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Spindle recommendations for a lathe toolpost grinder?

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  • #16
    make it yourself. its fun.


    • #17
      Like the rotozip idea, the things useless for Sheetrock, it's easier to use a knife or stab saw


      • #18
        I can fully appreciate building one, but recently searching the web for one myself
        this popped up for your size lathe;


        • #19
          "I can fully appreciate building one, but recently searching the web for one myself
          this popped up for your size lathe;"

          Unfortunately they do not provide any information, such as RPM, is it capable of internal grinding (I doubt that), what are the bearings? To produce good accuracy and good surface finish you need good bearings and dynamically balanced rotating element. To me the internal grinding capability is the most important, I can produce a decent OD by other means.


          • #20
            I'll add a few thoughts of my own here, possibly a little offbeat since this has been discussed here before. The main issue I have is with the bearings. First off, the ideal bearing would be an angular contact pair with some preload, possible close together, with a suitable rear bearing. All would be precision made to ABEC 27 or whatever the eyebrow-raising grade is these days. The better the more expensive, etc, and the contruction of the spindle would also be of exacting design and of some specific alloy. If something in this arena is desired, then I'd suggest a bought spindle- and it won't be cheap to say the least. But if we stick with more normal bearings, then high speed tools like routers, dremels, etc having shaft diameters ranging from 1/4 inch to perhaps 5/8 inch would be the pool to choose from. Either make your own, or modify an existing tool.

            The larger the shaft diameter, the more rigid the spindle is going to be, and the larger the bearings will need to be. The tradeoff is rpm. The faster you turn it, the more energy as heat is going to be generated and lost. If you have a pre-loaded front bearing pair, then more energy is lost there. Seals waste more energy than shields, so you can choose shields only, or one seal and one shield, or one seal and no shields, one seal on each bearing and one shield on each- you get the picture. I've seen router bearings with one seal and one shield, but usually they have shields only. This allows a higher speed with less heating, but is subject to intrusion of dirt and expulsion of the bearing lube. It seems that a good lifetime can be had using shields only, but nothing stops you from adding additional shielding in the form of a non-contact labyrinth type of construction. You can also forgo the bearing grease and use oil instead, as long as you include a method of adding oil often, and clearing away the inevitable ooze-out.

            Leaving that aside for the moment, a larger bearing will have a lower rpm rating in general. Hand held dremel type things don't have large bearings, so they will be deficient in rigidity although they might be capable of operating at 35000 rpm. You might need to stay with the smaller diameter bearings if you are grinding small diameter bores, but otherwise a larger shaft with larger bearings will give you better results. Two things here- with small bearings and lots of stick-out of the tool you will have flex regardless of how tight the bearings are. A larger shaft can stick out further before the flex becomes problematic. And secondly, there are two sizes of shank in common use, 1/8 inch as in dremels, and 1/4 inch as in typical routers. You can mount an 1/8 shank in a 1/4 inch collet using an adapter, but you can't go the other way. You do have the option to use a 1/2 inch router shaft, and if the collet end isn't too large to fit the work you'd be doing, then that will offer the most rigidity, the longest 'nose' before flex becomes an issue, the ability to accept all shanks up to 1/2 inch, but probably a reduced top end rpm. Personally, I have built and experimented with toolpost machines with 1/8 inch and 1/4 inch capable shafts- the only use I have for the smaller one is where I simply can't fit the larger one into the set-up. Small hole internal grinding is about it, but then the result is dubious anyway because of the lack of stability at the business end of the tool. My next tool post machine is going to use a 1/2 inch router spindle. I will get around the stability problem by custom making a tapered shank to carry the actual tool if I need the reach, etc.

            Something else I thought of- and the actual reason I'm posting this- the spindle shaft has a certain amount of weight inherent in it. If it's not part of an armature, that weight is minimal. If you are going to be using an external motor to drive it, you do not have the weight of an armature surrounding the spindle. Consequently you don't have the stabilizing effect that that mass would add to it. Perhaps a spindle design could include as much weight as you can fit within the small confines of the housing. Either add a steel sleeve to an existing spindle shaft, or if you make your own shaft from a blank of suitable steel, then start with a larger diameter piece and leave as much meat on it as you can. The extra weight of the spindle would help to stabilize the rpm at the end of the flex cable driving it, besides allowing for a better finish from the tool being used. This is conjecture on my part, but it makes sense to me. The only issue I see with leaving the spindle heavy is if its resonance point corresponds to a resonance in the lathe. I can see that being an issue. At any rate, having full control of the rpm though an electronic control would give you the ability to select a 'quiet' rpm to run at.

