surface ground part low in the middle
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    Default surface ground part low in the middle

    what is happening there? (i know when its high its from heat.) im talking sub micron level probably, i blued it up on an aa plate. its about 4x4x1", similar to o2 prehard.

    Edit: sorry, i got it wrong. the part is high in the center. low would not surprize me.
    Last edited by dian; 03-23-2020 at 10:02 AM.

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    Quote Originally Posted by dian View Post
    what is happening there? (i know when its high its from heat.) im talking sub micron level probably, i blued it up on an aa plate. its about 4x4x1", similar to o2 prehard.
    Take a mag stand and micron indicator to the grinding chuck, measure it across the surface while off, then on, map the difference. You might find an error there than "comes through" to the workpiece.

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    And could I clarify? When you say it's "low", are you saying that the part is thinner in the middle than the ends, so the test indicator needle dips down? Couldn't that be heat, as the part warms it swells in the middle, grinds away, and when cool leaves a depression?

    Ends of the stock should cool faster due to greater surface area than the middle.

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    For starters I would warm up the machine at least 5 minutes / *use about half magnetism / use smaller cross increments /overpass the part at both ends 1" / use coolant /might/likely slow down the long travel speed.
    It takes very little heat to swell a part one or a few microns.
    Measuring the part it should set on an iron plate for a half hour and only touched with wearing cotton gloves.
    This is the do something act to get the part done. It may be part heating and these means might correct that problem.
    On out, begin to think about wheel hardness, grit size, openness and wheel material. With good part cut down the end-over-travel needed to stay in tolerance.
    likely check the machine with using a granite parallel shimmed to zero and run a micron indicator over it on long and on cross..
    sometimes turning the part to make the long travel on the part shorted can help.

    I have had parts that I stayed off the part at both ends for a three second count.

    * with half mag you may need block-in at the go end, but it should touch at the part top and allow coolant to still run down the part on the go direction.

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    Next, I would use a precision granite parallel shimmed to dead flat on the chuck and run a micron indicator over at at long and cross travel to test a warmed-up machine, this to see if the machine runs dead flat.

    One might fresh dress then take a full wheel pass one a sacrifice part also set on the chuck before grinding the finish pass on the needed part, and come to the part with only touching at less the full clean-u / making perhaps three grids to full clean-up.

    On a stout part like 1"x4" I have placed two shims under at near the ends (about at the thirds) so to eliminate chuck flatness error, yes the half or so mag helps when doing this.

    You might give wheel specs and part hardness so the guys can add more suggestions.

    I have had parts that needed zero error and have used a lapping block when that took less time than careful grinding.

    I have steel flat parallels that I lay a part on when I don't have time to grind a whole chuck . They are grooved on the top so heat cant build up at truing.I just tickle grind these and and de burr with a few swipes of crocus cloth, then set the part on them for grinding the part dead true.

    OT:
    Good you asked, my answer adds 421 words to the book I am trying to write.

    Yes go back and read this over because I am still adding things as I think of them./ look at the count.

    Very important that all the grinder guys keep pitching in other ideas on this problem.

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    o.k., i should have written it the other way around: when the part cools, its high in the center (convex). as opposed to a part that bulged up on the grinder from heat and is concave after cooling. also its actually rather 3"x3" and 2.5" high.

    I lapped it flat and ground again with half magnet (this block woul not distort anyway, would it?) and on an other spot and its the same:

    3-399.jpg

    i grind dry with rather slow passes with long overtravel (more than 200% on each side) and about 3mm stepover. the grinder is used sparingly and i probably have a slight problem with the bearings. to my shame, i have no idea about the wheel. it came with the grinder and i have never taken it off. i always dress fast/rough (overhead).

    how do you put a bigger picture in here? clicking on "detailed" makes no difference. first its as above, then as below. ???
    Attached Thumbnails Attached Thumbnails 3-409.jpg  
    Last edited by dian; 03-23-2020 at 11:42 AM.

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    Check the part while on the surface grinder after grinding. Can you see the same error? If it looks perfect, the problem is in the grinder, not the technique. But your blueing technique doesn't quantify your error. What about your surface plate? Does it have an accuracy significantly better than that you are trying to achieve? The picture does not build confidence.

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    my cast iron block there might not look pretty, but its lapped flat almost to optical dimensions. i used it to take a print of the part. on the aa granite plate it blues up in the same way. im just playing around a little here, for most purposes the part is flat. im just curious why this is can happen repeatedly.

