OK, I love belt grinders just as much as the next person, but it's a tough outlay of cash to get into a new grinder. Many folks have asked me about building my variable speed grinder 2 x 72" and lots have used the basic design successfully. That's awesome and thank you for your comments, but can one make a grinder for "dirt cheap?"
My aim with this project is to build a reliable, useful grinder for under $200. It has to combine both a flat platen and a contact wheel. It has to use inexpensive, available parts and it has to be simple to put together without a welder. Most importantly it has to accept the standard 2" x 72" belts which knifemakers covet so much.
The motor you are going to have pick up yourself. I started with a 3/4 horsepower TEFC from a jet pump. I think 3/4 horsepower is an okay place to start for a sub $200 build.
Disclaimer: I offer this information for educational purposes only. I will not be held liable for any use or misuse of the details outlined on this page. It is entirely up to you to know and understand the risks of working with spinning objects, electricity, sparks and chemicals.
PROCUREMENTIn this build I've attempted to keep everything as simple and inexpensive as possible. For the most part I've tried to use parts that can be scavenged locally and avoided buying specialty items or things that can only be found at select vendors. Some web sites are listed for your convenience, however I cannot guarantee that an item will be on sale or is still for sale when you read this.
For the basic frame I found that 1 inch tubular steel with a 14 gauge 0.083" wall to be available and fairly sturdy, with 11 gauge (0.12") being preferred. As we do not need to tap any threads in the tubing wall we can get by with something lighter weight. The design calls for 94" so it will be likely that you'll end up buying 96" (8 feet) which will work out fine and allow for cuts and truing of ends. http://www.metalsdepot.com/catalog_search.php?search=T11114
Note the tubing I have is galvanized. Plain old steel will be fine especially if you want to paint the frame.
Longboard wheels are common to DIY grinder builds and you can change the bearings easily. To fit the bill they must be at least 2" wide. The ones I picked up on eBay are 2.75" diameter by 2" wide and are commonly sold as 70 mm x 51 mm. The inside dimension on the bearing needs to be greater than 5/16" (0.3125") as these are the bolts that hold the wheels on. The bearings I used are 608RZ, ABEC9 skateboard wheel bearings.
The contact wheel is from an 8" by 2" polyurethane on steel caster. These are available online or from caster supply places. Mine is rated for 1100 lbs., but the dimensions are more important than the weight rating for this application. It's best to go to the store see this in person and check the wheel for balance. I checked about five of them and gave them all a spin, then picked the best one. We'll be statically balancing and truing this wheel.
Drive Wheel and Belt Speed
If your motor is a 4 pole (1800 RPM), you may want to choose a larger wheel for the drive wheel. The formula for surface feet per minute is (RPM x Wheel Diameter x pi) / 12. In North America, a two pole motor with a 2.75" wheel is running at a respectable 2591 feet per minute. Half this for a 4 pole motor which I think is too slow. A 4" x 2" crowned caster makes a fair drive wheel and 1884 feet per minute.
The tracking hinge can be any 3" standard door hinge.
The tracking is adjusted with a bolt through a 1/4" coupling nut.
Other than the tubing, there are a few pieces of steel needed. This is common and can be found at Home Depot or a local metal supplier.
One of the biggest issues with building belt grinders is getting the belt tension correct. I've tried two approaches with this design: the pull spring with a fulcrum and that requires a 120 lb short spring, and the gas spring which is surprisingly easy to install and cheap at surplus places. Try to find one that is between 20 and 30 lbs force that extends about 14 to 17 inches.
Bill of Materials
I made a basic Bill of Materials and costed items based on local suppliers. These are in Canadian dollars and mostly retail. With a healthy scrap bin you can build this for cheaper for certain. Although in my build I used Item 1 as 14 gauge (0.83" wall), if you can get some 11 gauge (0.12" thick wall) go for it. This bill may be missing some a few nuts or washers. You are going to have to improvise a little.
The first order of business is to cut the square tubing. I was able to cut this on a bench-mounted portable bandsaw and it was quick and painless. If you do not have access to a band saw, use a hacksaw with a fresh 18 tooth/in blade, mark the lines with a square and cut in a vise. Each cut end should be deburred inside and out with a with a file.
