Tool Post Grinder Build Motor Question

[Rex Walters]

I recommend checking the hardness of the zone you intend to turn before you get too committed.

Good point. One could always use a toolpost grinder to grind down the diameter if it is hardened.

Oh ... wait.

I’m afraid that running even a small 3“ wheel at 8K max RPM directly in the motor’s ER collet will be too slow for the motor to make enough torque

Again: I'm definitely out of my depth, so take this with a grain of salt, but:

Aren't 220V 3-phase router spindles intended to run end mills and woodworking router bits? My intuition is that running a router bit through even soft stuff like wood would require more torque than spinning a small grinding wheel, especially once it's up to speed and inertia starts to help rather than hinder. It still seems reasonable to me to either use the router spindle you've selected for everything, or to go with a cheaper motor and use the belt-driven spindle for everything. Using one for OD and one for ID seems unnecessarily complicated.

As for wear, I only have my 6" bench grinder to compare from direct experience (my Quorn T&C grinder was completed too recently to have worn any of the wheels significantly). I use my bench grinder a LOT for a hobbyist, dressing the wheels frequently, but have only retired two wheels since I acquired the thing years (decades) ago. Small-diameter toolpost grinder wheels are fairly cheap, too. Unless you expect to use the grinder an awful lot (like for continuous production use) is wear really a significant concern?

In my case, I purchased a handful of wheels and grinder points from Dumore recently. I still haven't finished my grinder build (it's on my ever longer list of partially completed projects) but even if I finish it tomorrow, I don't expect I'll be replacing any of those in my lifetime. I mostly expect to use the toolpost grinder for various morse, jacobs, and other taper tooling that I have vague plans to make. My usage will be occasional and infrequent at best.

 

[Jake P]

I recommend checking the hardness of the zone you intend to turn before you get too committed.
Being that It's a square (or Acme) thread it will be an interrupted cut and may be pretty tough stuff.
You might be better off just salvaging the bearings (and spacers?) and making your own spindle. YMMV

Testing with hardness testing files set it looks to be somewhere between 45-50.

 

[Jake P]

Good point. One could always use a toolpost grinder to grind down the diameter if it is hardened.

Oh ... wait.


Again: I'm definitely out of my depth, so take this with a grain of salt, but:

Aren't 220V 3-phase router spindles intended to run end mills and woodworking router bits? My intuition is that running a router bit through even soft stuff like wood would require more torque than spinning a small grinding wheel, especially once it's up to speed and inertia starts to help rather than hinder. It still seems reasonable to me to either use the router spindle you've selected for everything, or to go with a cheaper motor and use the belt-driven spindle for everything. Using one for OD and one for ID seems unnecessarily complicated.

As for wear, I only have my 6" bench grinder to compare from direct experience (my Quorn T&C grinder was completed too recently to have worn any of the wheels significantly). I use my bench grinder a LOT for a hobbyist, dressing the wheels frequently, but have only retired two wheels since I acquired the thing years (decades) ago. Small-diameter toolpost grinder wheels are fairly cheap, too. Unless you expect to use the grinder an awful lot (like for continuous production use) is wear really a significant concern?

In my case, I purchased a handful of wheels and grinder points from Dumore recently. I still haven't finished my grinder build (it's on my ever longer list of partially completed projects) but even if I finish it tomorrow, I don't expect I'll be replacing any of those in my lifetime. I mostly expect to use the toolpost grinder for various morse, jacobs, and other taper tooling that I have vague plans to make. My usage will be occasional and infrequent at best.

I've got a die grinder "tool post" grinder that I could take the threads down with, but I don't think it's going to be a problem to turn them off should I choose to go that route.

I'm out of my depth here as well, so we're in the same position.

I guess the level of complication perceived in my plan is subjective, but I see your point.

The wheel wear issue is not financial, it's accuracy (as I see it). Seems to me one of the main points of a tool post grinder is not finish so much as accuracy. Attempting to attain less than say a tenth of taper over an 8" piece would require a very consistent wheel diameter for the entire grind. But having not done this as yet, I'm just speculating! I tend to over engineer.

 

[pontiac428]

Maybe back-calculating your ideal surface speed for your largest and smallest wheels first would give you a better sense of this spindle's usefulness as a toolpost grinder. The point stones will need higher RPM, obviously. Might make sense to not run direct at all, but run a countershaft only and just jockey ratios with sheaves to reach the extremes using both overdrive and underdrive.

 

[extropic]

IMHO, If your target is .0001 tolerance over 8" on a 10" lathe, you will be into lapping/polishing after you do the best you can with a tool post grinder.

I don't mean to be negative regarding your TPG project. I'm just trying to point out some of the difficulties in the approach, as I understand it.

 

[Jake P]

IMHO, If your target is .0001 tolerance over 8" on a 10" lathe, you will be into lapping/polishing after you do the best you can with a tool post grinder.

I don't mean to be negative regarding your TPG project. I'm just trying to point out some of the difficulties in the approach, as I understand it.

You probably know more about all this than I do!

I’m just having fun with my hobby and trying to build some tools that can perform to any level that I may likely demand of them. I am using a PM1440GT lathe if that makes any difference. I’ve been able to dial the lathe in to where I can turn within .0001 over 8”, and that’s with the lathe on a wooden floor. So I would think that a properly made tool post grinder could better that, but again, I’m FAR from the most knowledgeable machinist in the room!

 

[Jake P]

Maybe back-calculating your ideal surface speed for your largest and smallest wheels first would give you a better sense of this spindle's usefulness as a toolpost grinder. The point stones will need higher RPM, obviously. Might make sense to not run direct at all, but run a countershaft only and just jockey ratios with sheaves to reach the extremes using both overdrive and underdrive.

Forgive me, but I've read the last sentence multiple times and still am not sure what you are describing.

