restoring an abused surface plate
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  1. #1
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    Default restoring an abused surface plate

    Acquired this surface plate, it's a cast iron B&S 10x15.

    Someone apparently used it as an anvil or for peening or punching out something. It's not cracked and I'd like to restore it's flatness. Should I:
    1) lightly surface grind it, then scrape it flat
    2) stone it and then scrape it flat
    3) scrape it flat
    4) check it for raised burrs/ridges/etc., stone those off, blue a known "good" granite plate, print this plate against it, proceed with 1, 2 or 3 depending on the result

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    The answer depends on how deep the existing damage is. Looks to me (deep dings?) like you'll want to machine or grind (Blanchard??) it flat before taking on the task of scraping. That said, for a tiny fraction of the time / cost involved you can buy a small granite surface plate. If you just want the exercise and practice in patience, scrape away

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    I have a good granite plate to blue up and use as a reference. Since the inspection papers that accompany the cheap chink plates are basically bogus, I don't see buying one as being a viable alternative to fixing this plate.

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    I purchased an abused surface plate, but My purpose was for a weldment table... flat enough.
    given the cost of new plates finding other uses for old ones, and getting on with jobs is more important to me, YMMV.

    However, Id start with 4, at least you'll have an idea of how bad it is then.
    Probably then you'll end up with option 1, which would probably be fastest anyway assuming you have access to a large enough SG.

    Dave

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    Option 5:- get a Biax

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    How you go about reconditioning this plate depends on your use of it. You can hand scrape the thing to lab master quality if you go about it right.

    If you wish for a flat surface for simple not-too-accurate work a Blanchard grind is about all you need to do.

    If you wish to restore the plate to its original glory, you'll have to conduct a survey and maybe do some preliminary stuff before you get into scraping.

    First, examine the plate for weld arcs etc. Sudden local heating of cast iron creates hard spots a scraper will only glide over. If the plate has been subjected to heat, it needs to be heat soaked to above 1200F and allowed to cool slowly. Build a bon fire about 3X bigger than the plate in a safe place for fires. When the fire burns down to coals bury the plate in them and cover the top with more coals. Cover the coals with a couple of layers of batt house insulation. Leave the plate in place for a couple of days until it's cool.

    Surface grind, machine, what ever to clean up the defects. Round up scraping equipment and go to work. It will take you several days to scrape the plate flat to a good 8 spots per square inch. By the time you get done you will be stiff and sore and probably sorry you started.

    Work clean. Use a shop vac to pick up scraping swarf. Handle the plate in a way that protects it from heat input. There's lots more to know but this is just the beginning. I'm sure others will chime in.

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    I'd do #4 then #3. I hate scraping on ground surfaces. A Biax handles it a bit better.

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    The primary purpose of a project like that is to justify the acquisition of a Biax.

    It sounds like you are interested in developing scraping skills at the same time producing a useful tool.

    Scraping is most useful for a.) metal removal and refinement in restoring machine tool ways, surfaces, and alignments, and b.) generating or restoring high quality inspections tools.

    If you are developing the skill set and tool kit with an eye to machine restoration, get the Biax, and finish scrape gages or where appropriate by hand. Athough I think there is great merit to learning to shovel off metal by hand methods, too, when roughing down. The biggest inhibition I see to progress by new practitioners is a failure to aggressively remove the "bad" metal with efficiency and abandon before settling in to a finish scraping routine. A Biax speeds the process along when roughing down machine ways that are impractical or would be time consuming to set up to machine first. It is also an excellent tool for removing larger amounts of metal on an abused gage, though the gage may be more conveniently machined first if good machines are available.

    If you are going to hand scrape it, plane it off first on a good shaper or fly cut it with a sharp tool on the mill. If you have a biax, grinding will get you closer, then light all over with biax 'til it spots flat, then finish by hand. I hate handscraping a ground surface though sharp carbide tools make it easier to get started. But with a biax, it is not much an issue.

    smt

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    If grinding is all that's available before scraping, dress the wheel very agressively and don't go for a mirror finish. That gives the work enough "bite" to stop the scraper blade from sliding.

    My shaper is only 10"x10", my surface grinder is 8"x24" and my mill is a basket case waiting for a rebuild. The grinder sometimes gets used where a flycutter would be more appropriate

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    Okay, so the plate is clean, with no large perceptable burrs, ridges, weld arcs, etc. Should I check the depth of the dings with a straightedge and a fealer guage? What level of surface damage would warrent a cleanup grind, flycut, etc. before scraping?

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    "What level of surface damage would warrent a cleanup grind, flycut, etc. before scraping?"

    The answer depends on the time and energy you are willing to apply and is different for power scraping and hand scraping. .003" is easy to do by hand. So is .005" but it will take more time and sweat. Even .015" is doable by hand if you are a glutton for punishment and very dedicated. A strong man with a big, sharp scraper can take a gray cast iron surface down .0005" per pass. The rest of us succumb to fatigue, boredom, and lack of confidence so progress is much slower.

    Double check for burrs and ridges with a stone rubbed over the whole surface. Use a small surface gage that will allow you to maneuver on the unblemished parts of the plate and hang a test indicator into the pits. The deepest pit will tell you how far you have to go, at minimum. It would be very hard to get the same measurements with a straight edge and feeler gages. But, once you decide how to proceed, the measurements are not important at all.

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    A plate that size I would chuck in a lathe and take a face cut on it. No biggie on a 24" lathe with a 16" chuck. Grip it gently so you don't deform it. What ever shape you cut it to will have to be scraped off. I know just the local guy and he owes me.

