table lift in 15" 4 column planner--how accurate does it need to be?

[James Baker SD]

I have my Powermatic planer in a hundred pieces now as I prepare to replace the two broken columns (which were damaged in a freak tip-over accident). I have the screws that raise/lower the table out of all 4 columns. The screw that engages the hand crank for raising the table is supported at both ends of its column, at the bottom by a ball-bearing and at the top by a bronze bushing. The other three screws are supported only at the bottom by the ball-bearing.

Things I have noticed.
1. The ball-bearings have quite a bit of "slop" in them that allows the unsupported end of the screw to wobble.
2. The screws themselves seem to have all started life as the longer version, and the ones not intended for engaging the hand crank are simply cut shorter (no two of them are the exact same length).
3. There is no seating shoulder on the screws, so how far they are inserted into the ball-bearings is variable (they are close, but the eye is enough to see the differences). There is a shoulder in the bottom of the column for seating the ball-bearing.

Questions I have:
1. How much impact do these "features" have on the accuracy of keeping the table parallel to the cutter and rollers above?
2. Would it be worth buying three more long versions of the screw and three more bronze bushings to support all four screws at both ends? (would probably need to cut a little off each since they would not have the gear to engage the hand crank plus some work in fitting the bushings in the other 3 columns)
3. how much effort do I need to place in getting each screw at exact same distance into the ball-bearing? Is this critical (did not seem to be in the original assembly) or do you play with the chain engagement (different amount of initial rotation to get the lead nuts even) on the 4 screws and adjustments in the cutter head assembly to get the cutter and rollers parallel to the table?

It seems to me that the accuracy of table motion is critical to quality cuts at different thicknesses, but I do not see a lot of care in the original manufacture in the regard.

Since I am putting so much effort into getting this planer in top shape (it all started with the attempt to replace the cutter head with a Byrd Shelix), a few dollars more for the long screws and some extra time installing the screws in the ball-bearings seems OK to me if it will help.

James

[Don Jarvie]

What does the manual say? The extra long screws may not work once everything goes back together. One thing I learned in tearing apart and rebuilding my Belsaw Planer was once the planer was put together it all tightened up.

The 4 posts that hold the table on the Belsaw just sit in a washed on the base. Once the table is put on and the rollers attached it tightens them up so they move up and down correctly.

[James Baker SD]

The manual says to make the table parallel to the cutter head by removing the chain and rotating the screws individually until the table is good, then re-install the chain. But this is limited to increments of 1 tooth minimum rotation in adjusting the screws, which I felt was not extremely precise. The screw thread is pretty course and the sprockets do not have many teeth.

My hesitation on installing 4 long screws is that maybe the wobble in the unsupported screws is necessary to compensate for less than perfect machining of the columns and the table itself.

I was hoping someone with more experience in machining and knowledge on how these planers are designed and made (tolerances etc) could shed light on this for me.

James

[James Malcolm]

Wobble in leadscrew: As long as the nut on the leadscrew is not able to move in the horizontal plane, the wobble you see will have absolutely zero affect on position. For arguement sake, let's say that the nut is able to move horizontally and at the end of 6" of travel, the leadscrew is angled 5°. "Inaccuracy" = travel length - cosine 5° x travel length. Comes out to ~0.023", but to do so that nut would be shifted 1/2" of an inch. Let's say the bearing is able to move 0.025", that would lead to vertical "inaccuracy" of 0.00005". In the case of your jointer, that nut cannot move side to side because the table prevents it from doing so independently of the other 3 nuts, therefore no "inaccuracy". The only way to get inaccuracy out of the leadscrew itself is if the threads per inch is not consistent along the length of each shaft as well as being the same between all four. Unless you're having problems, you're better off staying with the set you have rather than introducing new parts into the mix. Its also much much easier to adjust the system if the leadscrews are not supported on both ends. If they were, you would be force to make the main posts, leadscrews and nut retaining feature on the table parallel, otherwise you would have binding in the leadscrew which would make the handle harder to turn and eventually premature wear of the nuts. At a previous job the president demanded that we support both ends of the leadscrew. It was constantly a problem; everything had to be adjusted perfectly. We ran a test under 10000x magnification and could see absolutely no difference between having it and not. We ended up going behind the president's back and opening up the bushing so much that you could see light through it. Never had a complaint. This was semicronductor test equipment with micron level accuracy.

Insertion into bearing: As long as the leadscrews don't shift in the bearing, it doesn't matter where the leadscrew starts at, it only matters where they nuts are as you stated. The chain is adjusted so that when you have tension on the chain at the leadscrew with the handcrank that the other three posts are exactly the same; tight against the chain and just about to move. If you then turn the hand crank the opposite way, the chain should engage the teeth on all 4 gears at exactly the same time in the opposite direction. This is where misadjustment happens. As the chain stretches, not all four gears engage at exactly the same time. It may necessary to rotate the gear on the leadscrew if it's not keyed in place or adjust the nut relative to the table. This should only be necessary if you are out of adjustment range on the chain idlers due to excessive chain stretch or because you don't put each leadscew and nut back in exactly the same position they came out at.

I hope this makes sense.

[James Baker SD]

I just measured the screw and the threads are 4mm apart. There are 10 teeth on the sprocket, so this gives the minimum adjustment at approximately 1/64" if I move the chain one sprocket on a screw. In another post, the OP was stating that he had a 1/64" error from side to side in his planer. This would be an error I could likely do nothing about unless I started moving the screw in the ball-bearing by less than this. Seems this places the absolute limit on the accuracy of a Taiwanese 4 post planer. Maybe this is OK for woodworking tolerances, but it "feels" a bit high to me.

James

[James Malcolm]

Remember, 1/64" of an inch is only 0.015". At the most you would only ever be off by 0.008" by only moving the chain. Say for instance you were off by 0.010. You could move it 1 tooth and only be off .005". If you were off by .008", it's not worth the hassle.

You can also loosen the recessed socket head screws on the outside of each post, in the nut area of the table that attach to the nut through the slot in each post. There should be at least a few thousandths of clearance. Between moving the chain and doing this, you should be able to get down around 0.005" without too much hassle. You may find that the table is loaded the direction you need to go and won't go anymore. Rotate the gear in the direction you need and them push the table the opposite direction. You may need to loosen another one or two up to get it to move depending on how it's preloaded. Don't go crazy though with loosening things up, fighting for that last little bit isn't worth having to start from scratch. One at a time. At a certain point you have to consider your measurement techinque, it can easily throw you off when you're talking a few thousandths.

You can also add an idler pulley to pull the chain the direction you need it to rotate the offending shaft, loosen up the other idler pulley a little to compensate and you're perfect.

[James Baker SD]

Over the weekend I reassembled the lead screws into the columns. I was wrong about the screws not having shoulders to seat the bearings. I had to replace one damaged bearing and the screw does have a seating shoulder. Columns are back in the base, table over the columns and cutter head assembly back on top of the columns.

Tonight I leveled the table with Lee Valley Set-Up blocks and feeler gauages. Got the chain back on and snug and the handle driving the whole system. I raised and lowered the table to the extremes several times bringing it back to the 3" height each cycle to test. I get between 0.007" and 0.003" error between the table and cutter head from side to side. Think I will be satisfied with that.

James

[James Malcolm]

Glad to hear to got it all back together. Just think, now you're an expert. Wasn't so bad was it?