Thread: Bench grinder extension spindle

Here is a whole thread about this

https://www.woodworkforums.com/f65/be...pgrade-177942/

Thanks Bob interesting thread, i was thinking of making up an extension spindle, approx 6 inches and running a couple of buffing wheels/mdf wheels. The spindle would be threaded m12 and replace the nut currently on the grinder. My concern was the safety side of things and would something protruding that far affect bearings etc on the grinder?

If it is a double ended grinder, it will have LH Thread one end and RH thread the other so both nuts tend to tighten with the rotation. If you want to make a pair of extensions or tapers, you need a LH thread and a RH thread unit, making them a bit outside the realm of DIY with a Tap and Die set unless you want to drop a bundle on a LH Tap and Die.

For a 6 inch hobby unit, my gut feeling is that the bearings and spindle would be prone to overload and potential damage if you work the machine hard enough to buff effectively with 6 inch extensions fitted. The machining on the extensions would need to be spot on to eliminate run out without load. Your buffs would need to be well balanced before mounting them the first time, if they were off balance there is a chance that you actually permanently bend the extensions or the spindle on the first start.

Then there is the question of whether the machine has enough guts to work a buff under load, and whether the body is strong enough to actual keep the unit secure under load. Personally I cringe at the thought of an 8in A&A working with extensions mounted, but my background is with 5HP polishing lathes with 70mm spindles and tapers. We could trip the overcurrent limiters on these beasts mirror polishing metal with 12-14inch diameter buffs mounts, about 30kg contact force at the buff, and about 20mm deflection at the end of the shaft.

I know that Creusen sell extensions for some of their grinders, but having been through the process of buying a grinder not too long ago, I believe that there is a fair difference between a Creusen and a Ryobi. I'd keep a careful eye on the shafts for any sign of play, and it's possible that you'll be in the market for a new grinder if it starts to came apart.

Originally Posted by malb

If it is a double ended grinder, it will have LH Thread one end and RH thread the other so both nuts tend to tighten with the rotation. If you want to make a pair of extensions or tapers, you need a LH thread and a RH thread unit, making them a bit outside the realm of DIY with a Tap and Die set unless you want to drop a bundle on a LH Tap and Die.

For a 6 inch hobby unit, my gut feeling is that the bearings and spindle would be prone to overload and potential damage if you work the machine hard enough to buff effectively with 6 inch extensions fitted. The machining on the extensions would need to be spot on to eliminate run out without load. Your buffs would need to be well balanced before mounting them the first time, if they were off balance there is a chance that you actually permanently bend the extensions or the spindle on the first start.

Then there is the question of whether the machine has enough guts to work a buff under load, and whether the body is strong enough to actual keep the unit secure under load. Personally I cringe at the thought of an 8in A&A working with extensions mounted, but my background is with 5HP polishing lathes with 70mm spindles and tapers. We could trip the overcurrent limiters on these beasts mirror polishing metal with 12-14inch diameter buffs mounts, about 30kg contact force at the buff, and about 20mm deflection at the end of the shaft.

G'day Mal, hope you're keeping well. You've pretty much summed it up, my grinder does have LH and RH threads. At this point I've only looked at the right side. I did a bit of experimenting and turned a 6 inch spindle extension. One end was tapped for the RH side and it was concentric straight off the lathe. I put it onto the grinder and there was a little run-out. I am running a white wheel as well on the RH side but there's still enough thread there for the extension so I looked further and found that the white wheel's bushing was just a little smaller than the hole in the white wheel. I turned a new bushing/spacer from some hardwood and now it's a perfect fit for the white wheel. Alas, there's still some run-out. Thinking my machining was somehow out of whack I did what BobL's previous thread reference suggested and that was cut down a RH tapered spindle to fit. Even that has some run-out and that is shorter than mine. There is a small wobble in the white wheel but not sure where it's coming from, only thing that I can see that might be askew is that on one side of the wheel, the thin cardboad with the wheel details has come away. As you've suggested, it's difficult to get everything balanced but I only have very little space and no room for another grinder. The main reason for doing all this is to power home not so much for polishing.

