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suggested milling speed/depth of cut

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Stovepipe08/02/2011 19:15:00
196 forum posts
May I ask for your suggestions and opininons, please, gentlemen.
 
I have a Warco Mill/drill rated at 550 watts. I need to cut a 10mm rebate on a 1"
steel bar. . It has a maximum low speed rating of 1200 rpm, and a maximum high speed rating of 2200 rpm. I'm currently using a 10mm cutter to cut the rebate.
 
My question is, should I use a smaller cutter to cut in more than one pass, and what depth of cut should I use ?
The advice of those much more knowledgeable than myself would be much appreciated. (Probably be hard to find someone less knowledgeable than me !)
 
Dennis
JasonB08/02/2011 19:35:43
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Hard to know as you have not said how deep the rebate is.
 
Given that its a small mill I would say use a 12mm cutter, cut the full 10mm width of the cut in 1mm deep steps and if the machine complains reduce the DOC. Run at about 400rpm in the low range, again if it complains slow things down
 
J

Edited By JasonB on 08/02/2011 19:36:31

mgj08/02/2011 19:43:58
1017 forum posts
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Well a slot drill should cut to a max depth of d/2. Cutting speed will be about 100fpm, and feed will be a tooth load of about 3-4 thou per tooth.
 
Rpm will be (3.5 x f/min)/D in inches. (It isn't actually 3.5 - but its close enough and 3.5 I find easier to remember.)
 
Feed will be tooth load x no of teeth x rpm x D. Thats dimensionless, as long as you keep to the same units all the way through.
 
Yes you can cut a 10 mm slot with a slot drill, in one pass for width. But you will end up with a finish like a badgers bum. I'd suggest something like an 8mm cutter, take out one trough to depth less a smidgen, get the slot tonear width less a smidgen, and then take a finishing cut on each side , and to depth - 5-10 thou wth a good sharp cutter .
 
Important if you want the slot to be accurate for width - do it in 5 cuts. One full width of an undersize cutter. This is a roughing cut. Measure. Next cut is half the error divided by 2, because you are going to do that to both sides. Measure, and then apply the final cut - again 1/2 the error on each side. What that slightly cumbersome system does is to ensure that any deflection in the tool on the final cut is approximately the same as the deflection before the last correction.

 
I'd also suggest you ensure you upcut mill on both sides as you finish, so you go up one side feeding aginst the direction of rotation, and then down the other to keep feeding against the direction of rotation.(Unless your mill has backlash eliminators and they are properly adjusted.)

Edited By mgj on 08/02/2011 19:45:20

Edited By mgj on 08/02/2011 19:47:06

JasonB08/02/2011 20:10:34
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I thought it was a rebate not a slot? Thats why I suggested a slightly larger and more rigid cutter.
 
J
Stovepipe08/02/2011 21:09:52
196 forum posts
Sorry, folks. Should have made  it clear that it is a 10mm x 10mm rebate..
 
Dennis
 

Edited By Stovepipe on 08/02/2011 21:16:18

mgj08/02/2011 22:54:00
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Sorry assumption. My apologies - but I'd still do it the same way if I wanted it accurate, but agreed, one could use a bigger cutter.A rebate is just 1/2 a slot
 
In which case one would adjust RPM and feeds according. You can speed up if one is using carbide or cobalt cutters too, so long as the job and the machine will carry the vibration. What you don't want to do is cut the tooth load, because that provokes rubbing.
 
Rebate you can use an endmill of ocurse, and the max cuts are d for depth, and d/4 for sidefeed. So for a 10mm endmill you'd take a cut 10mm deep top to bottom, and 2.5mm or so on the side helices
 

 

Stovepipe08/02/2011 23:20:18
196 forum posts
Jason, Meyrick,
 
Thank you both for your advice, it is is very helpful and very much appreciated.
 
Dennis
JasonB09/02/2011 07:25:34
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Posted by mgj on 08/02/2011 22:54:00:
 
Rebate you can use an endmill of ocurse, and the max cuts are d for depth, and d/4 for sidefeed. So for a 10mm endmill you'd take a cut 10mm deep top to bottom, and 2.5mm or so on the side helices
 
It very unlikely that a 550watt machine will cut at these figures, removing 10mm x1.00 is likely to be a maximum and depending on how the gibbs and spindle are adjusted is liekly to be less.
 
