Friday, November 20, 2015

A Ming-Inspired Cabinet (10)

This is the third project working with material from some big slabs of curly bubinga. Each time I delve back into this material I am brought face to face with one of the main challenges: the propensity of the wood to tear-out during processing.

Even with sharp knives, and four of them, in both jointer and planer, I have found I can only reliably dimension down to a certain point, beyond which I proceed at my peril. My shoebox Makita planer, given the high rpm, sharp knives and a shorter distance between the cutter head and the infeed/outfeed rollers, can obtain a slightly better finish, tighter to desired final dimension, than can the bigger planer, but I am still limited to dimensioning down to about 1/8" overside in a given dimension. Tear out, when it occurs, can pull fibers from nearly 1/16" into a surface, so if I want to be confident of obtaining a clean surface, I have to allow for that in the planing.

I've learned about this matter the hard way in the past, having to take some parts below intended dimension so as to produce a clean surface afterwards. I wanted to avoid that outcome this time around, and I have found that the tool that helps get me there is the FZ-5V milling machine with helical shell mill. Even longer pieces can be adequately supported and worked from each end to produce a flat surface.


Another view:
















A close -up shows the fine slices that can be removed, a layer at a time until a clean surface is left, with one round or two of final planing or super-surfacing remaining:


Above is a skimming pass taken closer to the dimension, though at other times I will mill off 1/16" or more. Generally speaking, it's not at all the type of milling, the heavier stock hogging, that one can undertake on the planer or the shaper. After all, we're talking about a 4 hp spindle, not an 11 hp one.

With these particular sticks, their final form was to be an inverted 'T' section, so in an effort to guard against unwarranted wood movement after the waste strips were ripped from the blanks to produce the 'T', I left the stock oversize in every critical dimension prior to doing the rip cuts:


I'll let the stock settle for a day or two before milling to final shape. The excess thickness in every dimension should allow me to realize the ideal stick hidden within - all being well also at dimension and straight and square. There is the ideal and then there is the reality. Just trying to shorten the gap between those two states.

Here are the short side pieces (top), with the pillow block stock (bottom) also being worked, step by step, down to the line:


It's certainly not a quick process to mill the stock in this manner, as the amounts taken per pass are rather modest, but it has allowed me to obtain clean surfaces without tear out, getting closer and closer to dimension, and that is really all that matters to me.

All for today- thanks for visiting! On to post 11.

6 comments:

  1. Several years ago I was in a shop doing a large job with sapele (doors, moldings, etc.) and we had an awful time with tear out on the jointer and planer with the more highly figured pieces. On some pieces we would be going to the wide belt sander to remove almost 1/8". Talk about a time killer!
    Eventually our shop foreman found someone who made retrofit helical heads for the machines. The difference was remarkable. The parts came out needing minimal sanding. As a bonus, the machines were much quieter. As far as day to day maintenance, there were pros and cons. If you chipped blades, turning a few would solve your problem. Once you had to rotate all the blades, turning, cleaning, and reseating them was almost an all day affair.
    You may already be aware of this, but I'll throw it out there anyway.
    On a somewhat related note, what made you decide to go with this machine as opposed to a three axis CNC?
    I've been reading your blog for quite some time now. Great stuff, keep it up!

    ReplyDelete
    Replies
    1. Unknown,

      have considered helical heads, and do have a Byrd Shelix head for my shaper. Retrofitting that type of head to either the planer or jointer would be prohibitively expensive, and the vast majority of the woods I work the Tersa knives give an excellent finish. So, doubtful I will head in that direction, but you never know.

      As for three-axis CNC, I have some interest in that, however for one-off tasks, drilling a few holes, milling tasks on 4 or 5 sticks, etc., the CNC makes less sense. In terms of the capacities that the CNC can provide, however, in terms of complex curved surfaces and so forth that can be realized, it definitely opens up some possibilities, but does so at a certain cost and a certain space requirement. I don't have that space available, for one thing.

      I would want a CNC machine with a good amount of z capacity, and would want at least 4' x 8' work surface, so that would tend to mean a rather expensive machine, or a used one with all the challenges that come with it. Maybe in the future I'll make a move in that direction, but for the time being the milling machine is a valuable asset to my shop. And, I can sub-contract any CNC work to a company in my area, and have found this has worked well in the past when needed.

      ~C

      Delete
  2. Have you tried milling a less tear-out prune wood on the milling machine, Just curious to know.

    Barak

    ReplyDelete
    Replies
    1. Barak,

      thanks for your comment. As of yet, the only wood I have milled on the Zimmermann has been teak and bubinga. I Think with less tear-out prone woods i would be using my jointer and planer to do the facing and dimensioning.

      ~C

      Delete
  3. One solution to minimizing tear out with planer/jointers is taking heavy cuts. This provides the material itself to backup the cut. Tear out is now at the exit surface, This works surprisingly well.......................Jack

    ReplyDelete
    Replies
    1. Jack,

      I'm familiar with that strategy. Funny enough, with the milling cutter, I found tear out only occurred when I took heavy cuts (about 1/8"), the opposite of what you would get with the cutter traveling the opposite direction.

      I still have to start off by jointing, and pushing a wider bubinga board over the jointer and trying to take a heaver cut is a bit demanding. Less of an issue for the planer of course.

      I mill in stages to let the wood settle, allowing at least a day between passes, and if the only strategy I had to work with is to take heavier cuts, that would mean leaving the final planing passes on the heavier side, and having a whole lot of faith that there would be no issues. If the wood moved significantly on the previous heavy planing pass, then how would one be confident that taking further heavy passes is not going to lead to more movement? What if a board moved- say it cupped or twisted- such that one side could have a heavy pass taken to obtain flatness, but that left only a skim for the other side, or else you would go under dimension? Not an uncommon scenario.

      I guess, if at all possible, I prefer to creep up on final dimension, and with each round of dimensioning progressively make the board flatter and flatter. It has worked generally for me, and with the curly bubinga I simply need to take extra precautions. The mill is making this much easier than in previous go-rounds.

      ~C

      Delete

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