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Math is hard!
开云体育I had a real problem with math in hi school , to the point where i got frustrated I just got up & walked out of the class room . It took me years to appreciate all the numbers & logical thinking . good luck animal On 3/30/24 12:57 PM, Nick via groups.io
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Calculating the bolt circle for a chinesium chuck of unknown dimensions... |
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Something strange happened when I tried to reply to the incoming email with this message.? It bounced because it seemed to have picked up the old yahoo groups address somehow.? Apologies if groups io delivers it twice.? What I was trying to say is this:
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The hardest part is measuring accurately the distances between the bolt holes. ?The way I do that is to measure the distance between?the inner edges of the holes and the outer edges of the holes using sliding jaw caliper such as a vernier, dial or digital type, then average the 2 dimensions. ?Call this Y, the centre distance for the holes. ? ? Unfortunately your trigonometry is too blurred to read - I'm guessing that the internet automatically reduces resolution when we post anything - but it looks like you have 3 equispaced holes, so the formulae for an equilateral triangle apply in order to find the pitch circle. ?Radius =?Cos 30 x Y / 2. ?(Cos 30 ?= ?0.866, near enough). ? Having drawn the circle using the radius found above, keep your dividers set at the same radius and you can then divide the circumference into 6 equal parts. ? Even numbers of holes is easier - just measure the spacing of the holes across the diameter. ?Mark the centre of your fist hole, draw a line from this through the centre, extrapolating to find the opposite one, then for 4, bisect from the 2 centres at the vertical positions to find the 2 at the horizontal positions. ?For 8, bisect again. ?For 6, proceed as though for 3. ? Eddie |
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I have gotten a bit lazy in my old age. Back in the past I would have calculated this by hand on paper. Later I wrote a BASIC program to do the calculations. These days I use one of two methods:?
If I am drilling holes in piece of metal that is not too big. I will just mount it on my rotary table, find the center of the bolt circle, move the table away from the center by the radius of the circle, decide my starting point and use the degree wheel to move the rotary table however many degrees it takes. 90 degrees for four holes, 72 degrees for five holes, 60 degrees for six holes, etc.. This would be the mechanical way of doing things and this works as long as I can fit the metal, wood or plastic on my milling machine.? CNC can also handle this with a servo driven table or by entering the center, how many holes and the first hole coordinates. I don't have a CNC mill so have to use some manual method, which is satisfying my desire to do things the more by skill than automation.? If I am going to have to layout the bolt holes on something that cannot be done as above, I would just use a web bolt hole calculator, then layout the measurements. Find the center of the circle, draw a circle with a divider, find my starting hole position on the circle and set the calipers for the distance between holes. Then scribe lightly from hole position to hole position, making sure the points on the circle exactly match the scribe distance. Doing this in reverse around the circle is a good check. Once I am sure the points are all correct, I center punch each and drill. Drilling a bolt circle in a business/production environment, I would probably choose to use CNC or a digital readout setup which can do hole position math. |
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Good info, thanks! I'll copy that for future reference.
My "math is hard" comment mostly comes from the backlash Mattel received a handful of years ago when they released a talking Barbie, and that was actually one of her preprogrammed phrases! Cue female STEM student shrieking... No, I actually got it right on. I got the bolt center measurement by threading the bolts in until they bottomed, tightening them square in their bores. I used the corners of their hex heads to scribe lines across their heads to find and strike a center in each head. I used the caliper to measure between centers and verify each to each. That's how I got my equilateral side measurement. I took this, divided one side in half to create a right triangle, and used the good old Pythagorean theorem to get a center line. The epicenter where each crosses each is 2/3c, so that gave me my bolt circle radius. I subtracted the register hub diameter from the calculated bolt circle diameter and divided by 2 to get the difference movement at the cross slide. I used a trepanning tool to indicate off the register hub of the back plate, then moved out that cross slide distance and inscribed a mark there. I then punched a mark at an arbitrary point on that circle, used the dividers to mark my other two points from that point, and punched those as well. But problem- the distance between points? A and B and points A and C were the same, the distance between points B and C was different from the other two! Slightly longer. Oddly enough, what hamstrung me was not taking into consideration the fact that the tip of the tool was 0.03125". This drew the tip of my center punch out away from my actual proposed bolt circle. Had I remembered this, my marks would have been right on. As it was, I went back to my calculations and used the difference in calculated bolt circle vs register hub diameter to measure out from the hub and discover my mistake. Okay, so now! Got my new marks made and punched, they are ACTUALLY equidistant at my actual bolt distance, time to drill. Went to the drill press, chucked up a center drill, and ... wait. Why does it look like it's wobbling? Put a dial gauge on it and discovered my chuck is THIRTY-SEVEN THOUSANDTHS OUT OF TRUE. Holy crow! Well, what do you want from a 1970s Craftsman press that was made for drilling wood... yeah, that weird one with the 1/8" belt and the 850 RPM "slow" speed. Use what ya got, except the chuck is pinned in place instead of being either threaded or Morse tapered... and the point at which it's out of true is right AT the roll pin, so it was drilled off square from the factory. Balls, there's no fixing that without a lot of surgery. Fortunately, a neighbor has a Shop Fox benchtop mill he is generous enough to allow me to use. It may not be a Bridgeport, but it should be precise and square *enough* to let me drill my three holes with reasonable precision so I don't have to try to make my own Adjust-tru chuck. So that's where I'm at. Plate is done, holes are marked, all I lack is drilling them so I can finally mount that chuck. Frustrating! So close. |
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One thing that threw all our talk of measurements and ascribing into a cocked hat is the fact that this circle *has no center*- the middle of the back plate is the hole where the spindle goes through. So even if I could find true center, I can't reliably indicate from it, as there's no material there.
