The Crossbow Buffer Problem: Part 2

I had a crossbow problem: the buffer from the trigger mechanism of my Crisbow Cheetah MKIV was broken. Without that buffer it would be unsafe to fire because there would be no retarding force on the string latch. I described the loading process in Part 1 here..

In order to design the buffer in 3D, I needed to get measurements from the Quadro buffer. The first thing I did was X-ray the Quadro buffer and then make some physical measurements with a digital caliper and add those to the radiographs. This was a rough plan:

There were some measurements I couldn't get with calipers, either because they were curves or they were recessed areas in the buffer. One of those recesses was curved and you can detect it on the radiograph. Here it is outlined in red:

My plan was to design the buffer as best I could with the available measurements and then see what options the software would give me to get around the missing measurements.

Next, I needed a 3D design program. In the past I have used Cinema4D, way back in the days of the Commodore Amiga. I am a beginner when it comes to 3D and raytracing but I understand the basics. I can visualise how a part should look and figure out a way to extrude or "cut" this using Boolean operations.

At this point I have to give Autodesk a huge thumbs up, because without their product Fusion and the non-commercial free license to use it, I would have had significant difficulties doing this on my own.

Fusion (or Fusion 360 as I first knew it) is a complete package for the creation of 3D objects which can then be printed or CNCed. The program allows you to create an object and then export a 3D file that you can email directly to a printer or machinist to produce. You can design the item in one country and have it printed in another!

I imported the radiograph into Fusion and used it as a guide for designing the buffer.

Here is one image from the design phase, this was when the program was known as Fusion 360:

From the above image you can see I had done all the easy bits, which is cutting pieces off a cube to get the basic shape. The curves had not been done yet.

In the next image you can see that I approximated the main exterior curve with an ordinary cylinder shape and merged it into the model:

Similarly I used a short cylinder with a greater radius to "cut" the curved recess out of the model. I made an assumption with this buffer, which was that I wouldn't need to put a hole in the foot of the buffer for the screw, because I assumed that I could simply screw through that foot because it would be only 4mm thick.

Fusion has a very nice rendering module and it can simulate various materials. Here is my first design, as seen rendered in Fusion:

There are two important points to make here:

1. I am aware that this visual "trial and error" fit based on a radiograph will have inaccuracies. The radiograph itself is created with a diverging X-ray beam, and there will be a small magnification difference between the structures that were nearer the X-ray tube versus those nearer the X-ray detector.

2. This buffer can be checked for fitment prior to use. Not all the measurements need to be exact as long as the device fits in the bow and can be secured by the trigger mechanism. What I deemed important was the inner channel that has to be clear for the sear arm, and the upper angled surface which has to support the string latch on firing.

If this was a component that had to be 100% dimensionally accurate, I would have been forced to get the buffer laser scanned by another company just so I could get the 3D model. This can be quite expensive and the model you are given usually cannot be edited. I was lucky that I didn't have to do this.

The next step was printing or CNCing the model.

I had to do some reading up on these two production options, because there are tolerances to consider. You can have a very accurate 3D model and end up with a bad product if it is printed using a technology and substrate with tolerances that are not tight enough. At the top end of these production methods is a component CNCed out of a polymer, for example. That will be expensive but the company (such as Protolabs) will have an engineer who can discuss your requirements. They often require not just the 3D model but a drawing of the component with dimensions that they can use to verify that the part has been accurately produced. So you are getting expertise on hand and paying for a precision product.

3D printing using an online vendor is the cheapest option for a one-off product. You provide them the model (they don't need the drawing usually) and you specify the printing material and the colour. There are other options such as the percentage infill.

I went with Craftcloud because they have an instant quotation setup. You upload the 3D file (I used STEP format) and specify the material and colour needed. They then have clients all around the world who quote on your model.

There's a handy description of material properties and printing tolerances when you order.

For the first print I chose ABS plastic printed with 100% infill. You can select various infills and I thought 100% would be the most durable. More about infill and material choice later!

That was around ￡17 delivered.

This is the product that arrived. Of course, the first thing I did was X-ray it and that's how I found out that it was definitely not printed with 100% infill:

I asked Craftcloud for a reprint at the requested 100% infill from the same company.

