Hollow grinding - 8 vs 6 inch wheels


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31 minutes ago, JacksonMaberry said:

A good friend of mine who makes furniture made his own massive belt machine powered by a 3/4 horse AC motor controlled with a VFD. The only thing I don't love about it is that at it's very lowest speed, the torque drops off hard. DC is better in that respect, but DC motors are significantly more expensive. Hence the $2k price tag on a fully loaded Tradesman grinder from Canada. 

For new machinery, yes, PM DC motors and controllers run expensive.  However, for DIY conversions, you can pick up motors and controllers on eBay occasionally for reasonable prices.  I have converted my 2 bandsaws, drillpress, and lathe to DC that way.

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5 minutes ago, Don Noon said:

For new machinery, yes, PM DC motors and controllers run expensive.  However, for DIY conversions, you can pick up motors and controllers on eBay occasionally for reasonable prices.  I have converted my 2 bandsaws, drillpress, and lathe to DC that way.

Thanks for the tip, Don. DC bandsaw sounds very cool indeed.

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Just one more motor-related tangent while I'm here... a caution about using a VFD on a single-phase AC motor:  beware of the starting capacitor.  Most larger motors have a starting capacitor, and a centrifugal switch that cuts it off at higher speed.  The capacitor is not meant for continuous use, and will heat up and melt or burn if it's run at a speed below where the switch clicks off.

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Thank you for that advice, Don. My wheel grinder lacks a start capacitor, but my belt grinder, which is just 1/3hp, has one and would undoubtedly be damaged by running at lower speed.

Another possible issue is cooling. I read years ago that ac motors designed to run on European 50Hz power would be safe  to run on American 60Hz power, they’d just run a little faster. But motors designed for 60Hz American power would run slower on European 50Hz, and therefore might not cool properly. And the centrifugal switch might not open.

Update: as I read more about VFD’s, I realize that they’re not really designed for single phase motors at all. There’s the capacitor switch issue, grounding issues that can damage bearings and other parts, and a list of other problems. VFD’s are not a panacea for those among us seeking to slow down our basic workshop machines. And now I realize why I’d never heard of them before.

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5 hours ago, Violadamore said:

Yup, about what I figured, actually.  You're using a grinder wheel for stock removal, but then shaping and sharpening with a succession of stones to arrive at a flat or convex edge bevel profile..  When I refer to a "hollow ground blade", that's not what I mean at all, nor do most other people who say nasty things about them.  I'm talking about blades which are primarily shaped or entirely finished on a grinder, leaving a concave "holiday" between the cutting edge and the body of the finished tool.  That concavity will interfere with efficient cutting.  To see exactly what I'm saying to avoid, compare a cheap hollow-ground chef or butcher knife with a classic convex-ground (teardrop cross-section) Marttini fish filet knife. 

Note well, in most of my dealings with blades, I'm cutting deeply into or entirely through something, rather than merely shaving the surface, which affects my perspective on shaping/sharpening.

BTW, overall, I'm a vocal opponent of producing blade profiles by stock removal rather than forging to shape, so with most blades I handle, this never comes up,  :)

 

 

Sorry, I could not know from whence you were coming.  You did ask the question in a violin making, and not fish filleting forum.  I suspect that most here do hone their blades after grinding (if they grind at all) regardless of the shape of whatever they are using for "stock removal", which will of course result in at least a flat surface at the edge.  I do my grinding on the periphery of a 6" diameter wheel and the surface I have as a result is nicely even and concave.  I hone only as much as I need to in order to remove the ground surface at the cutting edge using 1 or two stones and I do not intentionally leave it convex.  The majority of the bevel is indeed concave and I don't sense any problems with chips "impacting" this concave surface or that it interferes with efficient cutting.  I don't follow at all why you think the sort of concave surface you're referencing would interfere with efficient cutting and am curious about your thought models.  However as I'm no longer cutting flesh very often and the sharpening system I use has been serving me well for decades while cutting wood, it's not very important to me.

At this stage of my life, I don't think I'll be taking up the forging of bridge knives...

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36 minutes ago, David Burgess said:

Been using this grinder for about 30 years, made from whatever I had laying around at the time.

Just goes to show that nice machines aren't necessary for making nice violins.  However, to my mechanical engineering eye,  YUCK!  Don't let OSHA see that exposed fan.

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53 minutes ago, Don Noon said:

Just goes to show that nice machines aren't necessary for making nice violins.  However, to my mechanical engineering eye,  YUCK!  Don't let OSHA see that exposed fan.

Actually, those parts are somewhat covered during use.  I removed the cover to show what was underneath.

Oh, the platform it's mounted on is a rough-sawn rectangle from an old discarded formica-skinned kitchen countertop. :lol:

If I were to do it again, I'd probably use a discarded sewing machine motor and foot pedal, which would give me an enclosed motor and fan, belt-drive, and variable speed with a "throttle pedal". Motor-head heaven! :D

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2 hours ago, David Burgess said:

Actually, those parts are somewhat covered during use.  I removed the cover to show what was underneath.

