Mike's Slow Speed Carbide Grinder

[macardoso]

As I start tooling up for hand scraping, I came to the realization that there was no good way to sharpen the carbide blade without a slow speed carbide grinder. Looking online, I liked the diamond lapping plate concept many are using. I have some components laying around the house which would be perfect for one so my design will be central around these components. I hope this will aid my scraping experience and perhaps allow me to resharpen my used carbide lathe inserts.

For the motor, I have a 25W (1/30HP) 3 phase induction motor from Oriental motor. It is pretty tiny but has surprising torque so I think it will be more than enough for this limited application. Rated speed is 1600 rpm and I have run it up to 3200 rpm no problem.



This motor will be driven by an Allen Bradley Powerflex 4 rated at 1/2HP. This is a super basic VFD with V/Hz control only. I wanted to use it because there is a built in keypad and potentiometer that will handle start, stop, reverse, and speed select.



I plan to use 6" diamond grit lapping discs. They can be had for about $12 each on amazon. So far I have purchased an 800 grit, 1500 grit, and 3000 grit. I bet these will cover the range of my needs.

I'll follow this post up with some CAD models as I am in the very early phases of my design. The key features are a 6" lapping disc retained with magnets, tilting table (tilt axis in plane with the front of the disc), dust/splash guard, and contemplating some kind of MQL drip lubrication to keep the carbide dust down. I am designing this based on stock I have on hand as well so that is determining the size.

 

[jwmelvin]

Curious what sort of speeds you are targeting. I put one of those discs on my bench grinder (against the side of a stone disc) and it was great for a while (for TIG electrodes) but wore out pretty quickly. My grinder runs quite fast though (belt driven).

 

[Braeden P]

if you got the same disks as me the 800 grit wears to about 1500 so i only used the 800 and the 3000 grit ones.

 

[macardoso]

Curious what sort of speeds you are targeting. I put one of those discs on my bench grinder (against the side of a stone disc) and it was great for a while (for TIG electrodes) but wore out pretty quickly. My grinder runs quite fast though (belt driven).

I expect to run in the 400-800 rpm range, but honestly I've never tried this so I have no clue. The grinder will be infinitely variable between 0 and 3200 rpm. I intend to use this for very delicate grinding. The torque below 400rpm is not great, so it would only be good for polishing, not stock removal. Can always put a bigger motor on it, but this one is free and works perfectly with the VFD.

 

[bakrch]

I expect to run in the 400-800 rpm range, but honestly I've never tried this so I have no clue. The grinder will be infinitely variable between 0 and 3200 rpm. I intend to use this for very delicate grinding. The torque below 400rpm is not great, so it would only be good for polishing, not stock removal. Can always put a bigger motor on it, but this one is free and works perfectly with the VFD.

Accu-finish grinders spin at about 300 rpm I believe, and they are glorious.

 

[macardoso]

Here is an early CAD model of the grinder. I have two 6" x 12" x 0.5" aluminum plates which I want to use for the backbone of this design. I will be boxing in the sides with 1/4" aluminum plate as well. Should be really stable.

I currently plan on using the motor bearings as the only support for the platter. They are spring loaded axially as many many small motor bearing arrangements are, so I am considering adding a single angular contact ball bearing in the front plate to take the thrust loads from the platter (otherwise the plate can traverse about 1/8" when a few pounds of force is applied.

The VFD is hanging off the back. Motor wires will pass through a hole in the real bulkhead. A tilting table will be added in front of the platter, roughly about centerline. I really want to design the hinges so the pivot axis is inline with the platter face and the top edge of the table.



And from the rear.

 

[macardoso]

Here is it with the sides, top, and a mockup work table. I think I am aiming for a +45, -30 degree tilt range on the table.

There will be some sheet metal to cover the VFD (I am not concerned about heat since the motor is so undersized for the drive). That should also keep dust from settling inside. I unplugged the internal fan so it does not circulate. Again, I won't be running this anywhere near the rated load, so heating is not currently a concern.

There will also be a sheet metal plate below the table to help contain dust. The sides extend beyond the front of the wheel to catch spray or dust thrown by the wheel. The two holes in the side plate will be for the pivot for the table, and the locking screw to hold it in place. The work table is 5.88" x 3" currently, but it is up for changing. I might also add a shallow slot in the center for a fence. Table is 3/8" thick aluminum for rigidity (and I have some on hand).

 

[homebrewed]

I used a stepper/driver combination along with a very simple 555 based timer (to generate the step pulses), to make my low speed grinder. It runs about 120 RPMs and that seems to be plenty fast. I set mine up so I can put a disposable aluminum bread tray underneath the disk. I add water to the tray, enough to go about halfway up the disk. This limits the RPMs you can use because higher RPMs start to sling the water off the disk, but it does a good job of lubricating the blade and capturing the nasty carbide/cobalt dust.

The 3000 grit disk produces a really nice edge on the carbide scrapers.

 

[macardoso]

I used a stepper/driver combination along with a very simple 555 based timer (to generate the step pulses), to make my low speed grinder. It runs about 120 RPMs and that seems to be plenty fast. I set mine up so I can put a disposable aluminum bread tray underneath the disk. I add water to the tray, enough to go about halfway up the disk. This limits the RPMs you can use because higher RPMs start to sling the water off the disk, but it does a good job of lubricating the blade and capturing the nasty carbide/cobalt dust.

The 3000 grit disk produces a really nice edge on the carbide scrapers.

Nice Idea! I left room to slide a pan under the disc to catch drippings, but not submerge the disc. I imagined a refillable tub on top with a needle valve to drip onto the disc. I like your idea though.

 

[jwmelvin]

Designing the table motion to have a pivot axis at the wheel-table interface would be really nice. I'm imagining a four-bar linkage? I was playing around with this simulator.

I wanted the same action for my bench-grinder support, but no need for much precision, so I just used arc slots with a center at the support-wheel interface. It's a work in progress (slots here) but the prototype was good enough.