 

[pontiac428]

Sorry, I write funny. In order to do an external grind, the motor is in the way. You need a countershaft. If you are going to run a countershaft, you may as well run sheaves to expand your RPM range. A 1/2" diameter stone point needs 8x more RPM than a 4" wheel. You can reach the extremes of the RPM range with pulleys, which can be over- or under- driven. I did some math from Norton's databook, and saw that even that hot-rod spindle you linked may not make enough surface speed for a small internal grind, thus requiring overdrive. It might be smoking fast on an exterior grind, requiring underdrive. I don't know what the minimum smooth running speed is with that VFD, but if you're making sheaves, you can pick your min and max to cover it and fine tune with the VFD. If you are going to go to the trouble of making a countershaft spindle capable of 40k RPM, you might as well forego the idea of running direct drive for internal work- your countershaft spindle would certainly exceed the quality of your drive spindle at that point.

My experience with tool post grinders (Dumore, good and functional but not exactly Swiss) is that they leave a nice finish. I ended that sentence early, because I have not seen .0001 along the length get done on anything short of a cylinder grinder or centerless grinder. In fact, I'm not sure I can reliably measure ten spots on a turned surface to the tenth with tenths confidence, and I have micron scale Suhl gauge mics from Germany. I'm not being sarcastic in any way here. Grinding an OD with a TPG using power feed is a demonstration of harmonics. The lathe turns the part that, if it's longer than say 5-10:1, has it's induced harmonics from the lathe spindle. The grinding wheel has it's own spindle with it's own signature vibe. Put the two together and add a longitudinal feed, and most of the time you will get waves in your work. Grinding machines have follow rests supporting the work, very stout spindles, and make compound cyclic movements- those are the tools that make tenths tolerance. Tool post grinders are awesome to have, but we really shouldn't talk about anything smaller than thousandth finishes. Reality check aside, the TPG opens up a lot of different possibilities limited mainly by creativity, and gives you the ability to put a nice surface on all kinds of stuff. It's a great finishing touch for shop made tools and precision fits. Making tiny tolerances is going to be tougher than it might seem due to the real-world limitations of the setup due to stacked vibrations on the lathe.

 

[extropic]

Sorry about the 10" lathe reference. I must have gotten my wires crossed (again).

 

[Jake P]

Sorry, I write funny. In order to do an external grind, the motor is in the way. You need a countershaft. If you are going to run a countershaft, you may as well run sheaves to expand your RPM range. A 1/2" diameter stone point needs 8x more RPM than a 4" wheel. You can reach the extremes of the RPM range with pulleys, which can be over- or under- driven. I did some math from Norton's databook, and saw that even that hot-rod spindle you linked may not make enough surface speed for a small internal grind, thus requiring overdrive. It might be smoking fast on an exterior grind, requiring underdrive. I don't know what the minimum smooth running speed is with that VFD, but if you're making sheaves, you can pick your min and max to cover it and fine tune with the VFD. If you are going to go to the trouble of making a countershaft spindle capable of 40k RPM, you might as well forego the idea of running direct drive for internal work- your countershaft spindle would certainly exceed the quality of your drive spindle at that point.

My experience with tool post grinders (Dumore, good and functional but not exactly Swiss) is that they leave a nice finish. I ended that sentence early, because I have not seen .0001 along the length get done on anything short of a cylinder grinder or centerless grinder. In fact, I'm not sure I can reliably measure ten spots on a turned surface to the tenth with tenths confidence, and I have micron scale Suhl gauge mics from Germany. I'm not being sarcastic in any way here. Grinding an OD with a TPG using power feed is a demonstration of harmonics. The lathe turns the part that, if it's longer than say 5-10:1, has it's induced harmonics from the lathe spindle. The grinding wheel has it's own spindle with it's own signature vibe. Put the two together and add a longitudinal feed, and most of the time you will get waves in your work. Grinding machines have follow rests supporting the work, very stout spindles, and make compound cyclic movements- those are the tools that make tenths tolerance. Tool post grinders are awesome to have, but we really shouldn't talk about anything smaller than thousandth finishes. Reality check aside, the TPG opens up a lot of different possibilities limited mainly by creativity, and gives you the ability to put a nice surface on all kinds of stuff. It's a great finishing touch for shop made tools and precision fits. Making tiny tolerances is going to be tougher than it might seem due to the real-world limitations of the setup due to stacked vibrations on the lathe.

Wow, that’s a lot of great info! Thanks for the time you put into this response as you have given me some information that I was not in possession off.

I am going to back off the idea of using the table saw spindle/arbor/countershaft (I’m really unsure as to where each of these terms is correctly used). There is an individual on YouTube (Cough42) who made his own tool post grinder from the spindle up. I may have a closer look at what he did in making the spindle and go that route.

I also had to look up the difference between a sheave and a pulley. Seems to be a matter of preference according to this page:

Sheave vs Pulley: What is the Difference? | Illinois Pulley & Gear

Do you struggle to differentiate between a sheave vs pulley? Because the question is so common, we dive into the precise definitions of each.

illinoispulleyandgear.com

One thought I have regarding the motor I noted in my OP is that given a large enough wheel (6“ would do it) there would be clearance for OD grinding without the motor or mount getting in the way. But I don’t know if that motor could drive a wheel that large at the needed low (sub 4K RPM) with enough torque. But the motor is 80mm dia. and the mount would not need to exceed 15mm thickness making the OD of the motor/mount combo around 110mm or 4.4“ dia. That would give about .75” clearance from the mount outer edge to the face of the wheel. Of course if you need to work close to the tail stock then there is still an issue.

The simplicity of a motor/spindle combination is alluring, but may not be doable as yet until better motor technology comes along. But it appears we are getting close?