angled strait edge for cross slide dovetail
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  1. #1
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    Default angled strait edge for cross slide dovetail

    I am looking for some way to spot the guideding sides of cross slide dovetail and carriage of a SB 10L lathe for scrapeing. I purchased a "strait edge" from USSR EBAY and no big surprise it is actually a hardened ateel cutting blade, quite sharp but also warped by about 0.015", included angle of 61 degrees. I do not have a surface grinder so not sure if there is a decent way to make this into a strait edge? It was $40 + $30 shipping so not a huge loss to write off.

    I would like to find a tapered edge cast iron strait edge so I can maintain its flatness (or suitable piece of cast iron to start from). Some of the ones I have seen for sale seem pricy for a hobby but I may end up there anyway.

    I have also tried using the disassembled taper attachment as a strait edge, the dovetail on the bottom is long enough and appears in excellent shape. This seems to be workable but I have no way to verify its flatness and the the angle is not smaller than the dovetail like most tapered edge strait edges I have seen in books.

    I will attach a photo of the ebay listing so others can beware.
    Attached Thumbnails Attached Thumbnails strait-edge.jpg  

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    You might draw it, Hack it out on a mill, and have a local grinding shop finish grind it.
    Possible a local grinder guy could/would true up the USSR part.

    QT Op: purchased a "straight edge" from USSR EBAY and no big surprise it is actually a hardened ateel cutting blade, quite sharp but also warped by about 0.015"

    I was wondering if they (ussr straight edges.) were any good, thought about buying one a number of times.

    You might start a thread on them so the PM guys here would be informed about the USSR part.

  3. #3
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    You could post a WTB in the Commerce/Tools Parts and Accessories section.

    Denis Foster (dgfoster) has castings but you'd need to get it scraped.

    Ditto for Richard King. Castings but would need to be machined and scraped.

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    Scraping a new straight edge isn't a difficult job and offers good practice before messing with the machine you plan to do. If you don't have a mill big enough to do the initial cuts, You can likely find a local machine shop to do it. All you need to verify flatness during scraping is a granite surface plate at least as long as the surface you need to check (and for that matter, you'll need a straight edge as long as the way you need to check). Something like an 18"x24" surface plate can be found cheaper than the straight edge casting AND it's a good tool to have for checking parts and assemblies in the future. Finished straight edges that have been scraped and calibrated by someone who knows what they are doing won't be cheap, IMO. The DIY approach is the most economical, but you still need to make sure you do it right or you'll end up with a machine that might have pretty looking ways, but it doesn't move or hold position as it should and wears out fast (which are the very things you're trying to correct by scraping it!). You could have $1000 in tool costs by the time you are done, but that money invested into your machine makes a lasting improvement in it and you can always sell the scraping tools later to recoup part of the cost.

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    A friend of mine in Springfield has several scraped straight-edges. Maybe you could barter with him. Also Warren Jones who is a member here and lives in New Hampshire has several straight-edges. He also scrapes as he is my 3 time host, 4 time student. Also is a friend of the shop owner in Springfield is a machine rebuilder. Warren also has a friend in NH with a planer who could machine all parts of your lathe
    Barter with him for it's use. If you can wrench a machine apart, clean out a machine, paint a machine, etc. o is a machine rebuilder. Barter with him. Booth are Practical Machinist members too. If you only have 2 sides of a dove tail, why buy a SE? Hopefully you gave the Russian a negative on Ebay. You get what you pay for. Message me with your email and I will tell them

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    Common for such things NOT be intended to fit in the angle as a "master" controlling the angle

    The angle on the straight edge only has to be LESS than the piece being scraped - one surface at a time - to suit the mating part
    Last edited by johnoder; 10-07-2021 at 03:20 PM.

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    Like John says, you only need one side scraped and it should be something more acute than the dovetail. Scrape it flat, don't grind it flat. Ground surfaces are a bit of pita to print from. I've made them from durabar before. Its not fancy but once you get it flat, its amazingly stable

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    Perhaps the OP knows this, but I wouldn’t suggest scraping just the “guiding sides of a cross slide dovetail”. Would want to do the flats first.

    Second the idea of a bit of durabar. Lovely, messy, stable stuff.

    L7

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    Quote Originally Posted by lucky7 View Post
    Perhaps the OP knows this, but I wouldn’t suggest scraping just the “guiding sides of a cross slide dovetail”. Would want to do the flats first.

    Second the idea of a bit of durabar. Lovely, messy, stable stuff.

    L7
    Thanks to everyone for the suggestions. The Machine Tool Reconditioning book has details on what needs to be scraped in what order and the desired angle of the angle strait edge. I have made and scraped a strait edge from a piece of cast iron from an old paino so that was a good learning experience. I will look for some Durabar. I may try to straighten out this chunk of steel and lower the angle but it seems like it may be too hard to scrape and not sure if I have a way to soften it. I wonder if a big propane burner would get it hot enough. I have mostly scraped cast iron but I guess it is possible to hand scrape steel if it is not too hard?
    I don't mind spending some money on tools but not sure it is reasonable to spend a lot more on tools to rebuild a lathe than this 80 year old e lathe will ever be worth. I may make a little money using the lathe in the future but in reality it is more of a retirement hobby for me.

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    Regarding USSR straightedge:

    Quote Originally Posted by Gard View Post
    ... I may try to straighten out this chunk of steel and lower the angle but it seems like it may be too hard to scrape and not sure if I have a way to soften it. I wonder if a big propane burner would get it hot enough. ...
    I think the Russian straightedges are intended for stuff like scribing straight lines on things or checking flatness by holding them to an edge with light behind them, not for use as scraping masters. I wouldn't try to make it into a scraping master. Either use it for its intended purpose or sell it on to someone else. Trying to make soften it and then scrape it seems like a lot of work and will give a second class result. Just get some durabar.

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    Quote Originally Posted by jmkasunich View Post
    Regarding USSR straightedge:



    I think the Russian straightedges are intended for stuff like scribing straight lines on things or checking flatness by holding them to an edge with light behind them, not for use as scraping masters. I wouldn't try to make it into a scraping master. Either use it for its intended purpose or sell it on to someone else. Trying to make soften it and then scrape it seems like a lot of work and will give a second class result. Just get some durabar.
    I agree, there may very well be some russian strait edges that are a good deal and useable for scrapeing however the one I got I think is some kind of cutter, it is sharp enough to easily cut you if you are not careful and has a row of big holes for some sort of flat head bolts. I think some sellers assume any chunk of metal that has a strait edge on it is a precision grade machinist strait edge. There are some camelbacks out there now, shipping is pricey. There are some 3 sided strait edges but the flats appear to only come in about 1/4 inch from the edges, not sure what they would be used for, If I knew it was cast iron it might be a good thing to start from.

    I have seen the difficulty of checking flatness of ground and polished steel surfaces with ink. I often end up with a tenths indicator on a surface plate. But can a polished steel surface be used ok for spotting a cast iron surface that is being scraped flat?
    It does seem if I am going to make another strait edge it would be better to start with cast iron, is any particular grade of durabar better?

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    Scraped CI, and granite, seem to mark well. Polished, or nt so polished (ground smooth) steel tends to smear, I don't know why. If scraped, it works decently.

    I've had the dubious pleasure of scraping some steel, and did not enjoy it.


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