SNS 293: Inspecting and Surface Grinding Parallels
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- Опубликовано: 21 фев 2020
- This is some footage from when I was spending a couple days with my buddy Lance at his shop. We inspected several of my parallel sets and reground them using his Brown & Sharpe Micromaster surface grinder. Many of these parallels were shop made and have been passed down from one generation to the next.
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The thinner areas are effected by heat. The center lengths of the parallels stay stable because there is more metal to absorb the heat. A simple solution to this problem is to angle the parallels slightly on the mag chuck so the grinding wheel is not contacting the thinner areas of the work along its full length. Setting parts up parallel to a grinders travel is one the most common mistakes people make. Always angle parts when possible this reduces heat build up an wheel load up an wear.
Makes sense...
I've always wondered why people set parts on there angled. Now I know, thank you.
Aww this comments section is so frustrating. I posted the easy answer to this on the first page and yet still people are suggesting shims and what not. The easiest way without any doubt is to clamp the piece to an angle plate, then simply clock from end to end to find the mean. Once you've done that you can then grind the bow out. After that you simply turn them over and grind them all together directly on the mag chuck. You should never use the mag chuck for the first cut, it will just distort your piece. obviously the better your equipment the better the end result...If you have a AA inspection standard angle plate and a very good DTI you can easily get within 1 or 2 microns maybe even 0. My preferred way of doing this is to set up some packing against the angle plate then place a small dowel pin in the centre, then sit your work on the dowel so it pivots. Level it by eye, clamp it securely, then clock it up end to end. find the mean so you don't have to remove too much material. Make sure firstly your wheel is freshly dressed and use lots of coolant. Remove what you need to, to get it clean. Then the penultimate cut should be very small 1 tenth. Then let it (spark out) several passes without adjustment. The finish result will be as perfect as your angle plate and machine will allow. I have seen zero many many times. Sorry for getting all menstrual, Great channel BTW.
would using residual grinding methods still pull the bow down, I would think, i absence of having an angle plate that would be the best way to go.
I have never machined a single piece of metal in my life. I found TOT’s channel and from there came to yours I subbed both really really enjoy the videos I’m 50 years old and am amazed at the things you all do! Thank you for sharing your trade and skill sir!
The nice thing about having a little machine shop like mine where the only customer is me is that this kind of fanatical precision is never necessary.
Some people like to strive for the stars.
Cant let them touch each other while on the chuck. Non-square sides will influence them on the magnet when it sucks them down and lead to your side to side problems.
They were slightly off because the center was VERY strong, supported by the I-beam, and the outer edges were able to flex down a tiny amount.
i cant say it enough Adom is a good boy / man . his pop and granddad did themselves proud. they taught him right. America forever.
I like to precision flat grind with the parts at an angle to reduce wheel time on the part to prevent heating even though I run flood coolant. I also like to Grind with only residual on the magnet for a finish pass. I have heard you can hot glue bowed parts to a flat plate and let it cure before you mag them down to get the crowned side flat, then flip and mag down to knock the cup flat. For use on any machine besides a surface grinder I wouldn't mess with any parallels that weren't at least .0005" out.
bcbloc02 yep the glue trick works great! The glue supports the part so the mag doesn’t distort the part. Works every time! My uncle taught me that one!
Good call Brian - I like the idea of using residual mag for the finish pass, I think that can easily bow the parts a few tenths
I didn't mention this in my response I guess because it is a habit to me. Forgive me
Tool pressure (grinding wheel) will vary over those surfaces due to the force of the wheel on the surface of the parallel. As the wheel starts on the grind, the wheel has only a thin strip in contact with the wheel. So, assuming the wheel is pushing down with 10 pounds force, it’s pushing on, say, 1/8” wide surface. When the wheel is in the middle of the parallel, the contact is, say, 1” wide with the same downward 10 pound force. As the wheel exits, again the wheel works on a narrow strip. What that means is that the surface PRESSURE is changing as the wheel moves across the surface of the parallel. The resulting surface will be slightly crowned. That effect is magnified with the thin I-beam style parallels since there is less support under the outer edges and they will move under the downward force of the wheel. All of the effects I’ve just mentioned are SMALL but for very precise work, they can show up. For most everyday work, most people either won’t notice or won’t care because they’re small enough that they don’t affect the end result. Heck, I often use off-the-shelf keystock because it’s within .001” along it’s length which is plenty good enough for most of my work. As far as the longitudinal bow, a magnetic chuck will pull small parallels flat to the chuck which will spring back to a bowed shape when released. 3/4”x1 1/2” x 6” I beam parallels will do that. I have a pair. The only way that I know of to get them flat is to clamp them on the sides (lightly) and the ends so as to prevent distortion of the parallels during grinding. Once you have e certified flat surface, you can go back to the magnetic chuck for the opposite side.
