Dovetail myth regarding pin dimensions.

Gentlemen. Thank you for your indulgence on this issue!!
My take away on this is that for precision, I will need to use the pin method!
I am doing this in my home shop using an RJ round column mill, and a Chinese mini lathe.
So while the pin method is useful for micro scale work, my tooling is better suited to furlong scale work!!
I spent a lot of my life in the micron and sub micron world where we found we needed a professional scraper to be flown in to do some of the work. Mind you I was only his escort and get a ---- for me. But gained an apparition for the fine craft end of it all. Measurements on the scale of a 1/2 wave length of light? Really!!! I understand HOW that works now, and the results of the work, but WOW!!!
So thank you for your indulgence.

b
 
I have a follow on if I will be allowed. I kind of understand using the pins to get untold of accuracy to fit the dovetails together. Then I was playing with my lathe bed and all of a sudden it dawned on me "THE GIBBS!!!" These allow for adjustments due to wear, misfiting and all sort of other evils. As these are adjustable so the accuracy isn't that big of a deal, is it?
Most all of the "professional" dovetails I have seen have Gibbs in them.
Once again, just trying to make sure I really understand what is happening.
And Thanks again.
 
Bruce Billett said:
I have a follow on if I will be allowed. I kind of understand using the pins to get untold of accuracy to fit the dovetails together. Then I was playing with my lathe bed and all of a sudden it dawned on me "THE GIBBS!!!" These allow for adjustments due to wear, misfiting and all sort of other evils. As these are adjustable so the accuracy isn't that big of a deal, is it?
Most all of the "professional" dovetails I have seen have Gibbs in them.
Once again, just trying to make sure I really understand what is happening.
And Thanks again.
Click to expand...
The pins are just a means to measure the size. Provision for a gib strip can easily be added to the nominal dimension.
 
Yea, I get that. But a good deal of the above conversation revolved around getting the dovetail size VERY accurate. Once you throw a gib in there, doesn't kind of make getting the accuracy kind of moot?
Any inaccuracy say 2-5 thousandths would be taken up by adjusting the gib?
 
Bruce Billett said:
Yea, I get that. But a good deal of the above conversation revolved around getting the dovetail size VERY accurate. Once you throw a gib in there, doesn't kind of make getting the accuracy kind of moot?
Any inaccuracy say 2-5 thousandths would be taken up by adjusting the gib?
Click to expand...
The conversation was about how to measure the feature.
 
Bruce Billett said:
So I need a different conversation to understand WHY I want to measure the feature?
Strikes me as they are hand in hand?
Click to expand...
101. Whatever you make, you need to measure it, that is why you want to measure the part.
 
Parlo said:
101. Whatever you make, you need to measure it, that is why you want to measure the part.
Click to expand...
Well this is certainly true!
Making and measuring a part in the micron range can be useful The life span of two moving parts with that sort of accuracy is silly as temp variations alone render it useless, even in a very tightly controlled temp. and humidity environment. Not to mention wear and tear on the parts over a relatively short time. Now factor in the pride of doing something like that. Haven fallen flat on my face a (VERY) few times making and absolutely perfect part, well that is priceless!!! Especially if you do it at home, (I never have, only at work).
So to me at least, Getting VERY close is useful! But when one weighs in cost, time, finding a truly skilled shop willing to do the work, makes building in adjustability as an alternative becomes VERY viable in a most cases.
Perhaps this is the wrong forum to ask this question, however as a newbie, teaching myself, all this stuff, I like to understand the why of some things.
Should this be the wrong forum to ask this sort of question in, I apologize, beg your indulgence and if I may, ask to be directed to the proper forum.
Thank you.
 
Bruce Billett said:
Well this is certainly true!
Making and measuring a part in the micron range can be useful The life span of two moving parts with that sort of accuracy is silly as temp variations alone render it useless, even in a very tightly controlled temp. and humidity environment. Not to mention wear and tear on the parts over a relatively short time. Now factor in the pride of doing something like that. Haven fallen flat on my face a (VERY) few times making and absolutely perfect part, well that is priceless!!! Especially if you do it at home, (I never have, only at work).
So to me at least, Getting VERY close is useful! But when one weighs in cost, time, finding a truly skilled shop willing to do the work, makes building in adjustability as an alternative becomes VERY viable in a most cases.
Perhaps this is the wrong forum to ask this question, however as a newbie, teaching myself, all this stuff, I like to understand the why of some things.
Should this be the wrong forum to ask this sort of question in, I apologize, beg your indulgence and if I may, ask to be directed to the proper forum.
Thank you.
Click to expand...
Measuring a dovetail that has a gib in it, is perhaps not as critical as one without.

If making a tool holder like for a lathe, it's pretty important to get the dovetail dimensions correct. If the dovetail is too large (wide), the locking mechanism will rotate a lot more and be difficult to use. It doesn't take much error (a few thousandths of an inch) for that locking mechanism to swing an additional quarter of a turn or more. I have some tool holders that were made too large, and they are annoyingly different to use, compared to the rest. Generally one wants the tooling to lock up or tighten in a similar fashion. It's no fun if the locking lever is pointing into the work piece, rather than at say 4:30PM.

Using pins for the measurement is both convenient and repeatable. Just one of the many techniques we have to learn in order to get decent results. Cutting a dovetail to fit correctly isn't that difficult, if one uses these techniques. If a machining novice like me can do it, so can anyone else. If you want to try to invent a new technique, by all means, give it a try. There's many ways to solve a machining problem, but some ways are much easier than others. I try to learn the easy ways first, and only branch out if they are not working for me. Or I ask a bunch of questions here. I find I learn a lot by trying things just to see what happens. Other times, I'm perhaps intimidated by an operation and ask (sometimes) way too many questions. Either way, I find this is a tolerant place to ask questions and to learn. So perhaps rephrase your question and lets try again.

I think this spot is as good as any to continue your questions.

What is your question? What is the problem or issue you are trying to solve? This can give us some context to properly approach the question.
Even with your tooling, you are not in the furlongs domain - thousandths matter for somethings, like lathe QCTP dovetail fit, or tapers, interference fits, etc.
 
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