Thread: Anyone have any thoughts on the OBM grinder?

I'm gonna be pushing the buy button on one of these grinders later this week (hooray for car rebate!). I'm adding a 4" contact wheel and some other parts and I'll be driving it with a 2.5hp DC motor with variable speed.

A lot of my 'making razors' plans got badly stalled out by the shared-space shop I was using going under (AGAIN).

I'm on generally better footing this time, and I've got a 3d printer that I can use to make scale molds (probably with lost wax and aluminum or something, or maybe directly from printed ceramic resin -- we'll see if that stuff works).

But first up, GRINDER!

I bought his super budget bolt together jobby. It tracks well enough but I did put a 40lb gas spring to replace the 20lb it came with. If I were to do it again I would have bought that yellow one on his website and figured out a way to weld on a 56c patterned mounting bracket for my motor.

This is the one I bought, eBay only but same manufacturer:


I have an extra 40lb spring if you need it, you’ll just need to cannibalize the mounting the mounting hardware.

Originally Posted by jfk742

I bought his super budget bolt together jobby. It tracks well enough but I did put a 40lb gas spring to replace the 20lb it came with. If I were to do it again I would have bought that yellow one on his website and figured out a way to weld on a 56c patterned mounting bracket for my motor.

This is the one I bought, eBay only but same manufacturer:


I have an extra 40lb spring if you need it, you’ll just need to cannibalize the mounting the mounting hardware.

I’m still planning to make a housing for the motor since it’s air cooled and completely open. Then I plan to just mount it to whatever I use for a table (still looking around for that).

If I need the beefier spring, I’ll hit you up!

Also, if you need anything 3D printed... my resin printer can do pretty detailed stuff! With the resin I’m using, it’s not as tough as the usual filament printers, but it’s got 50 micron accuracy. So when you think 3D printed, think this:

Not a bad looking set up. Went to variable speed several years ago and wonder what took me so long. One thing I really like about it is the 1 1/2" tooling arms make it easy to interchange with my KMG. One thing I don't like is the yellow one has plastic idler wheels, go for the aluminum.

I love my KMG, and have a lot invested in tooling and such, but if it'd been available at the time, and knowing what I know now, I'd have went with something like the KMG TX. At the very least a grinder capable of tilting 90 deg. would have eliminated my horizontal edge grinder.

IT’S ALIVE!

Well, mostly alive. I could grind on it now if I wanted to, but for a variety of reasons I don’t want to yet.

Most importantly, the motor really needs a functional fan and ventilation system. The DC treadmill motor I’m using is fully open frame and was used in the treadmill with an integral fan blade to force air through the fan.

That will kill the motor dead as a grinder, and probably a lot sooner than later.

I have a plan of attack!

The original fan blade was integrated into a 5-10lbs flywheel, which I absolutely do not need. I already designed and printed a replacement.



Unfortunately, I was in a hurry to get it to my friend so he could start it printing and I could pick it up the next morning. It blows air the wrong direction (and kinda poorly at that). So I need to redesign the fan.

The motor has arbors sticking out of both ends (it’d make a great buffer — it’s only about 4.5 inches in diameter), the fan goes on one end and the drive wheel on the other. When it’s operating correctly, the fan pushes air through the frame of the motor.

What I plan to do is build a sheet metal shroud that’s air-tight. On the vent end, I’ll extend the shroud and stick the motor controller (since it needs air cooling as well), so that all the air forced through the motor then goes through the motor controller.

I’ll stick a short length of duct onto the intake, attach a dowel or something to the bench, and have the air intake up near the ceiling (with a filter on it), the idea being that the metal dust shouldn’t be going up nearly as much as it goes down. I’ve also got a couple of ideas for controlling the dust. It’ll include the usual bucket of water just below the grinder wheel as well as a couple of plates or something to deflect the most wayward sparks.

All that should keep the motor in its happy place.

The controller is an SCR voltage regulator running through a bridge rectifier.

