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It's hard to be parallel

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  • It's hard to be parallel

    I've posted on this before, but have never come up with a solution. Once again, I need to make parallel rails, this time for a lightweight CNC machine. The rails will be 12mm or 1/2" ground shaft on 2 1/2" centers. The corresponding holes in the rail supports and especially in the slide need to be REALLY parallel, and the center-to-center distances have to be exactly the same piece-to-piece, or the slide will bind. The holes for the bushings in the slide will be about 7/8" diameter. The holes in the support will be 12mm or 1/2". In both cases, they will be through holes about 3.5 diameters long.

    I've never been able to drill/bore/ream holes of that depth that come out anywhere near parallel. I have a lathe, a vertical mill, and a shaper. Suggestions?

  • #2
    Do you have a digital readout on your mill?

    Do you have a mill?

    Comment


    • #3
      Can you arrange all the parts clamped into a stack to bore them all in the same operation?
      First one hole, then the second without moving the part on the machine table or loosening the clamping arrangement in any way?
      That way you will keep alignment to the Z axis of the machine and even if your head is tilted or the table is off, it will still come out parallel.

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      • #4
        Or do you mean you can't bore parallel for a single part 36mm deep? In which case, the only suggestion I have is to take very light cuts towards the end and make a few passes to take out the spring you are suffering with.

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        • #5
          There are a number of ways to accomplish this. My Unimat lathe has a bed that utilizes similar, parallel rails. It has a cast iron bed and that bed was milled in, what I am sure was a single pass from headstock to tailstock ends with two Vee shaped milling cutters to form two Vee shaped notches for the rails to sit in. The result is a very parallel pair of round rails. Or perhaps it was two passes with one Vee shaped milling cutter.

          The other way that I would consider would be to use two plates with holes drilled and reamed in them. They would be tightly clamped together for these operations and both pairs of holes would be made at the same time. Drill them, then ream them to exact size. That way they will be dead on parallel. Well, probably within a few tenths anyway.

          A final method, which is potentially more accurate than either of the above, is to have at least one of the holes made adjustable. Then you can adjust the rails to parallel after they are assembled. This may be done with a number of mechanisms, including screws and/or shims.
          Paul A.
          Golden Triangle, SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #6
            I have a K&T horizontal mill fitted with a Bridgeport M head. My experience has been that if I drill/bore/ream two deep holes in a piece of work, the exit holes are not the same distance apart as the entry holes, and the direction that the holes wander is random. The deeper the hole, the greater the drift. It only takes .001" of deviation from parallel to cause a slider to bind. Either my boring is actually following the original drilled hole to some extent, or the boring setup itself has some random variations. Or perhaps .001" in 3" is too much to ask. If so, I may have to change the design to use slots instead of holes -- those I can make parallel. Or drill oversize, insert the bushings loose, insert the shafts in the bushings, align the shafts using v-blocks, and glue the bushings in. This has worked for me before, but it's a PITA. I'd rather drill/bore/ream and be done with it, if I can get the accuracy.

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            • #7
              You could try using a 'D' bit for drilling straight holes. They're very easy to make.

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              • #8
                Is the mill head trammed? How about the fixed jaw of the vise? Is the floor of the vise parallel to the table? Are the drills you are using straight and properly sharpened? Seems like this is a setup and tooling issue, not impossible to accomplish without a DRO.

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                • #9
                  I've done a long bore to size by making a boring bar that's supported on both ends. One instance was in the lathe with the workpiece mounted to the crosslide. The boring bar is supported on both ends and both holes will be parallel. On that job exact spacing wasn't critical, but if it were I'd get the second hole to a little undersize then check spacing and adjust as necessary. The leadscrew is just a rough approximation for distance, but it can be tweaked and re-locked while I'm taking additional cuts and re-checking the numbers.

                  In another instance I used a mill with a bearing support for the lower end of the boring bar. Not enough quill travel but quill + knee got me the distance required and assurance of concentricity. This job was a single hole, but again, if I'd needed two to accurate distance I'd have measured and tweaked.
                  .
                  "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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                  • #10
                    The line boring technique suggested by TGTool is the best way. Using boring bar that is supported in the chuck and tailstock will give you very accurate holes when you are done. Reaming to clean up the finish will be the last step.

