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  • Tool post grinder rookie question

    Does anyone know off-hand what the maximum safe rpm is for a 4" grinding wheel? I am planning on building the grinder that was featured in the HSM magazine back in '99. I have ordered the archived magazine but have to wait a few weeks to get it. I have looked at a few vendor websites and they do not specify. My concern is that I am going to power the grinder with a trim router that turns 25,000 rpm. Obviously I am going to have to gear that down but how much?

    If anyone has any input, I would be glad to receive it. I'm still pretty new at this.

    Thanks.

  • #2
    One of my 3 inch cutoff discs says 13,500 rpm max. I never spin them that fast, not even close. A 4 inch similar disc would probably have a rating of 10,000 rpm. That's not a grinding wheel though- you'd be best to look into it yourself just so you know the published figure for the wheel you would be using. Still, I'd run it slower anyway.

    Many grinders turn at 3000 rpm or so, and with 8 inch wheels. If that is safe for an 8 inch wheel, then a 4 inch wheel would probably be within the safety factor at 5000 rpm. Maybe check out the optimum surface feet per minute for the wheel you'd want to use, and calculate rpm based on that.

    The other thing that concerns me is your choice of motor. You might be better off starting with a slower speed motor so you don't have to gear it down. That saves having to come up with a reliable reduction drive that can handle the input rpm. Other than that, I'd say you need at least a 5-1 reduction.
    Last edited by darryl; 05-20-2010, 04:21 PM.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      Will depend on the exact brand/model of wheel, but for example....... McMaster Carr rates their "economy" 4" wheel at 6210 rpm, their "premium" 4" wheel at 8120 rpm. Those are wheels intended for bench grinders.

      They offer a "straight snagging" 4" wheel rated at 13,560 rpm.

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      • #4
        I found this TP grinder on this web-site http://www.strappe.com/hsm.html#grinder. They dialog says they used a router???? I will wait to read the entire article and plans before getting too serious about it but the question was bugging me. Like you, I was questioning the choice of motor.

        Thanks for your help.

        JM

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        • #5
          Sounds like you're referring to the TP grinder design by D.E. Johnson. His was powered by a Ryobi trim router, and the plans were published along about that time. I too had planned to build that, going so far as to obtain the bearings and belts. But to this day it remains undone.

          Be advised those plans were published serially over two or three issues. They're also presented in the Village Press hard cover book "Shop Wisdom of D.E. Johnson", along with many other interesting projects/articles Mr Johnson had submitted. If it were me, I'd just go ahead and get that book rather than the magazine back issues. Though that'd probably be a few bucks more.

          As to the RPM's, I think you'd want to get the grinding wheels first, to see what rpm's they specify, and then do your pulley calculations from there.

          But I'm looking in his book now, and he mentions 9930rpm and 8280rpm (max)for 2.5 and 3" wheels. More generally he is shooting for the range of 5500 to 6500 sfpm, so I'd think that would be a good figure to use in your calculations.

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          • #6
            find the wheel you want to use and use the manufactures recommended surface speed (feet per minute). Its the surface speed you should design to which will be a less than the maximum safe speed
            .

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            • #7
              JB, I have done this -- and it will work. The pic is of a 'Frankenstein' little grinder that I was working on when I came up on my Cuttermaster T&CG (it has since been abandoned and sits on a shelf in the shop) Its primarily parts of a Tinker and one by Glenn Wilson from Projects in Metal in 90-91?. The router in on a mount pretty much direct from Glenns description. He had used a small ac motor that I also tried, but found underpowered, so I converted to the HF trim router with their speed controller -- theyre sooo expensive!, what $25-30 and $12-15?

              I dont have any idea of the max rpm this thing will turn with that pulley ratio - no doubt way to much! That set-up was retained from using the small ac motor and just used to check viability with the speed controller, which performed very well and would vary the speed nicely, though, as you would expect, when dropped too low would lose power badly. At one point I moved the controller to the end of the cord some 6-8 feet away around the corner and turned the thing up to max and ran it for 10-20 seconds and it handled that speed with a yawn. I had planned to make up a stop to fit on the controller at the "sweet" spot to prevent over-revving, and of course a wheel guard, but never got around to it.

              Oh, in light of the recent discussion on air bearings, that was an attempt to convert a spin indexer to be an air bearing - it had a modicum of success...




              Last edited by Bill Pace; 05-20-2010, 05:30 PM.
              If everything seems to be going well, you have obviously overlooked something........

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              • #8
                Don't want to hurt your feelings. but this is a dumb question. There is no "safe" rpm for a wheel diameter. Each wheel will have a rated speed on its blotters or wheel surface or somewhere. A further consideration is that a lathe is not , and never will be, a grinder. What makes a grinder is an incredible level of precision that is not found in a lathe. Even if you had a lathe with this level of precision, a toolpost grinder would remove it in short order. If all you want is a ground appearance at the the expense of your machine,then a toolpost grinder is the way to go. If you need the accuracy of a grinder and have only a lathe, consider jobbing out the final finish.

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                • #9
                  Generally vitrified wheels run at 6000 ft per minute,so 6000 x 4 / dia = 6000 x 1. so 6000 rpm should be good. Use blotters and guards of course. Resinoid wheels usually run at 9,000 fpm. Peter
                  Last edited by Oldbrock; 05-21-2010, 04:12 PM.
                  The difficult done right away. the impossible takes a little time.

