How best to set P/T knives?

[ivan]

The recently reccomended Machine Woodworking for Hand Woodworkers makes an interesting valuable read for the self taught. Thanks Scrit!

From it I get the impression that the practice of setting planer blades with a dial gauge on an engineers magnetic mount, may be laudable but possibly over zealous?

I'm prompted down this mental path, as our realtively new Jet P/T (the one where both tables lift as one for thicknessing) does not have a perfectly flat out feed table. It's probably within 2or 3 thou, but unfortunately there's a dodgy bit of grinding quite close to the cutter block which makes the magnetic mount unsteady, and of couse the dial gauge is a few cms away on the adjustable arm where any movement is magnified. The table is probably well within spec, so...

Whilst you can easily set a knife this way, at any one point to +/- a thou, do you attempt to match the knife to the outfeed table grinding error? The traditional long bit of wood shifted along 3mm averages out the table surface but is very trial and error. Perhaps I should make myself a dial gauge holder to reference from the machined block surface?

Incidentally, Jet provide a short length of ali box section in lieu of the stick - the short length makes this too, position sensive on the out feed table. The book? (above) A precision cutter setter, a Heath-Robinson looking lever/pointer arrangement with scale divisions ofabout 0.2mm and a reminder that about 0.5mm difference in height is needed to make one knife to do all the work.

 

[DaveL]

Ivan,

Would you be referring to one of theses?



Sorry but I don't have a better picture of my Wadkin blade setting gauge, it may look Heath-Robinson but it works a treat.

 

[devonwoody]

Cant you have a go at Jet, faulty mechandise?

 

[PaulO]

Incidentally, Jet provide a short length of ali box section in lieu of the stick - the short length makes this too, position sensive on the out feed table.

The ali box section has a 3mm wide notch in it (at least mine does on a JPT310). Place the front of the notch level with the front of the outfeed table, then check that the blade advances the ali by the width of the notch. If this is the method you are using, then I am not sure what you meant by it being "too position sensitive". One thing I have found is that it is worth dusting the table and giving it and the box section a quick wipe with a rag to ensure that dust isn't hindering the progress of the box section. With those precautions I have no variability using that gauge. As a thought, are your tables waxed? Mine are, and might account for this trick working for me.

WRT the "dodgy grinding near the cutter block", if it is enough to make a magnetic mount unsteady, then I would definitely be on to Jet support about it.

 

[Nick.]

Hi Ivan, I can only echo what Paul0 has said above. If there is some 'dodgy grinding' on the table making the mount unsteady, then let us know and we will sort it out for you. I've just pm'd you my mobile number.

Which machine do you have by the way? Is it 10" or 12" width?

As you said, tolerances within 2 or 3 thou are acceptable at this price point - I'm sure our spec. on tolerances compares favourably with most other makes on the market at this price level.. If it was 'banana shaped' and 4mm out, then that would be a different issue.

If you want a 0.2 thou tolerance, then by all means we would be happy to sell you two 13k seperate machines :lol:

Ivan, let me know how we can help.

Cheers
Nick
Jet

ps - thx Neil too for the pm.

 

[ivan]

Thanks for your replies and PMs. But no one has had a punt on acceptable knife height variation! And yes, that's the boy, Dave; little weight and a gert long pointer. A real Woodworker's dial gauge....

Just to clarify:

Absolutely no slight on JETor the 310 intended! I think a +/- 1.5 thou surface (or thereabouts) is pretty good at this price point. I bought the JET in the knowledge of the fence issue discussed here, and would buy again with hindsight. "Grinding" was a slip of the fingers - actually I think the tables are finished on a (think it's called) shaper - by linear tool cutting.

The positioning issue for the JET short ali box "stick" is the same as for the magnetic mount. Local outfeed surface variation close to the block, prevents the magnetic mount sticking rigidly to the table, and gives a locally different forward movement of the ali box when tickled forward by rotating the block/knives.