Great video.
Thanks,
John
I wonder if that's why they were originally blanchard ground
Taylor Nicoletti
Makes sense to me.
You are correct sir!
@@davidgreen9957 My thoughts exactly.
I would just put them on the mag chuck at a slight angle to the direction of grinding.
I’ve learned a lot about surface grinding in apprenticeship and I’m grateful for it. I’m not the best at it, but I remember tips and tricks that make it better.
When you stoned the parallels in the beginning, you did it over a surface plate. Please don’t risk getting dust on your plate. You don’t have to stone them a ton, just enough so that burrs won’t mess you up.
I think even though you used coolant, the thin part on the I beams could have gotten warm and ground out of flat. Definitely grind the chuck to clean it up and make it flatter. When you put the parts on the chuck, tap them down with a soft metal knocker after the magnet is on. It’s an obsessive thing but it makes sure the parts are down.
Definitely grind them square on the sides while you’re at it making them parallel. It’s nice to be able to use both sides to have different heights.
I like this stuff and I’m still learning it. You and lance have more years in machining than me so feel free to do things the way you learned.
Two great guys doing the work they love doing. Awesome work, keep it up!
I would recalibrate the flux capacitor on the mag chuck. That has always worked for me
Po[p[po[]
If you originated that joke it still wouldn't be funny, so just shut up.
U show your intelligence Abom by knowing when you need a hand, humility is a valued treat, all your machining knowledge yet you still know when to seek help. Awsome
Shop work is always better with a friend. Great to work on working better. Thanks for sharing.
Enjoyed Adam and Lance!
ATB, Robin
Good video, its nice to see someone using coolant on a surface grinder most dont. I was taught by a very experienced guy that you use coolant and turn the parallels at an angle so the heat build up is minimised. He then would use less and less mag pull down and only use 1 tenth cuts. thank you.
My theory on variations on the I-beam parallels would be more to do with the tool pressure on the outer edges of the parallels, because that area is just unsupported. I believe the mag chuck had very little input on the deflection, if not any at all. If the I-beams had more of an integrated gusset from the outer edges to the center beam, the variation would be less. Also if they are not heat treated steel, the tool pressure deflection will be greater.
This is the right answer. Every material has a non-zero stiffness (modulus of elasticity) and will deflect under pressure. Lapping and polishing processes that apply less force than the grinder will give better results. However, the i-beam shape itself is the problem with using these parts as precision parallels. Any clamp-up will also cause deflection in the flanges away from the center web. That said, they are beautiful pieces even if they aren’t the best for uber precision use.
Couldn't agree more. I'm not a machinist but I work in a mechanical laboratory and this is spot on.
From the indicator readings I could see, the "wings," when measured in the free state, were lower. A better look at a mapped measurements would help answer this. If the wheel was exerting tool pressure then the wings would deflect downwards during the cut, and spring back up in the free state. That's not what I saw. Finally, the statement above about the deflection increasing if the temper is lower is not correct. The modulus of steel is very nearly the same, and independent of temper. The yield point may be lower or higher, but for a given load (tool pressure) the deflection will be virtually the same.
The mind boggles at the possibilities. What I do know, is that these are more precise than 99% of anything I'll ever own, this is amazing work.
Precision grinding is a skill set all on and of itself for a machinist... U can earn a living just by being a precision grinding expert.