    You see how a loose framework of exchanged favors soon morphs to a system of debts and obligations.

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    Quote Originally Posted by Forrest Addy View Post
    A plate that size I would chuck in a lathe and take a face cut on it. No biggie on a 24" lathe with a 16" chuck. Grip it gently so you don't deform it. What ever shape you cut it to will have to be scraped off. I know just the local guy and he owes me.

    You see how a loose framework of exchanged favors soon morphs to a system of debts and obligations.
    Debts and obligations - make the world go round, no ?

    I'm curious as to what you would see as the respective merits of facing in a lathe vs flycutting on a mill, for this particular endeavour?

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    Quote Originally Posted by Forrest Addy View Post
    A plate that size I would chuck in a lathe and take a face cut on it. No biggie on a 24" lathe with a 16" chuck. Grip it gently so you don't deform it. What ever shape you cut it to will have to be scraped off. I know just the local guy and he owes me.

    You see how a loose framework of exchanged favors soon morphs to a system of debts and obligations.
    Debts and obligations - make the world go round, no ?

    I'm curious as to what you would see as the respective merits of facing in a lathe vs flycutting on a mill, for this particular endeavour?

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    Quote Originally Posted by Troup View Post
    Debts and obligations - make the world go round, no ?

    I'm curious as to what you would see as the respective merits of facing in a lathe vs flycutting on a mill, for this particular endeavour?
    For one thing, quicker. It's hard to clamp a surface plate on a mill table without distorting it. You need a jack under each clamp unless you pinch it by an edge in a lath chuck clamped to the table. Also flycutting involves two edge crossings per spindle revolution, tool drag on the back side of the sweep, and chip recutting if they are not vacuumed up as generated. All that means shortened tool life and a dull tool makes a stressed finish.

    For another, in the usual R8 spindle with no positive drive a fly cutter with just over a 5" radius can be easily slipped even on light cuts. A fly cutter on a real mill spindle would be far better.

    Yet another given a constant SFPM and feed per rev a mill table has to advance a full 20" to cut a 15" long surface plate. The lathe has only to feed from the corner to the center - a bit less than 1/3 the time of flycutting.

    Flycutting is expensive. A consistent finish is no guarantee of flatness particularly when tool wear is a factor.

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    Quote Originally Posted by Forrest Addy View Post
    For one thing, quicker. It's hard to clamp a surface plate on a mill table without distorting it. You need a jack under each clamp unless you pinch it by an edge in a lath chuck clamped to the table. Also flycutting involves two edge crossings per spindle revolution, tool drag on the back side of the sweep, and chip recutting if they are not vacuumed up as generated. All that means shortened tool life and a dull tool makes a stressed finish.

    For another, in the usual R8 spindle with no positive drive a fly cutter with just over a 5" radius can be easily slipped even on light cuts. A fly cutter on a real mill spindle would be far better.

    Yet another given a constant SFPM and feed per rev a mill table has to advance a full 20" to cut a 15" long surface plate. The lathe has only to feed from the corner to the center - a bit less than 1/3 the time of flycutting.

    Flycutting is expensive. A consistent finish is no guarantee of flatness particularly when tool wear is a factor.
    Thanks for that, Mr Addy

    Interesting points, some I was unable to come up with

    Some of them can obviously be addressed, for instance I guess if a person had horizontally acting plate clamps, exploded vices, or Mitee Bites, and a 'proper' spindle taper the merits are more evenly weighted

    A shop with which I'm in just the sort of 'nothing reciprocates like reciprocity' relationship you alluded to, has a turret mill (Taiwanese) which in addition to an NT40 taper has a pitch circle of tapped holes going vert up around the mouth of the taper, to which they bolt a large circle of 1" plate with a lathe tool set into the periphery. Must swing about 16".

    Makes a nice job, great flywheel effect which is very beneficial

    I'm not sure how common this is; my Toolmaster doesn't have this feature, even though the quill is bigger OD than theirs

    - - - - -

    Are there any disadvantages of lathe facing, to your way of thinking? I wondered how many lathes would be set up to face slightly hollow, frinstance...

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    "Are there any disadvantages of lathe facing, to your way of thinking? I wondered how many lathes would be set up to face slightly hollow, frinstance..."

    Outside of time to perform the work and the effects of tool wear, theres no reason to prefer a faced on a lathe finish over one that's fly cut - or verse vise-a. If you have acess to a mill and no mid sized lathe handy that should settle it.

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    Quote Originally Posted by Forrest Addy View Post
    .....theres no reason to prefer a faced on a lathe finish over one that's fly cut - or verse vise-a. ......
    Thanks for that

    I should have been clearer - my question was not about finish, it was about geometry

    I had the impression (and the European lathes I've surveyed have borne out) that it was common practice by lathe makers to allocate a unilateral tolerance to the perpendicularity of the cross slide ways to the turning axis.

    1) So that facing cuts would not produce convex conical geometry on a faced cut, which would cause it to rock on a mating, flat face.

    2) So that clockwise wear, in plan view, to the slides and ways above and beneath the carriage would not lead to convex faces

    3) So that tool wear during the finishing cut would not lead to convex faces

    I haven't checked the Schlesinger acceptance criteria, but I'm sure I recall them bearing this out...

    Last lathe I refurbished I couldn't bring myself to do this, partly as I'm planning to fit a swinging column milling head at the back of the cross slide at a future date, so I go it as near to square as my skills permit...

    Of course, provided it's modest, convexity is probably not a bad starting point for a surface plate scraping cycle, better than a rocking horse....
    (which I guess table droop could easily produce on a turret mill or similar)


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