Practicalities of round indicator stand base
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    Default Practicalities of round indicator stand base

    Hi, (first post)

    I've recently got a 400x400mm (16x16") granite surface plate and I'm mucking around hand scraping the bottom of an old hand plane as a bit of a learing experience; I'm just starting but it's going well and is good fun. However, I figure it would be good to take some measurements to have a better idea of the results, and again, just for fun. I've got a dial indicator and (adjustable) stand/arm with mag base. Since the mag base is no use on the granite, I thought it would be good to make a cast iron base that either the mag base can attach to (on top), or the stand can be threaded into directly (after removing from the mag base). I've found a supplier who has cast iron round bar (but not flat) in various diameters and can cut to length. I was thinking that a piece 90mm (3.5") diameter and 25mm (1") thick might be a good candidate for a suitable base, and would be further scraping practice.

    My questions relate to using something like this in practice, since I've never used one. Do you think the dimensions I've suggested would be suitable? I guess too large a base would take up too much space on the surface plate and leave less room for parts under test, too small, and it could be unsteady or topple over. Is it better to be thicker/taller/heavier, or thinner/shorter/lighter for ease and effectiveness of use? Lastly, would a round base be problematic in any situation? The only ones I've seen images of are rectangular with grooves and other features. I look forward to your thoughts.

    Kind Regards,

    Alistair Witt

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    Alistar,
    I think you're on a good path with the idea of the round base. Till certain limits, the heavier and the larger, the better, since it would hold better an indicator off-balance without tipping or, anyhow, without having all the weight on the edge of the base.

    Instead of scraping the bottom, I'd use three circular carbide inserts: drill and tap holes for the screws holding them, put a dab of epoxy putty between the back of the insert and the base, hand-tight the screw till squeezing out the excess of epoxy and let the epoxy to cure with the base resting (on the three inserts) on the surface plate (it is advisable to cover first the surface plate with a thin of plastic film-e.g. Saran Wrap, plastic shim stock, etc.). Once cured, you can tight the screws and you'll have your base always sitting on three points.

    Paolo

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    I use a simpler solution. I have turned three "pucks" from magnetic stainless steel. These are about 120mm (5") in diameter and about 15mm (5/8") thick. On the bottom I have relieved all but a 8mm (3/8") annulus around the outside, and I surface ground this ring underneath and the top. I use two of these with magnetic base indicators and the third has a Noga arm permanently mounted to it (photo below is before I attached that arm to the puck on the left). They work great, and actually "float" on the table when I move them. I guess that air gets compressed underneath as they slide.



    Below is a photo showing the bottom coming off the lathe BEFORE I surface ground it, so you can see the annular ring.


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    The proposed three-point base makes some sense as it will promote stability of the base where ever it happens to be located on the plate since it will be in contact at 3 points only.

    But, might using such a 3-point method increase the variability of the levelness of the base compared to an annular or more traditional flat gage block base? I think it would since the flat base system has a tendency to “average” high points on the base and rest on the highest points encompassed by to gage block. The variation in height of high points should be less than the potential variation of ,say, one high high point and one low low point and one intermediate point.

    So the three-point base should oscillate in verticality and therefore height measurement more than a flat base. An annular base should be somewhere in between since it averages fewer high points than flat but more than 3.

    So there may be a trade off of stability (potentially greater for thre points) and consistency (greater for gage block). I’ll admit this may be a bit theoretical. And I may be off base. I’ll be interested in other’s thoughts.

    In practice a scraped gage blocks seems plenty stable for my use and gets my vote for the best compromise. But the dirrerence in performance between the annular and flat base is likely to be very small. So ease of fabrication favors annular

    Denis

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    Denis,
    In practical terms, you're probably right that scraped blocks are good enough.
    My point is that a plane is defined by three points and, in most cases you're better off by being you to decide their location (in my case ~120° radially apart) than leaving it up to the fate, since you can have two high points rather close by and one 180° apart, making a flat or annular base less stable (on very imperceptible levels).
    If you look carefully, most of the high precision instruments (even the ones supposed to sit on a surface plate) rest on three hardened pads.
    I tend to agree with Robin Renzetti in preferring sharp edges on the feet, in order to minimize the risk of wedging dust, sirt, etc. between the foot and the surface, when sliding the gage.
    As I have admitted at the beginning of this reply, for most practical uses, likely it doesn't matter much. However, I think it's much faster to prepare a base with my method than scraping or surface grinding it.

