Dovetail cutter on brass

[Martin Shaw 1Participant @martinshaw1]

Jason

That's very interesting, much better surface finish than I'm getting, so I have to conclude that either my revs or my feed rate are drastically wrong, possibly both. I am getting the swarf you describe so possibly depth of cut is right and I wonder if a high feed rate combined with a slow rev is allowing the cutter to take " bites" from the work with the attendant ridged finish. I'll have another go at it and see where it leads.

Regards

Martin

[HopperParticipant @hopper]

It's brass. Rev the billy-o out of it.

Standard formula for rpm is RPM = 4Cs / D, where Cs is the cutting speed in feet/min and D is the diameter in inches.

Cutting speed for brass is 300 feet/min. Your cutter's largest diameter is 20mm, ie nominally.75" .

So RPM = 4 x 300 divided by .75

= 900rpm.

Depending on the composition of the brass, cutting speed can be as much as 600 ft/min, so you might even be able to double that figure to 1800 rpm. Which makes Jason's 1500rpm right in the ball park.

500rpm is more like what you would run for cutting steel with that cutter. Way too slow for brass. Steel's cutting speed (ie surface speed) is 100 ft/min. Cast iron is about 60ft/min and ally is about the same as brass.

Edited By Hopper on 07/02/2018 23:53:47

Edited By Hopper on 07/02/2018 23:56:26

[Anonymous]

Posted by Hopper on 07/02/2018 23:52:14:

Cutting speed for brass is 300 feet/min. Your cutter's largest diameter is 20mm, ie nominally.75" .

So RPM = 4 x 300 divided by .75

= 900rpm.

Oh dear, somebody wasn't paying attention at school.

Andrew

[HopperParticipant @hopper]

Posted by Andrew Johnston on 08/02/2018 02:09:21:

Posted by Hopper on 07/02/2018 23:52:14:

Cutting speed for brass is 300 feet/min. Your cutter's largest diameter is 20mm, ie nominally.75" .

So RPM = 4 x 300 divided by .75

= 900rpm.

Oh dear, somebody wasn't paying attention at school.

Andrew

LOL. True that. (Can you tell I was an English Lit major at uni?)

Ahem, let me try again. That should be 4 x 300 DIVIDED by .75 = 1600 . So almost spot on wot Jason said in the first place.

Carry on.

Edited By Hopper on 08/02/2018 05:15:39

[JasonBModerator @jasonb]

I think on a cutter like this then maybe the average diameter should be used if doing any calcs. top of cutter is 12mm dia so maybe 16mm dia for calculations.

And it was Andrew who suggested 1500 before I tried it so credit to him.

Also being that the cutter has 8 flutes it needs to be fed twice as fast as a 4-flute to give the same chip loading so maybe that is why Andrews feed rate sounds scary to some.

Martin, any chance of a photo of your setup for cutting as that may show up something else that is causing the problem.

[Martin Shaw 1Participant @martinshaw1]

Unfortunately it will be Saturday before I can post a picture, but I will do so then.

Regards

Martin

[Martin Shaw 1Participant @martinshaw1]

Following on from Thursday, I had to pop down south to see my Dad, at nearly 94 he is ageing quite rapidly. However here are pics of the set up, as you can see the swarf is, from Jason's description, about right so I think the tooth load is about right, but the close up shows the ridged paterrn in the cutting surface. Having had a couple of days to think about this I'm reasonably happy that it is a too high feed rate with a too slow rpm, but if anyone thinks otherwise I'm happy to be corrected.

Regards

Martin

[JasonBModerator @jasonb]

If we are looking at the ridges across the work that can best be seen in the first photo at the left of the work then they look to be spaced at about the pitch of your lead screw. Could be caused by loose x gibs and as you wind the handle round there is a tendency to lift and then lower the table.

If it is the ridges that run along the work then that is down to the grinding of the tool.

I would also suggest a third clamp mid way along the work as it could be lifting a little. Your clamping method is also not ideal, if possible the studs should be as near to the point of clamping as possible and as far away from the stepped packers as possible to give the best mechanical advantage which may be letting the work flex slightly

[Martin Shaw 1Participant @martinshaw1]

Jason

Thanks for your thoughts, there is a bit of excess in the X gib but not for the full travel, so it 's probably part of the problem. Can't do much about the tool grinding however you have picked up the other major problem which is bad clamping practice. I have had a look and apart from the visibly obvious there is also a lift of the parallel at the rear because of it's location on the bed. Fortunately there is anough of an edge to realign when I have sorted a better arrangement. Thanks for yours and others help.