Oh, the platform it's mounted on is a rough-sawn rectangle from an old discarded formica-skinned kitchen countertop. :lol:

If I were to do it again, I'd probably use a discarded sewing machine motor and foot pedal, which would give me an enclosed motor and fan, belt-drive, and variable speed with a "throttle pedal". Motor-head heaven! :D

I've often wondered how formica might do as a baroque fingerboard veneer... :ph34r:

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https://www.amazon.com/Variable-Frequency-Drive,MYSWEETY-Converter-VFD-2-2KW/dp/B07D76365G/ref=sr_1_6?dchild=1&keywords=Phase+Converter&qid=1605327758&sr=8-6

Potentially, there is a much better way ( and cheaper) of doing this.  Get a 3-phase converter in which the frequency can be controlled.  The above model for $95 goes from 0-400 Hz at the output.  Single phase at 220 volts input and 3- phase out.

Used 3-phase motors are easy to get and are inexpensive because most homes and business are not wired for 3-phase--so they cannot be used.  There is a glut of them.

Mike D

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2 minutes ago, Mike_Danielson said:

https://www.amazon.com/Variable-Frequency-Drive,MYSWEETY-Converter-VFD-2-2KW/dp/B07D76365G/ref=sr_1_6?dchild=1&keywords=Phase+Converter&qid=1605327758&sr=8-6

Potentially, there is a much better way ( and cheaper) of doing this.  Get a 3-phase converter in which the frequency can be controlled.  The above model for $95 goes from 0-400 Hz at the output.  Single phase at 220 volts input and 3- phase out.

Used 3-phase motors are easy to get and are inexpensive because most homes and business are not wired for 3-phase--so they cannot be used.  There is a glut of them.

Mike D

Good thinking, Mike. I appreciate your thoughts.

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21 hours ago, Don Noon said:

For new machinery, yes, PM DC motors and controllers run expensive.  However, for DIY conversions, you can pick up motors and controllers on eBay occasionally for reasonable prices.  I have converted my 2 bandsaws, drillpress, and lathe to DC that way.

Back when I converted all these machines to DC, variable frequency drives were not widely available and inexpensive like they are now.  If you don't need the high torque at low speed that a PM DC motor can give, the VFD 3-phase is the cheaper route.  Some day I'll re-convert my big bandsaw from the 1 HP DC motor back to the original 5 HP 3-phase with a VFD, now that I have 220V wired to the shop.  But 1 HP gets the job done well enough.

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14 hours ago, Mike_Danielson said:

https://www.amazon.com/Variable-Frequency-Drive,MYSWEETY-Converter-VFD-2-2KW/dp/B07D76365G/ref=sr_1_6?dchild=1&keywords=Phase+Converter&qid=1605327758&sr=8-6

Potentially, there is a much better way ( and cheaper) of doing this.  Get a 3-phase converter in which the frequency can be controlled.  The above model for $95 goes from 0-400 Hz at the output.  Single phase at 220 volts input and 3- phase out.

Used 3-phase motors are easy to get and are inexpensive because most homes and business are not wired for 3-phase--so they cannot be used.  There is a glut of them.

Mike D

Mike Danielson, do you know of any similarly inexpensive 110 volt AC single phase (both in and out) induction motor variable frequency speed controllers? I would like speed control on some of my machines (particularly on my band saw, but it would also be handy if I didn't need to switch the belt between different pulleys on my drill press so often).  I would be willing to pay a little more for a unit which works with the motor(s) I already have, as opposed to the hassles and expense of finding, purchasing and mounting a different motor, with the expense of the speed controller on top of that.

 

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1 hour ago, David Burgess said:

Mike Danielson, do you know of any similarly inexpensive 110 volt AC single phase (both in and out) induction motor variable frequency speed controllers? I would like speed control on some of my machines (particularly on my band saw, but it would also be handy if I didn't need to switch the belt between different pulleys on my drill press so often).  I would be willing to pay a little more for a unit which works with the motor(s) I already have, as opposed to the hassles and expense of finding, purchasing and mounting a different motor, with the expense of the speed controller on top of that.

 

There are these, and maybe others too, that claim to do exactly what you’re asking for. Reasonably priced too. But this company has online videos showing how to modify your motors to work with their devices, including removing the start capacitors from the circuit and changing some other connections. I’m left uneasy about modifying the rather beautiful American industrial quality motors that my machines have. I wonder what the motor manufacturers (Baldor, Leeson, Marathon) might say about this. I’m inclined to let you try it first, and I’ll wait for your report. ;)

https://www.ato.com/single-phase-vfd

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3 minutes ago, MarkBouquet said:

There are these, and maybe others too, that claim to do exactly what you’re asking for. Reasonably priced too. But this company has online videos showing how to modify your motors to work with their devices, including removing the start capacitors from the circuit and changing some other connections. I’m left uneasy about modifying the rather beautiful American industrial quality motors that my machines have. I wonder what the motor manufacturers (Baldor, Leeson, Marathon) might say about this. I’m inclined to let you try it first, and I’ll wait for your report. ;)

https://www.ato.com/single-phase-vfd

Those are cool, but looks like nothing rated for 120v. =(

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David, I do not know the clear answer to your question.   Any motor which is single phase and runs at a single speed and has a start circuit is going to be a problem for variable frequency control.  

