I’ve always been curious as to how one tests jointer beds for warp and twist to thousandths of an inch, especially the newer 12″ jointers with beds about 45″ long. A 24″ machinist-grade straightedge is expensive, a 48″ about $200. But even with a top grade 48″ machinist straightedge and feeler gauges, how would one test for wind or twist? I can’t imagine shop made winding sticks would be able to measure to thousands of an inch.
Can someone detail the procedure for testing jointer beds for flatness? I’m dangerously interested in the Powermatic 12″ puppy.
Thanks.
Replies
Don,
I tested my new jointer by passing a board across it. It came out flat , and I was satisfied.
Regards,
Ray Pine
Just go ahead a get the PM. If you find it doesn't mill boards to your satisfaction, I'll be glad to take it off your hands, and I won't even ask for a certificate of flatness. :)
I can't see 1/1000", out of sight, out of mind!
Beat it to fit / Paint it to match
I repair machines and have a 4' and 6' machinist straight edge and a set of feeler gauges to check and adjust jointer beds as well as a special type of base with a dial indicator to check alignment. Winding sticks would not be appropriate to do such a job.
Would your dial indicator base be something that could be easily made and still be sufficiently accurate for the purpose? I have a dial indicator with magnetic base, but couldn't figure out a way to accurately move it over a 40" table. Thanks.
The indicator is to adjust the edge of the table to the cutterhead and set the knives. The straight edge is used to check out the tables alignment and condition.
Gotcha. That's how I've used mine on the jointer. Thanks.
A machinist grade straight edge will give you some information about flatness and twist, at least enough to tell that something isn't right. But for a better understanding I also use a machinist's level, which is extremely sensitive, to get a better sense of the condition of the table's surface.
To use the level, the machine needs to be sitting solidly on the floor and ideally the base has adjustment screws to bring the table surface to dead level at whatever point you decide to use as your reference point. The level is then used crosswise on the tables, moving from one end to the other, if the tables wind, the bubble will tell you.
John W.
Don,
A 50" aluminum straight edge can be purchased from Lee Valley for about $60. They are guaranteed flat to within 0.003" over the entire span, and that is fine for milling operations.
Also, you can make a pseudo-straight edge of any length, with 3 ridged boards and 9 screws. Screw 3 screws into each board at the middle and both ends. The screws should be adjusted in or out until all 3 boards align at the screws. You then have 3 straight edges. Well they are garanteed straight along each of their 3 screws.
This was in a mag about 6 years ago. Don't remember which one.
Todd
That screw thing was in FWW but it doesn't work as good as a straight edge. I heard about that years ago from an old machinist friend. You can actually measure twist with a good straight edge and feeler gauge.
Todd:
Thanks for the suggestion. I found the sidebar on the 3 screw "straightedge", and the accompanying article on truing a jointer, in FWW # 142, June 2000 pp 38-43.
how would one test for wind or twist?
Powermatic 12"
If it's a Powermatic I would not worry to much about it.. They make a good quality tool.. SOme better but MORE expensive..
Take it from a old metal worker.. Iron is sort of like wood.. It DOES change with temperature and age.. I have seen 1,000,000 plus machines warp and whatever! We made GOOD money fixing them!
Really, I think you will be safe with a Powermatic.. (NO I do not own one)
Old or new they can still get a warp on the tables. Over thirty years ago I bought my second jointer, PM 6" and I struggled with getting a good jointed edge. Bought a 4' Starret straight edge and discovered it had a warped bed. PM made it right but I had to send the machine in to get it reground and it led to a career of fixing machines. Over the years I've had old Olivers from the 50's reground and newer machines as well so there is no guarantee any age machine can't warp, but they are all fixable. The newer parallelgram machines have an advantage of being able to treat each table individually. The older style dovetailed ways jointers have to be reground as a complete machine. Parallelgram tables can be replaced or reground individually as they have four point adjustment. The plder Oliver and Crescent have inclined ways which allows the same individual four point adjustment. It takes a knowledgable grinder to do it correctly. Basically a large magnet holds the table down and it must be shimmed correctly or the magnet will distort the table and when ground it can be worse off then before. You can only grind it so many times.
This forum post is now archived. Commenting has been disabled