Tool Post Grinder Build Motor Question

[Jake P]

Hey all, I’m playing around with the idea of building a tool post grinder. I have various designs in mind depending on the motor I end up using.

I am wondering what your thoughts are regarding this motor:

https://www.amazon.com/gp/product/B0785QKYT6/ref=ox_sc_act_title_1?smid=A3E9T3KQ1YPBBY&psc=1

It would easily provide the necessary RPM for small wheels for internal grinding, and with the proper pulley arrangement to keep motor RPM up and wheel RPM down I wonder if it would have the power run a larger (4-6”) wheel for external grinding.

But not understanding everything I should about electric motors I suspect I may be missing something here that would tell me that this is not actually a good motor candidate for this use?

Edit: In case it is relevant, this will be used on a PM1440GT.

 

[extropic]

One comment about the ER20 collet chuck: ER20 is 1/2" nominal max capacity. I wouldn't like to run a 6" wheel on a 1/2" shank.
Maybe think 3-4" for the OD work. YMMV

 

[Jake P]

One comment about the ER20 collet chuck: ER20 is 1/2" nominal max capacity. I wouldn't like to run a 6" wheel on a 1/2" shank.
Maybe think 3-4" for the OD work. YMMV

That’s a good point!

But the way I plan on running the external wheels would be using this motor via pulleys and a spindle. Internal grinding would be straight off the ER collet.

 

[Rex Walters]

I plan on running the external wheels would be using this motor via pulleys and a spindle

I've never used a toolpost grinder, but I'm in the planning stages of a build. Take my comments with an appropriately sized grain of salt.

But if you're going to make or buy a high speed spindle and belt drive anyway, this CNC router spindle and VFD seem like overkill to me.

Seems reasonable to either attach grinding wheels/points directly to the router spindle for both OD and ID grinding, or use a belt-driven spindle for everything (with a less costly motor). Variable speed might be nice, but in practice, I think two speeds (two belt positions) should suffice. I'm designing mine for two fixed speeds, anyway.

I don't expect the cutting forces on a toolpost grinder to be too significant (it's not intended to hog off a lot of metal), so I'm designing around a cheap 24V DC motor (like this one) with a salvaged 24V computer power supply. Mine will also be belt driven. I plan to leverage a removable high speed spindle from my Gorton pantograph engraver. Unfortunately, the Gorton spindle has the belt pulley at the collet end, so I will unfortunately need to build shaft extenders for ID grinding.

Note that most Dumore grinding wheels are rarely larger than 3" and won't have larger than 3/8" bores (I expect to use 1/4" points for most of my ID grinding, not wheels). I'd think the capacity of an ER20 collet chuck will be far more than necessary and even a 4" wheel seems way oversized to me (I've only got a 10" lathe so cross-slide travel is definitely a factor for OD grinding).

 

[Jake P]

Seems reasonable to either attach grinding wheels/points directly to the router spindle for both OD and ID grinding, or use a belt-driven spindle for everything (with a less costly motor). Variable speed might be nice, but in practice, I think two speeds (two belt positions) should suffice. I'm designing mine for two fixed speeds, anyway.

Thanks for the thoughts Rex!

I did not want to go into all the details of what I am up to as there are variables as to how I may go. But the first item that differs between us is that you have a high speed spindle already. Dumore for instance wants LOTS of money for just a spindle, and yes one could build their own as an option. However I already have a "spindle". I bought a Powermatic 66 table saw some 30 years ago and had an issue with blade runout. Long story short, they sent me a new arbor.

This is the old one:


Turned out the problem was with the nut not the spindle/arbor (IIRC they called this part an arbor, I'll call it a spindle). The issue here is that the bearings are rated to 10K RPM, so it will not work for high speed internal grinding ops. But it will work fine for OD grinding as even a small 3" wheel is limited to around 8K max RPM, and a 4" to around 6K RPM.

Note that most Dumore grinding wheels are rarely larger than 3" and won't have larger than 3/8" bores (I expect to use 1/4" points for most of my ID grinding, not wheels). I'd think the capacity of an ER20 collet chuck will be far more than necessary and even a 4" wheel seems way oversized to me (I've only got a 10" lathe so cross-slide travel is definitely a factor for OD grinding).

The smaller Dumore is limited to 3" wheels, but the larger grinders go up to at least 5" wheels. Edit: I just looked again at their website and their wheels are up to 12" depending on the grinder model.

So my idea if I use the motor that I linked in the original post is to make it moveable/switchable on my mounting system to either run a pulley for the spindle I have (OD grinding) or move it to use small grinding stones in the ER collet mounted directly inline. Should be easy enough to design a mounting system to enable quick change positioning as the motor is just a round cylinder.

Edit: I just got off Dumore's website again. Here's a link to the page with the wheels I'll be looking to use with this spindle:

Grinding Wheels | Dumore Series 57 Tool Post Grinder External Spindle

www.dumoretools.com

 

[Rex Walters]

Ah. I see. What’s the advantage of using the saw arbor over the router spindle directly, though?

I’d have a hard time mounting even 3” wheels on either side of the work on my 10” lathe. It’s hard to figure out how to mount a motor and spindle with pulleys compactly for OD grinding. I don’t know how large your lathe is or how large a diameter of stock you plan to use, but a belt drive arrangement requires a surprising amount of space (and counter-balance/overhang) when you mount it on the compound.

If you never plan to grind tapers with the compound, I suppose you could mount directly to the cross-slide which would free up space for larger wheels, but if you use the router spindle directly the RPM limitation is moot, so larger wheels don’t seem advantageous.

 

[extropic]

Is the table saw arbor 1/2" or 5/8" at the threaded end?

 

[Jake P]

Ah. I see. What’s the advantage of using the saw arbor over the router spindle directly, though?

I’d have a hard time mounting even 3” wheels on either side of the work on my 10” lathe. It’s hard to figure out how to mount a motor and spindle with pulleys compactly for OD grinding. I don’t know how large your lathe is or how large a diameter of stock you plan to use, but a belt drive arrangement requires a surprising amount of space (and counter-balance/overhang) when you mount it on the compound.

If you never plan to grind tapers with the compound, I suppose you could mount directly to the cross-slide which would free up space for larger wheels, but if you use the router spindle directly the RPM limitation is moot, so larger wheels don’t seem advantageous.

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. I suspect that motor doesn’t get close to its rated power until it’s up over 20K RPM, but that’s just a guess. That‘s the kind of info I’m hoping to find here. But with a pulley arrangement I can run the motor near max RPM and power while turning the spindle at the appropriate RPM for the selected wheel size.

Yes I plan to mount on the compound, and I have a 1440 size lathe, so I think if I’m careful in my design for the spindle mounting I should be able to grind a pretty good size diameter with a 4” wheel. To my way of thinking it is desirable to spin as large a wheel as possible for the simple reason of wear on the wheel. As the wheel wears it will cut a slight taper, which of course will be more noticeable the larger the workpiece and the longer the grind. So the more surface area on the wheel the slower the surface shrinks.

 

[Jake P]

Is the table saw arbor 1/2" or 5/8" at the threaded end?

The thread OD is 5/8”. The best I’ve been able to measure tells me that the valleys of the threads are 1/2”, and that’s the arbor size I’m after. It’s also too short, so I will need to turn it down and make it a bit longer.

 

[extropic]

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