Grinder-In-A-Box Build

A description of how I built my 2"×72" belt grinder.

Choosing What to Build

I've wanted a good belt grinder for some time now. I wanted one that could do general grinding, but with the features needed to make high quality knives. I looked a a number of grinders, and picked out five that met my needs:

In the end, I decided to go with the Grinder-in-a-Box (GIB). I like building things myself, but cutting out the pieces for the EERF looked harder than I wanted to do, and I don't have much experience welding. I planned to do my build without any welds.

Ordering and Delivery

Once I'd chosen the GIB, I started making a list of the other things I'd need. I had some of them, and planned to order the rest. I decided on these options:

I ordered the GIB kit from Polar Bear Forge, the motor and VFD (a KBAC-27D) from online merchants, the wheels and some belts from USA Knife Maker Supply, a wiring kit from the very helpful Wayne Coe, and hardware and an aluminum bar for tooling arms from McMaster-Carr. I already had all the tools I planned to use (a drill press, drill press vises, drills, taps, screwdrivers, etc.).

package as it arrived packaging box contents motor unboxed
The package, as it arrived. The packaging. The pieces from the box. The motor, unboxed.

Soon, everything began to arrive. Everything except the actual GIB kit itself. USPS tracking showed the package being picked up and in transit to a sorting center in South Dakota, but that was the last entry. Jamie at Polar Bear Forge was very responsive, replying courteously to my (perhaps annoyingly frequent) emails and sending a replacement when the Post Office finally decided that the package was lost.

The GIB package is heavy! It had clearly been dropped at least once during transit, but the packaging is very sturdy and nothing inside had been damaged.

Preparing the Parts

Most of the GIB kit is separate parts, but a few of them are joined by a thin metal web. It's easy to break the web, and then I cleaned up the leftover edges on the bench grinder. A few of the parts had some rough spots where the cutter hadn't quite moved right. The only one that made a difference was a place in the motor mount hole that interfered with the motor face, but that was easily cleaned up with about 10 minutes of file work. I also ran a tap clearance drill in each hole I planned to tap. Some of the holes needed the cleanup, others didn't, and one had a few irregularities that left visible low spots in the threads.

unwrapped parts spacer cleanup drilling the arm tapping the support
The parts, unwrapped. Cleaning up a spacer. Drilling holes in the tension arm. Tapping a hole in the support.

Following the directions on the Polar Bear Forge website, I clamped the base and support together at a right angle (checked a few times along the way), transferred the hole positions to the support through the base, and drilled the holes in the support with a #F drill (the suggested size for tapping a 1/4-20 thread). It took a little work to get that set up, as the support was just tall enough that it couldn't fit on the table of my little drill press.

The other unmarked holes are on the tension arm, where the hinge bracket attaches. I clamped and transferred the hole locations, again following the directions on the Polar Bear Forge website, and then drilled the two holes all the way through the tension arm. This was much easier to set up, as the clamped tension arm fits nicely under the drill bit. I also countersunk the motor face bolt holes, as I had flat head bolts for those, and countersunk the four holes on the bottom of the base where the support attaches.

Then it was time to tap the holes. This was something I hadn't done a lot of, and I ended up learning more than I really wanted to. Some of the lessons were:

I ended up breaking two taps, one 1/4-20 and one 5/16-18. Neither one came out with needlenose pliers and penetrating oil, or with gentle percussion on the flutes. The larger one was easy to remove with a Walton broken tap remover and some Kroil penetrating oil. The smaller one broke the tap remover, when used with strict attention to the instructions (the instructions actually recommend that breaking the tap remover can be a reasonable outcome). I heated the broken tap with an oxy-MAPP torch until it glowed, and then carved out the center using a 1/8" carbide ball Dremel tool. That weakened it enough that I could finally whack it out with a hammer. The threads suffered a little, but running a tap through it again cleaned them up nicely.

All in all, I ended up tapping 25 holes: 14 with 1/4-20 taps, 4 with 5/16-18, 2 with 3/8-16, and 5 with 1/2-13. I broke two taps, and retired two that were getting dull before they broke (one of the dull ones had visibly broken teeth).


I attached the base and support before painting. I wanted to be sure they were at right angles, and wanted the paint to cover that joint. And I was a little impatient to see something assembled.

