Sunday, January 21, 2018

Tolerance Issues in Joint Fit

This post is a discussion of tolerance in getting parts to fit together well. It particularly deals with laser cut joints, water jet cut joints, and CNC routed joints.

Material Thickness

In fitting joints together the material thickness is a key parameter. You must not assume the thickness as advertised is correct. You have to measure it for yourself. It's a good practice to measure the material in several places because the thickness can vary across the piece. For a good fit you need to measure to 3 decimal places of accuracy using a digital caliper.

Fit Offset

Parts cut to exactly the same size will not fit together. Why? Because the cuts are not perfectly smooth, nor perfectly accurate. There needs to be a little extra material removed to allow them to slide past one another into place. To give this quantity a name we'll call it the Fit Offset. It varies based on the machine and the material.

Kerf

The amount of material removed by the cutting process is called the Kerf. For example if you are cutting a sheet of plywood on a table saw, the saw blade removes material. Often, a table saw blade is 1/8" thick... so that's the amount of material it removes. If you cut a 4'x8' sheet of plywood in half to get two 4'x4' sheets, either one or both will be less than 4'x4' because a kerf width of material is removed.

Laser cutters and water jet cutters also have a kerf although it is much smaller than a table saw blade.

To visualize the kerf we'll use this simple box joint as an example. On the left the parts join edge to edge. On the right they join perpendicular to each other.

In the drawing below the black lines show the part edges which are drawn at exactly the same size. The kerf is exaggerated and drawn in red. The laser itself is drawn as a red dot.

You can see how cutting right along the centerline results in two parts that don't fit.

You need to compensate for the kerf in the design of the joint to get a proper fit.

You can use a feeler gauge to measure the kerf in laser cut joints. Simply run a test cut of a line and measure the width of the kerf. In the sample below the kerf is 0.015".

Make sure when you cut you use the same machine and same power settings. Otherwise the kerf is sure to be different.

Kerf Compensation

Since a laser cutter cuts right on the line, you need to offset the original line to follow by half the Kerf, to the outside of the part. If you cut along that line it will exactly match the part as originally drawn. Then you need to offset that new line, towards the inside of the part, by the Fit Offset. Cutting along that line will result in exactly the Fit Offset being removed from the desired part size. Here's a step by step example... the designed cut is a solid black line.

First you need to compensate for the width of the laser. Here the dashed red line is offset from the original by half the kerf. If cut along that dashed red line the part would exactly match the designed size.

That's not what we want... We need to remove more material for the Fit Offset so there is a slightly smaller part. Then it can fit together with its mating part.

As above, here you can see the original line (black) and half the kerf width line (dashed red). The dashed red is offset, inward, by the Fit Offset resulting in the dashed blue line. If we cut along that dashed blue line the part will be slightly smaller than the designed size. And that's exactly what we want.

A simplified version showing only the fit offset line and the solid blue resulting size:

So in summary, offset to the outside of the part by half the kerf. Then offset that line towards the inside by the Fit Offset. That's the line to follow to cut.

Laser Cutter / Water Jet Cutter - Kerf Compensation

The laser cutter and water jet cutter have a relatively small kerf. The amount is very dependent on the specific machine and how well it's adjusted. A typical laser kerf is between 0.005" and 0.020".

Water jet cut kerfs in our lab at Taubman are usually 0.030" to 0.040".

There are two possibilities for compensating for the kerf:
  • Cut centered on the line. In this case the one-half of the kerf width is removed from the part. So the drawn part needs to account for this difference. It needs to be wider by half the kerf. Then when cut the part winds up the correct size. 
  • Cut offset from the line by the half the kerf. In this case the part is modeled to its desired size and the compensation is done in the CAM software rather than in the CAD model itself. 


CNC Router - Kerf Compensation

CNC routers typically are set to cut with the tool to one side of the cut line rather than centered on it. They automatically offset half the width of the cutter so the edge cuts exactly on the line. Therefore the kerf is not a factor in the size of the parts. However, the Fit Offset still needs to be used to get proper fitting joints. For CNC cut joints I usually use 0.005" offset per side. So for a mortise and tenon joint the tenon is 0.01" smaller than the mortise (0.005" is taken from each side for a total difference of 0.01").

The other issue with routers is that the cylindrical end mill cutters are not able to get into tight corners. Here's an example - the white circles represent the bit doing the cutting. It cannot reach into the inside corners.

The resulting cut material will look like this - the bit couldn't reach into the corners and thus they remain rounded. This joint obviously would not fit together.

In order to make the joint fit you need to route or drill out the corners. Here a 1/4" drill removes just enough material to clean out the corner for the mating part to fit.

After drilling the parts look like this:

Here's how the joint appears as the parts meet at a corner:

If you cut the parts with a smaller router bit, say 1/8" diameter, you can drill with a smaller bit as well.

This leaves a smaller gap where the parts meet:

Here's a comparison between the 1/4" end mill/drill on the left and the 1/8" tools on the right. The holes are much less noticeable on the right:

The CNC Corner Fix Utility I wrote for Rhino is useful for creating the geometry necessary to cut the corners out.


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