Ya where lmao.....i can grind fairly well and its tough finding a grinding gig
like hell you can I mop floors because nobody will touch me at 68. you have to live in the correct areas where they specifically like precision grinders and toolmakers
I don’t know what it is about seeing bare metal soooo shiney it just looks beautiful!!!
Awesome job on parallels. Nice surface grinder. Thank you for sharing.
Consider this guys. As the grinding wheel grinds, it is deflecting the "wings" of the I-beam while grinding. I think this is why you're seeing variation of the readings as you move from the center to the "wings".
The mag chuck is going to pull the parts flat during grinding and then they will spring back slightly when removed. If the chuck has an adjustable 'power' setting, turn it as low as you are comfortable with and butt them up against a larger steel plate to prevent them being thrown. Make sure the steel plate has a nice flat face to mate to the chuck and even if the 'power' is turned way down, it should still hold the plate firm enough.
I am amazed at what good condition your parallels were in after all those years of service. It is a sign of a true craftsman.
Careful use,as well.
When I used to grind parallels I ground them on a Jones& Shipman 540 l... brilliant machine..some magnetic chucks tend to convex or concave at different times of the day....any grinder needs 20 mins of traversing first thing in the morning...to warm up .. without a temperature controlled toolroom you find materials tend to fluctuate during the day...
I just purchased a mint condition 540, very happy after a full teardown to inspect. Glad you like these machines, I've heard a lot of good things since my purchase which is always nice. You mention dressing the sides of the wheel? why? and how is the best approach?
@@tristanpatterson3843 Use the diamond single point fixed at an angle and traverse the table. It will reduce vibration and will grind sharp corners/slots nicely.
540 definitely the best small surface grinder around .. your right chucks can flex a bit .. when thin parts were bowed did you take a chance and grind it with residual magnet or even wirh the magnet off.. Oops health and safety wouldn't like the flyers 😅😅
His shop is sooo killer - just looks like such a great place to work. And that surface grinder - for whatever reason just looks so great while its doing its thing. That was some fantastic grinding - definitely one of my favorite overall "machining" tasks to watch
Lance needs his own channel, your videos with him leave me wanting to see more of his work.
Probably be a pretty useless parallel, but a collaboration with one of the blacksmiths, machining a "Damascus patern" Parallel, would be sweet. Have them forge up some blanks, ruff mill them to size, shaper action (?), and finish on the grinder? Could be fun.
rough it with the shaper and I'm in
Amazing! Thanks for the post.
Now you know what ultra cool machine you need for the shop Adam! We'll be looking forward to the surface grinder and Lance coming up there to help you set it up like you did him for his shaper!!!
Grind parts at an angle,if they aren’t long like these parallels,make your cross feed index as small as possible,you want to only use the edge of the wheel to grind,not the whole surface of the wheel,it makes wheel dig in the middle, make a couple of light passes at .0001 or .0002 when finishing and let wheel spark out,and when flipping your parts,always flip from end to end,never flip parts straight over away from you. If you really want expert grinding advice contact Don at Suburban Tool he has a YT channel,the man knows his stuff.
Adam
I have mentioned this before when you showed and talked about precision ground slip gauges. This is the effect of human sweat on polished ground metal surfaces. I have acidic sweat and if I was handling those beautiful finished parallels tomorrow they would have my finger prints all over them. Lance was wearing protective gloves some of the time but not during the final handling process. I strongly recommend the use of cotton gloves at wipe down and prior to stowage.
Milling machine or shaper for the first side then surface grinding ,because the magnet will straighten out the steel but once it is released take back its form.
Apologies if someone else said this, but with 412 comments, I just scanned the first page to see if my thought was addressed.
Caveman's Mancave's answer is closest IMO to the problem. A lot of focus was on the mag-chuck, but from a physics standpoint, it's pretty unlikely that the problem is from magnetic deformation. The forces from a mag-chuck aren't going to warp the metal in an I-beam structure because they will cancel each-other out, and no, I'm not going to give a physics lecture on why, message me if you're that much of a masochist! What's more likely (I think) is the "depth of cut" on the grinding wheel. The i-beams are true across the center line because they resist much more compression force from the grind wheel, while the edges of the i-beam are (mildly) susceptible to flexion from the downward force of the grind wheel.
