An Alternative Bed for my Probotix CNC

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4DThinker
Vectric Wizard
Posts: 1700
Joined: Sun Sep 23, 2012 12:14 pm
Model of CNC Machine: CNC Shark Pro, Probotix Meteor 25" x 50"

An Alternative Bed for my Probotix CNC

Post by 4DThinker »

I previously made a couple of frame bridges to support an assembled student project for some CNC joinery cuts. Lately I've been using these bridges more often to support flat work. They have proven their worth in every case and these are the advantages they offer:
Frame Bridges.jpg
1. They are stiff and flat and remain level to each other and plane-parallel with the CNC's X/Y travel.
2. They have a horizontal side to provide a clamping edge for c-clamps to hold the work down.
3. They have aligned holes for pins to set the project board against for perfect alignment with the CNC Y axis.
4. They attach quickly into the side frame rails with one screw on each end into a drop-in twist-nut.
5. They can be positioned anywhere along the side rails.
So far two have been enough, but potentially a 3rd might be useful for long boards that may sag or have a bow in them. The 3rd bridge could also be used to clamp a stop block on for repeated identical cuts using the same Y origin. Small C-clamps have been enough to hold work to them, but with a t-slot track embedded in their top any other t-track clamp could be used.

MDF slats between t-track on another MDF panel that span the CNC frame tend to sag in the middle over time. Plywood bridges in an L configuration remain stiff and straight with no deflection under the weight of projects boards or flexing when clamped to warped wood. With a single t-track on each bridge only 2 or 3 lengths of t-track are needed as opposed to several to cover the bed area.

Even my thin bed vise: http://4dfurniture.blogspot.com/2016/12 ... -vise.html that is designed to slide into t-slots can be simply held down by small c-clamps. With t-slot rails in each bridge any alignment or other clamping jig could still be used.
While my past experience with holding parts down to CNC tells me this won't be a cure-all-perfect-solution, it may prove to be the least expensive reliable strategy for holding down the majority of the flat work I do.
Bridges are possible with any CNC that has nothing running under the bed, and a way to remove the bed and attach the bridges.
My strategy when using C-clamps is to use the center of the board as origin and make sure the toolpaths don't go anywhere near the clamps.
4D

Bobtail Farm
Vectric Craftsman
Posts: 283
Joined: Mon Dec 01, 2014 3:53 pm
Model of CNC Machine: Homemade

An Alternative Bed for my Probotix CNC

Post by Bobtail Farm »

splendid. thanks for posting.

4DThinker
Vectric Wizard
Posts: 1700
Joined: Sun Sep 23, 2012 12:14 pm
Model of CNC Machine: CNC Shark Pro, Probotix Meteor 25" x 50"

Re: An Alternative Bed for my Probotix CNC

Post by 4DThinker »

Thought I'd post a photo of the updated design.
Before cam levers arrived
Before cam levers arrived
With cam levers
With cam levers
Cam levers make moving the bridges and locking them in place far simpler than fetching a screwdriver to loosen the screws initially used. More peg holes for pins to align project boards to. BTW I used to use the CNC itself to line up boards true to the Y axis when alignment was critical. Not a complex process but it took a few steps and some time I don't have to spend now. I recommend that no matter what you have for a bed let the CNC cut a few aligned holes along both axis for short dowel lengths you can drop in when aligning your project board. The holes don't have to be deep. I just shop-vac them clean if a job fills them with chips/dust. Same for the t-track.

My original bed has 6 parallel t-track sections each 4" apart running the long length of my CNC. Between each was a strip of sacrificial tru-pan that I would replace once a year or so. Under that a 27"x52" sheet of MDF that sat on strips of plywood screwed into the sides of the perimeter frame rails. All that has been replaced by two (or maybe three) movable plywood bridges with one strip of t-track each.

A bonus potential is that the bridges can be bolted to the underside or the frame rails to gain 78mm of z axis space. The Probotix Meteor I own has 5+ inches of Z travel but only barely 3" of Z room above the bed.
4D

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