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Cutting dovetails in cast iron

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Ron Laden05/02/2019 09:48:47
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1206 forum posts
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Having never cut a dovetail before I was thinking about the best or a good way for the pair I need in the cross slide. I have read and watched quite a lot on dovetail cutting but there seems to be a number of different approaches.

Would it be best to go with a single main cut followed by a finishing cut or go easier and take a number of cuts.

As I now have the blank good to size and shape I would obviously prefer not to mess it up, so any tips or advice would be helpful.

Thanks

Ron

Andy Carruthers05/02/2019 10:13:19
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Not being an expert on this and having cut only two dovetails recently here's my $0.02 worth...

I suspect the best approach is to use an end mill to remove the hard outer layer and cut to depth / near desired width of dovetail after which cutting the actual dovetail should be easier. I did mine with a series of progressive cuts to approach final size with several trial fits as I approached final dimensions

Hopper05/02/2019 10:38:53
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You will get a better finish on the final cuts if you can manipulate the job so you are cutting on only one side of the cutter at a time. As most dovetails will have either a clearance slot in the sharp corner of the "female" dovetail or the corner knocked off the male counterpart for clearance, there is no need to try to cut right into that corner on both surfaces in one cut.

So for instance you would cut the 45 degree surface to size with the cutter lifted a few thou off the horizontal surface. Then drop the cutter a few thou to take the final cut along the horizontal surface with no contact between cutter and the 45 degree surface. Then you run a mini hacksaw or the likes down the point where the two surfaces meet so any step is removed and clearance is created. That way your dovetail is sure to ride on the two flat surfaces and not bear on the point in the V of the dovetail.

As to how many roughing cuts and finishing cuts, that all depends on many things such as size and rigidity of machine, nature of material, quality and type of cutter. In general it takes longer but is usually safer to take a number of roughing cuts then a number of finishing cuts so you can get the "feel" of things before you get to the critical stage. If your machine sails through it all like butter, you can up the ante. If it chatters and complains, you can slow things down.

It might pay to do a few test pieces first to try some different cut depths and feed rates to find what suits your machine and set up.

Edited By Hopper on 05/02/2019 10:51:59

Chris Trice05/02/2019 10:57:27
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1362 forum posts
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I've done this a number of times and while I don't claim it is "the way to do it", I've removed most of the metal with a conventional end mill and then used the dovetail cutter advancing it about .030 at a time until the finishing cuts. Use a slow rotational cutting speed and the iron should machine beautifully. Have an old vacuum cleaner handy to suck up the mountain of iron dust.

Neil Wyatt05/02/2019 21:21:05
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After roughing a central slot, I usually cut a 6mm high by 4mm deep dovetail in three or four passes of the cutter: two (CI) or three (steel) steps to make the full form (at progressively smaller depth to keep the volume of cut each time similar, judged by eye) followed by a single light finishing pass. It's a mistake to do lots of small cuts, you are just wearing out your cutter for no gain. Plenty of feedrate as they have lots of edges compared to endmills.

Always break the edge of the matching dovetail so it can't 'rest in the corner'.

After much use you can stone off the corners by a fraction of a mm to keep the cutter working freely, as the tips talk 90% of the wear. This gives the cutter a second life which will possibly be longer than its first life. Just break the matching dovetail a little more aggressively.

Neil

Ron Laden06/02/2019 07:26:01
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1206 forum posts
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Thanks guys for the advice, appreciated.

Ron

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