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Woodworker needs machinists advice!

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  • Woodworker needs machinists advice!

    I'm a woodworker who's putting together a Powermatic table saw. You machinists out there would appreciate the casting and grinding of the table top! Anyway, it's a precision machine, and I'm attempting to set it up as best I can. I bought a Starrett two foot ruler to help with aligning the cast iron extension wings properly. I did the best I could, using shims under the wings to get it lined up as well as I could. There are still some slight discrepancies, and I'm debating about how anal I should get on this. The problem with the two foot Starrett is that it's not long enough to go from one extension wing to another--I can only line up each wing from the center table, not the entire length.

    To compound this, I'm in the process of mounting the wooden extension table, which ostensibly needs to be lined up perfectly as well. Using the two foot Starrett becomes a bit problematic since the extension table itself is four feet.

    Another issue that's arisen while setting things up is that I began experimenting with turning the ruler upside down. Much to my surprise, I found that there were gaps in different places! That was really frustrating! How can you be precise when you question your measuring device?

    That brings me to the question. My wish is to have the 48 inch Starrett straight edge, but for price of $186 for one without any measurements on it, it's a hard price to swallow for a woodworker. Are there any other brands out there that make an acceptable four foot straight edge? My four foot straight edge is a $10 jobbie--good for measuring, but no good for this purpose.

    Oh, and as far as the gaps I'm finding, they're miniscule in woodworking terms: the smallest feeler gage I have is .008, and it's less than that (on the cast iron table itself anyway. Moving onto the wooden extension table is another matter).

    Anyway, sorry for the long post from a newbie, but I'd sure appreciate the advice/input from guys who need to know precision for their jobs!

  • #2
    As a machinist and a woodworker, I would suggest doing your best with what you have, and leaving it at that. The precision of your Starret rule greatly exceeds the straightness of the finest 9 ply plywood. Over such a span, the wood will either be off by much more than your error, or will yield to the surface anyway. The farther you get from the blade, the less the absolute flatness of your whole table matters. I set my extension tables -slightly- low.
    Don’t get me wrong, you can fit wood to within a couple of thousandths if needed, but not on a piece of wood that covers your table.

    Have you ever used Top Cote? The stuff is magic on saw and jointer tables.


    • #3
      One thing we learn pretty early on as machinists is it is important not to obsess about making parts to closer tolerances than necessary. In fact, the principle of "interchangable parts" rests on intentionally designing clearance between mating parts so that they assemble easily but not sloppily. To spend extra time making parts with all dimensions within .0001" when the blueprint tolerance is .005" will usually go unnoticed by the customer (unless it's in a bad way, as in late delivery).
      Consider this in woodworking. If you spend hours of extra time insuring that the width and depth of a dado allows the mating parts to fit so close that you don't need filler, you will have the satisfaction that you can do it, but the end user will probably never notice. Plus, once the humidity changes the wood will shrink and expand anyways, erasing any proof of your efforts.
      You would probably benefit more from insuring your fence is square to your blade and be very happy that your table is within .010" of flat for now. By the way, .008" drop over two feet translates to .0019 degrees! That's pretty damned good for woodworking - definitely better than the wood will ever keep permanently.
      I would think that using any decent 4' level for a straight edge should get your table flat enough to allow you to do very fine work on a table saw. You can always check it later if you run across a machinist's straight edge.


      • #4
        As another machinist and woodworker I'm here to tell you that you can't tell a perfectly flat table from one that 0.015 convex over the surface merely by useing it.

        One that's concave can be a PITA when you're trying to make tight straight rabbet joints because the wood will tend to bridge the concavity.

        So Joel, Venito, and I tell you that close enough is close enough.


        • #5
          What the heck do you think they make wood putty for?
          I think that if you had a table that was gound perfectly flat, you would have a hard time finding wood that would lay flat on it. If you did happen to find a peice that was perfectly flat, then you would most likely have drag issues, because of such a large area of contacting surfaces.
          The tablesaw that I use gets a lot of ribbing, but it has ribs designed into it to both strengthen the cast aluminum top and to reduce the contact points with the work. It also allows somewhere for the sawdust to go. It has a sliding miter table and it is engineered a tad higher than the main table. I have not noticed this difference effecting any of my projects. Some of the projects that I have done require as much precision as you can muster. Here is an example done on a inexpensive tablesaw with surfaces that aren't in the same plane.