A much longer stick bridges local surface aberations and is consistent, (Paul0, the tables are soaked in carnauba wax) but I find the stick method a bit tedious compared to setting with a gauge, and I have one in the cupboard. I actually finished the job by referencing from the infeed table (no problems here) and tweaking the outfeed to the knives rather than vice versa. Not long after the faffing about I read the book Scrit mentioned, and then began to think "just how accurate do you need to be?" Self taught, I have to ask at the Forum!

 

[David C]

Ivan,

I don't know if your out feed table is adjustable? This was a desirable feature on older UK machinery.

Blades should be referenced to out feed table as this is what supports the cut surface of the timber.

I used wooden carry forward stick, with a series of pencil marks near the end which just covers the knife block. in feed table is lowered.

I guess my pencil marks were 1/32 apart.

If the target for a particular hardwood was a 'carry forward' movement of 3 marks, when the blades were rotated by hand, I would happily accept a variation of plus or minus one and a half marks.

Have never known exactly what this would represent in blade projection measured in thou".

see page 41 of my first book.

David Charlesworth

 

[devonwoody]

Myself on my SIP P/T I find the blade level with the outfeed table is the best position for me. I gave up letting an ali bar ride upto 5mm across from the outfeed to the infeed.

 

[David C]

Fine if that works for you but technically it is not correct

PTs cut a series of crosswise scallops. It should be clear that it is the intersection of the scallops which ride on the out table not the deepest points of the hollows which more or less represent the top dead centre of the knives. (Springiness of timber can have an effect here, denser timbers need less carry forward than soft pines.)

best wishes
David Charlesworth

 

[devonwoody]

David I know what you say is noted to be the correct proceedure, but if my blade is above the outfeed table I get a snipe when planing across to top.

Picture the blade 10mm above that outfeed table you would definatelky be in trouble so why does that minute amount not also give that snipe at the end of a cut.

 

[David C]

See explanation above. or pages 40 & 41 of my first book or any competent machining book.

The cut surface is munutely scalloped.

Interestingly Knapp do not understand this basic principle in the appallingly bad manuals that came with my 12" P/T.

I find the 12" Knapp P/T a most irritating machine and will replace it as soon as possible.

The actual setting is a matter of one or very few thousandths of an inch, nothing that you could see easily.

Hence the use of the carry forward stick or dial gauge.

David C

 

[devonwoody]

your quote answers my views. (perhaps its that bit I cant get to)

your quote

The actual setting is a matter of one or very few thousandths of an inch, nothing that you could see easily

 

[woodbloke]

DW - I use the same method as David C and use carry forward marks that are knifed 1mm apart. I have a carry forward of 3mm which seems to work well for my kit - Rob

 

[David C]

Interesting things happen if the knife dead centre height to out table relationship is not correct.

Out table too high will tend to produce a convex edge.

Out table too low will produce excessive snipe, and possibly a hollow edge as well.

These results assume perfect allignment of the two tables, which is exceedingly rare.

David Charlesworth

 

[ivan]

Thanks for your input, DC.

Call your accepted variation in stick movement of 3x 1/32" (around a target of 3x 1/32") 2.5mm. If we assume a 45mm radius block in the Knapp a bit of Pythagorus suggests your target knife height (of between 3 and 6 stick divisions) calculates to the range of 0.02 to 0.08mm, or something like 1 to 3 thou. The maximum accepted error in the height would be ~ 2 thou. (more, if the block was smaller)

This seems to fit well with the estimated precision of DaveL's setting gauge. It also confirms my suspicion engendered by Scrit's reccomended book, that to use a dial gauge was OK, but to seek engineering dial-gauge precision might smack of overkill.

The optimum setting of a thou or two above the outfeed table varies with species, as DC says. In general I suppose softer woods can compress more than harder, so need the knives set a fraction higher. What ever the exact reason, the true setting for a particular machine and timber variety can only be verified by running two samples through and checking the two planed surfaces against each other and tweaking outfeed table height, as per DC's preceeding post.