Notes on cutting:
For items J and K, I started with a 5" square piece of 1/8" steel and cut it into two right triangles.
For item L, I cut a piece of 2" x 1/8" flat stock at 11-1/2".
For the M pieces, I cut a piece of 2" angle iron into four 1-1/2" pieces on the bandsaw.
For item N, I cut a small piece of 1/8" steel to 2-1/2" x 3/4". Thicker steel will work here too. 1/4" is okay.
Mark the pieces after cutting, so they don't get mixed up when drilling.
For all the drilling I used a drill press. It's important to have good right angle holes going through the tubing at 90°. This is key to proper alignment of the wheels. I think there are about 44 holes that need to be drilled and the drill press makes for less work.
NOTICE: Thanks to Anthony for pointing this out. The holes shown at 3 and 6" on item B are incorrect on the drawing above. I will correct the drawing shortly. The holes on B need to be 5" and 10". January 21, 2106.
Notes on drilling:
Most of the holes are drilled for 1/4" bolts to go through, this hole should be clearance fit for 1/4", and will likely be an F bit or a 17/64" fractional. For the few 5/16" holes (identified as 0.33") a 21/64" fractional bit works well. The 1/2" hole in the items A, J and K can be oversized for clearance at 17/32".
Item M is showing four items in the cut and drill drawings. Only two of the four angles need to have slotted holes. The two that are used to attach the platen L to the vertical frame member B should be slotted to allow for platen adjustment. The other two, M1 are for the front feet of the vertical frame member B to mount to your base. M1 only needs 17/64" holes, no slots.
Item L requires a countersink to accept a flat head 1/4" NC bolt. I used an 82° 1/2" countersink and kept drilling and checking the bolt until it was flush to slightly below the platen face.
I intentionally omitted the 3/16" gas spring mounting holes in the horizontal member A. The location of these holes will depend on the length of the gas spring you source.
To make the slotted holes in two of the M1 pieces I drilled two 1/4" holes about 1/4" apart and used a round file to remove the extra material to make the slot.
Once all the metalwork is cut you can start assembling the pieces for a test fitting. Refer to the series of photos to test fit. If you wish to paint, you should disassemble after your test fitting, paint and re-assemble. Note that for most of the test fitting I did not use nylon locking nuts. I used nylon lock nuts for final assembly. Also, some of the photos show the assembly after paint.
Wherever possible, use a square to make sure the vertical and horizontal frame pieces are at 90°.
Let's start with the tracking arm is it's the most complex part and should be assembled before going on the rest of the frame.
Tracking Arm Assembly
After squaring out the hole, I cut back the opposite side of the hinge. This makes it fit better on the tracking arm. I drilled two holes 1-1/2" in the middle. These holes should match the spacing of the holes in the tracking arm C and the coupling nut hold down N.
The tracking wheel itself is bolted on the 5/16" carriage bolt. Some washers and a nut will be needed to space the wheel 1/2" away from the tracking arm C. I have added some electrical tape to effectively crown the wheel and aid in belt tracking.
Any 1/4" bolt of suitable length can be made into the tracking adjustment bolt.
The handle is a 3" bolt with some black tubing slipped over the shoulder and attached with a nylon lock nut.
Assembly of the frame pieces are pretty straight forward. As a rule, for final assembly, when bolting two 1" pieces together I used a 2-1/2" bolt, flat washers and a nylon lock nut.
The gussets J and K are attached with 2" bolts.
The contact wheel is attached with a long 1/2" bolt, flat washers and a nylon lock nut.
Here we also see the gas spring being mounted with some 10-24 by 1-1/2" machine screws and nuts. The position of the gas spring may vary depending on the length of the spring you get.
The pivot for the tracking arm is a 4" length of 5/16" threaded rod with washers and nylon lock nuts. Just below this a 4" length of 1/4" threaded rod holds items D and E to the main horizontal member A. Suitable 4" bolts can work in either of these locations.