Tiger, if your white wheel has run out already, it will be one of these things;

The grinder spindle has runout. If this is the case, theres not a lot that you can do unless you are well practiced in the art of straightening and have the right gear to do it, or have someone who is and is willing to do it gratis. Paying to have it done would cost more than buying a decent 8in grinder new.

The mounting flanges may not be true. Once upon a time they we always turned, more recently the are often pressed metal and could be out of true a fair bit. If the issue is the flanges, you could possibly have someone either skim them to true them, or machine a turned set.

The wheel bushings may be slightly out of kilter or the wheel may be. This is not uncommon, and the manufacturers assume that users will dress the wheel insitu to have it run true and concentric in use. But most hobbiests don't have either a steel or diamond dresser to do this and put up with wheels that aren't properly dressed. Again the tools to do the job can cost more than the grinder and wheels. Two things that you did mention are that the card is missing from one side of the wheel, and that you made a new bushing from timber. To my mind, both are dangerous and could cause you grief. With the card missing, I assume that the flange is bearing directly on the side of the wheel which is a no no.

The wheel is supplied with card either side so that the flanges can compres the card to ensure an even degree of compression of the wheel by the flanges, ensuring that there is no point with significant stress to start a wheel fracture. If the original card is completely missing it can be replaced with a couple of layers of old style blotting paper cut to a circular shape and larger than the mounting flanges. They don't need to be perfectly shaped or positioned as they will very little effect on balancing in comparison to the wheel which is much larger and heavier.

A timber bushing would be prone to uneven expansion and contraction with variation in moisture content and could give rise to localised or generalised outward contact pressure within the wheel mounting hole, again potentially creating conditions conducive to wheel fracture and explosion.

As I see it, if the wheel has runout when mounted, it will telegraph into an extension that installs to the shaft either because the the shaft has runout itself or the mating surface between the wheel flange/nut/extension shaft is not perfectly perpendicular to the shaft axis, and is causing stress and flex in the shaft/extension interface. This would be aggravated by any size mismatch or taper in the threads on the shaft and extension.

Do you have access to dial guages and stands etc and verniers etc to check everything to locate the source of the runout? You can borrow mine if you need to.

There's not a huge difference between honing and mirror polishing other than the area to be covered and the size of the toys and effort involved. I'm sure I could mirror 1m x 100mm x 10mm stainless with a small buff on a small grinder, but I doubt that the boss would want to pay me for the week it would take, hence the big beasts, big wheels, and high contact pressures. With a commercial machine mirroring all over from a 600 base would probably be around 15-20 minutes. The polishing lathe was mentioned basically as an example of the loads and flex that can be placed on the machines, to give an insight to what the grinder may have to handle.

Thanks Mal for your response. The amount of lateral runout is small but nonetheless present, i do have dial indicator and verniers but i'm pretty sure there's no or very minimal runout on the grinder shaft, but thank you for your offer. I used klin dried tas oak for the bushings, would have used aluminium but did not have any. The timber is very dry but i think i'll hunt down some ally or some nylon and turn some bushings, the plastic spacers that came withthe wheel were very sloppy and would have been difficult to true. Is there an alternative to blotting paper for the wheel sides as don't have any and not sure where i'd get some.

Making the bushing from very dry timber is possibly the worse approach as it will expand more than damper timber as it absorbs moisture in the shop, so more likely to generate stress that can crack the wheel. The original bushes that came with the wheel being loose is not an issue, if they are loose they won't start a crack.

Wheel positioning is supposed to be controlled by the mounting flanges, the bushes can normally get the wheel close while positioning nut and flanges, then once the wheel is properly dressed it is running true and very close to balanced.

Sorry, I am not aware of an alternative to blotting paper for replacing missing cards. You basically need something that is thick and able to compress locally while maintaining its overall thickness over a wide area. This ensures even clamping pressures for the wheel to minimise stress concentration and potential for failure.

Think I'll have to take off the wheel. The original bushings were so sloppy that I thought anything would be an improvement. Have a look at Creating Bushings for Your Grinding Wheels | Articles | Woodworkers Journal.