    Paolo

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    The type of surface on which the block will be used might matter some. In the OP's case, he is going to be using it on a ground granite plate which should have minimal local variation from highs to lows. So, the concern I raised about averaging is probably moot. But it may not be so moot when using such a block on a scraped surface which inherently has a fair bit of local variability an more so on a machined surface when comparing two machined surfaces for parallelism.

    Ease of fabrication definitely favors 3 points and, I agree, likely would work fine for the OP.

    Other features for the OP to consider might be including some sort of straight section on the side of the block (perhaps as a bolt-on item) to be used when he wants to run the block along a guide since doing so can, at times, be convenient. Doing so with a round base might be a little challenging. An doing so while maintaining a constant angle of rotation would be nearly impossible. Here is a recent video I made of testing the straightness of an 18 Featherweight Straight Edge after I milled it an prior to scraping. I used a simple 1/4 by 1" flat bar as a guide to slide the standard gage block along the SE. The SE is up on 1-2-3 blocks and the DTI probe is pushed up against the under surface on which the SE is supported. The flat bar just guided the probe along a more or less straight path making it easier to do this quick assessment. YouTube

    Denis

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    Why not screw it into one face of a tapped 1-2-3 block. Might come in usefull having something square to run in a straight line when needed down a part.

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    Quote Originally Posted by ballen View Post
    I use a simpler solution. I have turned three "pucks" from magnetic stainless steel. These are about 120mm (5") in diameter and about 15mm (5/8") thick. On the bottom I have relieved all but a 8mm (3/8") annulus around the outside, and I surface ground this ring underneath and the top. I use two of these with magnetic base indicators and the third has a Noga arm permanently mounted to it (photo below is before I attached that arm to the puck on the left). They work great, and actually "float" on the table when I move them. I guess that air gets compressed underneath as they slide.



    Below is a photo showing the bottom coming off the lathe BEFORE I surface ground it, so you can see the annular ring.

    That'd be my solution also.

    Regards Tyrone.

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    Thank you all for the helpful comments! I do have access to a lathe and had thought the turning would be necessary before scraping anyway, so the annular ring option could work. However, I don't have a surface grinder for use per ballen's post. I suppose I could make the ring width a little larger (maybe 20mm? (25/32")) and scrape it, which would reduce the scraping work involved compared to the 'full puck'. I'm tempted to do this as a compromise between scraping practice and speed to make.

    The 3 x circular carbide insert option does look like the fastest option to make. Would the carbide cause accelerated wear on the granite plate, with the smaller contact area, and the increased hardness of the carbide? Also, is the epoxy putty to allow the three inserts align themselves to be on the same plane (while curing on the surface plate) when otherwise they might conform to the rough finish of the base to which they're attached?

    Actually a photo of an arm attached to a 1-2-3 block was what set me down this path, but at the time I figured it would be too small and light (with all the holes) to be stable. Perhaps this assessment was wrong. Does anyone have much experience with this?

    On a related point, I figure it would be prudent to turn down the outside diameter of the puck since it's supplied with a rough cast finish, but how best to do this (hold the work-piece) in the lathe? I've seen examples of work pieces being glued with CA to a faced chunk of aluminium that's smaller in diameter than the work-piece. However, I think that was for lightweight machining of brass; would it be pushing the limits of CA with cast iron, and indeed cutting through the cast skin?

    Regards,

    Alistair

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    Quote Originally Posted by alistairwitt View Post

    On a related point, I figure it would be prudent to turn down the outside diameter of the puck since it's supplied with a rough cast finish, but how best to do this (hold the work-piece) in the lathe? I've seen examples of work pieces being glued with CA to a faced chunk of aluminium that's smaller in diameter than the work-piece. However, I think that was for lightweight machining of brass; would it be pushing the limits of CA with cast iron, and indeed cutting through the cast skin?

    Regards,

    Alistair
    Just do the OD last. The top and bottom faces don't technically have to be parallel unless you're obsessive and just want them to be. And the recess in the bottom doesn't need to be concentric unless you have that same handicap. So with the top surface finished trap the workpiece between the closed faces of the chuck jaws with whatever cardboard or masonite spacer you deem appropriate and hold it with the tailstock center in the center hole. You can do the OD with light cuts. If you need more friction, pieces of friction tape or emery cloth will keep it from moving.