I would posit that if progressively less aggressive passes were made prior to the finishing passes (Say, instead of taking X depth every pass out of 20 passes, you take X-1/20) then I THINK you might end up with it running true on both axis.
Then again, we should also consider how they're used. The highest point (center line) is going to be the only important part because that's how they're used. Stock rests on the highest point, and if that's consistent across the length of the contact with the stock, it's not going to matter if the edges are off by the thickness of of a warm breath!
Sorry Mozi but that's wrong. When you're grinding two parallel faces you have to take out the bow first...it doesn't matter what the profile is or the gauge you're grinding...The bigger it is the more force that will be exerted on it by the mag chuck.
Good stuff man. Great work.
Thank you Adam I always enjoy your videos I am from South Africa
I remember making several sets while in Machinists school. I purchased a set of Starrett and a lot of other tools from a man that retired. Then I joined the Navy and sold a most all of my tools because I wasn’t a Machinist in the Navy...
fascinating--great video
wont be much longer i think abom will be buying surface grinder, scraping videos and now using friends grinder... LOVE THE CONTENT!!!
Get outta my chair meat head 😂😂😂😂😂😂😂😂
I love all the precision.
All steel has stress in is so it changes as you remove material, also the magnet will bend the part if its all not perfectly flat on the chuck side. We used a couple of tricks to get one side flat, reduced magnet power and or placing the parts on paper on the chucks that did not have reduced power settings. When doing stright edges they were ground with just a stop to keep them from sliding with the magnet off. You could not do them after using the magnet at all because it got warm which would bow a 14' stright edge by 5 to 8 thou.
thinking a optical flat would tell you what to do if you were wanting anything tighter. I'D'ed the trouble spots if you didnt mind the time it took to polish them out dead nuts flat is possible and oooooh would it shine.
“I just clocked back in so I’m a little slow...”. Best excuse ever when having a brain fart!!😬😬🤣🤣 “... I couldn’t afford my own work...” OMG Adam, you guys are perfect together!
Hi Adam, no doubt very accurate, interesting video and thanks for sharing.
Nice job on the grind guys,
Fully automatic is the only way to go!!!
Love the final clip, The reflection from the lighting made them look like Files.
That is one sweet mirror finish on them👌
A good way to get something perfectly flat is put a few sheets of newspaper under the part and then turn the magnet on. It doesn’t allow to magnet to have as much grab on the part. Should get you within 0.00005 every time
You know what kind of day it's going to be when the biggest guy in the room says you're making me nervous 😂😆 great job guys
I worked at a shop that made the parallels for Bush, one guy did most all of them over the years. I did a lot of the stright edges and large angle plates on the 20' surface grinder.
You should dress in one direction only. Drop your wheel down when you traverse back. When you run the diamond back across the wheel without engaging the diamond, you're dulling what you just sharpened. That may be why you got the variation in the I-Beam parallels. The wheel is pushing instead of cutting. Also, I would hold them in a grinding vice to grind the first side. The magnetic chuck can pull down any "warp" there may be in the parallels.
I’ve got to tinnitus and I could still hear the difference in how course it sounded when you first hit the I-beam parallels.
That first finish looked absolutely gorgeous! On the parallel I-beam.
@paul beenis --- is that Sarcasm 🤔 or self-deprecation 🤪
Gorgeous finish on those parallels!
BAD finish. waste of money for no reason. surface ground parts should be satin finish and true has nothing to do with mirror finish.
I know it’s getting real when Adam wears his butcher shop apron! Great video brother 👍😊❤️
I miss surface grinding. I haven't done it in over a decade since I went through tech college. We made a set of 1/4 wide by about 1 inch high parallels as a class project and I remember in order to get them as flat as possible we qualified the first side in a grinding vise before sticking them to the magnet because they were super warped from heat treating and the magnet would just pull them flat. We weren't working to the tolerances you guys are here, though.
my thoughts on the i beam ones are the area in the middle where there is metal under it does not get as hot as the metal on the edges. Therefore the metal at edges expanse more than in metal in the middle where there is metal under it to acts like a heat sink.
