Dovetail Cutter...Failed...

[Darren]

This project failed...but I thought I'd post it anyway. To put it simply I was pushing the boundaries a tad too far, but I was trying to work with what I had and it was a good practise run anyway. Can't win at everything can we.....

I'll let the pictures do most of the talking......





Used the RT to set the angle, here I'm squaring up the collet block with my indicator to set the RT to zero.









You can see I was struggling a bit to get it all to fit in the slot...



Instant disaster and I barely kissed it..........



Like I said, I knew I was pushing it from the start, but heck, gave it a go anyway. Basically the tip was too large for the slot I was trying to cut. Larger dia bar with more meat left on it to support the tip would probably have worked. But then it wouldn't have fit the slot. I'll try a smaller tip next time, or make a bigger dovetail......

[jim]

did the tip have clearance for the back edge?

just a thought

[sbwhart]

Darren:-

That may have worked if you hadn't under cut the holder, if you look at the last but one pic you can see where you can have more meat in the holder.

With a dove tail cutter you really load the cutter by using its full length to take the cut, not like a spiral end mill cutter, where is takes the cut progressively in a slicing action, thats why you need to take small cuts and keep the tool as strong as posable.

Good luck

Stew

Edit:- Just read Jimmy's reply and he's correct you need to back the tip off.

Stew

[websterz]

A little positive criticism Darren? The problem is two-fold. The insert you chose and the depth of your pocket (on the cutter, not your breeches) are to blame. By putting the cutting edge on center as shown you leave the backside hanging out further than the cut, that's why it hung and twisted off. You need to use a TCMT style insert:

http://www.carbidedepot.com/SearchResult.aspx?CategoryID=2801

That's how I built my inserted dovetail cutter and it works great! With the wider cutting face you can put the cutting edge on center and still have clearance so the heel doesn't rub. Even better is to put the centerline of the insert on the centerline of the cutter, splitting the difference gives better clearance and still acceptable performance.

[Darren]

Thanks Guys, I knew there was a good reason to put the post up, never gave the back edge a thought......

Dunno why not.......

I'll try to get some different tips, smaller ones too so the holder is left with more meat on it ( a lot more) and have another go......

[websterz]

Darren:-

That may have worked if you hadn't under cut the holder, if you look at the last but one pic you can see where you can have more meat in the holder.

With a dove tail cutter you really load the cutter by using its full length to take the cut, not like a spiral end mill cutter, where is takes the cut progressively in a slicing action, thats why you need to take small cuts and keep the tool as strong as posable.

Good luck

Stew

Edit:- Just read Jimmy's reply and he's correct you need to back the tip off.

Stew

No matter how deep or shallow the pocket, when you swing a zero rake insert like this in an arc the backside will always take a bigger swath than the insert I have referenced. The 7 degree clearance is critical. By cutting the pocket far enough back to line the bottom edge of the insert up with the cutter body you have plenty of tip extending beyond the cutter for doing the job. I can put up a pic of my cutter if you'd like.

[jim]

you could just back off the tips you've got to get enough clearance

[websterz]

you could just back off the tips you've got to get enough clearance

The backside moves the same proportional distance as the cutting face...I don't think there's a point where the insert shown will work. Pics of my cutter coming up in a sec.

[websterz]

Cutting edge on centerline...sorry for the blurry pic.


half-inch shank...plenty stout


Cutting edge extends about .020" below the bottom of cutter body so it cuts without rubbing. You can see the light under the cutter body but none under the insert in this pic.

[websterz]

Thanks Guys, I knew there was a good reason to put the post up, never gave the back edge a thought......

Dunno why not.......

I'll try to get some different tips, smaller ones too so the holder is left with more meat on it ( a lot more) and have another go......

PM me your address Darren. I have a few extras in 2 different sizes...parting gifts from a former employer. I'll be happy to share.

[tinkerer]

  Just reading threads is instructional. Now I know what all of those things with steps are for. I have several of them and couldn't imagine what I could do with them. Now I know.

[John Hill]

You get a nice blue star for effort Darren but I doubt I could have done any better myself!

I did think though that the chip was rather big for that application?

BTW, why are you making a dovetail cutter?  Are you not one of the shaper fraternity?

[Darren]

Indeed I am John.....indeed......

Having failed to make a milling dovetail cutter ( I will come back to that at a later date when I have some more tips to try) I decided to go back to basics and fire Old Faithful up.

Instead of angling the head over to cut the dovetails as I did on "The Thing"  http://madmodder.net/index.php?topic=908.msg6429#msg6429 I decided to use the same tip in a lathe tool holder I made some time ago http://madmodder.net/index.php?topic=963.msg7088#msg7088


I used the tool on the left in the shaper



Sorry for the poor picture



Note the groove in the centre, this is important to lessen the size of the cut to reduce chatter



And this is what it's all been about, to fix my parting tool to my QCTP







Did I ever mention how much I like using the shaper.......