Working outdoors, I wiped down all the parts with acetone to remove any oil and dust. I put the parts on top of a cardboard box, supported by a pair of sticks, and gave them a coat of Rustoleum enamel primer. After they had dried, I did the other side. This worked adequately, but left some marks where the sticks had supported the parts.

primed parts painted parts
Primed parts. The parts, with final paint coat.

For the top coat, I didn't want any marks. I made up small hangers for each part from coat hanger wire or election sign rod, and supported some conduit between two pool ladders. I took each part and sprayed it with Rustoleum enamel, then hung it to dry on the conduit. This worked much better at keeping the paint from being marred.


Once the paint was dry, it was time to put it all together.

I'd picked out a spot on a workbench where I wanted to mount the grinder. It had a stud near it, where I would put the VFD. I put together a mounting board of 1×4 oak, which attached to the stud and held the four mounting screws for the VFD. With the board in place, it was easy to mount the VFD to the wall.

The motor came with one knockout on the bottom of the wiring box. I couldn't see how to set up the wires so that the knockout would be usable; I was mounting the motor too close to the baseplate, and the wires would have been under continual strain. So I cut a hole in the side of the wiring box, and installed the strain relief "cable gland" there.

I then wired the motor to the VFD, and tried out the electrical supply for the grinder. It turns out that the outlet I'd picked wasn't up to the load, and the GFCI popped immediately when I started the motor. Since the VFD is a model designed for a GFCI, I expect I will to need to talk with an electrician to get the problem fixed. At least the wiring turns the motor in the correct direction.

I attached the motor to the support with four 3/8-16 flat head bolts, and then to the base with four 1/4-20 round head bolts, using Loctite 242 thread locker on all bolts. I'd tapped six holes for the base attachment, but two of the holes are under the motor's wiring box, and I can't get the bolts in under there. The four face bolts should be sufficient, so the base bolts are just insurance.

VFD mount VFD and motor spring shackle Complete frame
Mounting board for the VFD. The VFD and motor, ready to wire. The upper shackle for the spring. The frame, assembled.

I put together the various pieces in the order that seemed most convenient. That meant the three 3/8" bolts that hold the upright tension arm support and part of the tooling arm housings, then the rest of the tooling arm housings, with the spacers, the 16 ga. shims, and so on, all held together with 12 1/4-20 bolts. I used nylon-insert locking nuts on all of these bolts, so I didn't use the Loctite. I put together the tension arm and its tracking wheel supports, and attached that to the upright.

The spring that provides the tension for the tension arm came next. I improvised a shackle for the upper support from a piece of 3/16" rod, bending it into place so that both ends met inside the hole in the tension arm. I attached the spring to that shackle, and to an eyebolt I'd put near the bottom of the upright.

I found two problems when test fitting a belt to the two mounted wheels. First, the bolts I used to attach the upright tension arm support are too prominent, and the middle one is directly in the path of the belt. I removed the bolt, countersunk the hole, and replaced it with a flat head bolt. Second, the spring I used wasn't strong enough. Replacing it with a stronger spring made the belt tracking more stable.

First Tooling Arm

I'd bought both platens as part of the kit, but only the two 2" wheels had been available, so I only built the standard platen. I cut a length of aluminum bar and rounded one end, then counterbored and tapped a hole in the end to hold the pivoting bolt for the standard platen. After mounting the platen I added a tapped hole and a bolt in the curved slot, locking the platen at the chosen angle when tightened.

Trimming the arm Standard platen tooling arm bench
Trimming the tool arm. The standard platen tooling arm. The bench with drying glue.

Bench Mount

After considering the bench location, I decided I really didn't want to mount it permanently at that location. Instead, I built a sturdy table out of scrap lumber, with a post at the back to support the VFD. The table is just large enough to hold the GIB grinder and a small bench grinder. With the table, I can move the grinders should I decide to rearrange the shop, or I can move them temporarily for those times when I need more room. The garage is decidedly crowded.

I plan to add a backing plate to the standard platen to support the belt for flat grinding, make another tooling arm for the multi-platen, and make a work rest arm. However, it's already usable, for slack belt grinding and grinding with a 2" radius, and I'm very happy with the result.