Yeah but how hot can they possibly be getting with all that coolant/wash?
Great videos.
I've learned so much from watching your videos.
Getting an initial flat on an object distorted by being drawn down to a magnetic chuck. Let me suggest lifting the object on two small transfer blocks under the Bessel points. This will minimise the distortion from both gravity and the magnetic attraction of the chuck. You may need a stop to prevent endwise motion if the small transfer blocks don't provide enough grip.
In my very limited experience, when using 2 supports, or even a grinding vise, the part can heat up on the top, causing it to bow upward into the wheel.. if you are not really light and careful with it, this can run away with the part bowing up into the wheel more and more as it heats and gouging the part... Coolant would probably help a lot with this. On my little dry grinder I was able to very carefully take the bow out of a part, taking very light cuts & letting it cool between passes. Great stuff.. a little tricky to get a feel for what can cause big problems, good to be able to learn from others! Love the precision stones.. I have a couple soft Arkansas stones I lapped on a piece of granite with some lapping compound.. I seem to have gotten them pretty flat, and they are amazing for cleaning up precision surfaces. I hardly put anything questionable on a surface plate without checking the surfaces with a precision stone.
on those ibeam parallels, the side to side variation could be heat related. the wheel is slowing down some, which means heavy cut, and possibly warping during the curt.
Lance - NICE SHOP, there!
I love your theme music. It is just perfect for you. It matches your style and content perfectly. When I worked in the General Motors press metal factory, the dies were made of precision machined metal blocks bolted together. The also stuck together. Metal that is free of rust, and surface grime, AND which is really flat, will stick together.
Wrings together like guage blocks do.
All I can think of seeing those finishes is the Rick and Morty true level gag.
Holy ...the finish.
I like the Abom set too. They have more meat, they'll stay more stable edge to edge.
Make and sell them Adam, they'll be a hit!
Adam and Lance nice work on the parallels. Adam it great meeting you at the Florida Flywheelers. Next time see if Lance can be there it would great to meet him as well.
You might try setting the parallels on the table at a slight diagonal.When I was in a machine trades class years ago, we used that method to get better results on unsupported areas of a part. Also the two that seemed to bow after being ground, is most likely from the finger groove cut in the side causing uneven stress in the material. Love the videos. Surface grinding has always been on of my favorite machine processes. Your channel rocks.
Nice!! High precision... :-)
Tips for shop made parallels. Back in 1962 starting my tool and die apprenticeship I was in need of tools and setup stuff (parallels). To produce cavity, core or inserts for an injection mold, we milled tool steel blocks oversized, squaring them oversize on a surface grinder, milling rough oversize forms, heat treating the 'tool's then final shapes by EDM, carbide cutters and grinding to fit into the mold base.
I was able to source 2 pieces of cold rolled steel 1/2" x 1" x 7" to make a pair of Bridgeport milling vise parallels. The shop foreman sent them along with another job to our heat treater for nitriding.
Two MISTAKES by now. First do not use CRS for tools. Second do not use a surface hardening process.
Use tool steel that can be hardened throughout!
I would use them on the 1" dimension in the vise but occasionally could position them using the 1/2" direction. No matter what grit wheel, how many times it was dressed or flooded with coolant could I get the warp out of the thickness. Caused by stresses produced manufacturing the steel, stresses from the surface hardness and stresses by the touch of the grinding wheel and it would bow, never less than .0015".
However the 1" direction was far more stable, I was able to get it to .0003"-.0004" which was good enough for milling. They are still in my machinists took box!
I left them solid but finger grooves could be added. Or a series of through lightening chambered holes would permit use for bolting for setups. Or several short I-beam portions over the length to leave solid webs top to bottom.
But use a quality tool steel which can be hardened.
Amazing.