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                  • #10
                    Originally posted by tdmidget
                    Don't want to hurt your feelings. but this is a dumb question. There is no "safe" rpm for a wheel diameter. Each wheel will have a rated speed on its blotters or wheel surface or somewhere.
                    That is right enough.

                    the idea of assuming in the absence of other info, that a sfm of around 5000 is attainable is reasonable. Personally, I prefer to see the number actually marked on the wheel............



                    Originally posted by tdmidget
                    A further consideration is that a lathe is not , and never will be, a grinder. What makes a grinder is an incredible level of precision that is not found in a lathe. Even if you had a lathe with this level of precision, a toolpost grinder would remove it in short order. If all you want is a ground appearance at the the expense of your machine,then a toolpost grinder is the way to go. If you need the accuracy of a grinder and have only a lathe, consider jobbing out the final finish.
                    Most of this should be simply ignored.

                    A TP grinder will give a finely finished surface of at least the same accuracy that the machine will 'turn" a part...... Which means that you can avoid the "fuzz" and still get a good turned part to finer diameter tolerances than available with turning.

                    Far from being an 'appearance only" matter, as TDMIDGET implies, it will be a perfectly functional ground surface. I have ground journals for bearings etc this way and it works VERY well. better than turned surfaces, and better than filing or holding up a strip of sandpaper etc.

                    As for "removing it (the accuracy) in short order", that is also pretty extreme, even approaching "bunkum".

                    Machines wear.... unless you never use them, in which case they may rust away with no benefit to anyone.
                    If you allow grinder dust on your machine it will wear faster, so drape and guard the machine, and don't make a habit of grinding on the lathe.

                    But when you need to, if properly done it is a good accurate technique that you would do well to not ignore.

                    Myself, I hate doing it, and I am setting up a grinder in a far corner specifically to avoid the necessity of doing it on the lathe. But I can, and have, used a TP grinder on the lathe. I got a very good result, and you can too.
                    Last edited by J Tiers; 05-21-2010, 11:51 PM.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      Just be careful

                      +1.

                      I agree with JT and McGyver and others similar.

                      If a router is to be used why not use one with a variable speed setting?

                      Keep the wheel well-dressed.

                      An issue not to be ignored is the length and diameter of the wheel spindle "stick-out" from its nearest bearing.

                      Don't go "cheap" with wheels. A "Norton" wheel is almost a "must" as they are true and well-balanced right out of the box. There is nothing worse than a wheel that is out of balance or has a "side-ways wobble" - or both as it will not only do your grinder or the job no good but is potentially quite risky/dangerous.

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                      • #12
                        Thanks all. My main goal is to be able to grind the tapers of shop made mandrels and arbors. The second goal (maybe first) is just another project. I have been doing this for about 1 year now. I have figured out that this is a perpetual hobby. My friends ask "what do I make with my lathe?". My answer...............Make tools, so I can make more tools!

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                        • #13
                          you can half the speed of a grinder/router by connecting a diode in series with the supply, sounds complex but can be done in 5 minutes.
                          The speed of a grinding wheel varies depnding on the application, material its made from and the manufacturer, the speed should be quoted in the spec for it.
                          Dont forget to do the tap test with a spanner to make sure the new wheel doesnt have any flaws.
                          A wheel exploded at leyland motors where I used to work, it was on a crank grinding machine, it was 60" dia and made a hell of a mess.
                          Build it, bodge it, but dont buy it.

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                          • #14
                            You cannot make any assumptions about safe wheel speed. There is no rule of thumb or formula that can be applied. The maximum speed for a wheel is what the manufacturer says it is. The way this is determined by them is to spin test the wheels to destruction in a large chamber with walls of steel 1/2" thick. The chamber at one manufacturer that my wife has toured looks like the Incredible Hulk was locked inside and tried to get out. That particular manufacturer spin tests 100% of thier vitrified grit wheels because those wheels are 100% hand made by expert wheel hands. Sometimes one has a bad day and his wheels explode.

                            They didn't test the one my wife made because it had an intentional defect.



                            A much more reliable test for cracks on a vitrified wheel is to strike a tuning fork and hold the handle to the wheel. If it is cracked the tuning fork will go dead immediately.
                            Last edited by Evan; 05-23-2010, 06:38 AM.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Although most tool post grinders are made with a single speed, there is some logic behind using a multispeed or variable speed arrangement. If you ever want to grind a bore or an internal taper, you will be forced to use a small diameter stone and then a higher speed will be nice to have.

                              I have made adaptors to allow using my Unimat headstock as a tool post grinder and it has speeds from a few hundred to about 7500 RPM. I also have mounted a fan speed control for use with it so the range is essentially continously variable. This seems to be a nice range which can match most available wheels.



                              This picture shows an internal taper being ground at max RPM.

                              And before you jump on me for using a drill chuck for holding the stone, the stone was dressed after being chucked so it was true. Besides, I was making a collet holder for the Unimat and that is what I would use now.
                              Last edited by Paul Alciatore; 05-23-2010, 11:47 AM.
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!

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