            I'm not going to discuss the obvious issues of making your own spindle at this time. Suffice to say that if it's not well-machined for least run-out or balance, then it likely won't be satisfactory.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #21
              DICKEYBIRD - that is a nice project you have going there One of the things I want to do is internal grinding of roulette curves, so I will need small bits and high speeds for that. Cheers for the tip on the vortex cone, those things look great for general dust collection.

              polaraligned - yeah, T=r*F, the reactive cutting force would ramp up as the diameter ramps down

              boslab - thanks for the extra details

              J Tiers - Why is it that you don't like grinding on a lathe? Is it because of abrasive dust getting all over precision mating surfaces? Or do you just find it sucks?

              Paul Alciatore - that looks pretty versatile, and if it gets the job done to required tolerance, who cares what people think of the methods?

              mikey553 - This die grinder?
              I was looking at a very similar (though more expensive) die grinder by Makita. - and I have a Bunnings gift card we won in a local raffle stuck to my fridge.

              Did you gut the unit and make your own housing? I can't see any way to secure and position them otherwise.

              dian - Like many here I imagine, I am not exactly short of fun ideas for projects. Rather, we are short on time, resources, knowledge, and skills to get to all of them. An accurate spindle is beyond my skill+equipment.

              QSIMDO & mikey553 - ahh, they look like little Sieg Industrial 10131 units, which they don't have an actual page for, but are listed in the optional accessories for their mini lathes.

              Grinding attachment
              Spindle speed 0-6000±10%rpm
              Wheel size 80*20*10mm
              Motor output power 250W
              Net/Gross weight 4/5kg
              The price is definitely low. Haven't seen them offered in Australia though. And yeah, not suitable for inside grinding or small bits, but interesting nevertheless. I assume they would be of similar quality to their other products.

              darryl - sorry, I haven't had enough time to properly consider your post, I will get back to you soon, but I have to go eat


              • #22
                darryl - Thanks for this detailed post. Your explanation on spindle designs in particular is very helpful. And thanks for reminding me about thermal considerations and resonance.

                I hadn't properly considered the reduced moment of inertia that results from not having an armature. I was just thinking that for a given sized housing, I could use a spindle with the diameter of an armature, increasing my load bearing capacity and spindle momentum, while still having reasonable clearance with smaller wheels. But of course, if I throw a large wheel on, it will have far more momentum than the spindle. Perhaps that is why the TP grinders for larger wheels use a pulley on the back of the spindle, to increase gyroscopic stability, and the belt adding a small counter moment to the cutting forces.

                And yeah, I'm definitely not up to maching my own spindle. What did you end up using for you 1/4" one?


                • #23
                  How about using an ER16 straight shank collet chuck? Add a couple of bearings, a housing, and a motor and you're ready to roll.


                  • #24
                    Swarfer, I only said the Unimat spindle beats the pants off a Dremel. And yes, I have tried both.

                    Grinding chuck jaws with Dremel:

                    I like this photo:

                    Paul A.
                    SE Texas

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


                    • #25
                      I'm not sure I am 100% on-board with all Darryl said.

                      With regard to diameter of shank.... The theory of small = deflection is attractive, but should really not be considered too serious for a toolpost grinder. The forces used in grinding, particularly with small shanks, are, or SHOULD BE, small. Therefore there should not BE enough force to get involved with deflection. If the shank is long enough, it might get "whippy", and unstable, but I will assume most folks have reasonable "machine sense" and won't get involved with that.

                      "Grinding" has a popular image involving pressure and forces to wear away ("grind away at") the work. But in reality, it shouldn't, for any reasonably precise operation. It takes very little pressure to cut well when there are several million cuts per minute taken by the grinding wheel. (10,000 rpm Dremel, and a small wheel with a thousand or more grains on the OD).

                      It makes sense to use the largest spindle that you can work with. A 3/8" spindle is fairly common with Dumore grinders, for instance. That size could allow you to make, or attach, a collet type adapter for mounted "points", and still use wheels if so desired.

                      As for bearings, there are some pitfalls and cures. Angular contact are the obvious go-to type, of course. But Dumore, for instance, does not use them in the "non-cartridge" spindles. Instead, they use a normal or deep-groove ball bearing, and preload it just as if it were an angular contact type.