When anything but a mirror finish is roughing. I love perfectionists. I wish I was one.
you are mistaken surface grinding is not mirror finish he wasted valuable time.
Having had zero surface grinder experience, it seems to me that if you want to surface grind a bowed piece, it would be best to use non-magnetic clamping on the first face, otherwise the magnetic force would just straighten it out while grinding and it would spring back when the magnet is turned off. Once the first face is flat you can go back to magnetic clamping. I'd be interested to know if there is another way of doing it. Excellent video - thanks Adam.
A very good way to flatten something on a magnetic chuck is put newspaper under the part and turn the magnet on.
I have no clue but I don't think the mag chuck is the problem on the I beams, I think it's thermal expansion, I think the beam of the I is thermally sinking the centre, and the wings expand locally as they're being ground...
But I honestly have no idea whatsoever, this is pure intuition...
My guess is the I beams are ground in a fixture... Might be a trick to sell more, actually, you try to grind them you mess them up...
Just wanted to say how much I hate Adam. Having no machining experience, I got hooked on his videos and ended up buying a Monarch lathe, tooling, chucks, etc. Now I'm gonna have to get myself a surface plate and grinder, it just goes on and on. His videos are great for my enjoyment but are hellish on my back account.
Be glad you have the room for all that stuff
Adam booth is a great person and master machinist his job aweosme he has got alot of experience. Thumbs up 👍 to you abom. 😀
The ones with the "wings" to each side have the problems that: (1) The grinder has to push down at the point of contact at an EXTREMELY strong force to tear off the hardened material it is cutting off; (2) The sparks indicate a very high local temperature at the contact point that will expand and then contract the material AND very locally change its heat treatment on a very thin surface film (you may actually be seeing molecular size differences here); and (3) When you release the grinder force, the material will spring back up and, in these unsupported, rather thin wings, this is not a small matter. Since the wings are not buttressed by under-material, they will react to this as the various internal crystals at each point vary in size, shape, and orientation.
Surban tool showed a grinding episode, with a item they were grinding that was rocking, they placed a wet paper shop towel under the peace, and used half of the magnet force.
After all that work, you need a velvet lined case to transport them in.
Is that a cam mag chuck? Those thin cross sections, as said, will conform. Need to run very low mag and put stop on backside as precaution. And as said, grinding across heat effected zones the stone could deflect.
Wow how amazing. Man if you could just have leather sheath for them would look cool. Thanks Adam and Lance
I agree with the other posts here; the magnetic chuck made the non-flat hidden surface conform to be flat, twisting the exposed surface. After grinding the exposed surface flat, releasing the magnetic chuck allowed the parallel beam twist back. Somehow the beam needs to be attached to the surface of the chuck and only restrained along the X and Y axis and not the z axis.
Every so often grind your mag chuck. Grind one side of parallels in vise or angle plate, then put on magnet and grind them together. Good results. I have enjoyed watching your videos for awhile and this is my first comment ever on your site. I have been machining since 1983 and I notice you do alot of things just like I would do. Even the language. Bustem dead nuts abom79.
Your apron reminds me of my uncle's butcher's apron. As you probably guessed he was a butcher. 🙂 I found this video both interesting and informative. Cool
Ive had the Mag Chuck get warm and bow slightly , influencing the flatness. Like others said , you could off mag grind with a stop .
That third I-beam sounded rough when you started cleaning them off with the precision stones! 🤨🤨 Hurt my ears just hearing it! It’s amazing how much I’ve learned from just watching your awesome content Adam, and I’m not even in the industry(I’m in IT). I’m learning the basics just watching you work and it’s fucking awesome, thanks a million for the awesome content!! 👍🏻👍🏻
Abom79 You have to shim the parallels to keep the bow on the mag chuck so that when you grind it puts a flat side on it, after that you can flip it and no shims are required as your pulling down on a true flat side.
Very cool.
All of what you guys are doing is good information to,know.
fancy blue cool-aid mixer ya got there.
Great work!!! On the bow(ing) see Don Baily`s take on that at Suburban Tool i.e. set the mag plate at very light pull ;-)
Full on,then down to half. Operates under half magnet. Relieves any bow,lessens it.