Jason
Ian S C09/02/2011 08:33:35
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Can you perhaps cut out some of the metal by hack saw/ band saw, or even drill ot a good percent of it, then mill it, take it .5mm at a time, keep the feed up, and about 400/ 500rpm. Ian S C
mgj09/02/2011 08:55:05
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Jason - agreed.
 
I was really just stating the maxima for a cutter. These are Tubal Cains figures for long cutter life - One could, if the machine had the HP/rigidity drive them harder at industrial rates at the expense of cutter life.
 
Stovepipe. Could I suggest you buy, if you don't have it already, The Model Engineers Handbook by Tubal Cain. It has all this stuff, and almost everything else in it too. o ring sizes for seals, piston ring fits, milling cuts, basic electrics, rotational fits, keyways. All that stuff. Very useful book.
KWIL09/02/2011 09:09:04
3681 forum posts
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Dennis, I have PM'd you about a useful chart.
Stovepipe09/02/2011 11:06:08
196 forum posts
KWIL,
PM replied to, thank you.
 
Dennis
KWIL09/02/2011 12:03:36
3681 forum posts
70 photos
Glad they were useful
 
K
Stovepipe09/02/2011 13:36:29
196 forum posts
Gentlemen,
Many thanks to you all for your help and advice on this topic. It is indeed appreciated for a new boy to milling.
 
Dennis
Jeff Dayman09/02/2011 14:49:38
2356 forum posts
47 photos
I would try a 1/2" dia slot drill at 700-1000 rpm, full 10 mm width of rebate, with .010-.015" depth of cut per pass, feed it as fast as you like until the finish suffers or it starts vibrating. I have a similar machine but a bit bigger and these settings work great for general milling on it, and have for many years. I'm still using many cutters I bought years ago, cutting mostly steel and cast iron, and they last fine with above settings. I do buy good quality HSS cutters though, no cheap junk.
 
If you can saw some material away first, should always do so, there is no faster way to remove metal than sawing chunks away. Hacksaw or bandsaw are ideal for this.
 
Forget the mgj theory. The numbers and rules he quotes are for big industrial machines, not for light, low power mills.
 
JD
mgj09/02/2011 18:15:06
1017 forum posts
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My authority for what I said. Tubal Cain and the Model Engineers handbook - where, as an engineer of some experience, he specifically says that the rates he quotes are for good tool life and light machines.
 
I'm delighted that you know so much better than he - I'll be sure to ask you for advice on such matters in the future. The rest of us just used those figures when all we had was a Myford and a vertical slide, and found they worked very well.
 
Now I have a proper tool and cutter grinder plus a lot of indexable tooling on a big mill I use the industrial rates suggested by the people who make the tools. Given your obvious knowledge, I was surprised to note that they are substantially higher than those suggested by Tubal Cain - if you were right, they would not be?.
 
The difference is not generally in depth of cut or cutting speed. What does make the difference is the tooth load, or thickness of cut. (feed rate) in any given material
 
The real answer is of course that with a bit of experience you get to know when a cutter/material combination is going right, and that it will vary from machine to machine, and with the amount of overhang and support. What you can do with a normal cutter say compared to a long series one, and so forth. However experience is what the beginner doesn't have, and he has to start somewhere.
 
TCs figures are a pretty fair place for the amateur to start, and have deen doing so reliably for some years.
 
Incidentally, if you do the maths for the example I gave you'll find TC suggests 760rpm, and my example suggests 700, no higher. That is low on your recommendation. So much for industrial rates?



Edited By mgj on 09/02/2011 18:21:18

KWIL09/02/2011 19:05:52
3681 forum posts
70 photos
And so say all of us!
Nicholas Farr09/02/2011 20:50:33
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Hi, if you have access to MEW No's 10, 11 and 12, milling for beginners 1, 2 and 3 may be worth a read.
 
Regards Nick.
michael cole21/02/2011 18:16:54
166 forum posts
Use the biggest end mill you got.

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