Producing Stable Part Stock

Other Versions
With little time and no system in place, can this shop produce parts that won't move over time? October 20, 2004

(Solid Wood Machining Forum)
A three stage process consisting of rough milling, stickering and stacking, and finish milling seems to be most acceptable to produce environmentally stable stock. Unfortunately, that process requires a week or so of stickered stabilization time.

Iím being asked to produce a quantity of solid wood parts that will soon be used in a multi-million dollar installment. I donít have the lead time, storage space or advanced planning to stack and sticker parts for a week or two. My best effort consists of rough facing, ripping and cutting to length a series of blanks that will immediately be turned into finished parts.

By initially rough milling the parts slightly oversized, I can compensate for major wood movement that occurs in the first few minutes or hours after machining. With a little luck, any movement, after the initial stresses are released, is less noticeable.

Generally, do you stack and sticker your parts? Do you take the time to rough and finish mill at all, or do you just mill parts to final dimensions?

I realize an accepted method is to first rough cut to length, then width, and finally thickness. After some stabilization time, the process is reversed. Thickness, then width, and cut to length when finish milling.

By rough milling thickness, width and length, and finish milling in the same order of thickness, width and length, I feel I am giving the maximum time allowable for each dimension to stabilize before the next round of milling within my machining cycle time.

Forum Responses
I have a small shop and make a wide variety of products, mostly high-end. When we look at the choice of single or double milling, we consider a lot of factors.

Lumber: Is it a stable species such as mahogany, or a wilder one such as birch, or does it have figured or twisted grain? Is there a lot of curve to the end grain? (As wood dries, the rings tend to flatten.) Is it reaction wood, sawn from more horizontal branches? (All the boards will be crooked and the heart will be off-center.) Are there signs of case hardening or other dryer defects? (Saw a 1" long piece and bandsaw it into a clothespin shape. Look for the former outer faces to move together or apart.) Is the wood at a stable moisture content, or will it dry or moisten onsite?

End product: Are the pieces especially long or wide? Are they supported along their length? Is reinforcement and stabilization built into the design of the product?

Process: How fast will you be putting things together? Are parts going to hang around your shop, or worse, go to a jobsite with uncontrolled conditions, before assembly/machining/stabilization? Are subsequent machining or assembly operations particularly tight tolerance?

Go through this checklist and see what makes sense to you. The more times you flirt with danger, the more you are asking the wood to do, the better you have to treat it.

From the original questioner:
Typically, we work in cherry. When resawn, all the boards tend to crook. There are occasionally signs of case hardening in one of some number of boards, but it is not typical. Also, the wood is typically at some specified moisture content when it arrives. However, after as it spends a number of days, weeks or months waiting to be consumed, it moves with our weather in the great, and rainy, NW.

End product: Yes, the pieces are often 8í long or longer. Often, the pieces are consumed in 30Ē or less lengths, but I cannot control the consumption. They are often applied and finished soon but can be scribed raw and fit on site.

Process: How fast? Anytime from yesterday to next month. They will most likely hang around the shop until they are used on a product. Are they tight toleranced? Yes. They are used by jointersÖnot cabinet makersÖ not carpentersÖ not Joe Home Improvement. Jointers.

Iím currently doing as little as possible to the material, unless specified, and Iím just waiting to be bit.

"What is this? How is this used? How many sides need to be finished? What will cause an issue in joinery?"' Iím working part by part and trying to learn what not to do before I do the wrong thing. Iím looking for input just like yours to steer me toward a more acceptable part every time.

Your job appears fraught with danger. At every turn you are being asked to do things with wood that can go wrong immediately. What are the possibilities of showing the postings of this thread to your supervisor? In the presence of all this knowledge about wood and its movement, maybe the expectations of the management will become more realistic. On the other hand, going ahead with what they ask will soon convince them that their methods need revising.

I agree with the above - you are being asked to take responsibility for way too many things that are beyond your control.

I will be frank. This is way beyond my 30 years experience in the wild and wooly NE. The combination of time to apply, variable humidity, tight tolerances, and demonstrated problems all scream to me that your shop needs to take a comprehensive look at what it does regarding humidity and wood movement and design.

If you doubt what I say, including the bit about design (I bill myself first as a designer) consider this: So called 'contemporary' design, with its flush tops, tight tolerances, and clean lines, did not happen until we acquired both widespread climate control and the arrogance to think that we could apply that control to all of nature.

From the original questioner:
I donít doubt what you say. I have a few years of engineering logged with a high-end office furniture manufacturer (JFK and many other presidents used this stuff in the Oval Office). I have a few more logged in steel manufacturing centered around grinding shafts to bearing tolerances and electro/mechanical/chemical device development.

My current challenge initially appears overwhelming, and my supervisor appears to know as little about design for manufacture as my fish. Fortunately, I know better than to take ownership and accountability for things that are beyond my control.

I also realize these parts in my hands are not the same as the impossible, dead straight, cyan, green and white CAD lines that I loved to send to other folks for manufacture. The mix of experience will eventually help me produce parts we can all live with.

Dry your rough stock to 6% moisture content. Then mould it and store each moulding in a plastic sleeve which is sealed at the ends. Store it then in a room at 35% moisture content and you will not have any complaints. This will require you to meter each board before you start machining to cull out any wild boards.