Dovetail cutter.

I'm thinking of making a new dovetail cutter using 2 TC** inserts , only problem I can see is to avoid cutting on the bottom I would need to "tilt "the insert by about 0.5 ° only problem with that is the dovetail would not be a true 60 °. There are inserts TDEX , I've never studied them closely, but they must only use one edge as there can't be three at 60 ° or were back to the old problem anyone got any suggestions. I will order up a box of TDEX if that's the only solution.

Cheers.

Just had a look at TDEX inserts and there ARE three cutting edges. So they must be at 60 ° but then that takes us back to cutting on the bottom as well as on the "dovetail edge ". Am I missing something here ?

Possibly, my HSS dovetail cutters are sharpened on both the sides and the bottom.

Rod

From pictures (I don't have one to hand) it looks like the inserts are mounted so they 'lean forwards' slightly to restore the angle. Sounds like scary trig is needed to me if you don't have one to copy - or a 3D cad wizard.

Neil

Hi Rod, It's not a problem to cut on the bottom It's just that I thought maybe in [ error ] that an indexable dovetail cutter should only cut on the dovetail face. So if it is acceptable to cut on the bottom as well as the face then I can use TC** inserts.

Cheers.

Hi Neil, That was one of me thoughts. so they only cut on he face. In other words the insert is "layed down ".

Decided to go ahead and make the cutter I set my self a tolerance of 0.01 mm for the inserts in relation to each other [ outside that then scrap and try again ] Got them to 0.005mm on the bottom and 0.007mm on the cutting face. 0 would have been perfect, but in my world what I managed was close enough for me. It's made from a piece of EN24T . Tried it out on a bit of 4140 and got the full depth dovetail [ 11mm on the face ] cut in 4 passes and surprisingly soft cutting action It doesn't "hammer " like single insert ones. And leaves a very smooth silky finish. The inserts I chose are TCGT 1104 although any TC insert will suffice. I initially had reservations about the TCGT inserts [due to their perceived fragility ] after the test piece I checked the state of the inserts and no chipping had taken place. It is I think vital, to get the 2 inserts as near perfect in position to each other and the use of the correct Torx screw. This cutter will fit into a slot of 24mm in width I am now going to make one that uses a 09 insert to allow it to enter a narrower slot [should be able to get into 16mm slot ] and one using 16 size insert. Maybe even try a 3 insert design on the bigger size.

Well done, Raymond

MichaelG.

Thanks Michael, It was something that I wanted to try a while back. Actually works better than I thought it would. .

Excellent work, Raymond! That is something I've been wanting to try so it's great to see that you've made it work so well. Very encouraging!

Pete

Hi Peter, As you will probably realise already it is VITAL that the inserts are as near as perfect in relation to each other or else one insert will be doing most [all ] of the work. That is easily the hardest part of making the cutter,.the rest is straight forward. I could easily have done a three insert one using the 11 02 inserts but I wanted to keep the core strength as high as possible. 3 inserts of that size would have meant a bigger Ø body to keep the strength. I will go 3 inserts on the 16 size insert which should fit into a 30 /31 mm slot. It was a nice little exercise.

Cheers.

I was idly thinking about that. It seems to assume that the higher (prouder) insert cuts first and is followed by the lower insert cutting air.

I could just as easily assume that the lower insert cuts first - making a partial cut - followed by the higher insert finishing off. Which would seem to be quite a good cutting action.

There's probably something I'm not considering ....

Hi Bandersnatch, I get your thinking but It does not work like that. I have tried once before [ thinking the same as you about a slightly lower insert. ] but the performance was not good, too much "hammer " and the surface finish was ok, but nowhere near as good as in the above pics. So yes, they must be very very close [ the closer to identical the better ]

Cheers.

I am curious about the back edge of the inserts. They look to me like they will have a larger radius than the cutting edge and as a result will rub. Is there any sign of this?

Martin

Martin, I'm not sure what you mean about the "back edge" do you mean what would normally be the"bottom" of the insert if it were mounted in a lathe tool ? Because they are positive inserts they cut only on the leading edge [s] There is no rubbing. The back of the inserts are exactly on the edge of the mounting surface so only the leading edges project below the base / rim of the cutter.. If they project too much then that would weaken the support for the inserts. They project the same as they would on any quality lathe toolholder.. Hope iv'e grasped what you mean..

Cheers.

Well, I'll take your practice over my theory any day, Raymond.

On further thought, at relatively low feed rates - as you would likely use in cutting dovetails - since the higher insert is leading the lower it would presumably always be cutting first with the lower insert cutting air.

Edited By Bandersnatch on 28/08/2016 20:39:12

To clarify, the cutting edges should be at least half an insert thickness "higher" in their seat than you show them. The insert you have used has perpendicular edges, so the trailing edge of the insert is at a larger radius than the edge you suppose is doing the cutting. It's possible that this is why the surface finish is so good! Does it get hot?

Also, if using conventional rather than climb milling you will get a burnishing effect anyway. Would be interesting to see the chips produced. This might help to understand the actual cutting action here.

There are special inserts to avoid the problem resulting from the square front edge. Alternatively as alluded above, you could "raise" the insert at least half the cutter thickness but this will adversely affect the cutting (top rake) angle. Easy to try.

Rather than going from 2 to 3 inserts, you may actually be better off going down to 1. Although the cutting may be more interrupted, using a proper chamfering insert rather than a general purpose turning insert may give a smoother cut and lower cutting forces and avoid the requirement to match the positions of the inserts with such deadly accuracy.

Murray

I have made and used a dovetail cutter with a single insert, see:

http://mikesworkshop.weebly.com/dovetail-cutter.html

Carbide inserts like to cut fast so you can compensate for having only one insert by running at twice the speed.

Mike

Even further thoughts (I was doing a lot of work on slow power feed today so I had time on my hands):

Let's say the difference in height (protrusion) between the two cutters is 1mm (probably not realistic but it's an easy number):

When you start a cut the higher (leading) cutter will get there first and cut. If the feed hasn't moved the part by 1mm in a half-turn of the cutter, the lower (trailing) cutter will cut air. At 1mm feed it would rub and anything more than that, the lower cutter will cut some material thereby lessening the cut to be made by the higher cutter on the next rev.

That suggests that the "hammer" could be lessened and the finish improved by increasing the feed somewhat. There may be a sweet-spot in there.

Edited By Bandersnatch on 29/08/2016 01:06:12

Edited By Bill Wood 2 on 29/08/2016 07:01:56