I’ve gone over and over this part because it looks like the SCR/bridge method is not the greatest. I would normally have just scrounged the controller out of the treadmill I took it from since it was fully functional, but it’s an old model and it’s super picky and insists that it has everything else connected or it just won’t run. So if the incline motor and it’s sensor aren’t there, it won’t even power on. If the speed sensor isn’t there, it’ll spin up briefly then shut down and beep loudly. Also it’s parts are spread over two PCBs that are both nearly 3 feet wide, and all the controls are on a floppy membrane panel.

TL;DR — original controls non-viable.

Other tweaks:

The motor seems to really want to be raised up about 4 inches from where it’s at.

It’s currently mounted to a cheapo Harbor Freight workstand. It really doesn’t feel as secure as I’d like, however, and I’ll probably get a better stand later.

Other thoughts.

I’m really happy with the build quality on the OBM grinder. It’s a sturdy chunk of kit.

I opted for 1 each of:

VSM 36 grit ceramic
VSM 60 grit ceramic
VSM 120 grit ceramic

A160 (120 grit) Trizact
A65 (280 grit) Trizact
A30 (600 grit) Trizact
VSM cork

Gonna see how I feel about those before getting any kind of quantity.

Let me know what you think of the VSM belts. I picked up one Trizact (A16), the finish is rather impressive and consistent, I think you will like those belts.

I couldn't help myself and played with the A30 trizact and the cork.

The cork, even when loaded with polish, was meh.

The trizact, then a dressed leather belt should get me to a very passable imitation of Sheffield crocus. There's a particular buffer polish I use that I might possibly want to figure out what it was so I can re-order it because I'm almost out. I got them a while back and I find them noticeably better than the stuff you find in hardware stores.

And speaking of hardware stores, I needed some bolts to mount the grinder and motor. My local Lowes was impressively useless for that task. All the bins were horribly disorganized and they just don't carry any sort of bolt variety packs. I had it in my head that they carried assortment packs of bolts in a small organizer, so I didn't take measurements. Luckily, I guessed right, but the only ones I could find that were close to the right size were stainless, which was perhaps slightly overkill.

I think I may buy a leather 2x72 belt and try and load it with greaseless glue and then try and stick some of the metal shavings from post heat treat grinding to get that burnished/glazed look. Speaking with Glen about how they achieved their polished finished, I believe, whale oil or fat and sticking grinding dust to it. There was a name he called it but I don’t recall. The finishes I’m getting are far too consistent to look period correct.

If the fan spins backwards, just install it backwards....

Grinding steel goes everywhere. When I was in the finishing department for a way cover manufavturer, we would have the days between Christmas and new years to clean. On top of lights, fans, etc.

Reconsider any "long" duct work or many elbows, this increases the load on any systems. This is called "head pressure" in fluids texts. Any filter should be fine, but a cheapo car or home filter is easily replaceable.

Originally Posted by dinnermint

If the fan spins backwards, just install it backwards....

Grinding steel goes everywhere. When I was in the finishing department for a way cover manufavturer, we would have the days between Christmas and new years to clean. On top of lights, fans, etc.

Reconsider any "long" duct work or many elbows, this increases the load on any systems. This is called "head pressure" in fluids texts. Any filter should be fine, but a cheapo car or home filter is easily replaceable.

I haven’t done anything with ducting yet, and don’t really have anything invested in the idea — it just seemed like a decent step in controlling intake. Just having a filter over the intake simplifies things a lot, so I’ll give that a shot and see.

As for reversing the blade, I managed to design a fan that only really works in one direction. If its reversed, it pushes air sideways instead of forward.





I went back to the drawing board and started over from scratch, this time taking into account how it would be mounted and which direction it needed to turn. This is what I came up with:



(The weird treebranch stuff is supports for the printer, so it all remains appropriately attached to the build plate and the model).

It’s printing now.

Fun trivia: the filament based FDM print of the first model was done on a friend’s printer, and printed out fully in about 7 hours.

The SLA resin print I’m currently running, despite being exactly the same size model, will take ten minutes shy of 18 hours, which is why I had my friend print it! Flip side is my resin print will be much smoother. To flip to the third(?) side, my resin print will also not be anywhere nearly as tough.