The blotter is far more important than I thought so thanks Mal for pointing that out. Have had trouble locating some blotting paper. From what I've been able to glean, cereal cardboard box is an accepted substitute according to some sources but Norton abrasives warn against it. All in all far cheaper to just buy another wheel except that Carbatec which is my usual supplier are out of them. Mal, your technical knowledge as usual is spot-on and appreciate your help .

Tiger, I viewed the Ernie video and agree with everthing he said except making the bushing out of wood. As you are probably aware, change in moisture content will not produce an appreciable change in dimension along the grain, but produces different expansion/contraction rates for radial (with the rings) and tangential (accross the rings). If the bushing moisture content changes (as it would over time in Melbourne) the bushing will change shape and could either create local stress points due to expansion, or leave the wheel bore unsupported locally due to contraction. If there is a timber bush, the moisture content can change as the cards on the wheel are porous like blotting paper and allow the changes, though they may not be as rapid or extreme as they are in the atmosphere.

It's probably 15yrs since I purchased and mounted a wheel, and the bushings supplied with them had some freedom, but not as much as shown in his video. There needs to be enough freedom between shaft, bushing, and wheel to ensure that when the flanges clamp the wheel there is no pressure point caused by the shaft and bushing contacting the wheel bore unevenly when the flanges clamp the wheel in position. I have 100mm length of 30mm dia black Delrin that you can use to turn a replacement bushing if you can solve the card issue. Come back if you want the Delrin.

In the meantime I suggest you remove the wheel and check the runout of the shaft and of the inboard flange. That way we might be able to eliminate them from the equation. The only way I can think to test the outboard flange is to turn a spacer about the width of the wheel, and with a greater diameter than the flange, and sandwich that between the flanges, then fasten the assembly with a small ring nut so you can probe the mating contact surface of the flange where the nut would normally be.

The wheel has been retired for now. Officeworks sell blotting paper but you need to buy 25 sheets at around $20, cheaper to get a new wheel. I have some old white wheels which have worn down, they have the blotters, if I can get them off I'll attach them to the retired wheel. Given that a bit of slop is ok I can probably make do with the plastic spacers I have for now, must get myself some delrin it's handy stuff for other projects. One thing I'd add, when I put on my timber bushing the grinder was quieter and I didn't notice as much vibration as before, still I prefer the safer option. Will get the dial indicator out, am curious to see where the run-out is on this grinder.

One other thing I failed to mention originally. Commercially when mounting tapers, the wheel is not installed, the taper screws all of the way to the machined step in the machine shaft, and obtains its registration from that (similar to a threaded lathe chuck, the thread provides the grip, the shoulder the registration).

This is because the wheels are moulded from an amalgam of binder and abrasive, then cured using processes I know nothing about. I know they used to be aged for a substantial period before shipping to 'normalise'. The end result is suited to grinding but not guaranteed to have parallel faces perpendicular to the bore. The card on the side of the wheel is intended to absorb any variation in mounting pressure, but the outer flange can be cocked slightly if the outer face is not perpendicular to the shaft. Screwing an extension or nut on will then cock that once it tries to register against the flange during tightening.

I haven't been able to come up with an alternative card, but agree with Norton that wheeties box cardboard would not be a good alternative. It needs to be something of similar thickness but not to highly compressed so that there is scope to crush it locally to even out mounting pressures around the flanges. Have looked at older books, building plan copies from the seventies and a variety of other stuff but haven't come up with anything that looks/feels right so far.

Thanks Mal, take the point about the tapered attachment, I did face it so reasonably confident it was true concentrically. There are plenty of people that have used the cereal box but that is on both sides of the wheel therefore getting even compression, I won't be taking the risk when a wheel is only $20 or so. I do have a couple of worn wheels with blotters in place, going to try to remove the blotter intact otherwise guess the wheel goes into the bin.

Found the blotter, it had peeled off and was sitting in the drawer where the wheel was kept. What type of glue is required to adhere it back to the wheel?