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Vector patterns and modifying them for double bevel cutting

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Blog entry by jfk4032 posted 04-01-2014 10:17 PM 746 reads 2 times favorited 2 comments Add to Favorites Watch

It was requested on another forum for me to explain how I capture my intarsia patterns digitally, create vector artwork, then modify that artwork for accurate double bevel cutting…so here it is for my LJ friends:

I’ll try to explain the techniques I’ve developed for digitizing my patterns and using that pattern to expand the artwork for accurate double bevel cutting. Keep in mind, I just started intarsia less than 2 years ago, only have 7 projects under my belt and only tried double bevel cutting on my most recent project. So, by no means am I an expert, I’m just beginning with all this, but I do have a graphics and printing background to give me perhaps a different perspective than most, especially with pattern modification.

You would first start out by capturing the pattern in digital form. The two ways I know to do this are to scan the pattern as a whole or in pieces if too large for your scanner or take a digital photograph of the pattern. If taking a picture, you’re best to stand far away as possible to reduce the distortion of taking a picture.

Once I have the base pattern digitally captured, I’ll take that file into Adobe Illustrator where I begin the painstaking, but necessary task of redrawing all of the lines over the pattern and then applying numbering, grain direction arrows and other notes if needed. This is in vector form, so it is infinitely scalable which is a real nice benefit. The next project I’m working on started off way bigger than I realized, so it was very simple to start scaling it down to a reasonable pattern before sizing up the wood I’ll need for it.

I’ll then print out multiple copies off a large format printer as reference and to start some of the cutting that doesn’t require double bevel cutting. For the ones that will be double bevel cut, I’ll use the following steps.

I’ll select test pieces of the two woods I intend to use and start experimenting with the angles and thicknesses of the top piece to get the desired height or recess of the top piece into the bottom piece. Once I determine that is satisfactory, I’ll measure the distance across the top of the top piece test cut hole and the distance across the bottom of the top piece test hole. Refer to the diagram to follow along.

Diagram 1

In this example I’ll use 3º as the angle cutting clockwise. The measurement across the top piece test piece (inlaid wood) is .75” at the top and .71” at the bottom. This means if I were to use the existing pattern without modifying it, I’ll get the desired height I wanted by following the trial and error process above to get the correct angles, but the piece itself will be shrunk inwards, smaller than desired relative to the actual pattern. This is where having it in digital form makes for an easy adjustment. If the total distance shrinkage is .04”, that means I need to expand the pattern .02” all the way around it to get that .04” back. There’s a simple command in Illustrator to do this. I’ll color the expanded line in red so it’s easier to see which line to cut.

I’ll just print these out in sections off my standard printer at home on 8.5×11 paper. See the next picture for that result.

Enlarged expansion
IMG_0598 copy
As you can also see in that picture and the next, the amount of expansion is greater as the height of the piece gets taller. The teeth for example stick up the highest and accordingly have the greatest amount of offset, whereas the tongue is almost level with the background, so it’s expansion is much less. In this case the thickness of the holly for the teeth was also thicker than the pink ivory for the tongue…but the testing of angles with thicknesses I described above takes that into account.

If you don’t have Illustrator or another drawing software, you can still do this technique the old fashioned way. Do the test cutting, and measurements as I described above and then carefully outline the pattern piece with a pencil to the correct offset and you’re good to go.

The position of multiple double bevel cut pieces into an intarsia pattern also requires tight “registration”, using a printing term, of one piece to another. I figured out to do this I need to cut the shape of the top piece to fit over reference points of previously cut patterns to the base piece. You can see this in the next picture, how the the outside shape of the hind leg top piece (the grayish blue pine) was cut to fit over the already cut pattern of the base piece (the darker wenge)...this ensures good registration of the piece to be cut.

IMG_0570
IMG_0597 copy
I hope this sheds some light on what I’ve just learned on this latest piece.
Joel

Here are some Q&A’s from that other post:

When taking a photo and stand as far away as possible, do you use the zoom feature?
Yes

Do you save the photo as .jpg?
jpg is fine, or any format you can open in a drawing program like Illustrator. pdf or tiff should also work fine

When you cut the expanded line (red) don’t you have the same issues, just larger. If you are cutting double bevel it tapers no matter what size the pattern is.

Not sure I’m following you on this question. When you cut the red line on the top of the top piece, the angle tapers in so that the cut at the bottom of the top piece (where the fit actually takes place in reference to the base piece) is back to the original pattern shape…or close enough to it that the pattern integrity is good enough by your eye.

Are you marking a “registration” with a pencil or using a image point on the pattern itself? Do you use only one point, or two, say corner to corner?

Not a specific image point at all, but rather an edge, curve or better yet a corner or sharp arc. If you can find opposite edges or corners to align to, you can be very accurate…check out the 4th picture and see how there are cut features around the perimeter of the grey wood that allow me to lay it over and fit it to areas of the piece below that were already cut. You always need a first piece cut into the base where you won’t have anything else cut yet. In that case you can use the outside contour of the base piece to find where to make that first cut.

I presume you don’t have to use shims?

You could use very thin shims, but keep in mind the main purpose of using the double bevel cutting, is to make a glove tight fit of the top piece into the bottom piece. The higher you shim the piece up, the further away from that perfect fit you take it away. If you want a higher top piece you would increase the top piece thickness and accordingly probably need to increase the offset to get the bottom side of the cut top piece back to the original pattern shape.

Once you have your table angled to the initial offset, do you change the angle depending on the height you need for the piece?

Yes, each piece or group of pieces may need to be set to a different angle depending on the thickness of the top piece and the end height I’m looking for. On the Duke dog, for example, I cut most of the gray body pieces at the same angle, because I started with the same thickness wood and wanted the same starting height for all of the pieces. I then sanded and shaped the various gray pieces down to create the height differentials in the final piece. Only on several of the gray pieces did I change the table angle. The lower jaw pieces were raised up the least, so their offset was less than the other gray pieces.

-- ---Joel; Central MD...rookie empter nester and getting back into woodworking!



2 comments so far

View Monte Pittman's profile

Monte Pittman

15450 posts in 1085 days


#1 posted 04-02-2014 12:06 AM

My brain is completely overwhelmed just trying to think my way through it. Very impressive.

-- Mother Nature created it, I just assemble it.

View TopamaxSurvivor's profile

TopamaxSurvivor

15071 posts in 2423 days


#2 posted 04-03-2014 07:56 AM

awesome procedure .

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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