Note this photo shows the rear pull spring option installed. This can be used on lieu of the gas spring if you can find a suitable spring in the 120 to 140 pound range.
A different view of the back, post assembly.
Two 1/4" x 1" flat head bolts are sunk flush or below the platen face
When setting up the final platen adjust it so that it pushes the belt away from the top wheel and contact wheel about 1/8".
Note that I chamfered the top and bottom of the platen.
The platen can be replaced as it wears.
Mounting The Frame
The frame must be mounted to a sturdy surface. If you use wood, make it at least 3/4" thick plywood. For my tests I used a length of 3/4" plywood that was 26" long and 12" wide. I would expect this to be the smallest surface that can be used. I recommend using a 26" x 18" and mounting your motor directly to the base.
Items G and H along with the angle iron pieces M1 are used to mount the frame to the wood. You may use 1/4" bolts with nuts (preferred) or lag screws and washers. The recommended height for the base is 28" to 30", or the tips of your fingers when your hands are relaxed at your side. This height will position your elbows about half way up the platen.
Balancing the Caster
The caster wheel will likely be out of balance as purchased, so we must improve it by statically balancing it. This is done by letting it come to a stop and marking the bottom point. Using a larger, say 1/2" drill bit, remove some of the steel in the hub. Give it spin and see where it stops. If it stops with the same spot at the bottom, remove some more steel from the area. Once enough material is removed from the heavy side if the wheel it will randomly anywhere due to friction and not always at the heavy spot. Remove a little from each side as you balance. Although this is not perfect, this will be a huge improvement over the stock unbalanced caster.
Truing the Caster Wheel
If the face of the wheel has a wow (out of round) in it you can true the face when the belt is on and the motor is driving the wheel. With the unit off, clamp a steel bar or 1" angle iron to the vertical frame member B just above the wheel to act as a tool rest. With a belt on the grinder, power it on and check tracking. Using a sharp chisel against the improvised tool rest, very carefully move across the wheel. Only a small amount of polyurethane should be removed at a time. When buying your caster, checking the wheel face for minimum wow can greatly reduce the truing effort. In the case of my wheel, no truing was required as I chose the best of the half dozen or so wheels at the store.
MOTORSo far there has been little mention of the motor. I am expecting that many different motors will be used in the construction of these types of belt grinders. In the earlier section entitled Drive Wheel and Belt Speed we looked at the speed of the belt in relation to the size of the drive wheel and the motor RPMs as related to the procurement of parts.
You should be on the lookout for something along the lines of a 1 horsepower motor. If your motor says its RPM are in the 1750 area, then you will want to use about a 6" drive wheel. If your motor says the RPM are in the 3450 range, then you will be looking for a 4" drive wheel. Most general purpose motors in this size will have a 5/8" diameter shaft. Second hand table saws are a good source of decent electric motors.
Motors that have a good starting torque winding are preferred for this grinder design due to the mass of the steel caster it takes a little grunt to get things up to speed. When you power up a motor and after a second you hear a click, that is the starter winding dropping out. A centrifugal switch operates when the start winding and capacitor, then the motor goes into its normal run winding when it's at speed.
I live in a rural area and everyone here has water wells. Used jet pump motors are free to cheap. These are almost always fast 2 pole (3600 RPM) kind of motors.3/4 to 1-1/2 horsepower are typical.
TEFC or ODP
A Totally Enclosed Fan Cooled (TEFC) motor is preferred for any grinder. The insides are sealed from dust and moisture. The alternative Open Drip Proof (ODP) is a second choice. This doesn't mean the ODPs cannot be used. It's easy to make dust shields to keep metal dust out of the motor. Regular cleaning with compressed air and vacuuming helps prolong the ODP's life as well.
I could pay the $53 and freight and order a nice machined 4" aluminum wheel, but your drive wheel doesn't have to be store bought. Here's where this can get interesting and creative. In fact my robust 2 hp belt grinder uses a 4" nylon caster.
How about turning a drive wheel out of wood?
Working on a page for that now.
This page will be updated shortly.
November 23, 2016