[bogstandard]

Nice job Darren, but if I could make a suggestion.

I have attached a c-o-c to explain things.

I suspect you have just bolted the two together. If so, no matter how many bolts you have used, the tool is liable to move under the cutting stresses.

If you just mark where the tool is sitting now, and machine a small recess (say 3mm deep) for a tight fit when the tool is bolted up, it will transfer the cutting forces to your toolpost rather than just the tool holding bolts.


John

[Darren]

No bolting as yet John, it's just sitting there where I finished last night. I am a bit concerned about just bolting and rigidity and that's quite a bit of overhang.

I could just weld the bits together and be done with it, sure would be rigid then......

I will follow your suggestion, or I may just cut a dovetail in the tool shank itself. The dovetail I made yesterday started out as just a bit of practice, but then I got carried away.............turned out ok really.

But I do have one prob with the shaper that I still have not resolved. It's to do with the return stroke on undercuts ie dovetails. On the tools return it lifts and damages the finish, quite badly in fact.

I'm thinking that I need to lock the clapper with undercut work......could anyone confirm this?

[John Stevenson]

Below is a drawing of an insert laid out in two positions.

The green insert has its leading edge [ green X ] on the centre line, it trailing edge can be seen to be greater that the swept yellow circle of the leading edge.

now if the insert is placed so it's seating is on the centre line as in the blue insert the trailing edge with always have clearance over the leading edge [ blue X ]



John S.

[Darren]

Thank you for taking the time to show that John. So my biggest mistake (apart from the insert being too large in the first place) was positioning relative to the centre line then.

Very interesting. I wonder which line the cutting edge has the best advantage. What I mean is do we loose any efficiency having the cutting edge forward of the centre line?

I deff will have another go......

[John Stevenson]

No problem darren, it's a common mistake and the drawing has been on the computer for a while, it's alos been posted on a few forums, as i say it's a common mistake.

John S.

[sbwhart]

Darren can you tilt the head of your shaper ?.

If you can tilt it over to the angle you want and feed it down to shape the dove tail, don't have to worry about having the form on the tool and taking a cut across its width.

Stew

[Darren]

Hi Stew,
Yes that's how I did it last time, but it's not the easiest way to do it or to get right. I have to manually down feed while it's bouncing backwards and forwards. This does wonderful  things to accuracy...not...!!
Then there is matching both sides equally.....again not that easy to do.

And still the lifting of the clapper presents the same problems on it's return.


By having a form tool, the carbide tip is a perfect 60 deg...I only have to worry about feeding in one direction which is a simple matter and having set the depth it will be equal on both sides.
The actual size of the cut is no problem, shaper copes without a sweat and could easily manage larger I assume. Making the central slot not only reduces the cutting area but it also gives you somewhere to feed from. This is quite an advantage.

My only real problem with undercutting like this is the lifting of the clapper on its return stroke. I'm going to try to lock it. After all the carbide tip is identical on both sides so it should cut in both directions anyway. That might be an advantage? Forward stroke take the main cut, backwards stroke should take a skim off.

I will give it a go and report back, but it won't be today......got building work to do......

[kvom]

If you can clamp the work vertically, then lifting on the return stroke wouldn't matter, I believe.

[Darren]

I've been thinking about this one all day since I read it earlier......I'm not sure, would it make a difference?

[Darren]

I decided to have another go at this tonight....practise make perfect as they say.....well making these things certainly doesn't faze me now.....


Do ya think this is a bit big for my lathe?



Only kidding, I'm going to cut a bit off the end for a bit of stock.....after visiting the miller for a bit of prep it was looking like this. No idea what grade of steel this is but it didn't leave as good a finish as the last piece I was using. But good enough for now.



I decided to stick my milling vice on the shaper to see if it would improve over the pillar drill vice I was using.....not 'alf, much more rigid and the workpiece never jumped out once... The vice looks a bit oversized in the pictures but in reality it was perfect. If/when I get a new vice for the mill this one has at least found a new home...



Close-up of the cutter. This time I locked the clapper and what an improvement. No more damage to the dovetail by having the clapper lifting on the return stroke. What's more this cutting tip cut on both forward and return strokes quite nicely.



Ended up with a nice smooth dovetail and a good fit......



A bit more meat on this holder so I can machine a slot for the parting tool shank and secure with grub screws. I might reduce the parting tools shank from 25mm sq to a more reasonable 20mm to make life easier.

[websterz]

Is that blood I see on top of the new toolholder block?

[Darren]

Oh yes indeedy, it bit me....savage little Blighter that one.......


Well spotted........