As a designer and manufacturer of t-slot framing projects (see full disclosure below), we are often asked how to fasten t-slot aluminum extrusions to maximize strength and rigidity by CNC frame builders. For the CNC frame builder, these characteristics are extremely important as they will ultimately help you minimize overall mechanical slop and maximize the accuracy of your machine.

The basic mechanical connection on your frame is the 90 degree connection between two bars. For most CNC frames, you will be using larger 1.5" x 3" extrusions, 3" x 3" extrusions, or 3" x 6" extrusions (or their metric counterparts).

When joining two t-slot extrusions, you really have a lot of ways to fasten them together - including internal fasteners, external fasteners, joining strips/plates, brackets, gussets and even mitered bars. While I know of no scientific comparison test of every available option, the 80/20 line of t-slot extrusions does offer a fastener strength chart and our CAD frame designers, with their many years of experience, have told me this is their preferred method for CNC frames:

First, use machined counterbores and anchor fasteners on both sides of the bar (see attached photo). Anchor fasteners provide very good vertical and horizontal strength, and when used in combinations (both sides), they provide excellent torsional strength as well (prevents twisting). When using larger extrusions with multiple slots, you will need to use an anchor fastener for each slot on opposite sides of the bar (so a double slotted bar will require 2 anchor fasteners on one side, and 2 on the opposite side). Multiple anchor fasteners is the core of a strong 90 degree bar connection.

To add further strength, our designers usually add external flat plates with at least 2 holes/fasteners for each bar (see attached photo). This is the most common CNC frame configuration - anchor fasteners & external plates.

External flat or joining plates add additional strength to the overall frame, and also gives it that super-strong industrial look. Use black bolts/screws to make it really stand out

To add further strength, a full width gusset will provide further strength and rigidity (see attached photo).

While there are literally hundreds of ways to attach t-slot extrusion bars when building a CNC frame, tool bench or cart, these are the methods we use and have found to be the most reliable over the last 13 years building of hundreds of t-slot framing projects. Hopefully, this will help many of you get started if you are not buying a tested t-slot frame plan.

DISCLOSURE: I work for a company called F&L Industrial Solutions, Inc. ( that sells t-slot extrusions and designs/builds t-slot extrusion frames commercially. We are an independent company and free to use any brand of t-slot aluminum extrusion we want, but we use the 80/20 brand almost exclusively because we think it is higher in quality (extruded in the USA) with a better design, better QA process and superior anodizing versus the small discount copy-cat companies or the Asian imports. 80/20 also offers the advantage of over 8,000 tested, compatible parts in the product family - which is very valuable in our business. Our posts on designing and building t-slot extrusions are written to be universally applicable and useful, even if you do not use 80/20 branded extrusions. We offer all CNCzone readers discounts on 80/20 bars and parts - just use the discount code: cnc10off at checkout at our shopping site: or give us a call.