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    Quote Originally Posted by TGTool View Post
    So with the top surface finished trap the workpiece between the closed faces of the chuck jaws with whatever cardboard or masonite spacer you deem appropriate and hold it with the tailstock center in the center hole.
    I hadn't considered a centre mark, but I suppose there's really no good reason why not, especially since it could be hidden on the bottom. That does make it all simpler and indeed possible. No risk of it flying off as in the case with the CA.

    Cheers,

    Alistair

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    I suggest to follow Deni's suggestion to mill a large-enough flat on the circumference. Make sure that it is perpendicular to the base.

    Regarding the epoxy, yes, the idea is to both lock in place the inserts and to make sure that they're flat against the surface plate.
    Pretty much any precision instrument has either carbide or hardened steel feet. If those surfaces are kept clean and placed against the plate carefully, they don't damage the granite. One of the reasons why those surfaces are hardened is that it is much less likely that an abrasive particle embeds itself in something hard than in softer material.

    Paolo

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    Quote Originally Posted by Paolo_MD View Post
    I suggest to follow Deni's suggestion to mill a large-enough flat on the circumference. Make sure that it is perpendicular to the base.

    Regarding the epoxy, yes, the idea is to both lock in place the inserts and to make sure that they're flat against the surface plate.
    Pretty much any precision instrument has either carbide or hardened steel feet. If those surfaces are kept clean and placed against the plate carefully, they don't damage the granite. One of the reasons why those surfaces are hardened is that it is much less likely that an abrasive particle embeds itself in something hard than in softer material.

    Paolo
    Fair point regarding the flat side; I need to get a friend with a milling machine!

    I hadn't considered the embedding of abrasive particles in feet, as you describe; makes sense.

    Maybe I'll have to make one of each option! If I can pull this off in any decent time frame, I'll post back with some photos.

    Cheers,

    Alistair

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    Alistair, I will suggest another method, not necessarily because it is better, but to offer an alternative that you may or may not like to try.

    I am assuming you do not have a faceplate or drive plate (If you had one the plate could be used rather than the face of the chuck as outlined later.)


    Maybe you have a common 6-inch 3-jaw chuck with reversible jaws. If so, reverse the jaws as shown in
    the photo:



    so you can grip the puck and let it sit against the next lower step in the jaw.

    Now you can cut one face and maybe make the recess as well. And cut to clean up the side of the puck beginning at the face and extending about 1/4 " down the side.

    Now through drill the puck with the hole you will later be using to tap and attach the arm and DTI. . This center hole, if well centered (could be bored to true it if desired), will be fairly concentric with the outside cut. Use a file or lathe cutting tool to generously chamfer the side/bottom junction.

    Now flip the puck and face the other side. By having the first face register on step of the jaws, the second face will be pretty close to parallel to the first (depends on how good the chuck is.)

    Now remove the jaws entirely. Use a live or dead center in the tailstock in the puck through-hole to center the puck on the jawless body of the chuck (which probably runs pretty true to the spindle.) With light pressure from the center pressing the piece onto the chuck face run some thin cyanoacrylic glue around the edge of the chuck allowing it to wick under the puck. Now spritz some cyano accelerator onto the glue and it will kick off instantly. That glue along with pressure from the center will hold the puck in place and allow you to make light cuts as long as the puck does not get hot to touch. I have found the glue to hold remarkably well if the surfaces were clean when applied---use isopropyl alcohol or other solvent.

    At this point it would be good to convert your through hole to a 60 degree center hole for better bearing by the center. (If using a dead center, don't use much pressure, lube often, turn slowly (100 RPM maybe) and feel for heat build up. Stop, reduce pressure, relube if heating at all is felt in the center. It is easy to burn up a dead center.

    Now cut down the sides to the prior 1/4 inch side cut so the entire side is shiny and smooth. The first side cut and second may not match perfectly unless you are extremely lucky and good. BUT, you can hide that by making a few evenly spaced shallow grooves one of which just happens to overlay the junction of side cut one and cut two. The grooves will look good and will aid in gripping the puck.

    By removing the center from the hole and using a plastic mallet and giving a sharp whack to the puck it will pop free. Heat can also be used without concern for the puck as it only has to heat to 400 to weaken the glue and those 400 degrees will not cause any problems.

    Replace the jaws and finish by tapping the center hole and chamfering the hole and edge of the puck. The cyano glue is easily removed from the puck and chuck with acetone or similar solvents or simple scraping.

    Make sense?

    Good luck and show us your results!

