Dovetail Day !

Coming right along with the tool post build , and today I started cutting the dovetails .The ones on the post are a couple of thousandths different , but I don't think that's going to be a problem . I have one tool holder cut (and 2 more slotted) and it's an easy sliding fit on both of the post dovetails . I'm wondering though just how much clearance I should allow . I know they don't need to be sloppy loose , but how much clearance is enough ? And where to measure ? A thousandth between the dovetail faces is a whole lot different than the same between the flat faces ...

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I've made three tools with dovetails, which means I know less about them than before I started and thought I did know something. They are a lathe milling vise, a boring head and a centering indicator holder that fits the ways of a small lathe.

The Machine Tool Reconditioning book describes measuring between cylinders pressed into the dovetail angles. You can use geometry to locate the angled faces, or just compare one with another to size them identically, then make the mating parts to fit. If you use drill bit shanks remember that they are undersized. Ground drill rod works well and may be useful for gaging or fixturing other jobs. If they are cut longer than the dovetail you can secure them in place with rubber bands.

At first I tediously machined them to a press fit that became looser with filing and stoning and use but leaving a gap for a spacer or gib is easier and allows adjusting for errors. Once two parts almost telescope together you can lightly bevel an edge, press them together and use the marked line of contact on the bevel to know how much more to remove, by measurement or short trial cuts.

I suppose the answer to how much clearance is: less than the throw of the cam that locks them together. Start small and try it, you can always remove more metal.

The trigonometric relationships of 45 and 60/30 triangles are worth remembering, because those angles are so common.

I took a semester of college geometry in night school in which all the homework problems involved 45 and 30/60 triangles, so we practiced and memorized the relationships in our heads without needing a calculator. Then I applied it at work on phase angles in aerospace digital radio modulation schemes where it fit perfectly, again without a calculator. I did have to learn to think of angles in radians, as 2*pi is a full circle, pi/2 is a quadrant etc, but we didn't have to calculate their numerical values in degrees.

And where to measure ? A thousandth between the dovetail faces is a whole lot different than the same between the flat faces ...

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For a 60 degree angle "a whole lot" is the ratio of 1 to 1.732, the square root of 3.

Someone posted the measurements using 3/16" pins , I used drill shanks knowing they were undersize . Got me pretty damn close , close enough I could fine tune them . This is my 3rd project with dovetails , and the first that didn't use a gib ... The first was a ball turning device for the lathe , the second was a micrometer adjust boring head .

These ended up with about .008-.010 between the faces with the cam locked , I'm happy with that .

I bookmarked that page , that's some very useful information ! I always sucked at "higher math" but geometry/basic trig was pretty easy to understand .

"exabell equals two pie eff ell" ... 'Bout 50 years ago I could calculate phase angle relationships and vectors ... with a slipstick .

That ratio is very useful ... and worth remembering . IIRC that ratio for a 45° is 1 to 1.414 .

That ratio is very useful ... and worth remembering . IIRC that ratio for a 45° is 1 to 1.414 .

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I've found many uses for knowing the sides of common triangles, such as figuring the lengths of rafters and fascia boards when I'm in a lumber store and remember that I need to replace one. Having a (long) list of memorized numbers helps when you need a new padlock code or PIN number you will remember. In programming 86,400 seconds per day comes up often. BTW spreadsheets count time in units of 1.xx day. If you memorize reciprocals you can use them for mental division.

I was testing some terminal equipment on a military satellite link for suspected errors when the senior engineer asked me how long I'd be tying up the channel. Since I know that 1/24 = 0.041666... I mentally divided 10 million bits by 2400 bits per second to get 4166.7 seconds, or 1 hour (3600 sec), 9 minutes (540 sec) and 26.7 seconds. He couldn't set up the problem on his calculator to check me.

There were no errors, the reported issue was somewhere on the ground in Iraq. It was weird to realize the signal had been floating through nothingness for a very noticeable time interval, yet came back perfectly intact.

Mostly I used the memorized reciprocals to convert between frequency and period for engineers who couldn't without a calculator. My old slide rule training to figure the mantissa and exponent separately was very helpful.

1/24" is how far the crossfeed on my small lathe advances per turn. I graduated a dial for it with a gap between 0 and 0.040".

Being a lowly non-EE lab tech in a think tank full of Ph.Ds I had to earn respect and opportunities to design circuits however I could. Fortunately they usually kept to their theories and delegated the dirty job of creating the hardware. They gave me that problem because I had a reputation for solving hard ones. The techs' running joke was that our job was cleaning the bottom of the think tank.

Ideally they should be so at the right tightness the locking lever swings back to somewhere between 4 and 5 o'clock. Most of my dovetail tool holders do just that, but a cheap batch I bought off Ebay one time let the handle clock all the way around to about 8 o'clock. That gets in the way and hits the safety shield when turning close to the chuck. When I use one of those tool holders I take the handle off so its out of the way. Kind defeats quick change. LOL. I may make a shim for those someday. I probably won't.

You may not be aware, but with many dovetail QCTPs you can weld round rods to your tool holders instead of cutting a dovetail. I don't suggest making a bunch of those, but if you need to make a special low usage holder and time is of the essence it is a quick and dirty option.

Ideally they should be so at the right tightness the locking lever swings back to somewhere between 4 and 5 o'clock. Most of my dovetail tool holders do just that, but a cheap batch I bought off Ebay one time let the handle clock all the way around to about 8 o'clock. That gets in the way and hits the safety shield when turning close to the chuck. When I use one of those tool holders I take the handle off so its out of the way. Kind defeats quick change. LOL. I may make a shim for those someday. I probably won't.

You may not be aware, but with many dovetail QCTPs you can weld round rods to your tool holders instead of cutting a dovetail. I don't suggest making a bunch of those, but if you need to make a special low usage holder and time is of the essence it is a quick and dirty option.

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The handle on my Multifix is 12 point splined to drop onto a tall hex head locking shaft, so it can be positioned out of the way however the toolpost may be rotated, or removed for safety when the carriage is beside the chuck for drilling. You could cut down a cheap offset box or swivel socket wrench since the torque is low.

But that might take me five minutes. Like making a shim for those other tool holders. I could do a lot of things... LOL

I've got a big "variable jaw shape - Its not fractal") vise I need to make a handle for first. I've had the vise for over a year. ;^)

I need to start fixing or start scrapping the Hurco mill too...

On 8/27/2022 2:37 PM, Jim Wilkins wrote: ...

But that might take me five minutes. Like making a shim for those other tool holders. I could do a lot of things... LOL

I've got a big "variable jaw shape - Its not fractal") vise I need to make a handle for first. I've had the vise for over a year. ;^)

I need to start fixing or start scrapping the Hurco mill too...

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It's very nice of you to be making parts for Snag.

Actually it just came to me. The simplest (maybe) solution would be to drill and tap a second hole for the handle. As long as the handle doesn't get in the way of the tail stock its all good. Or maybe just see if I can throw a washer in there. This is the CAM lever. Not the hold down lever, so I don't think a washer will do the trick. This lathe (my main lathe) came new with the QCTP preinstalled, and I don't think I have ever taken it off. The 4 way was in the tool box with the wrenches and change gears.

Right now the swing on the locking cam is about 1/2 turn locked to locked . I think I'm going to put more than one threaded hole for the lever . I'm actually getting pretty good at cutting these . I have one more blank to cut the dovetail then I'll do the bit slots then holes and threads . Cross that last dovetail off the list , just finished it .

The thing is , I can put the handle anywhere I like ... I get like 270° rotation or as little as around 90° if I swap faces. I may or may not put more than one hole in it .