I suppose I could have made a temporary hub center from a piece delrin or whatnot, let me just chuck that up and... <facepalm> |
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I bought a pair of these for each my 6" & 12 " Dial calipers 30+ years . Now I have several more caliper's that were my dads I should put a pair on one of them Y leave them on . Pretty handy if ya have more that one caliper .
? Sorry for some reason I couldn't get just the pic to send .? This set in much more user friendly on the ole budget . ? animal |
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One problem to address is getting the exact center of the backing plate to exactly line up with the exact center of the chuck jaws when they are clamped on something tight. This might be a few thousands or more, off of the center of the hole in the back of the chuck, depending on the quality of the chuck machining.?
Two choices: You could try mounting the backing plate accurately to the machined back of the chuck, and then carefully check and machine/grind the jaw gripping points to make them grip exactly on the center of a metal rod. Alternately you can center a pin in a rotary table or mount a rotating pin on a mill table centered under the mill chuck. Flip over the new chuck and grip the pin tightly as you would if you were mounting a rod in the chuck on a lathe. This will remove any slop and give you the exact center of the chuck when holding a round object. Move the table over to the radius of the listed bolt circle and as you spin the new chuck you can determine if the chuck jaws are really centered when under pressure. If it is off, you can see that the bolt holes will be off a little one way vs. the circle when the new chuck is rotated. You could take a dial indicator in the mill chuck and line it up with a machined surface on the new chuck to see how much off center it is and how much adjustment in the position of the bolt circle on the backing plate will be necessary to get as close as possible to the center point of the jaws when the new chuck and mounting plate are assembled an chucked in a lathe. So a slightly offset bolt circle on the mounting plate will correct the position of the jaws center position.? If it is a minor offset, a few thousands of correction might be possible with just clearance in the bolt holes. Point here is that if you line up the bolt holes perfectly with the chuck center line, mount the chuck in the lathe and chuck a machined rod in the jaws and tighten, there may be a slight offset to the chucked rod as it spins. If it is a lot, it might be hard to adjust out. Good to know this before you get too far machining the face plate. You will also want to mount the face plate in the lathe and machine the back and then the front to make sure the face plate runs true before drilling the holes and before the chuck is mounted. Worst case the back of the chuck might need a slight smoothing if it seems slightly off of true when mounted to the backing plate.? The farther the chuck jaws are from the pressure edge of the spindle the more any run-out is amplified.? You probably already know this but this thread might be read in the future by a person new to setting up a new chuck with a separate mounting plate. I have done a couple of these and it takes patience to assure the chuck runs true when finished mounting to the lathe. |
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Almost everyone has the drilling accuracy problem. It is impossible to drill a 0.500 hole with a 1/2" drill bit in a drill press. All drill press chuck assemblies have a little play, or they would not be able to move up and down very easily, or not at all. Drill chucks are not guaranteed to grip perfectly true. Drill bits generally have some wobble or flex, and have a diameter tolerance range.?? Reamers in collets on a lathe or mill will get you a lot closer but it still may take a lot of effort to even get close to 0.500.
In the case of your face plate adventure, you could put hours of exacting machine practice and science into getting the chuck almost exactly aligned. Or you can accept the realities of the tools and equipment you have and make allowances to calibrate the assembly, the best you theoretically can, after assembly. The little bit of slop in the holes might be needed to slightly move the chuck to get the center of the jaws aligned with the centerline of the lathe spindle and tailstock center. Having no tolerance will mean you may never be able to get it right. |
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Mike is right about the usefulness of the center adapters. Guessing just does not cut it. Even the trick of putting a bolt in the hole and using the hex head and surfaces is not always accurate as most bolts have some tolerance on the edges and straightness. The bolt trick is however a good one in many cases.