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    The most likely cause for the part being high in the centre is that the saddle ways are more worn in the centre part of the travel than the ends. Have you rebuilt the machine or it it original?

    Use Michiganbuck's technique with a granite parallel/flat, or an iron one (with the magnet off!), shimmed to give the same reading at the ends of the travel, and an indicator on the wheel head to see if the ways are worn.
    Last edited by Mark Rand; 03-23-2020 at 05:33 PM.

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    Quote Originally Posted by dian View Post
    what is happening there? (i know when its high its from heat.) im talking sub micron level probably, i blued it up on an aa plate.
    given the (apparent) scalloped finish you are getting on the ground part, and the condition of the plate, doubt you are seeing anything "sub micron"

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    Quote Originally Posted by michiganbuck View Post
    Next, I would use a precision granite parallel shimmed to dead flat on the chuck and run a micron indicator over at at long and cross travel to test a warmed-up machine, this to see if the machine runs dead flat.

    One might fresh dress then take a full wheel pass one a sacrifice part also set on the chuck before grinding the finish pass on the needed part, and come to the part with only touching at less the full clean-u / making perhaps three grids to full clean-up.

    On a stout part like 1"x4" I have placed two shims under at near the ends (about at the thirds) so to eliminate chuck flatness error, yes the half or so mag helps when doing this.

    You might give wheel specs and part hardness so the guys can add more suggestions.

    I have had parts that needed zero error and have used a lapping block when that took less time than careful grinding.

    I have steel flat parallels that I lay a part on when I don't have time to grind a whole chuck . They are grooved on the top so heat cant build up at truing.I just tickle grind these and and de burr with a few swipes of crocus cloth, then set the part on them for grinding the part dead true.

    OT:
    Good you asked, my answer adds 421 words to the book I am trying to write.

    Yes go back and read this over because I am still adding things as I think of them./ look at the count.

    Very important that all the grinder guys keep pitching in other ideas on this problem.

    "One might fresh dress then take a full wheel pass one a sacrifice part also set on the chuck before grinding the finish pass on the needed part, and come to the part with only touching at less the full clean-u / making perhaps three grids to full clean-up."

    you lost me there.

    also: " With good part cut down the end-over-travel needed to stay in tolerance." can you explain that, please?

    what kind of grooves do you put into those parallels? how deep/wide? you mean magnetic ones, right?

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    "One might fresh dress then take a full wheel pass one a sacrifice part also set on the chuck before grinding the finish pass on the needed part, and come to the part with only touching at less the full clean-u / making perhaps three grids to full clean-up."
    -> A fresh dress may leave the wheel too sharp so a light skim on a similar material that is not the needed part can make the wheel better. Having a small slug on the chuck near the part size can be a quick way to freshen a just dressed wheel.

    you lost me there.

    also: " With good part cut down the end-over-travel needed to stay in tolerance." can you explain that, please?
    -> When you find you can do with less over travel to keep the part cool reduce that over travel on the next part to save time. The part has to pass inspection but every minuet costs money.

    what kind of grooves do you put into those parallels? how deep/wide? you mean magnetic ones, right?
    -> No just steel about 3/8 thick or less, any kind of groves across at perhaps a 1/2 flat and a 1/4" grove 1/16 deep..tickle grind at the time of needing precision for one part or a few.. slide the part on with the mag left on, holding is light so high touch block in required, easy grinding so not to throw the part.
    Yes, one might make them with magnetic parallels..

    This for when you need a near dead zero flat part and you chuck can not be trusted so grinding the set-on -pads is quicker than grinding the whole chuck.
    Often the close to you area of a chuck is the most flat so with no set-on-pads, the close to you chuck area might be used.

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    Now you say the part is high in the middle. With that poor finish I wonder if you can down to checking to a micron. Is that a coolant-loaded or dirty out of balance wheel. A new wheel might do better, or if you can check balance..?
    You should be able to make a slow wet pass and see no chatter if all is good. or a slow dry pass if having no burning.

    QT: i have no idea about the wheel. it came with the grinder and i have never taken it off. i always dress fast/rough (overhead).

    That could be a part of the problem I have seen grinders thought to have a bad spindle when it was just a problem wheel.

    How the part sets on the chuck is also important, if the down side is domed you will have problem grinding the other (top) side. A two shims under first side can make one side flat.

    Some times one can wash a vitrified wheel with soaking it a few days in solvent and then in dish water then adding new blotters ..but best to just break and dumpster a bad wheel.

