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Finishing piston o.d. with toolpost grinder

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  • Finishing piston o.d. with toolpost grinder

    This is something I haven't done before, and I'm soliciting information from anybody that has. I have a new toolpost grinder from Little Machine Shop. (It doesn't actually fit on the toolpost but bolts to the topslide---Go figure???). I want to make a new cast iron piston, and hold the o.d. to +/- 0.001". Can I use the toolpost grinder to grind the o.d. of my piston to finished diameter? I would leave the piston attached to it's parent metal and turn it from piece of 1" diameter stock, then turn it with conventional mill tooling to 0.880" diameter, then without changing my set-up, grind the o.d. of the piston down to the size I require. ---About 0.876". It sounds logical to me, but again, it's something I haven't done before.---Brian
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    That is the basic approach. Include dressing the wheel and if you can best not to turn the grinder off after dressing.....make sure you cover up up everything well for dressing (paper towel and pot magnets is safer than rags). That's when the abrasive grit goes everywhere.

    lathe rotating slowly in the opposite the direction of the wheel.

    If you are grinding for accuracy, its not going to be really any better than using a very sharp tool and creeping up on the final OD. This is because you still have the same infeed resolution challenges. Attaching a 10ths indicator to the cross slide is one way to improve things. I use a digital one which makes easy to switch from imperial to metric.

    Also, lock the compound and tighten the cross slide gib as much as you can stand. That's probably hard on the lathe but its for a short duration. The reason is the vibration of the thing adds a new factor to the operation and can cause the cross slide to walk a bit, or least the flank of the thread can come off the flank of the nut....that can mess you up but if you've got an indicator mounted it should catch it.

    Personally I avoid it unless I need the finish or the material is hardened as its additional time to set up and given the choice I'd rather not subject the machine to abrasives. Except for the those two situations, imo, it doesn't get you much further ahead
    Last edited by Mcgyver; 01-16-2022, 01:09 PM.
    in Toronto Ontario - where are you?

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    • #3
      TP grinders are way over rated IMO. They're a hassle because of the need for constantly dressing the small diameter wheels.

      Practice a bit on a variety of materials. And try to get a wheel recommended for cast iron.

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      • #4
        Lots of good advice above. I would like to add, the best way to dress the wheel is with a diamond nib clamped to the workpiece or other round bar stock. In the picture below, it's the C clamp looking part just above the wrench. The nib sticks out opposite the clamp screw on the spindle centerline and faces the operator, with the clamp screw in back. The lathe is off for this operation. Then you turn the grinding wheel on and move the carriage side to side, lightly running the face of the wheel across the diamond. This makes the face of the wheel round and parallel to the lathe bedways. You can buy or make this item. Diamond nibs are pretty cheap these days. You want a wheel that won't break down quickly. A smaller wheel with the same specs as a surface grinding wheel would work nicely.

        https://www.ebay.com/itm/30408336257...MAAOSw3ExhAC~l
        Kansas City area

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        • #5
          Dumore TP's came with a diamond dresser holder that clamped around the tailstock quill. That was pretty good so for small diameter work you didn't have to crank the cross slide far to dress.

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          • #6
            Not to upset the applecart on this but why can't your lathe turn to better than +or- .001 just with regular tools? Or is it more about the final surface finish? Or just because you want to play with the new toys?

            From the bit of grinding I did to true up the 5C shanked ER32 collet chuck I got a while back using a Foredom clone as my "tool post grinder" I found that just taking off a half thou was a fair bit of work for the small low power and small stones. So if the goal is 0.876 then I'd suggest starting with 0.878 and just use the grinder for that last thou of material off the surface. Even with that it'll be something you likely find needs a few passes along with multiple final "dusting off" passes

            And then there's that other clever trick for more control over the DOC for the grinding passes if you haven't run across it already. Set the top slide at 6° to the axis and use the top slide's control to have a reduced feed for the cuts you make. The sin of 6° (well, technically 5.7° actually) is 0.1 so each thou of movement on the top slide control is 0.0001 feed on the grinder's DOC. Makes it easier to sneak up on and nail the final size.

            With the ER chuck I had the top slide set to the angle for the collets so I couldn't do it that way. But I used the angle to my advantage by putting a dial gauge on the carriage. So each .001 of advance towards the chuck resulted in .001 x sin 8° or 0.001 x 0.14= .00014. And even that caused some obvious stress to the flex shaft tool and multiple passes to ease the load before taking another .00014 bite. So it was pretty slow going just removing the roughly 5 to 7 tenths of material to true up the taper.
            Chilliwack BC, Canada

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            • #7
              I had the similar question when I turned my collet holder.
              The answers from that came out to:
              you can turn an OD as accurate as you can grind
              grinding leaves a mess
              grinding is suited when you have to turn a heat-treatable part oversize, then heat treat it, then grind to final size
              You can turn your compound to such an angle that the thou on the dial relates to tenths in reality diameter

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              • #8
                Ringo, agreed on all counts.

                When I did my 5C holder which is just mild steel I turned it then lightly sanded the taper with a suitable size backing bar. The idea being to just flatten the crests of the surface texture from the cutter so there was a smoother riding of the collets in the taper The actual "valleys" were left visible.