                    Before you drill anything, think of how you will index the part when you move it to do the second hole.

                    Dan
                    At the end of the project, there is a profound difference between spare parts and left over parts.

                    Location: SF East Bay.

                    Comment


                    • #11
                      Originally posted by alsinaj View Post
                      I've never been able to drill/bore/ream holes of that depth that come out anywhere near parallel. I have a lathe, a vertical mill, and a shaper. Suggestions?

                      Change the design to use commercial parallel rails for alignment with an adjacent rail for support. If you had a location in your profile and it was anywhere near me I'd let you have a 5 foot length of rail and some bearings for the price of a pint of beer.
                      Location- Rugby, Warwickshire. UK

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                      • #12
                        I can see where drilling and reaming 1/2" ّ holes 3 inches deep and maintaining parallelism within ±.0005" could be problematic.

                        A boring operation would conceivably cure the parallelism problem, but trying to run a boring bar that would fit into a 1/2" ّ hole that is 3" long would be problematic by itself. I think the only way you could do this is with the previously suggested bar supported on both ends. If you had the room to make the holes in your brackets bigger, say 1", or whatever size you though you could get a decent size boring bar through, I think you would have less problems achieving the parallelism that you need, and then you could then use bushings to get the resulting oversize holes to hold the rail material.

                        Another option to try would be a piloted reamer. I don't know that it would be an off the shelf item, but I know that Chadwick & Trefethen would make one for you. If you said what the bracket material is I didn't catch it, but if it was anything other than steel you could use the adjustable feature to start with 2 drilled holes and use successive passes to sneak up on your desired bore size, and with both ends supported it would tend to correct any out of parallel condition. I say anything other than steel as using the adjustable blade reamers in steel can be a problem. If this is the case on of the expansion reamers might be a better option.

                        I just checked their website, look at the MB and MBB style reamers. http://www.chadwickreamers.com/products.php

                        Dave

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                        • #13
                          Well if your getting into a fix, redesign, you need two parallel rails, into end plates or blocks.
                          Instead of drilling cut vee grooves in 4 bars, at the same time, clamp the rails using two of the end blocks, a 90 degree vee, round rail then cap with the other upturned block with vee cutout if you see what I mean.
                          The vee can be milled or cut on the shaper.
                          When it comes to the actual rails you could cut a recess in each end of the round rail that corrisponds to the width of the end clamp.
                          I know it sounds complex but imagine a vee block with a round bar in it and another on top, if you get my drift.
                          Personally I would just stick 2 plates together, tack weld them, drill and ream a couple of holes, separate the plates and it will be parallel when assembled, always is,
                          Mark

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                          • #14
                            Hmm- are you stuck with using round rods? How about square- I don't know, but possibly you can get accurately ground square rods. If so, I would be highly tempted to do that. You could machine parallel v grooves in a single block of material, then cut that into three parts- one becomes the slide and the others are the mounting blocks. Taking the concept a bit further, you machine the v grooves in a single block of material that's long enough to make both mounting blocks, the slide, and three other matching pieces. Two of these other pieces would be the clamps that hold the rails to the mounting blocks, and the third would be the bottom for the slide. You would be able to adjust the spacing of this third piece to basically take all the play out of the slide while leaving it just free enough to move as easily as you want. You eliminate all the potential problems of holes not being perfectly sized for the rods, in addition to eliminating parallel error.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              Another thing to consider is that you have a relatively light M head attached to one of the most ridged machines you can find - with it's own spindle. Horizontal mills are excellent machines when you start thinking sideways.

                              Consider turning the work axis 90 degrees and using the main spindle for this. You don't have a lot of daylight in that orientation but this job sounds like it will fit. Takes some creative setups sometimes - angle plates and strap clamps ext. and it is hard to see what you are doing. Still, you just can't get much more solid than that. If you want to try the line boring idea you have the arbor supports to hold the tailstock.

                              This is assuming that you don't also have a K&T universal milling attachment. Keith Fenner has one for his K&T and uses it like a mini-HBM

                              You can tell I'm a fan of those old beasties even though I do miss the quill sometimes.

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