          • #6
            I agree with the above thoughts on tolerances for your table saw. But if you must have a better straightedge - buy a straightedge not a rule. Here is a good link for a straightedge -



            • #7
              Not really any suggestion for your problem, other than to agree with the others.

              I also have a 12" powermatic that I purchased in a school auction. Had problems with the fence (broken/missing parts) so I built my own version of the cam lock style fence I used lengths of 2" square tubing for the rails and made tham long enough that I can easily rip sheets of plywood. Great saw have fun and good luck.


              • #8
                I like Vinito's idea of using a 4' level. Assuming it's of any decent quality at all, it will be straighter than any board will ever be.
                Try to make a living, not a killing. -- Utah Phillips
                Don't believe everything you know. -- Bumper sticker
                Everybody is ignorant, only on different subjects. -- Will Rogers
                There are lots of people who mistake their imagination for their memory. - Josh Billings
                Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                Don't own anything you have to feed or paint. - Hood River Blackie


                • #9
                  Nice work, Lee! Ever do any raised panels using the Bench saw alone?


                  • #10
                    Thanks guys--I'm going to go with it's "good enough." I tend to obsess over this stuff a little too much, and all I want to do now is make sawdust! I've yet to do anything with the saw because of this, so it's time to trust what I've done and go with it. And like you guys said, wood changes with the climate, so these tolerances shouldn't worry me too much.

                    As I've been working on aligning this table, it's dawned on me that there certainly has to be plenty of woodworkers who never really obsessed like I have on this set up, and their equipment is doing great work. "Here is an example done on a inexpensive tablesaw with surfaces that aren't in the same plane." Lee, that's some fine work, and makes me not worry so much! :-) I'm going to go out and buy a 4' level, as vinito suggested, and call it good.

                    "One thing we learn pretty early on as machinists is it is important not to obsess about making parts to closer tolerances than necessary." That's great to read and hear. I'm still a relative newbie to woodworking, and I know my tendency towards perfectionism needs to really be put on the backburner if I'm going to actually enjoy woodworking. I'm a professional orchestral musician, and we're trained to play perfectly, and it's just not possible--the phrase "optimal playing" is something I work on in my career, and lord knows I'm going to need to cut myself some slack in my hobby if I'm going to enjoy myself! This little jaunt setting up the table saw has taken WAY too long, and it's time to just go out there and start having fun. So thanks again--I appreciate it. I'm going to cut and paste this post and save it, since this topic of tolerances when setting up woodworking equipment comes up quite frequently on the WW forums.

                    You guys have set me at ease, so thanks!

                    Thanks for the suggestion on Top Cote. I tried WD-40 at first, but that's not a good rust inhibitor (found out the hard way). I heard good things about Boeshield, and it seems to do a good job, but then I heard rumor that it can mess up a finish, so I took that off. Surprisingly, a tip to just wipe waxed paper over the surface has done a great job preventing rust, and apparently adds to the ease in which you can slide the wood across the table. I'll have to look into Top Cote now though.

                    [This message has been edited by TboneDano (edited 08-25-2004).]


                    • #11

                      I was checking out your site, and was pretty impressed by your workmanship on the shark guard. I like how the splitter is circular and very close fitting to the blade--more like european saws. Have you ever made any for Powermatic saws?



                      • #12
                        I think everyone at some time or other gets overly meticulous about something.
                        I'm reminded of this humorous analogy I once heard: "..measure it with a micrometer. Mark it with a piece of chalk. Then cut it with an ax."


                        • #13
                          I do both woodwork and machine work and here is my suggestion. Take a good straight-grained piece of wood (say 3/4" x 4"-6")long enough to span across the cast iron and wood extension. Joint it (on 3/4" Dim.) on the jointer taking a very light cut. Use this piece of wood to align your tables.

                          [This message has been edited by Joe H (edited 08-25-2004).]


                          • #14
                            As mentioned above a good 4' level will do it.
                            Top Coat, absolutely, wonderful stuff. Use a light coating on metal surfaces, table saw, jointer bandsaw, etc, let it dry and give it quick buff with a clean rag you won't believe how muck it reduces friction.

                            Paul G.
                            Paul G.


                            • #15
                              I agree with the comments about not chasing accuracy that doesn't matter. I would just do the best I could with the ruller you have. Check it with both sides if you want and mentally average the results. On a table saw I would definitely want the wings either even with or slightly below the main table. Never above as that would tend to lift a plywood or other large piect off the table in it's center and change the vertical angle of the cut.

                              Paul A.
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!