Delta Jointer

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Forum topic by Eloyd posted 10-10-2017 05:47 AM 1683 views 0 times favorited 4 replies Add to Favorites Watch
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37 posts in 2166 days

10-10-2017 05:47 AM

I have an older Delta Jointer that I bought at an estate sale. It work great for getting me a straight edge to work from. The only problem is that it leaves tiny wave lines in the wood, even though the edge is now flat. I don’t think that the cutter head is spinning fast enough. I don’t know if the pulleys are the correct ones, or if they are something the seller had in his shop to install on the machine. Does anyone know the math formula to figure the speed of the cutter head with a motor pulley 2 1/2 inches in dia., and a cutter head pulley 7 inches in dia. What is the math calculation to figure cutter speeds? I could use this math to also check my step pulleys on my drill press if I knew what the math formula was.

-- Eloyd

4 replies so far

View Loren's profile


10477 posts in 4420 days

#1 posted 10-10-2017 06:12 AM

First thing is you have to look at the motor
plate to see what the rpm is.

Then find the circumference of each pulley.
It’s 3.14 x r(2). A 7” pulley is about 21”
circumference. Figure out how many revolutions
the smaller pulley travels per revolution
of the larger one and multiply times the rpm
of the motor. That will give you the cutterhead

You want the cutterhead to spin about 5000
rpm on a 3-knife jointer.

View MrUnix's profile


8089 posts in 2971 days

#2 posted 10-10-2017 07:11 AM

First off… if you have a 2.5” motor pulley and a 7” cutter head pulley – I can tell you right now that it’s not spinning anywhere near fast enough regardless of what motor you have. You don’t say what jointer or motor you do have… but for the ubiquitous Delta 37-205 and 37-207 jointers, the recommended cutter head speed is ~4200 rpm. Are you sure you didn’t mean a 7” motor pulley and a 2.5 inch cutter head pulley?

Anyway… cutter head speed (in rpm) = motor speed (in rpm) * pulley ratio

So for a 1725 rpm motor w/2.5” pulley and 7” arbor pulley, the cutter would be spinning at 616 rpm
A 3450 rpm motor would be twice that (1232 rpm) – both WAY too slow.

With the pulley sizes reversed (7” pulley on the motor), the cutter head would be spinning at 4,830 rpm when using a 1725 rpm motor – a little faster than recommend, but much better than the reverse.

For reference, the 37-205/37-207 jointer manual recommends a 1725 rpm motor, with a 7” motor pulley, and the jointer ships with a 2.75” arbor pulley.

You can play with motor speeds and pulley sizes using the arbor speed calculator over at the VintageMachinery site.


-- Brad in FL - In Dog I trust... everything else is questionable

View Andybb's profile


2762 posts in 1376 days

#3 posted 10-10-2017 07:23 AM

When you say “wave lines” do you mean a line that runs the length of the flat face?? If so it might be small nicks in the blades and they should be sharpened or replaced. Have you replaced them since you got it from the estate sale? Run your finger nail lightly along the blades.

As far as speed goes… RPM Calculator or Brad’s from above.

To verify the speeds download one of the free RPM apps for your phone. You put a white dot on the pulley and your phone light strobes and reads the RPM.

-- Andy - Seattle USA

View sciotomike's profile


12 posts in 1001 days

#4 posted 10-10-2017 01:17 PM

This may not be a math problem (calculating RPM), it may be a knife setting problem.

The RPM calculations above are correct, however the effectiveness of the joiner is determined by CPM (cuts per minute). If one knife is set higher than the other two, your CPM have been reduced by 1/3. Thus leaving a wavy finish.

Feed rate can also be an issue (too fast), but not likely.

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