I have an indicator holder just like that. Nice and heavy.
I got an ad for Motion Industries before this video...nice!
16:36 - 'Roughing?!!?"
My thoughts exactly lol
19:58 - “I can’t afford my own work, even though I pay for it” lmao 🤣 This is the beauty of precision machining. Turn scrap and make it into something that costs an arm and a leg.
Weekend motivation!
Those I-beams have a little bit of twist in them and when the mag chuck pulls them down they conform to that surface being impossible to make them completely flat using that method of clamping.
Although I like Bestest indicators even better than Interapids, chasing tenths with it on that little light transfer gage I never found to be very repeatable. I liked using a Starrett 252 height transfer stand with a lever probe and an electronic gage amplifier and then found height differences using a height gage or jo blocks to transfer the readings to. That grinder however looked like it would be a pleasure to use.
I am a big fan of Bestest indicators too. The indicator is a .00005" indicator not a "tenth" indicator. When I needed one of that accuracy I went twice with B&S. When you get that accurate, I agree that the inspection base is rather cheesey. You use what you have, but that setup was substandard for that indicator.
@@paulhunt598 The "test" style indicators do not necessarily give accurate readings, just move the angle of the lever probe and read it again and you'll likely get a different size. They should only be used to compare to a known standard. So when you see .0002 on the test indicator you should check the high reading and low reading with both standards to know the true difference. With a lever probe and a gage amplifier you can adjust the difference to the standards and be pretty confident it's reading properly, altho I still like to go to the height gage and find the true readings to be sure. The round "AGD" style indicators do give accurate readings, if their in good shape and in certification. So a better way would be to have a height stand with an AGD indicator to read the variations. Also after you clean your surface plate, keep your hands off it!! The oil on your hands can easily raise your parts a few tenths.
As some some have suggested, you need to measure the bow then shim it while grinding, using residual magnetism, to get one side flat because the chuck is pulling the bow out. You'll need plenty of extra clamping for this. Then flip it over and grind. If you still have problems, your chuck may not be flat. At that point check the flatness of the chuck and grind it flat. If you're in a hurry, you'll need to use shims (for bows) and rotate the work as you grind. A circular grind gives you the best chance for a flat grind in that situation. For the I-beam, I'd do a circular grind and lighter passes plus hold it in a vice because it's probably rocking side to side as you're grinding.
To start I know absolutely Zero about surface grinding but how do you account for ware in the wheel while grinding a surface ?
You will notice that he is advancing the part along the y axis so that the part isn’t being ground by the entire wheel, so small parts like these are not eating away the entire true edge, how ever multiple parts or larger parts will ware down the trueness and the wheel will then need dressings. Hope my terrible explanation helps.
The easiest way to grind the first surface flat is, to clamp it in a precision vise. So there is no tension on the upper surface.
Temperature variance on the I beams from the center line out to the edges? Maybe even influenced by the cooling effect of the liquid. This kind of precision is fascinating!
Not a machinist, but my take is that the unsupported horizontal may be resonating at a natural frequency of the part as it is ground thereby influencing the surface finish. Anybody have any thoughts on this?
Would it not be advantageous to flip end for end when you are checking with the indicators for consistency same as you would with levels?..
Comment on the I beam parallels: many variables here to get through. What wheel is being used? A vitreous bond wheel is where it's at for micron grinding. The B&S has an electric chuck. Are you using the residual setting? Drawn I beam structures have high torsional values inherently and are difficult to grind out. In finishing items like this I have put a piece of saran wrap under the work piece to tweak out the distortion. The I beam structure itself is unstable in the flat as it was originally designed for longitudinal loading. My best results have been with the mag in residual mode and with a cool mister rather than flood coolant. Speed is another consideration and usually a medium speed gives the best results since high speed and low speed can generate tremendous amounts of heat. In conclusion, precision grinding is a very fine line of balancing everything that matters, material, speed, wheel, magnetic attraction and deflection, and coolant or lubricant. My 2 cents worth. Thanks and carry on!