Dovetail Cutter - Fail..

[mahgnia]

Gents,

Attached pictures of my first attempt at making a dovetail cutter for the mill.

As can be seen the cutter worked well for the first 2 mm from the bottom, machining mild steel(test scrap pictured), but then decided to chew itself up.:(

I suspect that there were a couple of issues that may have caused this:
1. The cutter was not at a high enough temperature when I quenched it in oil to harden it, so only the ends of the teeth were properly hardened.
2. Not enough back clearance from the cutting edge was machined in the teeth.

If anyone has any further suggestions or advice it would be appreciated.:)

Andrew
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[Steamwhisperer]

Hi Andrew,
good try I thought.
It is only the tips or outer edges that do all the cutting and so only need to be hard/er and don't forget primary and secondary clearance when sharpening.
The only other thing I could suggest is to keep at it. There is a lot going on with regard to tool geometry.
good luck.

Phil

[nearnexus]

It's always interesting making milling cutters.

I can only comment as I see it:

- The edge finish is not ground so it's not great.
- it's best to sharpen AFTER heat treating
- the steel may not be hard enough (although as dovetails are machining into cast primarily, tool steel should be OK)
- there should be two rear facets on the cutters
- you may have been spinning it too fast fo the limited number of faces
- the test object you milled looks a lot like steel and may be too hard for the cutter.

Not wanting to sound negative on this, but those are my observations.

To avoid the hardness issue, I like to buy and re grind big old reamers or similar into different style cutters - but you need a TPG for that.

I hope this helps.

Cheers

Rob

[KBs PensNmore]

I think that there is not enough clearance behind the cutting edge, have a look at a 4 face milling cutter to get an idea of what it should look like, plus the actual cutting edge doesn't look all that good, as if the tempering process ruined the actual cutting edge, also the bottom needs more relief, a slight taper to the centre would be advisable.

[Techo1]

What material did you make it from Andrew?

[Machtool]

It looks like you went straight too work with that cutter. You might think about running a normal end mill through first. Just at the neck or small diameter of your dovetail cutter. That cleans out the bulk of material, then leaves the dovetail the detail of cutting the angles only. You have to do that for Tee-Slots obviously, it works just as well for dovetails also.

Regards Phil.

[morrisman]

Hi

I am not one to offer advice here, but the commercial dovetail cutters I have seen have many more cutting edges , compared to your four . They normally have 6 or 8 teeth on them . Maybe you could buy a commercial cutter and study it with a view to making your own e.g, study the angles needed . I agree with the poster who wrote , you normally use a end mill first to remove the bulk of the material , then go to the dovetail cutter to finish off . Mike

[mahgnia]

Thanks Guys for all your helpful advice and suggestions.

Under close examination of the cutter it seems clear that secondary relief behind each tooth was required. The primary relief machined on each tooth was barely adequate (can be seen in the photos), but the rears of the teeth were only just rubbing at the bottom and this would have become worse further up the teeth as the cutting radius reduced but the tooth thickness remained constant. It certainly is complex geometry here!

I guess the best approach would be to machine the each tooth at the rear so the thickness is tapered in proportion to the cutting radius, and the primary and secondary relief machining should leave proportionally tapered land widths. (Not sure if I explained myself clearly here!).
More primary and secondary relief may be needed at the ends of the teeth as well.

The end was machined slightly concave (approx. 1 degree) to allow the tips to cut at the bottom.

I'm pretty sure my hardening process was flawed due to lack of an adequate heat source (LPG torch). The bottom tips/edges of the teeth seem to have survived usage, but the teeth became soft further up from the end.
The steel used for the cutter was an old piece of bright steel bar of uncertain provinance, but it was very hard. A good file would not touch it, and it threw some very bright sprinkly sparks on the grinder. It was hard right through, not case hardened.
I annealed it by air cooling from bright red hot, heated in the slow combustion wood fire, and from there it was very easy
to machine.

The cutter was only being pre-tested on the sample piece pictured in the OP, it was not being loaded heavily and was operating at about 150-200 RPM, with oil for lubrication. Before use I cleaned up the cutting egdes lightly on an oilstone, as further cleanup was going to be done only if the cutter worked successfully.

I think I have a lot of things to consider for Dovetail Cutter Mk. 2, which will hopefully work better and avoid the above pitfalls.:)

Andrew

[Stustoys]

Would it be fair to say that the more metal removed with an endmill the less secondary clearance is needed on the dovetail cutter?

Stuart

[nearnexus]

Thanks Guys for all your helpful advice and suggestions.

I think I have a lot of things to consider for Dovetail Cutter Mk. 2, which will hopefully work better and avoid the above pitfalls.:)

Andrew

Yes, don't give up on the project.

We all learn as we go, and failure is a normal part of this until you get the wrinkles out.

When it all comes together, you think, well that wasn't so bad after all.

Hang in there.

Cheers

Rob

[GZBMW]

Homemade dovetail cutter. - Home Model Engine Machinist (http://www.homemodelenginemachinist.com/f13/homemade-dovetail-cutter-1213/)

This looks like a good alternative.

George

[morrisman]

I am sure there is a web site hosted in West. Aust., that has info on this topic. The web site is run by a chap makes his own milling cutters from old big end cap bolts or something similar ?... Mike

Found it Milling Cutters (http://home.iprimus.com.au/stevor/milling_cutters.htm)

[Ueee]

Good work and lotsa :2tsup: for having a go!

The only thing i will add is when you said cutting oil, did you brush a bit on, or use flood coolant? With carbon steel cutters you really need flood coolant so the cutting edges cannot get hot and loose their temper.
Ok maybe 2 things....as for hardening, use a ferrous magnet to test for the steels curie point as it heats up. As soon as the steel looses its magnetism quench the cutter. Use a ferrous magnet as i have found RE magnets loose their power permanently if they get too hot.

One good article in MEW lately was the making of counterbores from silver steel, Mikes link reminded me of it.

Cheers,
Ew

[mahgnia]

The only thing i will add is when you said cutting oil, did you brush a bit on, or use flood coolant? With carbon steel cutters you really need flood coolant so the cutting edges cannot get hot and loose their temper.
Ok maybe 2 things....as for hardening, use a ferrous magnet to test for the steels curie point as it heats up. As soon as the steel looses its magnetism quench the cutter. Use a ferrous magnet as i have found RE magnets loose their power permanently if they get too hot.

Ew

Ew,

I simply used a squirt of SAE 30 oil on each pass. I don't think it got hot at all as usually the oil will cloud if this happens.

The use of a ferrous magnet to test for temperature is an interesting idea. I WILL try this one:)

The link by morrisman to the WA site has some intriguing ideas, as well as milling cutters.

Andrew