Life's Forge

Los Angeles, California

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©2018 by Life's Forge.

Building a
Grinder-In-A-Box

The grinder-in-a-box, with a 5" contact wheel on a slack belt tol arm.

Here at Life's Forge, I had wanted a good belt grinder for some time. 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:

  • Grinder-in-a-Box from Polar Bear Forge: A frame kit based on the EERF grinder, set up to be buildable with some precision drilling and tapping; some of the tapping operations can be replaced with welding. Just the frame, doesn't include motor, wheels, and hardware. PBF has updated the design, and the version I built is no longer available. The new design looks like an improvement.

  • KMG Grinder, by Beaumont Metal Works: Not too expensive for a ready-to-use grinder, and everyone who has one seems to be happy with it. I got to use one during blacksmithing classes at Adam's Forge, and really liked it.

  • TAG-101, from Wilmont Grinders: Another ready-to-use grinder, looks perhaps more versatile than the KMG, but not as well established.

  • EERF, also from Wilmont Grinders: Plans for a grinder similar to the KMG grinder; requires machining or welding to build.

  • No-Weld Grinder: Plans for a grinder somewhat like the KMG grinder; requires nothing harder than drilling holes. Doesn't look as solid as the EERF or KMG, but people who own it have few complaints. Parts and kits available from USA Knife Maker Supply.

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. Since then, I've seen several other grinders from other companies that looked good and that I would happily have considered, but at that time these were the ones I found. After 5 years of use, I'm still happy with the quality of Polar Bear Forge's kit and the result, and would cheerfully buy from them again.

Ordering and Delivery

Once I'd chosen the Grinder-In-a-Box (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:

  • Platen: both platens, as I expect I'll want them eventually.

  • Power: 115V 60 Hz AC, which is what my garage is wired for.

  • Speed control: VFD, for variable speed without needing to change wheels or drive belt position.

  • Motor: 56C frame, TEFC, NEMA enclosure. Keep that metal dust out of the motor.

  • Two 2" contact wheels for one platen, a 3" and an 8" contact wheel for the other, a 4" drive wheel, and a tracking wheel. The 3" and 8" wheels were out of stock when I purchased, so I didn't get those until later (and ended up getting a 3", 5", and 8", for a the slack platen and an extra one-wheel tooling arm).

  • Dust-tight passthroughs for the wiring.

  • Various bolts, nuts, screw handles, washers, etc.

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.).

The GIB package, shipped in a USPS Priority Mail box, with damage from shipping.

The package as it arrived.

The GIB packaging inside the USPS Priority Mail box that protected the shipment from damage.

The packaging it came in.

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.

Inside the protective plywood box, the pieces were wrapped in plastic to protect against moisture.

The pieces, wrapped in plastic, from the box.

The motor, as shipped within its box.

The motor, unboxed.

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.

The pieces for a GIB, as shipped.

The parts, unwrapped.

Cleaning up the webbing from a GIB spacer.

Cleaning up a spacer.

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.

Drilling a hole in the GIB tension arm. Accuracy and alignmen are important, so a drill press is helpful.

Drilling holes in the tension arm.

Tapping a hole in the GIB support, to connct to the base.

Tapping a hole in the support.

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:

  • Use tapping fluid. Light oil is a lot better than nothing, when tapping steel, but a purpose-designed tapping fluid is even better.

  • When a tap gets harder and harder to turn, it's getting dull. If you keep going, it's going to break.

  • Replace dull taps before they break.

  • If a tap gets dropped on concrete, it can develop micro-cracks which make it break much more easily. Don't drop your taps.

  • Clean holes are easier on taps than irregular holes.

  • Take it slow and easy, and don't be in a rush.

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).

Painting

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.

GIB parts with primer.

Primed parts.

GIB parts primed and painted.

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.

Assembly

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 left the knockout in place, cut a hole in the side of the wiring box, and installed the strain relief "cable gland" there.

Mounting board for VFD.

Mounting board for the VFD.

A VFD and electric motor wiring and box.

The VFD and motor, ready to wire.

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.

Homemade upper shackle for spring on tension arm.

The upper shackle for the spring.

The frame of the grinder-in-a-box, assembled.

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.

Flat Platen Tool Arm

I'd bought both wheel plates as part of the kit, but only the two 2" wheels had been available, so I built the flat platen tool arm first, and left the slack belt tool arm for later. 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 flat platen wheel plate. After mounting the wheels I added a tapped hole and a bolt in the curved slot, locking the platen at the chosen angle when tightened.

The tool arm with flat platen, with wheels but no lock bolt or platen.

The tool arm, partially assembled.

The flat platen tool arm, fully assembled, with the adjustable tool rest.

The flat platen tool arm.

I made a flat platen of 2 1/4"×2 1/4", 1/4" thick angle iron, polishing the face and rounding the edges with a file and with some India stones. Very carefully, I drilled holes to fix it to the platen bracket about 1/32" proud of the plane of the wheels. It works well, but I may want to make it adjustable at some point, or add a glass face. For now, though, it's good enough for letting me make flat surfaces.

Slack Belt Tool Arm

After a while, I was able to order a pair of wheels for the slack belt tool arm, 3" and 5" diameter 70 duro. The tool arm is designed for use either as a slack belt, between the wheels, or for either wheel to be used as a contact wheel. The same aluminum bar provided the arm, with a rounded end and a hole tapped to 1/2"-13. The wheel plate can be rotated to present either wheel for grinding, or both wheels can be lined up to use the space between the wheels for slack belt grinding.

Trimming the end of the tool arm bar with a hacksaw, before rounding the end of the bar.

Trimming a tool arm.

The GIB with a slack belt tool arm.

The slack belt tool arm.

Bench for Mounting

After considering the bench location, I decided I really didn't want to mount it permanently at that spot. 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.

The GIB bench, clamped and with glue drying.

The bench, with glue drying.

The grinder-in-a-box with a 5" contact wheel on a slack belt tool arm.

The bench and grinder.

The bench and grinder have even made it out to several Adam's Forge classes, giving the students an extra belt grinder to work on.

Work Rests

I first made an adjustable work rest arm with 3 degrees of freedom. It's an aluminum tooling bar (1 1/2" square bar, from McMaster-Carr) with one end rounded and a half-inch hole drilled through the rounded end. In front of that I tapped a hole for a 1/4"-20 knob, which pins the half-inch steel bar and keeps it from sliding or rotating (2 degrees of freedom). The other end of the steel bar has a tapped 1/4"-20 hole, and a piece of the same angle iron as the flat platen is trapped against the bar by another knob. I was very happy to not have to grind unsupported. It works fairly well, but will rotate if the knobs aren't really tightened strongly.

An adjustablework rest for the GIB.

The adjustable work rest

The fixed work rest

After working with the adjustable work rest for some time, I wanted to have a fixed work rest I could lean the work on without worrying about it tilting. I had enough of the aluminum bar left over, so I took a piece of that and bolted a flat half-inch steel plate to the top of it with 3/8" flat head machine screws. This made a very solid fixed work rest, and I have little worry that I'm going to have it move. When I need a rest at an angle, I'll use the adjustable one, but most of the time I want one that's solidly horizontal.