Raised Grain on Mouldings

Pros chip in to help solve a problem with grain showing through a vinyl overlay applied to mouldings. November 13, 2005

Question
We are having issues with grain telegraphing through a smooth .008" thick vinyl overlay that we profile wrap onto a moulded profile. The moulded core is a finger jointed Eastern white pine that doesn't seem to clean up as well as we would like. We don't seem to have any problems when running a Radiata pine core and the occasional problem when running a ponderosa pine core, but primarily when running Eastern white pine.

The following are some of the machine parameters:
-Weinig 23C Hydromat
-6000 r.p.m.
-15 hp motor on sides
-20 hp motor on tops and bottoms
-6 wing heads
-20 degree hook angle
-knives sharpened at 28 degrees with final grind at 25 degrees
-weinig M3+ steel
-moisture content 8-10%

We have tried running at slower and faster speeds (started off at 160 fpm) and tried shear style heads with same weinig M3+ steel but not much luck with getting past the telegraphing issue. Does anyone have any suggestions on how to overcome the telegraphing?

Forum Responses
(Solid Wood Machining Forum)
From contributor R:
You gave excellent information in your post. I don’t have a lot of experience with finger jointed material, but I do with the solids. Does the grain feel raised on some sections of the boards and does some feel real smooth? Is this causing the problem? Look at a few pieces of lumber and tell me if the pieces that are raised, is this the heartwood or the bark side of the tree?

In some species of wood if your getting raised grain it is from the heart side of the tree you need to flip the board over and mill bark side up. But in finger jointed product this is nearly impossible to get all the pieces running the correct way. I would suggest doing a few tests. You might also rough grind at 30 degrees and finish @ 27 degree.


From the original questioner:
In talking more with the moulder operators, we have tried flipping the boards but no such luck with solving the telegraphing issue for the very reason you mentioned. With finger jointed material it is near impossible to rely on the consistency of how the wood is orientated from one stave to the next. And in talking with our tooling guys, we will also try rough grinding at 30 degrees and finish grind at 27 degrees and see what happens.

It may come down to a material issue as far as species, geographical location species is from, and possibly the supplier and how they dry the material. But we would like to exhaust all options before no making the decision to no longer use this material.


From Gene Wengert, forum technical advisor:
One possibility is that the grain is pushed down when machining it. Put a small amount of water on a machined surface with a wet rag and then observe the grain raising pattern. Is this what you are seeing? If you machine too aggressively, you will push some of the wood into the softer wood and then the surface will appear smooth but will recover (get wavy) when moisture gets to the surface.

So, consider this a machining problem accentuated by a moisture change. One way to correct the problem (in addition to using HSS knives that are freshly sharpened and not feeding too slowly) is to take a fairly heavy cut but then follow it with a light cut. The light cut will not push as hard as a heavy (deep) cut.


From the original questioner:
In response to Gene's suggestions and after doing some research on the web, one thing I have read a number of times is making sure the finish cut is a light cut. On one particular part that we experience the grain telegraphing on has a raw core nominal thickness of .750". We mould both the top and bottom and have a finished dimension of .670", so this gives us .080" to work with.

Assuming we center the part and remove .040" from the top and .040" from the bottom while knowing we have a rough cut and then a final cut, where on the final cut we use a .020" shim. Is .020" considered a 'light' cut? I guess my question is what's considered a 'heavy cut' and what is a 'light cut'?

More times than not our first top head (#4 head) we use a insert spiral ('whisper quiet') with Carbide inserts for hogging or rough cutting. Would using the freshly sharpened HSS knives be better in this situation? Is there a suggested feed speed we should run at? We currently run at approximately 160 fpm.


From contributor R:
In my opinion .040" is a very light cut. The reason you have a .020" shim under the last bottom is any less of a cut than .020" and you won’t create enough wood chip flow to keep the knives cool, and it burns up your tooling. You could try to pre-cut the profile but make sure there is at least .020" for the second top to take off. Try to loosen up the pressure on the chip-breakers.

From the original questioner:
Another question I thought of is, are there a certain number of knife marks per inch we should be targeting? Our customer once suggested for optimum performance on a moulder, that we should target 17-21 knife marks per inch. We would have to run fairly slow; at least slower than we would like, in order to hit that target with our current moulders.