Problem Measuring Dovetail Parallel on Compound -- Scraping in the Compound
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    Default Problem Measuring Dovetail Parallel on Compound -- Scraping in the Compound

    I am working on scraping the compound on my EE and have run into a problem measuring for parallel on the dovetails of the compound. I am following Connley's book and following the sequence he recommends. The scraping itself seems to be going fine. But when I tried to check parallelism of the compound dovetails a standard mic is too thick at the frame/lock side so that it does not allow the anvil to make good full contact with a dowel pin in the dovetail. I have jury rigged a solution that seems to work. But, I am wondering if there is a better way to do this. Obviously a caliper will reach across the pins and make good contact, but there is no way I could rely on that measuring device for a tenths reading. It looks like I am within a tenth of parallel using this system.

    I do not like the fact that you have so many potential sources of error with the vise jaw on which is stacked the dowel pin and then measured with a tenths DTI on a surface gage. I have switched things around left to right etc. to try to make sure that errors are not adding up to throw me off. The vise jaw is parallel within a tenth. The dowel pins measure spot on the same.

    Please see pics.

    Suggestions?

    Denis
    Attached Thumbnails Attached Thumbnails dovetail-1.jpg   dovetail-2.jpg   dovetail-3.jpg  

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    Quote Originally Posted by dgfoster View Post
    I am working on scraping the compound on my EE and have run into a problem measuring for parallel on the dovetails of the compound. I am following Connley's book and following the sequence he recommends. The scraping itself seems to be going fine. But when I tried to check parallelism of the compound dovetails a standard mic is too thick at the frame/lock side so that it does not allow the anvil to make good full contact with a dowel pin in the dovetail. I have jury rigged a solution that seems to work. But, I am wondering if there is a better way to do this. Obviously a caliper will reach across the pins and make good contact, but there is no way I could rely on that measuring device for a tenths reading. It looks like I am within a tenth of parallel using this system.

    I do not like the fact that you have so many potential sources of error with the vise jaw on which is stacked the dowel pin and then measured with a tenths DTI on a surface gage. I have switched things around left to right etc. to try to make sure that errors are not adding up to throw me off. The vise jaw is parallel within a tenth. The dowel pins measure spot on the same.

    Please see pics.

    Suggestions?

    Denis
    .
    the side where the micrometer cannot get low enough put a gage block in front of it so micrometer can get low enough. use the same gage block by the one dowel pin on all measurement. you will have to use a bigger micrometer

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    Quote Originally Posted by DMF_TomB View Post
    .
    the side where the micrometer cannot get low enough put a gage block in front of it so micrometer can get low enough. use the same gage block by the one dowel pin on all measurement. you will have to use a bigger micrometer
    Yes, good idea. Right now I have a "hole" in my micrometers in the needed 3-4" range. I am working on finding a good mechanical mic in that range. There are many on eBay.

    Denis

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    Try using just the surface gage, if the dowel pins are a diameter and length that will allow.

    Push down the pins that can extend out of the bottom of the surface gage. See if the distance from the pin to the edge of the surface gage is greater that the radius of the dowel, and the dowel is long enough to span the pins.
    If it is you can use the surface gage to measure directly off the dowel- if it will clear the compound "above" the dowel.

    Put the upright arm of the gage closer to parallel so you can have the indicator reach the other dowel.
    You may want to rest the surface gage on the compound on a shim about 1/2 the diameter of the dowel as you get the indicator set in place to save some trial and error.


    To use- rest the pin end of the gage on the dowel with the pins holding the dowel in place.
    Rotate the surface gage around the dowel to get a direct reading with the indicator from the other dowel.

    Remember that the measurement is of both the sides of the angle. You cannot tell if the dovetail is parallel unless you know the "flat" is all in one plane. It does not look like you have started scraping the flat yet. Having more area it should not be as worn as the dovetail. I assume you are getting some preliminary measurements and will scrape the flat, then the angle.

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    I knocked together the following dingus:
    sany0007-002.jpgsany0008-002.jpg

    The pin at the aft end rides on one pin, the two legs stabilize and the DTI probe rides on the second pin. Sweep for minimum reading.

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    As you've shown, the surface plate/indicator check is preferred imo. While scraping, you can mic (with a parallel or sometimes the interchangeable anvil style will work) but the final judge is the indicator/surface plate.

    I prefer the tenths reading from an indicator as more reliable than the vernier scale on a micrometer. Yes there are a few sources of error, but if the top the parallel doesn't flicker a tenths indicator needle, and then the pins all indicate the same, imo you are good to go. If you thought the pins not being straight was a a source of error, you could change interchange them left to right and see if you got the same reading.

    As JR says, the horizontals of the dovetail must be parallel to the bottom first. Then scrape one dovetail flat, then when scraping the second do it iteratively with checks for parallel.

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    Thanks to all the above.

