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Cutting dovetails, how hard can it be...

 
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  #1  
Old 03-22-2012, 09:12 PM
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Cutting dovetails, how hard can it be...

I set Bridgeport for some dovetail cutting using 1 3/8" cutter by F.K.D, I was running 100 RPM and feeding 3.0 IPM for plain 1020, I had radius comp on, which initially was 250 thousands more then actual tool radius and conventional milling, anyway after first pass which took .100 I decreased radius comp by .05 and then it happened, my coffee mug felt from work table, it was like the whole table with my part was beating against the cutter, I reduced feed rate almost to 0.5 IPM and only then beating stopped, but it still chatter. So what have I done wrong, I was running HSS tool using optimal speeds and feeds...
Here few images when chatter started:





here I reduced feed rate, but it still chattered

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Old 03-22-2012, 10:16 PM
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Re: Cutting dovetails, how hard can it be...

Were you really only running at 100 rpm? That seems super slow to me.
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Old 03-22-2012, 10:59 PM
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Re: Cutting dovetails, how hard can it be...

yes I was... Optimal cutting speed for 1020 is like 35ft/min RPM's should be even less like 97. Also I was using .0022 chip load and that thing got 10 teeth...





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Old 03-23-2012, 12:03 PM
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Join Date: Jan 2012
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Re: Cutting dovetails, how hard can it be...

I use tooling like this on an old Brigport , your cutter looks like one is tooth is chipped .
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Old 03-23-2012, 12:08 PM
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Re: Cutting dovetails, how hard can it be...

Quote:
Originally Posted by Jinx-) View Post
I set Bridgeport for some dovetail cutting using 1 3/8" cutter by F.K.D, I was running 100 RPM and feeding 3.0 IPM for plain 1020, I had radius comp on, which initially was 250 thousands more then actual tool radius and conventional milling, anyway after first pass which took .100 I decreased radius comp by .05 and then it happened, my coffee mug felt from work table, it was like the whole table with my part was beating against the cutter, I reduced feed rate almost to 0.5 IPM and only then beating stopped, but it still chatter. So what have I done wrong, I was running HSS tool using optimal speeds and feeds...
Here few images when chatter started:





here I reduced feed rate, but it still chattered

1020 CRS dosn't cut well with a lot of contact area. (material is very gummy like in nature) There's a couple ways to get around a lot of this, and really the best way is to use a different cutter. I used to grind the desired dove tail onto end mills (I think you can buy them already ground). You need a good spray mist coolant to keep the temps down and lube the bottom side of that dovetail cutter. But an even better material for your use would be generic 8620 (you can buy it in a ground finish to save some work). Still remember your milling the dovetail, and it will never be that quality of a dove tail cut on a shaper with single point contact, or wiring the shape out. We kept an old G&E shaper in the shop just for cutting precision dove tails as it was far more accurate.

Next time look around for a piece of 8620 bar stock, or even O-1 gauge stock. Make a roug cut with about .0075" stock left on the bottom flat face. Use something like grinder coolant with a little bit of cutting oil added to it (maybe one cup per gallon). There is a way to regrind the dove tail cutter if there's not enough bottom relief, but the end mill already has it and cuts better.
gary
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Old 03-23-2012, 12:16 PM
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Re: Cutting dovetails, how hard can it be...

Quote:
Originally Posted by Jinx-) View Post
yes I was... Optimal cutting speed for 1020 is like 35ft/min RPM's should be even less like 97. Also I was using .0022 chip load and that thing got 10 teeth...





that's a good cutter. I was thinking you were using one of the old style ones that had little reliefe on the bottom side. Another thing that will really help your dove tail is to relieve the female corner of the dovetail about .06". Most guys don't like to do this as it involves two setups. Make a rough cut leaving about six to eight thoundths stock to be cleaned up. Then cock the head about five degrees and go back into the female corner with a .06" saw cutter about .04" deep. Then realign the head and make a finsh cut. This method really helps cutting the angled part.
gary
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  #7  
Old 03-23-2012, 01:09 PM
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Re: Cutting dovetails, how hard can it be...

I think I need to resharpen primaries and secondaries on the bottom and add more relief, just like on endmill where its cutting just on the edges when facemilling so it would have more clearance and not rub and chatter like it does now.
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