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    MBuck, I think he grinds without coolant, and the block in question is too thick and short to be bowed by any significant amount by the magnetic attraction even if the contact face isn't flat itself.

    Now, if it could rock back and forth with each pass that would be an issue, not to mention scary as heck!

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    QT: [Now, if it could rock back and forth with each pass that would be an issue, not to mention scary as heck!]
    That would be reason to 2 shim the part...but to get sub .00004 (perhaps .000020) with that poor finish/likely on the other side also.

    Wonder id Dian is just pulling out chain.

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    no pulling here. as mentioned i have no coolant on the sg and i use it sparingly for shims and other small stuff. the largest part was an anvil for my vice (hardox 450). its extremely difficult to get a single wheel over here. they want to see a large turnover to sell to you at all. the last thing i heard from the local suppliers (now owned by st. gbain or 3m) was 10 wheels of a kind. thats why the wheel that came with the machine is still on there and i dont have any other adapters anyway. the scalopping is only cosmetical btw, i cant detect anything with the 0.2 mu millimess. i ground that part on both sides several times, btw.

    i have been planing to check the grinder for a while, but i dont own a long parallel of known precision. now it occured to me that i could use a 400mm din 00 beveled straight edge with a large radius (60mm from memory) tip on the indicator and probably get acceptable results.

    well, the result is as follows: in the middle half of the chuck things are somewhat flat. there is a dish of 3 mu over 150mm. on the ends however it rises by about 15 mu. so my grinder is a rocking chair. i also see there is a "hickup" of about 30 mu on the right side just beyond the chuck. i didnt notice anything when i had the table off.

    i also checked the flatness of the chuck with the beveled straight edge by placing a 5 mu tape under it. this "pull testing" makes me believe the chuck is flat to 1-2 mu, except on the very ends (around 20mm) where its low.

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    the whole situation seems to make sence. the overhang of the table makes the ways wear concave. this grinder probably has been used more on the left side. the top of the table stays flat, so it rises when the ends reach the wheel. as the chuck is flat it copies this motion. yes?

    this means the chuck has not been ground on the machine and if you were to grind it in it would copy the shape of the ways and become convex, right? now the question is: is it better to have a flat chuck in this situation or a convex one? if it were convex and you grind a long, flat part that gets sucked down by the magnet it would come out flat, yes? (after flipping several times.)

    so my conclusion is that the measurements are consistent with the ground block high in the middle. if the chuck follows a concave curve the part becomes convex. i didnt notice this before, probably because heat tends to make the part concave and compensates for the worn ways.

    (i have no idea if my thinking about all this is correct.)
    Attached Thumbnails Attached Thumbnails 0-117.jpg  

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    So to do better with your machine it sounds like you're going to have to address the ways. Be cautious, if you're not experienced at scraping you may find yourself worse off when you're done.

    Regarding your wheel and adapter limitations - perhaps you can find a kind soul here to help acquire and ship some parts to you from their country.

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    looks like a decent grinder from here. Has it oil scraped long travel ways or ball ways?
    I would post you a wheel but the cost likely more than a wheel costs.
    There must be another grind shop , perhaps buy a used wheel. perhaps take that wheel on mount to a grind shop and have them check balance...Do,you have or can make a wrench to pull the whee or wheel and mount?

    That blue guy looking at you is likely the problem.
    Was the indicator check with mag on or off? best would with mag off.

    Flat chuck even if you have to hon/scrape it flat.

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    the grinder has hydraulic traverse. that makes it diffcult to take precise measurements. even my 20mm rods dont prevent a noticeble hysteresis when changing directions. it has a flat and a v way. the scraping marks are visible almost everywhere from what i remember. thats why im surprized the table kicks up by about 30 mu in one spot to the right of the chuck. (glad im aware of this now, it would ruin a part and maybe destroy the wheel.) i checked the straight edge turning the mag on and off in several spots and the indicator doesnt move. that doesnt mean there is no difference, but its irrelevant in this case.

    i was planning to touch up the chuck because i know there are a few high areas (5-10 mu), so thats a bad idea i understand? i have been reluctant to take it off, was afraid of what i might find underneath, but in the current situation i might decide to find my biax, attack the ways, recondition table and chuck, finally take off the wheel and make some adapters.

    michigan, i might take you up on the offer concerning the wheel. a wheel in the 200mm size costs upwards of $100 locally anyway.

    (btw, you have to understand the business culture in switzerland. there are no tool shops, no automotive shops, you cant walk into a grind shop and talk to the owner, its all big companies making big business. thats what makes us so efficient, i guess.)


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