How Much Over-Width Should Stock Be Ripped?

A discussion of ripping precision, straight-line ripsaw performance, and how much extra width to allow in order to compensate for wood movement. January 8, 2010

I have just purchased a used 4 sided moulder. I was wondering how much you should cut stock overwidth when doing S4S faceframe stock? If it is to be finished at 2" what do you guys rip it to first?

Forum Responses
(Solid Wood Machining Forum)
From contributor E:
If I'm running long lengths, I rip a 1/4" over size. With shorter lengths you can tighten it up to 3/16" over size.

From contributor T:
Same as contributor E - 3/16" over except for long pieces, then 1/4" over.

From Dave Rankin, forum technical advisor:
I agree with the other posts, 1/4" oversize. You may find that as you learn the moulder better you may be able to reduce this down. Many moulding operations run 1/8" over. This requires a precise rip and a properly aligned moulder. I have several companies that I work with that allow 1/32" total oversize in width. One thing to remember, you need to take at least .0167" to produce a cut. Less than this and you pull the grain instead of cutting it. Also, the tooling must be sharp.

From contributor U:
To rip really close, you need a gang rip If you have enough production, the savings in lumber, better feed rates, etc., will pay for the gang.

From contributor E:
To contributor U: the reason I run my moulder blanks a 1/4" oversize in width is to make sure I have enough material left on the edge for the side heads to take out the bow that results from the tension being released after the board is ripped. Can a gang rip do what a straight line can't? I would agree that at times a 1/4" is excessive and I have been ripping closer than that, so I'm interested in your take on this.

From contributor U:
If you have a gang saw with saws on centers such that the kerf allowance results in the saws being 2" apart, there is no way any rip width can be more than 2". The only way a rip width can be less than 2" is if the board is scant on one edge and does not clean up. When you rip on a single saw machine of any kind, you are relying on two things: The fence being accurately set and that it is rigid.

Second: the operator never ever makes a mistake in the way he uses the fence. The rip will still be straight if those conditions are not met, but they will not be parallel. The real reason you have to allow so much moulder allowance on the single saw machine is that you get wedge shapes. You do not get parallel rips. You just think you do. With a gang there is no way they are not parallel. A roll feed gang rip will follow the fence and if a board has side bow or a bad edge next to the fence, the rips will mirror that defect.

A chain fed gang rip such as the Mereen-Johnson or Progressive will not follow the bow because the chain controls the lumber, not the fence. The fence is a reference and not a guide.

From contributor T:
To contributor U: I understand your points on the gang rip vs. straight line rip. I never considered the wedge shape issue. But I think what contributor E was asking about is the wood movement that occurs when drying stress is released by ripping. The tendency is for the stave to bow laterally so that there is a crown in the ripped part. To allow the moulder to straighten this part, the side fence has to be offset.

In my experience, I need to remove about 1/8" from the underside of this crown on the worst boards, and leave about 1/16" for the other side head to hit. I guess that if each blank were exactly to dimension, then you could eliminate the extra 1/16" on the left side. Does a gang rip produce a long blank with less crown than a straight line saw can?

From contributor U:
A moulder is not a straightening machine, unless it is a grooved bed machine. The largest moulder manufacturer in the world admits this, whether they mean to or not is another topic, by the fact that they manufacture a grooved bed machine. If you are going to guide the piece through the machine with a grooved bed, you can straighten but you cannot straighten on a smooth bed machine forcing the piece to the guide.

I heard this topic argued over and over and it is a chicken and the egg argument, long on technical if's and why's but short on practical application. For instance, what is the piece machined to be used for - long length millwork mouldings for architectural application? It is not straight after it lies around Lowe’s or wherever. Furniture parts? Cut into shorts found in furniture the bow or crook is within useful limits. Glue up? The clamping forces overcome any side bow.

From contributor J:
I hope you are enjoying your new machine! I thought I would just add to this interesting discussion. Great points on the side bend (crown) of a moulder blank. I would just like to remind you of the flatness issue as well. As you know, wood manipulation is important and the better it is coming off the machine, the better it will be after sitting around a while.

You can control the flatness of your finished piece using the bed of your moulder. If you’re feeding in flat blanks and they are coming out bent like a U shape, move your table down so your taking more off the bottom and less off the top. If your blanks are bowed the other way like a rainbow then you want to take more off with the top head. By leaving the thickness over by 1/4" allows you to control this to an extent. Keeping an open communication between the feeder and tailer is important.