                Mind you... the siren's call of a new tool.... that's hard to ignore....
                Chilliwack BC, Canada

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                • #9
                  Originally posted by Ringo View Post
                  You can turn your compound to such an angle that the thou on the dial relates to tenths in reality diameter
                  Yeah, in theory a bit, but it only sort of works. The reason is there has to be clearance in the compound slide for it to work, and further it is away from a high quality perfectly scraped new condition, the worse its going to be. and you now you've made that clearance almost perpendicular to the axis you want to have really exact control over. Lock or remove the compound and set up a tenths indicator on the cross slide when need a lot of infeed accuracy. That and razor sharp tools. And hold your tonque just so.
                  in Toronto Ontario - where are you?

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                  • #10
                    You can't turn a diameter to as true and round a surface as you can grind. If that were so, nobody would bother to grind anything. Tool pressure with a grinder is as near to zero as you're going to get. Can you get pretty close with proper and careful turning? Yes.

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                    • #11
                      To each their own but I found it was easier than trying to bump the cross slide dial by that small an amount. And with how I like the compound gib to be set a hair on the stiff side at least on my lathe the final results worked out according to the mic readings. It's worth a try in any case.

                      And yeah, it's certainly the time where a nicely honed HSS cutter will shine for removing scant tenths on demand too.
                      Chilliwack BC, Canada

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                      • #12
                        Grinding is way superior.... here's my reasoning.....

                        The grinding wheel has a high SFM. And it is covered in thousands of cutting edges that are self-sharpening to an extent.

                        So, yes, the high SFM allows removing meaningful material in a short time, with a great finish. And, you do not need a lathe that turns at 25,000 rpm.

                        With a grinding wheel only 3" diameter, turning at say, 3450 rpm, your SFM is 2700FPM. If you want that SFM on a 3/16" diameter workpiece to turn it, your lathe must spin at almost 58,000 rpm. Maybe yours does, mine does not.
                        4357 2773 5647 3671 3645 0087 1276

                        CNC machines only go through the motions

                        "There's no pleasing these serpents"......Lewis Carroll

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                        • #13
                          Originally posted by eKretz View Post
                          You can't turn a diameter to as true and round a surface as you can grind. If that were so, nobody would bother to grind anything. .
                          Grinding on what?

                          When people bother with grinding (the 99.9% of machining that isn't home shop machining) its done a cylindrical grinder. While it has considerable advantages (and I regularly take advantage of them) its not really comparable to grinding vs turning on the lathe. Outside of using a cylindrical grinder, just who is bother to grind anything? I don't think many or very often, outside of what I said - if you need an improved finish or the work is hardened.

                          I think it an erroneous conclusion that grinding in the lathe is going to produce a more true and round surface than turning. The those will depend on on the accuracy of the ways and spindle rotating the work (same as cylindrical grinder). You do with grinding get a better finish, or less deviation of highs and lows on the surface, however even that's not always the case with a tool post grinder; they're light, vibrate, don't always have the best spindles etc.

                          Tool pressure with a grinder is as near to zero as you're going to get.
                          yes and no. There is definitely some force when grinding, and how much is there with a few tenths DOC turning? Long skinny work can be tough to turn, and bloody tough to grind.

                          Then there is the heat grinding generates; you pretty much need flood when cylindrical grinding but most guys lathes aren't going to have that. Remember, temp where the cut is happening is a function of tool speed and without flood the part will grow a lot more with grinding than it will with slow sub thou lathe cuts with a very sharp tool
                          Last edited by Mcgyver; 01-16-2022, 06:06 PM.
                          in Toronto Ontario - where are you?

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                          • #14
                            I am somewhat familiar with cylindrical grinding and would like to add one more reason as to why cylindrical grinding is being done by the industry. This is the accuracy which can be achieved on a consistent basis. It is relatively easy to produce 3/4" diameter journals on a grinder with .0003" tolerance. Try to do it on a lathe in production quantities.

                            In addition most of the cylindrical grinding is done between dead centers, which is the most accurate way of doing it. So grinder does not suffer from the typical lathe problems like spindle bearings and live center inaccuracies. To be fair there are multiple issues with grinder accuracy too, but they can be solved.

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                            • #15
                              The reason it is so easy to hit size with the grinder is because of the lack of tool pressure. If you use it properly for finishing it takes off what you move in, every time. On a finish cut at the grinder, you don't have to worry about tearing, lines in the finish, or any of that jazz. You get better bearing with interference fits, etc. Long list.

                              If you use the grinder only for finishing, with the proper wheel and a good dress, the part does not need to be flood cooled, and there won't be much heat generated. If you're hogging off a lot of material and/or using the wrong grit/grade/porosity of wheel, running with a poor wheel dress, etc. that's no fault of the grinder.

                              A toolpost grinder can do easily 90% as well as a "real" grinder, but it requires a little more know-how to get around things like chatter and needs good dressing etc. because it won't be as rigid. The nice thing about the low tool pressure is that you don't really need every last bit of rigidity to do a good job. It just makes things easier. The infeed can be got around by kicking the compound or by the use of fine infeed mechanisms that some toolpost grinders have built-in. I've got one that can feed in tenths, that is built-in as part of an eccentric adjustment mechanism. I generally use the cross silde for roughing-in, then the fine adjustment mechanism for finishing.
                              Last edited by eKretz; 01-17-2022, 01:24 AM.

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