could think of was that the angles were different. So I pulled out my rough scale and sure enough NOPE!! So I pulled out my rather accurate scale and what I cut was in fact 30 deg. which is what the cutter said. So next I measured the angel on the tool post thinking it might be off. Well that came in at 26 Deg. So now my quest is two fold. 1) find out if it's the drugs, and it is supposed to be like this. Or 2) find out how to correct this, either by buying American made tool post and holders that are 30 Deg. or find a 26 Deg. bit. I'm not finding a 26 Deg. bit? Don't relish the idea of paying >$100.00 for a custom bit. AND I have NO where near the ability's to grind my own!
So I'm fishing for thoughts on the matter.
Thank you.
 
If your QCTP is non-standard, maybe you should be looking to replace it, so you can get standard tool holders? Just a thought. I don't know what makes sense for you. It would stink to buy this odd cutter and still run into issues down the road with tool holders. 26 degrees just doesn't seem right...
 
WobblyHand said:
If your QCTP is non-standard, maybe you should be looking to replace it, so you can get standard tool holders? Just a thought. I don't know what makes sense for you. It would stink to buy this odd cutter and still run into issues down the road with tool holders. 26 degrees just doesn't seem right...
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Yea, that's what I thought you (someone) would say. The custom tool bit seemed a bit much.
I do find it curious that I can buy tool holders on line that match this. I suspect from the same outfit, So I doubt it's a mistake as it were.
Thanks again for the help.

b
 
Bruce Billett said:
Yea, that's what I thought you (someone) would say. The custom tool bit seemed a bit much.
I do find it curious that I can buy tool holders on line that match this. I suspect from the same outfit, So I doubt it's a mistake as it were.
Thanks again for the help.

b
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It seems strange that a manufacturer would use an atypical dovetail angle, requiring custom cutters. The only reason that I could think of for such a practice would be to prevent customers from buying from competitors. Even at that, it would be simpler to change the distance between the dovetails.

Can you provide a link to your source for the QCTP?
 
My formula was for 30 degrees only.
For any angle dovetail.
( Cosine of angle divided by [ 1 + cosine of angle ] ) x depth of dovetail = common pin diameter.
 
Parlo said:
My formula was for 30 degrees only.
For any angle dovetail.
( Cosine of angle divided by [ 1 + cosine of angle ] ) x depth of dovetail = common pin diameter.
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And perhaps it was not totally my ignorance? Unless one wants to count my not checking the angle first, Just thought that was a given.
 
RJSakowski said:
It seems strange that a manufacturer would use an atypical dovetail angle, requiring custom cutters. The only reason that I could think of for such a practice would be to prevent customers from buying from competitors. Even at that, it would be simpler to change the distance between the dovetails.

Can you provide a link to your source for the QCTP?
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Sorry, can't provide a link, It is what came with the tool when I opened the box. I did buy an "extra" set of holders after the fact, This might explain why I had to buy and return 2 sets before getting the right ones. I had thought in those cases it was just me not really understanding the nomenclature, suppose that really is the case but....?
I have dealt with cases like this before but only on VERY expensive tool sets where they don't want anyone messing with their I.P. For some odd reason, I don't think that is the case here?
 
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