Dovetail technique

I have the recent 25-year anniversary issue and in Harold Hall's article about the grinding rest he describes a method for cutting dovetails.

I'm a little confused by it. It seems to indicate that a dovetail is to be cut at two different depths, instead of going with full depth from start to finish. Is that right? Is this done to minimize cutter load?

I would suggest several passes with a straight cutter that is the width of the smaller dovetail dimension. This will remove the majority of the waste. Follow with dovetail cutter at full depth.

I think he's aiming to get a better finish by keeping the second cut smaller.

Neil

My interpretation of Harold's instructions is to rough out the slot with a straight cutter then cut half of the dovetail, leaving 0.5mm of the finished width. Then lower the dovetail cutter to the full depth to machine the bottom half of the dovetail. Finishing with light cuts to finish the complete dovetail (removing the 0.5mm at the top). This reduces the load on the cutter. The tips of the dovetail cutters are small and could easily break.

Paul.

Like this?

That's how I machine dovetails

Neil

GHT described cutting dovetails which stated that it is best not to cut the slope and the base at the same time, because of "cutter flutter". His recommendation was to lift the cutter off the base by about 5 to 10 thou for a very light finishing cut along the slope.

Alan.

Thanks gents.

Sounds like pussy footing about to me. Thinking now about tee-slots, is there supposed to be some way of doing them in stages too? Can't see how that would be possible short of using 2 different sized cutters and endless passes. Just send it in!

Murray

> Sounds like pussy footing about to me.

Depends how big, rigid and powerful your machine is - and the depth of the dovetail.

Neil

If need or want to do it accurate, mill the slot to depth -0.1mm.

Flash dovetail cutter over the top and zero. Then full depth offsetting cutter to get the fit, easy no faffing do loads where 0.05mm offset will render the dovetail sloppy.

Murray, mill slot to depth, change cutter for T slot and zero. One pass straight in at depth clearing debris with air line and use coolant.

Well, yes, my point precisely. It would be difficult to do it any other way unless you had a whole set of smaller sized cutters and a spare week of your life. I cut a 10" long trial 5/8" tee-slot as you described from scratch in 2 passes - end mill and tee slotter. Loads of coolant and power feed, nice clean result in one pass.

Murray

Can i ask some more questions re dovetails, please?

One of my first projects when i kitted out was to make a few extra QCT-holders. The female dovetail is quite wide on those so after milling the relief slot i fed in to each side seperately and had issues...

If I fed in too far then lots of vibes and chatter and at the end of each pass unless very careful it would tear out and bounce. I finally succeded i making them all by the tedious process of multiple fine passes. The other problem was the tendency for the cutter to try and pull the machine down or pull the cutter out of the er32 collet and I suspect flex in the cutter causing the leading edge to snag at the bottom despite being a tad above the base of the relief slot.

It may well be that i was trying to feed too fast? It was a 25mm dovetail HSS 60deg cutter - not a cheap one - with lots of coolant hand misted during the cut.

Current project needs dovetails again.. shallower at least - at 7mm deep and therefore 4.05mm undercut. If I understand correctly you'ld feed full width leaving a finishing pass? What sort of speeds and feeds (feed will be manual as will coolant and air blasts)? Chester lux mill. Piece of hot-rolled.

Any benefits in indexable cutters here?

Edited By pgk pgk on 27/08/2015 10:31:50

What about using a shaper, ? I have made loads of dovetailed forging hammer die blocks,

With a shaper you can cut any size or angle of dovetail that is required.

Richard.

I am sorry Jon and Murray but you are trying to advise Mike without knowing the machine he is using, I doubt if one in a hundred workshop owners have power feed, loads of coolant and an airline to dispose of swarf.Also, my experience, through my web site, is that less than one in four have a really robust milling machine.

You are of course not the only ones to fall into this trap, it so often happens on this web site. On the other hand, the OP could provide more information as to the machines, etc. being used.

Also Murray, cutting a tee slot is quite different to cutting a dovetail as material is being removed on both side so there is very little attempt to force the cutter sideways. Of course, one side is climb milling whilst the other is conventional milling so it is not perfectly balanced.

My advice Mike is, if you have a largish cutter and a robust machine, then attempt it in one pass, but if you run into problems then revert to the two stage method as you so clearly illustrate in your drawings. These, prove the saying, a picture is worth a thousand words.

Harold

I just cut the male dovetails on my current project - bit the bullet and went full depth (7mm) at a slow 170rpm for my 25mm cutter with a handheld airblast. Feed rate was held very slow - just below the point of rumble and I was turning my handle to a slow count of 20 per rev. If I had to guess that's probably no more than a few mm per min and every time the compressor started up I started too. But it went through with a very neat cut, hardly touched the sides on a spring pass and the cutter was cool at the end of the first pass. I'm happy to have learned something.

Thanks Mike for the info I think perhaps you will need to use the two step method.

Incidentally, when I mentioned more info from the OP I was not aiming at yourself specifically, I often think the question being asked should be back up with a more detail.

Also, as you are making the item from the 25 year special you may find some additional detail that I have included on my web site worth considering. There is also an expanded drawing of the upper section that enables the fine detail to be more easier understood. Links to the various comments can be found on the rests main page here

Harold

No Harold I wasn't advising how to do it without knowing the machine. My point was that some milling operations such as tee-slots can't be crept up on in this way - for operations like that you can only succeed if you have a robust enough machine to start with. If you find you have to creep up on a dovetail, you'd be advised not to waste your money on a tee-slot cutter.

The downside of including a lot of detail in a question, Harold, is that it encourages (some) people to argue (nitpick) the detail rather than answering the actual question. Which in turn can hijack the whole thread. Happens here (and elsewhere) quite a lot.

For that reason I've often pared a post down to the bare bones in an attempt to get answers to my precise question.