PRO4848-ish with an aluminum top plate and tangential knife

Build Logs
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My brother and I shopped for quite a while for a gantry machine to support our composites projects, mainly machining foam masters and occasionally trimming finished parts.

Short requirements list:

  • 4’x4’ bed
  • More than 6" of gantry clearance/Z-travel
  • Has to fit through a conventional door
  • No steppers

That pretty much sealed the deal for Avid over any other manufacturer.

We ended up buying a used PRO4896. It was “used” in the sense that it had been ordered and delivered to someone else, but never assembled or even removed from the Avid packaging. It’s a newer EX machine with ClearPath servos and a 4HP spindle.

Since we assembled it as a 4848, we needed to off-road a little bit from the included assembly instructions. We also opted to double up on the lateral supports since they came with the machine anyway.

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We spent a decent amount of time leveling, squaring, and adjusting the machine before we were happy with the alignment.

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We weren’t psyched about only using an MDF sheet as a baseplate as the stiffness and dimensional stability leaves a lot to be desired. After trading some different options we came across a 48"x48"x0.75" sheet of Mic6 aluminum tooling plate from a local scrap dealer. This got counterbored and bolted down. After tweaking the cross beams we ended up with about 0.008" of total height deviation across the entire work surface.

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Now that we had a new tool in the shop we started having intrusive thoughts about how to automate some of the more tedious tasks we have in front of us. As mentioned, we’re doing a lot of composites work right now. This includes templating and cutting tons of carbon fiber fabric. We decided to try adding a tangential knife to the machine in support of this.

My brother designed a mount for an additional ClearPath servo motor. The spring-loaded blade holder is made using a ballspline and a 3D printed bearing block. All the other mechanical bits are from McMaster. We’re using the spindle as the toolholder since the ClearPath can’t cope with the axial loads. This has the added benefit of not requiring any additional shafting or X/Y tool offset weirdness. Both the motor and homing switch are wired to the EX controller’s A-axis ports, with power Y-cabled off the X-axis servo. I found a tangential knife post processor for Fusion 360 on GitHub that I re-wrote and customized to work with the Avid. The rotary homing scripts in CNC12 also required some mods to make them work with a C-axis instead of an A-axis.

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The crank handle on the left lets us pull the entire assembly as a unit. There’s also a stop collar to index the mount axially and a dowel pin that acts as a rotational stop.

Video of the first test cut:

We have since added a 48"x48"x0.093" LDPE sheet as a cutting surface. That’s backed by a sheet of mild steel. This way we can simply hold the cloth down with magnets.

My only gripe with the machine is how Z-axis and spindle tramming is handled. I think Avid missed the mark a bit on this. The assembly manual mentions nothing about Z-axis tramming, there’s no effective means of adjusting spindle nod, the tilt adjustment is finicky, and moving the spindle up or down on the Z-carriage means you have to re-tram the entire spindle (making the base spindle mount alignment dowels largely useless).

I want to be clear that overall we’re very happy with the machine and the more or less open nature of it’s design lends itself well to customization. We’re excited to keep putting this thing to work.

6 Likes
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Impressive!!! I have two questions:

  1. Would a normal drag knife work for you? If not, why?
  2. My 2019 machine came with tramming instructions from Avid. Here is a video version: Machine Calibration. Very easy to tram so spindle is perfectly vertical.
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Thank you!

  1. We considered using a drag knife, but the weave on a piece of carbon fabric is quite delicate. Without a vacuum table I don’t believe it would provide a good result. This whole setup only cost us a few hundred dollars in eBay, McMaster, and laser-cut components. Certainly cheaper than a vacuum hold down system.
  2. I’m well aware of those tramming instructions. We found them to be quite lacking. You can certainly get the tilt adjustment close, but to adjust nod you must shim, and Z-axis tram is never even mentioned. Our Z-axis required pretty significant adjustment. I can understand that most people may not care about it if all they’re cutting is thin sheet goods, but we’re fully machining parts that require every bit of the 8" clearance.
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How much did you find that your z-axis needed to be trammed? Did you shim it? Going to be moving to a new shop shortly so I will have a chance to do the same.

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The Z-axis itself did not require shimming, but the tilt was quite bad. Note that the lack of an absolute reference surface on these machines makes adjusting them a potentially brain melting game of chicken and egg. Our solution (as shown in the photo) was to check “table” level against four points, then suspend a cast iron straightedge across two of those points with a 24" machinist’s square on top. This let us adjust Z-axis tilt using feeler gauges.

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I don’t know how thick the carbon fiber cloth is that you are using, but I’ve cut it with the 45w laser I have on my AVID. For any flimsy material, the laser is a lot nicer since you don’t have to worry about catching and bunching up the fabric. You can make a simple honeycome vacuum table that uses a shop vac or regular dust collector to hold the material down flat (and remove smoke) that is a lot easier to make than the typical one to hold down sheet goods. You can also make it a removable unit so you can remove it when cutting other types of material with the spindle.

This is why the drag knife I got with my AVID is still brand new sitting in the box.

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“We’re going to need a bigger engineers square…” :open_mouth:

Could the CF cloth be cut with the AVID laser attachment?

1 Like
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@jjneeb mentioned that he could cut carbon cloth with the laser he had. If you have a suitable laser and smoke extractor then that could be a good option. W don’t have anything like that, so the cutter seemed like an easier method for us.

Here’s the github link for the post processor if anyone decides to give it a try: GitHub - basketofkittens/tangential-knife

1 Like