    Yes, I am aware of the need for both sides of the dovetail vee to be parallel and flat. As Connelly predicted, the horizontal portions of the compound wore "straight down." The headstock side wore down more than the tailstock side by almost a thou, but along the length I was surprised at how horizontal and straight they were. So, I am figuring there may need to be some minor correction of one of the angled dovetail surfaces after the horizontal surfaces are in. But it SHOULD be very little as they are parallel but just higher on one side than the other.. Time will tell.

    Denis

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    Quote Originally Posted by gernoff View Post
    I knocked together the following dingus:
    sany0007-002.jpgsany0008-002.jpg

    The pin at the aft end rides on one pin, the two legs stabilize and the DTI probe rides on the second pin. Sweep for minimum reading.
    Do you have nay pics of the device sitting in a dovetail as you set it up? Looks interesting.

    Denis

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    Ill snap a few shots tonight.

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    The Kingway type device also works for that sort of measurement.

    Here it is on a male dovetail with front side bearing, but it would also work for the reverse, you would bear on teh flat ways and check against the dovetail.


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    Here are a few more pics of the dovetail indicator:

    dsc00458.jpgdsc00461.jpgdsc00462.jpg

    Note that this is not an original idea. I copied it from someone, as I recall, I saw the idea on their youtube page.

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    Quote Originally Posted by gernoff View Post
    Here are a few more pics of the dovetail indicator:

    dsc00458.jpgdsc00461.jpgdsc00462.jpg

    Note that this is not an original idea. I copied it from someone, as I recall, I saw the idea on their youtube page.
    I see. Nice. A picture is worth a whole lot of words. Thanks for going to the trouble of making the photos and posting them. I read you description above and understand the aft pin is riding on a pin which is not visible. Looks like a good setup and would make the desired measurement.

    Denis

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    Yep, there is a pin in the other dovetail. You then simply slide the unit back and forth on the feet to get a minimum reading which tells you your perpendicular.

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    Look for Nick Mueller's YouTube videos on scraping. He shows what he does, talks about why and shows this kind of dovetail checking gage.

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    Click! Now I remember who I copied....

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    Quote Originally Posted by TGTool View Post
    Look for Nick Mueller's YouTube videos on scraping. He shows what he does, talks about why and shows this kind of dovetail checking gage.
    Thanks, TGTool. I think I found the one you are referring to. Scraping in a lathe's top slide (with tapered gib) - YouTube

    It is a very nice and informative video. I will add my thanks to Nick for making and publishing that helpful video. It does show his, as he dubs it, "dovetail-o-meter" at about 6 mins 10 sec into the video.

    Denis

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    Saw this thread yesterday and wanted to post pictures of my dovetail-o-meter (didn't find a more stupid name), but I couldn't find good ones from all sides.

    I only have this side view:
    pc111323.jpg

    That reminds me, that I wanted to make a video about the dovetail-o-meter showing it in detail ...

    Nice to see my idea copied!


    Nick

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    Here's one I made a few years ago.






    The end of the fixed point is a ball bearing press fit in a bore.

    The tool can be used for the style dovetails as in your example; but the reference pins have to be large enough for the flats of the gage to ride on

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    Quote Originally Posted by Nick Mueller View Post
    Saw this thread yesterday and wanted to post pictures of my dovetail-o-meter (didn't find a more stupid name), but I couldn't find good ones from all sides.

    I only have this side view:
    pc111323.jpg

    That reminds me, that I wanted to make a video about the dovetail-o-meter showing it in detail ...

    Nice to see my idea copied!


    Nick
    So, Nick, the one thing I do not understand is that you go to the trouble of making a high quality video showing the device in actual use including the meter's construction. Then, for no personal gain, you put the video on YouTube. And now you are apologizing for having only one good still image of the device?! I have an idea there will b e a few more copies made.

    Have you made other instruments to solve measuring problems encountered in lathe and mill scraping?

    Denis

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    Quote Originally Posted by stephen thomas View Post
    Here's one I made a few years ago.






    The end of the fixed point is a ball bearing press fit in a bore.

    The tool can be used for the style dovetails as in your example; but the reference pins have to be large enough for the flats of the gage to ride on
    Thanks, Stephen. That looks like a pretty robust tool and is another way to skin a cat. Having pins large enough for the flats to ride on in my application would require pins that would need to be of a greater than ideal diameter imo. BUT, it looks like your device could be used with the size pins I show in the OP, if your tool underwent a simple modification. On one end two slightly under-size 1/4" through holes could be drilled vertically locating one to the "right" and the other to the "left" of the bar that runs horizontally and is the beam of the tool. Lightly press in a piece of drill rod with a rounded foot-end in each hole. Do the same for one piece of drill rod on the other end of the tool. When the newly-installed three feet might get in the way, press them out. The pressed-in nature of the feet allows for easy height adjustment of the tool as well. The adjustable feet are not quite as elegant a solution as the height adjustment Nick incorporates, but is quick and dirty. Rather than a press fit, the typical shallow diagonal cut into the holes with clamping screws could be made and would be a better design.

    Thank you for posting. I like the simplicity of using a round beam as construction of such a device is relatively easy. The modification I am proposing is results in a tool combining your and Nick's features.

    Denis


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