There are a lot of challenges for machinists to measure things accurately. Cabinet makers working with hardwoods can have some challenges as well but not quite as bad. As far as calculating things goes, I have a binder filled with exhaustively calculated measurements for things I expect to do in my shop, in both metric and imperial sizes. A computer and usually a spreadsheet did all the tedious calculations.? I just open the binder and look at a chart for most anything I cannot do quickly in my head or simple calculator. I have spreadsheets set up for more complex things where I just enter a couple or few numbers and it shows the answer. Been adding to this for 40 years so I do less and less math over time.? If I run into anything outside of this, I just look up a calculator on-line. Am I lazy? Maybe. But it took some time to collect this info. I would rather spend my time now making or fixing something rather than sitting with a sheet of paper grinding out math problems. |
开云体育At the risk of posting something most people here already know: Our machinist (this was long ago) at work told me one day that if you drill a pilot hole first followed by a slightly undersize drill bit of the desired size to finish, ?the result be a hole just less than the drill size. He couldn’t explain why that happens. Part of the reason is I think that the drill cuts so little it’s acting like reamer and there is nothing forcing it off Center. I do this frequently.?You probably also know that if a drill is sharpened with the point off Center it will cut a larger hole than nominal. I do that too when I need a little clearance.? On Mar 30, 2024, at 6:56 PM, tgerbic via groups.io <tgerbic@...> wrote:
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开云体育Oh boy I need to edit that posting! The third step I omitted was finish the hole with the final drill size. The hole size you really want.?Me bad On Mar 30, 2024, at 7:20 PM, Jerry Cc via groups.io <Pfaff1222@...> wrote:
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Pilot holes are a good idea in most cases, especially if the ultimate drill size is vary big. Large drills alone may not track due to cutting torque.? Accurately positioned drill points matter a lot as well. It is also a good idea to use a center drill to start a hole that need some accuracy as they tend to be much more accurate for centering the hole and much more rigid compared to most drills people use.?
Your point is good since not everyone knows this that I have run across. You are commenting on a forum that has new people just starting out with metal working and need to pick up any good info we can provide. My dad was a machinist for 40 years or so, and also a hobby woodworker. I was lucky to be able to learn interesting things about both areas as I was growing up. Now many seem obvious to me, however there is always something to learn just around the corner no matter how much you think you know. |
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Yes i should have added using a Center drill prior to the small pilot hole. I do this mostly on the lathe.
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Another tip I found on the www is using a steel ruler to find the Center on a shaft. It works best I found if you use a pointed shaft mounted in the chuck. Balance the rule over the shaft, lower the point. When the ruler is level you are darn close center. Quick and dirty method. There are some very good YT videos out there. On Mar 30, 2024, at 7:45 PM, tgerbic via groups.io <tgerbic@...> wrote: |
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Another cheat I use on the lathe that sort of works, to stop a drill from wandering is use something in the tool post pressed near the tip of the drill bit when you begin the drilling operation.
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On Mar 30, 2024, at 8:12 PM, Mail <Pfaff1222@...> wrote: |
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I may not have understood the original question but I thought it referred to finding the hole centers rather than drilling them. As likely as not before NC stuff became so common. die makers frequently had to find hole locations and it was done with transfer punches.? I have fitted backing plates to chucks several times with no mathematical pain.? Most chucks have a large round recess in the back.? To fit a plate to the chuck it will be required sooner or later to machine a round boss in the front of the backing plate to? fit precisely in that recess.? Usually by trial and error and sneaking up on it.? Real transfer punches are carefully machined to an exact diameter with a conical tip on them and they are hardened.? I have used setscrews as poor man's transfer punches.? Once the boss on the backplate is a snug bit in the back of the chuck, put the appropriate size setscrews in the holes in the back of the chuck just barely above the surface, maybe put some machinist's blue on them and set the plate on the back of the chuck.? Tap the plate and gently remove it and you should have your locations.? Drilling the holes slightly over size is a good idea as the boss and recess should hold the plate and chuck in position, not the screws.
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I hope this helps, Jack On 3/30/2024 10:17 PM, Jerry Cc via groups.io wrote:
Another cheat I use on the lathe that sort of works, to stop a drill from wandering is use something in the tool post pressed near the tip of the drill bit when you begin the drilling operation.On Mar 30, 2024, at 8:12 PM, Mail <Pfaff1222@...> wrote: |
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