Summary:
This page covers my ongoing project to build a 2” by 72” belt grinder for knife making. The grinder is now completed, and is by far my favorite piece of shop equipment at the moment. Most of the design was waterjet out of 1/2in aluminum extrusion, but there are also a number of parts which were cut on a CNC mill or conventional lathe.
Thus far I have been delighted with the performance, my most recent knife took 30% as long to grind as my last knife with the older grinder. There is some minor resonance at certain motor frequencies, but that does not seem to impact the grinding behavior.
Key Features:
1.5hp variable speed motor.
Can accept industry standard tooling arms for both the work rest and main grinding attachment.
Built-in pivoting mechanism can shift the grinder from vertical to horizontal.
Tension hold-down allows single handed belt changes and provides repeatable belt tension (currently set to 35lbs).
Project Files: The project files are too large to host on this website. Shoot me an email through the form (under “about”) and I’ll send them to you.
Funding: This project was largely funded by the Housner Fund, without whose support it would not have been possible. More information about the fund can be found here: http://www.deans.caltech.edu/Grants_Funding/gwhfund/about-housner-fund
First finished part! The tracking pivot plate.
Really like how the handle turned out.
Resources:
This build draws heavily on the KMG and Reeder grinders with inspiration from the Sayber project and a number of other grinders. I have included some of the resources I found helpful when designing this grinder. It should be noted that in the case of company websites, I have only used my own grinder and thus cannot speak to the actual products listed.
Versions:
V1.0 is very nearly a clone of the standard KMG design. Not terribly original, but I figured it'd be a good idea to start with something known before diving off into a custom design. A few minor modifications have been made to eliminate any need for welded joints. The two pillow block bearings have also been switched out for a pair of press-fit bearing. A clamping plate (opposite side, not shown) and specially cut drive shaft provide pre-load. The tension arm, tension arm block, and platen supports are all chosen to use the same 1.5" square stock used by the interchangeable attachment arm.
The wheels are positioned relative to each other using brass spacers. These spacers make it easy to align all of the wheels correctly on their 1/2in bolts, and provide a light pre-load as needed.
The tension in this design is provided by a 40 lb/in spring on the rear side of the tension arm. This requires a larger spring than a forward mounted tension system, but allows for the use of standard hardware store trampoline springs.
The platen attachment shown provides platen and slack grinding as needed by loosening the horizontal support bolts. The platen itself is intended to be made from 1/4" hardened steel. Although more expensive than an angle-iron platen, this allows for edge cuts on both sides of the platen.
V1.1 keeps the same basic structure as V1.0, but adds a number of major changes to improve the feature set and ease of fabrication. This version complies with all of my design goals in the "need" and "want" categories. It is also a the first fully fleshed out model (in the sense of having all of its fasteners and tolerances).
The big functional improvements for V1.1 are tilting, direct drive, and the interchangeable support arm (not shown). The base material has also been changed from steel to aluminum for ease of fabrication. This roughly doubles the materials cost of the machine, but reduces the overall fabrication cost by 20% when water-jet time is considered. As with version 1.0, this machine can be fabricated using only two sizes of stock.
The idler wheel posts remain standard 1/2in bolts. However, each tilting pivot is composed of a 1/2in shoulder screw with a fanged bronze bushing in the base plate, and a bronze washer between the plates and bolt head. This setup is about 14$, but my hope is that it will confine major wear to the (easily replaced) bronze bushing.
For structural fasteners I've standardized on 1/4-20 socket head screws in two sizes; two inch for the tooling arm assembly, and one and a quarter inch for everything else. This requires a number of fasteners on some joints, but makes assembly easier.
Last updated: 1/4/2021
Reflections:
Although I would have no reservations about suggesting that someone copy this build without alteration, there are naturally a few tweaks which I feel would improve the overall experience. Below, I have included a number of lessons I learned while designing the machine, along with several pieces of information which I found it difficult to acquire while designing the grinder.
There are two different types of pivot in the design. The first, used in the tilting pivots, passes a shoulder screw through a stackup of plates and washers, with the tension provided by tightening a bolt placed on one side of the stack. The second style, used in the platen assembly, screws the pivot directly into one of the plates. Under that system the tension is provided by a belleville spring, and set by shims. Both styles work, but the second works better. In my estimation the gains in reliability, and in smoothness of motion, greatly outweigh the marginal increase in cost.
The tracking adjustment does not need very much range of motion. As configured on this machine a .02 Radian deviation from nominal produced a 0.64” shift in belt location at the platen. The setup that I have, with a 1/2-13 adjustment screw and a pivot works well, although if I were to do it again I might opt for a finer pitch thread.
The arm pocket tolerance can be pretty tight. I went with 40 thou of gap on each axis, and that was fine, but produces a noticeable bit of wobble when inserting a new tooling arm. I’d go with 20thou or less if I were to make the grinder again.
Placing the tension arm pivot above the travel line of the two interchangeable tooling arms would have allowed for longer arms to be used. This would have let me use a single standard size rather than needing to manufacture different sizes for different attachments. A similar observation might be made about the placement of the motor relative to the lower tooling arm.