    Denis

    PS If you want to be more anal about concentricity and parallelism there is a way to do this job with a piece of sacrificial aluminum to be used as a face plate in the 3-jaw. But, I don't think that is necessary---just could be fun.

    ADDED: since I started plodding along with the keyboard I see there have been a couple more posts.

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    Thanks very much for the highly detailed reply. Yes, I have a reversible jaw 3-jaw chuck and live centre, so should be able to pull this off. I really like the idea of the grooves to hide the side junction and provide grip/decoration; clever.

    To be honest, I previously had in my mind that the mounting hole for the arm would need be off-centre for maximum stability while the arm is oriented in that direction. It hadn't occurred to me to put the hole through the centre (). Explains one of my previous comments about the centre mark.

    Cheers,

    Alistair

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    Alistair, if you don't have a surface grinder but want to make pucks like mine, there is another simple solution. After turning the pucks on the lathe, put a piece of 400 grit wet-or-dry paper on your surface plate with the abrasive size up. Put a few drops of water and a drop of dishwasher soap on the abrasive side, and then lap your puck against it for a minute. This will give you a surface that is flat enough that you won't be able to detect any rocking, even if you extend the indicator arm parallel to the surface plate and put the indicator tip on the plate. It will also be smooth enough not to damage the surface plate.

    Yes, it is correct that when a softer and harder material rub, the softer one tends to abrade the harder one, because the softer material embeds abrasive grit which then laps the harder surface. This argues in favour of carbide feet. So if you are using your plate and sliding the indicator base around it for six hours a day, every day, this might matter. But if you are a hobbyist then I don't think you will see any difference over your lifetime compared with the simpler solution I am suggesting.

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    Another solution to the "three-point" mount is to take a ball-nose end mill and machine three equally spaced
    holes in the base and then glue in three identical bearing balls. The idea of the three round inserts doesn't
    make complete sense to me--I doubt that all three will lay with their faces on a perfect plane. You could easily
    end up with high and low spots. The three balls, on the other hand, should create a near-perfect three-point
    mount...

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    Quote Originally Posted by ballen View Post
    Alistair, if you don't have a surface grinder but want to make pucks like mine, there is another simple solution. After turning the pucks on the lathe, put a piece of 400 grit wet-or-dry paper on your surface plate with the abrasive size up. Put a few drops of water and a drop of dishwasher soap on the abrasive side, and then lap your puck against it for a minute. This will give you a surface that is flat enough that you won't be able to detect any rocking, even if you extend the indicator arm parallel to the surface plate and put the indicator tip on the plate. It will also be smooth enough not to damage the surface plate.

    Yes, it is correct that when a softer and harder material rub, the softer one tends to abrade the harder one, because the softer material embeds abrasive grit which then laps the harder surface. This argues in favour of carbide feet. So if you are using your plate and sliding the indicator base around it for six hours a day, every day, this might matter. But if you are a hobbyist then I don't think you will see any difference over your lifetime compared with the simpler solution I am suggesting.
    I used to work on marking out and inspection. One of the first things you learn is to keep everything scrupulously clean. Before using any instrument that's going to slide along a table you pick it up and wipe the base clean with the palm of your hand.

    Before you put it back down you wipe the table where you intend to put the instrument with the palm of your hand. Where you're working gets wiped, always with the palm of your hand. It gets to be second nature after a while. Helping guys set up big jobs on planing machines I've wiped the whole of a 20ft by 8 ft table with the palm of my head.

    Grit shouldn't be getting anywhere near your instruments.

    Regards Tyrone.

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    Quote Originally Posted by ballen View Post
    After turning the pucks on the lathe, put a piece of 400 grit wet-or-dry paper on your surface plate with the abrasive size up. Put a few drops of water and a drop of dishwasher soap on the abrasive side, and then lap your puck against it for a minute. This will give you a surface that is flat enough that you won't be able to detect any rocking, even if you extend the indicator arm parallel to the surface plate and put the indicator tip on the plate.
    Interesting thought; though, I've had bad luck lapping a couple of small items, using abrasive paper on plate glass. I ended up sanding the surface convex such that it rocked more than when I started. Perhaps I was using an incorrect technique.

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    Quote Originally Posted by LKeithR View Post
    Another solution to the "three-point" mount is to take a ball-nose end mill and machine three equally spaced
    holes in the base and then glue in three identical bearing balls.
    Yes, and if the ball bearings would ever wear, the worn faces would be coplanar, though that point is probably academic. However, it would also have even less contact area than the carbide inserts.


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