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Subject: Re: Scallop depth

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Message Thread:

Scallop depth

3/28/19       
Buck

Can anyone share with me the formula for calculating the scallop depth of a cut with tool&spindle when finish moulding wood components? I have seen calculators online but I want to know and understand how the figure is arrived at. I want to see the actual formula. I know the variables of tool diameter, rpm, feed rate, and number of knives are involved but I haven't been able to solve the equation.

Anyone?

4/3/19       #2: Scallop depth ...
Russ

Website: http://www.mr-moulding-knives.com

Buck

I have attached an article that may help you better understand knife marks per inch (KMPI).

All the best Russ @ MR

Article

4/11/19       #3: Scallop depth ...
Joe Barr

Depth of a cuttermark gullett in inches=
distance between 2 cuttermarks in inches squared divided by 4 x cutting circle in inches.
Distance between 2 cuttermarks in inches =
Feed speed in FPM x 12 divided by # of knives x spindle revolution in RPM

4/20/19       #4: Scallop depth ...
Henry J.

The OP's question about how deep are knife marks on moulded or planed wood can be addressed using the formula shown above.

H = L2 divided by 4D where H = height (or depth) of the scallop left by the knife, L = distance from crest to crest of one mark, and D = cutting circle diameter of knives.

Given a 6" cutting circle and 20 marks per inch: At 20 mpi, L will be 0.050"

0.050 squared is 0.0025.

D=6, so 4D = 24

0.0025 divided by 24 = 0.0001042", round off to 0.0001". That's a tenth of a thousandth of an inch.

That means that, no matter how carefully you grind your knives, if one is just 0.0001" longer that the others it will be the one leaving a mark in the wood. The others will do their share of the work, but only the longest will leave a mark.

Now, if we double the feed rate, we'll get only 10 mpi. Work through the formula and you'll find that the depth of each mark is now 0.0004". Not twice as deep but 4 times as deep!

And if we stay at 20mpi but use a smaller cutterhead, say 3" cutting circle, the depth goes from 0.0001 to 0.0002".

Marks Per Inch:

The formula for marks per inch can be simplified for most of us.

'Rpm times number of knives', divided by 'feet per minute times 12' seems simple enough, but look at this: if you are running 6000 rpm, non-jointed cutterheads, then the top of the equation is 6000. The 12 will always be at the bottom. So divide through by 12 and you have 500 divided by feet per minute = marks per inch.

And that's all you have to remember: 500 divided by any feed speed will give marks per inch.

500 divided by any marks per inch will give the feed speed.

If you are running a machine with 3600 rpm cutterheads, the constant will be 300 (3600 divided by 12)

 

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