At this point I acquired a new-old-stock Crisbow Cheetah MKIII frame and I wanted to test it. This is when I made a discovery about the differences between Crisbow versions and trigger mechanisms. The MKIII has a sear arm with a "swell" on the underside and it cannot work with the buffer from the MKIV. Here are X-rays of all three trigger mechanisms, with the fatter sear arm of the MKIII arrowed:

Here is a photo of the MKIII trigger with the replacement buffer that Craftcloud sent, the one with the 100% infill. Note that the sear arm cannot go into the inner channel of the buffer, which means the crossbow cannot be loaded with this combination

To make matters worse, I found out that a 100% infill buffer cannot be screwed through manually, not if it is ABS plastic. It really needs a hole incorporated in the design. Otherwise the user has to drill a hole for the screw, in the foot of the buffer.

As a matter of interest, this is the original buffer that was in the MKIII Cheetah. Notice the different neck geometry on the buffer to accommodate the fatter sear arm:

So it was back to the drawing board. I now had to make a buffer that would work in any of the three Crisbows I have. I redesigned the buffer in Fusion, this time including the wider neck for the fatter sear arm of the MKIII. I also added a hole in the foot of the buffer and a cylindrical relief cut so that the factory screw could be inserted easier than previously.

This is the new design:

I ordered two 100% infill prints of this new buffer using ABS plastic, from the same company as before. This time I chose the colour red. Here is one of the new printed buffers alongside the original buffer from the MKIII:

And here it is in the crossbow. This is with the MKIII trigger, showing where the screw is that fastens the buffer underneath the trigger mechanism to the frame:

I was less than amused to find out that the new prints were also not 100% infilled. I asked Craftcloud for reprints and I X-rayed those when they arrived. Here are the three buffers, the one on the left being less than 100% infill and the other two being 100%:

Just for giggles, I tested the print that was not 100% infill and it worked just fine. This is what it looked like after 60 shots, it held up well even though it sustained latch imprints on it:

Then I tested the two 100% infill prints and there was an unexpected failure. In both cases the left side of the buffer cleanly detached from the base. For one buffer it took 22 shots before it happened and for the other it happened on the very first shot.

Here are the two failed buffers, which were printed 100% infill using ABS.

First the main part of buffer 1, showing where the left side sheared off:

This is the underside of the part that sheared off:

This is the main part of buffer 2, showing where the left side sheared off:

This is the underside of the part that sheared off:

It may well be that for this application, a buffer that is printed in a solid straight "laminate" configuration is prone to separation in the manner shown. The other print that wasn't 100% infill clearly survived much better, although you can see latch imprints on the buffer.

I had a discussion about this buffer with some gunnies and I showed them these photographs. They were of the opinion that I should use a material that has some "give" and see how that goes.

So I had another look at the materials that Craftcloud offers and settled on TPU. Here's how it was described on Craftcloud:

I picked another company, based in Italy, and had two more buffers printed. These were TPU at 80% infill. I chose green this time:

These prints had a rough finish with wisps of TPU evident. Despite this homely look, the buffers performed well in the crossbow. Here is one of them newly installed:

And the same buffer after 70 shots

That buffer has just enough "give" in it to perform well. It fits in all three crossbows and was fired in the MKIII and MKIV Cheetahs. I don't want to fire the Quadro, but the buffer accommodates all the sear and latch positions when manually manipulated. It took less of a latch imprint than the ABS print.

Some time after this green buffer success, I wondered whether this wispy look was a manufacturing error or not. I gave an American company the same model to print, also specifying TPU at 80% infill. For giggles I specified glow-in-the-dark green.

This was more expensive mainly because of shipping.

However it was worth it. Look how neat this buffer is (and it does indeed glow in the dark):

In the bow it was a perfect fit:

And this is what it looked like after 50 shots:

What is interesting is if you compare the radiographs of the Italian and American, they are not filled the same.

On the radiograph the Italian is on the left and the American on the right.

I don't know which company has the correct fill, but I can tell you that the Italian weighs 6.8g and the American is 6.5g. The difference in weight is small and both buffers seem to hold up well.

If anybody needs this buffer, let me know and I will email you the STEP file. It is one way to keep these ageing crossbows operational!

So, what did I learn here?

1. You can design and create a model for 3D printing relatively cheaply with good software (thank you Autodesk/Fusion!)

2. Results can vary depending on printing materials and infill percentage.

3. There seems to be significant variation in print quality and you need to verify that you are getting the right infill. I am fortunate that I have X-ray to do this!

4. The method I chose (both to model and to print) suits an application where you don't need high tolerance. For more exacting dimensions I think CNC is far superior, but you will pay a lot for that.

5. You may have to make several versions of a model and do a lot of trial and error before you succeed.