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David - Machinist in wood
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Discussion Starter · #1 · (Edited)
This may be common knowledge amongst you CNC guru's but I just figured it out today. I don't research cutting profiles and tips often - ok, rarely - and it would probably be in my best interest to do so from time to time.

However, assuming this little issue is one you've encountered or not thought of then this little tip will save you a bunch of time and a fair amount of sanding.

My normal cutting profile on 1/2" BB (12mm) for Longworth chucks is to cut the slots in a rough pass climb cut leaving 0.007" on the side walls (radial) and 0.020" on the bottom (axial). Then I come back and clean up each slot with a conventional cut at full depth and that has left a very clean slot. Except for the bottom face veneer sitting on the spoilboard. That has splinters/fibers/fuzz and I have to hand sand those. All this is at 175 ipm and 18k rpm.

Now this may not be a big deal if you're cutting one Longworth chuck. But on days like today, where we had a 16" going to the UK and three 12" sets for the States, then that adds up to 64 slots! That's way more hand sanding than I want to be doing on these given the low cost we charge and I don't want to increase the cost just because I feel the need to do some sanding to make a better product.

However, time is money and as I was cutting the plates today I began to finally think about how I can eliminate the fuzz. It dawned on me before I cut the final of eight plates that I need to cut the full depth in one pass rather than leave 0.020" because what's happening is there's nothing to support the upcut of the compression bit in that final 0.020". So I quickly modified the cutting profile in Fusion 360 and cut the final plate.

Turns out my thinking was correct, albeit 7-8 months late (we've cut over 270 Longworth chucks and I've had to sand slots on most of these). So now I can't wait to cut more chucks and NOT sand slots! LOL!

Leaving 0.020" for final pass -
Rim Finger Electric blue Bangle Fashion accessory


Cutting to full depth in one pass -
Rim Acoustic-electric guitar Metal


David
 

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Wow David!! That's a great "tip of the day!" I got my 1/2" compression bit and cut 3/4" BB in 3 passes at .25 each. At 150ipm at 18,000 and had lots of sanding and the uncut tip was blue. Hope I didn't burn the bit too much.
I'll try the single pass dave maneuver tomorrow. What would your ipm be in 3/4" BB? Thanks.
 

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David - Machinist in wood
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Discussion Starter · #3 · (Edited)
Probably the same, Joe - 175 ipm. I've never cut 3/4" BB so I would have to test it first. According to some on other forums, guys who cut more aggressively than me, I should be cutting the full pass of the 1/2" BB at 250 ipm to 300 ipm. I accidentally cut at 600 ipm on the outer profile of a Longworth chuck and had zero pushback from the bit, spindle, or machine. So maybe 175 ipm is really conservative!

Without doing any checking or research (a trend for me... LOL!) I would think a 1/2" compression bit should cut at 250 - 300 or faster with no issues. Well, depending on how many flutes the bit has, how much power your spindle has, and how rigid your machine is, I guess.

What's the depth of the upcut portion of your 1/2" bit, Joe? Whatever it is you should probably set a depth of cut to be at least 1/16" greater, and probably 1/8" would be better as a minimum. So if the upcut portion is 0.200" and you're cutting 0.25" then you're not even 1/16" past the upcut section. That may be why the tip is blue.

David
 

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David - Machinist in wood
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Discussion Starter · #7 ·
https://youtu.be/dyMbbx9XjyM

Well I did 3/4" doc 350ipm at 22,000rpm.
I think the 2.2kw spindle might be a little underpowered for 350ipm. Maybe 250 next time. But it cut much cleaner than 75ipm.
Good job on that, Joe! And the chips look good coming off the bit. They should be carrying the heat away.

Yesterday I cut four Longworth chucks; 8 plates back to back to back and didn't stop any longer than it takes to mount the new blank on the spoilboard (64 slots, 32 finger holes, 8 perimeter cuts). When the last cut finished and the spindle stopped I put my hand on the bit. It wasn't any warmer than room temperature but the spoilboard was slightly warm from all the BB being cut. And I could feel each chuck plate was ever so slightly warm when it came off the machine.

David
 
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