Regards

Martin

[RobinParticipant @robin]

I am really worried about posting on these things because there is a huge risk of teaching Granny how to suck eggs and getting accused of all sorts of Smart Alecry. I've only been on here a few months and I didn't read the whole site, but In for a penny in for a pound, here we go…

I would guess machine and tool shank bending. We all know the difficulty in machining off that last thou. You advance .001" and make the cut, no difference. You repeat, nothing and then blammo, .004" undersize.

The tool puts up resistance to the cut, it would rather rub and bend everything than make the cut. Only when it can no longer resist the cutting pressure does it make the cut and then it takes everything available.

There has to be enough pressure to get your cutting edge under the surface. I have a notion that enough cutting teeth means one is always engaged and hold the tool against that surface when the next one arrives.

A scalloped surface may be indicative of a tool that is oscillating between rubbing and cutting. Everything bends while it builds up the cutting pressure and then blammo. To counter an oscillation you change feeds and speeds hoping to find a natural damping. You reduce overhangs wherever possible. You cut down hill to try and minimise the overshoot. If this were a production environment you might even consider unequal helix tooling.

[HopperParticipant @hopper]

Posted by Martin Shaw 1 on 10/02/2018 19:13:18:

…

Can't do much about the tool grinding…

YOu might be able to carefully dress each cutting edge with a slip stone, taking even amount off each edge.

I think some of these low-cost dovetail cutters are more suited to woodworking purposes than metal work.

Edited By Hopper on 11/02/2018 10:52:04

[John HaineParticipant @johnhaine32865]

Just a thought, are you sure that the dovetail angle is 60? I ask because I made a brass jib for an X1 mill and the angle was 55! Necessary to use my method of clamping the material in the dovetail to get the same angle.

[John HaineParticipant @johnhaine32865]

Here is a quick staged photo of how to hold a brass strip (1/8" in this case) in a dovetail so you can machine one of its edges to the dovetail angle.

On the left is the gib I made using the method. On the right I've clamped a length of silver steel into the dovetail angle using a toolmaker's clamp so that it clamps the gib material – in practice you would use 2 clamps, one at each end. Of course the slide itself would be clamped down to the mill table. To make the angle you just have to align the strip approximately parallel to the table X axis and machine off the square corner with an end mill. Then unclamp, turn the strip end for end and refit with the sharp corner you've just milled snug into the dovetail corner, with a mite of packing (~0.5mm?) underneath, and apply the clamps again. Mill down the second edge until very nearly flush with the top of the slide. You can smooth down the bearing face of the gib by draw filing.

[Martin Shaw 1Participant @martinshaw1]

Thanks for the explanation John, logical and simple. Anyone want a dovetail cutter, v little use? I was going to have a go this afternoon, but after our trip both my wife and I have picked up a nasty lurgy, so enthusiasm for the workshop is low.

Kind Regards

Martin

[Martin Shaw 1Participant @martinshaw1]

A number of folk have been exceptionally helpful, and as a thank you , I though I would post some pics of the Mk2 approach.

The first shows the revised set up, thank you John Haine, which works well. The second and third pics are the result of a few passes of a 14mm end mill, it looks a bit rough because it was, certainly somewhat better than the last attempt, but the cutter was putting a lot of vibration into the exercise. I suddenly twigged that the cutter was blunt, I can't recall what I might have used it on, but blunt nonetheless. I was going to use a 16mm cutter, regrettably the brand new, never unwrapped R8 cutter holder wouldn't take a 16mm cutter shank, so it'll have to go back to the supplier for a new one. A new 12mm cutter of course cut properly. The fourth pic shows the final surface finish, which in high enlargement looks rough but in practice isn't. What you see is really the striations of the cutter tips, which polish out on a bit of wet and dry. The cutting was done at 1200rpm, it felt right even if 1500rpm is a better speed, and the feed rate was a bit lower, either way it's all worked out quite well, so my grateful thanks to everyone who has contributed their thoughts, it's appreciated.

Regards

Martin

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