                      The bearings Dumore used in the two grinders of theirs that I have had, have been open types, with the shielding against grit being [art of the housing, and not the bearing. They are both made with oiled bearings, as well, having an internal "tube wick" to supply oil. Grease vs oil is something you need to decide. Shielded and sealed (sealed not recommended) are supplied with "lifetime grease", typically. Open bearings are often supplied with only a preservative grease, and need some means for either greasing or oiling them.

                      High speed bearings do not need, and do not want, a lot of oil. Oil or grease in excess gets stirred up and created a good deal of frictional heat, which is bad for bearings. There is much to be said for a shielded type with manufacturer supplied grease, as well as a maximum speed spec that you can count on them tolerating as-shipped, grease and all.

                      Bearings have a spec known as "clearance", which is the looseness, or slop, in the bearing. If you handle a lot of different bearings, you will notice that some are looser than others. Obviously clearance will allow your spindle to "rattle around", cutting slightly deeper and shallower on a random basis, and that would be undesirable if you want precision.

                      So, a "wave washer", a Belleville spring, or other means are commonly used to set some preload. This is what Dumore does, and is probably a good plan for any lightly loaded spindle such as a toolpost grinder. Atlas lathes, and other machines handling heavier loading, may set preload by a nut that is tightened, or, in higher class machines, by different length spacers that are inserted between the outer races vs the inner races. They also use angular contact bearings, or, in the case of Atlas, tapered roller bearings.

                      The preload should not be "heavy", because that lowers lifetime. If you look at the detailed bearing specs, they will generally show a number for axial pressure, and may have a graph of axial pressure vs lifetime. A preload way down on that curve is best, it really should not take a lot. Perhaps a couple percent of the axial load limit.
                      CNC machines only go through the motions


                      • #26
                        When I'm using my TP grinder on small diameter parts, the finish I get is quite dependent on whether or not I damp the part as I'm grinding. If I don't damp it, it will vibrate to some extent, usually not visibly but enough to affect the grind. The usual result of this is a part that becomes too small in diameter and visually rougher. It follows then, in my mind, that if the grinder spindle can also vibrate, or whip, that the same result could occur. The amount of flex would not be much, but it represents a varying distance between the grinding wheel and the work piece, which translates into a lack of precision.

                        Even though 10,000 grains are touching the work piece in a sort of random fashion, each time one makes contact it would tend to push the wheel away from the work piece- even though by only a tiny amount. Letting it spark out should be giving you a nice finished result, but you also don't want to have thousands of slightly deeper gouges left on the surface by the previous heavier contact grinding. I don't know- some of this is from actual experience, some is conjecture. Personally I would go with the more rigid spindle every time, and that would be the larger diameter one, other factors being equal.

                        My 1/4 inch capable grinder uses the shaft from an old Makita trim router. I have also gone partway on a project using the spindle from an air die grinder. I can't say which one would be better, but in all the spindles I've used and saved for future use, there sure doesn't seem to be much meat around the collet end. Mostly they look like they could break off or at least bend if you just looked at them wrong. Looking at it this way, the 1/2 inch shank capable spindle out of a larger router would seem to offer the best strength. The one I saved will be the one in my next tool post machine, unless I take on the task of making my own spindle. So far I've stayed with using the existing spindles to take advantage of the already-machined collet, the nut, and threads. It is a bit daunting to machine all this to a high degree of concentricity.

                        I did actually build my own spindle once, using a needle roller bearing for the front end and a deep groove ball bearing for the rear, but the front bearing was noisy and I didn't complete that project. I understand now that it's possible to control the radial play of such a bearing by pressing the outer race into a precisely bored hole to shrink the outer race by just the right amount- but I think you'd have to work to a tenth or so in order to remove all play but not have it tight. The intent at the time was to restrain radial play to a very high degree while keeping the bearing OD as small as possible. The housing for this actually fit within the slot on my four-way tool post, but that too was impractical because the fixture bolts on the tool post would eventually have distorted the very thin housing and probably would have destroyed the bearing anyway.