If I was trying to do this, I would identify each type of motor in my machinery and then do the internet search to find out if variable frequency speed control is feasible.  There are a huge variety of electric motors.  The cheaper ones are capacitor start because they have good starting power (but I suspect they will not work for this modification).

Mike D

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I got one of these 1HP 110V/220V cheap VFD's to play around with.  I wasn't sure it would work for slowing down my 1-phase dust collector, but it does (barely) by just connecting 2 wires to the output instead of 3 (but then there's the capacitor starter, but that's another issue).  I don't think it's very happy about it, but since it only speaks Chinese, I can't tell.  It works fine for very low-power motors without capacitor starters.

I would say that if your existing motor has a capacitor start, forget about it... or go with the full motor & controller route.  

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I was passing through a farmer’s market this morning, and there was a person operating a mobile sharpening service. I thought some of you might find one of his machines interesting. I asked him if he made it, and he said no, but it was highly modified. It’s two 1”x30” belt sander/grinder machines side by side. 1800 rpm motors with small drive wheels resulting in 800 ft/ min., which he said was “why I like it.” He had other machines but this one caught my eye. And he admitted that he couldn’t sharpen plane blades or chisels, mostly knives, hedge shears, pruning shears, scissors, etc. But the machine is interesting.

A28BE279-472B-408D-996F-18F763C9D43C.jpeg

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For a small grinder like a 6" wheel diameter, a 1/4 HP motor will be adequate.  That means it doesn't require a lot of electrical power.  Here in the United States we have 110 volts, and the unit Don Noon has posted will provide 3-phase power with 110 volt input.  Thus, you do not need the 220 volts input that most of these 3-phase units require (220 volts may not be conveniently located to the grinder). 

Here in the US, 220 volts is saved for the clothes dryer, water heater, furnace/AC, and electric stove--so every home has 220 but there is usually not an outlet for convenient use.

Mike D

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A ridiculous/expensive solution that might not work:  110/220 step-up transformer to run the VFD, and then another 220/110 step-down transformer to go to the motor.  The first half will work, but I'm not sure how will the converter will handle low-frequency AC on the output.

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I think it will be by far less expensive to stick to DC variable control or AC 3 phase VFD.
I've not followed this thread closely, but it looks like what Don has done with his experiment is put a 3 phase VFD on a single phase motor, which isn't working terribly well.  I suspect the reason for this is that the AC output of the VFD isn't evenly driving the motor as the current is being fed to the windings at 0 and 120 degrees of rotation rather than at 180.  Don connected 2 of the three wires of the drive output.

There are single phase VFDs but they are expensive and only work on some types of AC motors.
https://www.wolfautomation.com/blog/vfds-for-single-phase-motors/
The link in this blog for single phase VFDs is broken, but this one should get you there:
https://www.wolfautomation.com/products/ac-drives/single_phase_output/

3 phase motors are available used with a bit of scrounging or on the surplus market fairly inexpensively and 3 phase VFDs are pretty cheap.  DC motors and controllers are pretty available as well, this is one source:
https://www.surpluscenter.com/Electric-Motors/DC-Motors/?page_no=2

If one is working on retrofitting a motor to an existing machine then you have to sort out the type of mount etc, which can be a bit daunting apart from the electrical issues.  The good news is that they are fairly standardized.

Modern AC and DC variable speed controllers maintain torque fairly well at low speeds, but if you want to get low speeds that are near the limits of the motor on your machine, such as on a grinder or bandsaw, and still have useable torque, then it's probably best to use mechanical means as well as electrical for finer control.  I've done this with a small lathe I use for peg bushings and cello endpins.  It has a DC motor and a variable DC drive which unlike the readily available modern electronic ones, does not have speed control feedback and so does not maintain torque or speed at low motor speed.  Most of the time I use that machine in its middle (of three) pulley positions.  If I want to use it at really low speed and low or high torque then I have to change the pulley position.  Sometimes low torque at low speed is useful, such as while doing tapping while holding a tap chuck by hand.

One thing to keep in mind with trying to slow down motors, especially if you plan to use them for long periods of time,  is that extra heat may be generated, and if the motor has an integral fan, the cooling air flow will be reduced.  Motors do normally generate heat, but too much does shorten their lives.

On to the next phase...

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26 minutes ago, Mark Norfleet said:

I've not followed this thread closely, but it looks like what Don has done with his experiment is put a 3 phase VFD on a single phase motor, which isn't working terribly well.

The marginal result is that I'm using a VFD advertised as "1HP" and using only 1 phase of the 3 to run a 1HP single-plase dust collector.  It actually works fine at about half-speed, where I want to run it... but I have to run it up to near full speed to get the starting capacitor switch to cut out before lowering back to what I want.  I have to very carefully ramp speeds up and down, otherwise the VFD loses consciousness, and I have to block off the collector intake to reduce the power draw until I get it down to half speed.  Smaller 1-plase motors work fine, but pushing stuff way beyond where they're supposed to be used is often problemmatic.

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