                        What I really wanted to use was a small-sized tapered roller bearing, but I couldn't find one small enough. And JT you could be right- perhaps there's no need for such a high degree of rigidity in something like this.
                        Last edited by darryl; 12-22-2016, 03:52 AM.
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                        • #27
                          I've rattled on quite a bit here, but I also wanted to mention something about my conversion of a dremel shaft to a TP machine. Obviously it takes only a 1/8 shank tool, but apart from that I somehow destroyed the front bearing when the collet tightening nut contacted the work piece at one point. From that very moment the thing runs with an audible knock. The bearings are very small and in my opinion not really up to the task. As a handheld you could have the same problem, but there's going to be enough give in your hand holding to allow the peak bearing pressures to be lower. With something like this rigidly mounted in your tool post, you could encounter the same kind of peak instantaneous pressures that could overwhelm and damage a bearing.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                          • #28
                            I'm using a Rotozip Rebel as a tool post rotary tool, I do use it for grinding but with a carbide endmill it will mill steel nicely and is great for all sorts of irregular and interrupted slots on odd-ball parts.

                            - Nick
                            If you benefit from the Dunning-Kruger Effect you may not even know it ;-)


                            • #29
                              "mikey553 - This die grinder?
                              I was looking at a very similar (though more expensive) die grinder by Makita. - and I have a Bunnings gift card we won in a local raffle stuck to my fridge."

                              Harbor Freight sells two similar die grinders. I could choose which one to buy and I selected 60656 one. It has a regular collet with no threads or hex on it. It appears that the internal construction is different too. You can look at the user feedback at the HF site and see that both grinders have quite horrible ratings. The most of them are due to motor becomes unoperational and grinder overheats in operation. I took mine completely apart and can testify that grinder overheats due to the fact that spindle bearings are assembled with too much preload. There is nothing in the design to adjust the preload and all depends on the parts dimensions and assembler skill. There is not much material in the aluminum spindle housing to work with, but I still hope I will not need to make a new one. I am thinking about machining a portion on the housing OD to create a journal I can use for clamping in the lathe.

                              I did a similar machining to the Rotozip Solaris tool and it is working very well for clamping. Makita grinder has a rubber cover over the spindle housing, So I cannot tell what is under this cover and how suitable the housing is for clamping. Just don't expect any miracles from Makita - it will not have a real grinding spindle by any means.

                              I have to make a few remarks about an interesting subject a few of you have touched. It is called rotordynamics. It studies the behavior of rotating assemblies. Our grinding spindles definitely fall into this category. Every rotating assembly has its own critical speeds, at which a small external force (such as unbalance) will cause a very big amplitude of vibration due to resonance. Normally you do not want to operate in this region. You can operate below first critical speed or above it, but not directly at it. Critical speed depends on bearings, distance between them, shaft rigidity and weight, attachments location and weight. We should probably talk only about the 1st critical speed, because the other ones are normally too high to interfere with our projects. With all other things being equal the bigger shaft diameter will make it more rigid and will increase the critical speed, the bigger weight will decrease it.

                              While the spindle shaft is normally well machined and does not cause a lot of vibration, the attachments (such as grinding wheels or stones) is a different story. The long and slim arbors produce a lot of flexibility and grinding wheels naturally have an unbalance. This creates a potential for the grinding wheel to run above the first critical speed. There is nothing wrong about it as long as you are not running directly on top of it.

                              I also want to say a few words about bearings. I have ABEC-9 angular contact bearings for my project with 12 mm ID for the front bearing and 10 mm for the rear one. It is overkill, but I have them already. Believe it or not, but even these small bearings are close to speed limit at 28000 RPM. This is what I have measured at no load and full voltage on the motor. So going for a bigger spindle with 1/2" collet most likely will force you to reduce RPM.
                              Another consideration - is parts quality to accept precision bearings. We are talking about a few microns tolerances for spindle journals and housing bores and alignment between them. I do not know if I can measure the parts that good, leave alone machining them to that level of accuracy. That is why precision spindles cost a lot of money and the ones, that are cheap, are only called "precision", but in reality they are not.
                              The amount of preload, I am going to put on these bearings, is about 5-10 lbs. That is all what's required since axial forces in operation will be very small. I will use a regular compression spring and one sliding bearing to accomplish that. Sliding fit is needed to compensate for thermal expansion of spindle and housing. I have to admit - this is not my design, I borrowed it from the literature I found online.


                              • #30
                                Bearings can have different speed limits.

                                I once saw a grinder head for small mounted points. The spindle was somewhere in the 3/8 to 1/2 inch range, 10-12mm say. The RPM for this grinder head had a limit at 150,000 rpm. I think I can guarantee that the bearings in it were the best grade that could be bought, and possibly selected from among them.

                                Makes sense, since a point of 1/8" diameter should be going only at about 5000 FPM at that rpm. About equal to a 6" wheel at 3600 rpm.

                                I suspect nobody would spend that money required to get those, but they exist
                                CNC machines only go through the motions