G.Day each,here we go again.
I did promise that i would explain what is happening during Slot Cutting.
Now its not my intention to teach Granmar how to suck eggs but what i am trying to explain is what is missing from Technical Journals.
I am not saying that it is not there but that i have never seen it,so here goes.
In diagram No.1 we have a 4 bladed milling cutter cutting a slot in a piece of metal.
This would be the view looking down on the work.
Cutter blade 'A' is cutting and it is also traveling into the material.(remember this)
Cutter 'B' has finished its cutting.
Cutter 'C' is doing nothing.
Cutter 'D' is just about to start cutting.
The forces set up by cutter 'A' will force the cutter to move over to the left causing 'D' to start its cut deeper into the side of 'L'.
At the same time cutter 'B' is relieved from its cutting of side 'R'.
Now this is not a sudden condition but is gradual during the whole rotation and movement of the cutter into the work.
This operation is the cause of rough sides to the slot and an increase in the width.
Now have a look at diagram No.2
This is what is happening when the cutter has broken out from the slot cutting and is still advancing only there is now lighter forces in play to prevent the cutter from advancing so it is usual to find that the cutter moves forward further than required.
Now this is the most dangerous time for the cutter.
Look at diagram No.3
This is the shape of the remaining material during the brake out.
The cutter 'A' is about to come onto contact with the remaining material on the right.
See how much it is expected to cut on contact,plus the fact that the whole milling cutter is moving forward unopposed.
Now i will lay you odds on that this is where your cutter teeth broke off and you were at a loss to know why.
Its happened to me and to every on else.
This is one of the reasons for not using a milling cutter to cut slots.
Now i will not use the word 'never' as there is always the exception.
You may have come across a 3 bladed milling cutter,these are ok but not very common.
The other time to use a milling cutter is when you have not got a slot drill of the required dimension.
You can cut a slot using a smaller diameter milling cutter to cut the slot leaving the sides to be cut to the finished size.
Of cause this will entail extra work,time and patience but it can be done.
Now what happens when we use a Slot Drill.
Slot Drills are what they say they are,cutters for cutting slots and will also drill,but be careful here they are not like your common drills.
Where as a milling cutter will not drill out any thing above 10 to 15 thou the slot drill has no qualms about depth,but remember it is not a drill.
It comes into its own when cutting a slot that ends within the bounds of the material therefor does not come out at the edges.
Here you allow the cutter to cut to the depth you set for the first cut and away you go.
When you get to the end you cut a little deeper and cutting a deeper slot return to the start.
The depth of cut will depend on many factors that you will have to adhere to ,material,size of slot,quality of the cutter,milling machine,finish required,
cutting liquids( not cast iron) removal of swarf,etc,etc.
Lets take a look at diagram No.4.
Here we are set up as per the other but using a two bladed cutter,or slot drill.
When i first came across the slot drill i thought that it was a cutter intended for routing wood and the like.i mean who would foolishly try cutting steel with a cutter with only two blades when milling cutters have at least 4.
I was to learn!
As you can see the two cutters are 180 deg.apart meaning that when one cutter is engaged the other is free.
This is simply a double fly cutter.
Due to the fact that only one cutter is engaged at a time any displacement of the whole cutter does not cause any other cutter to engage in the side of the work.
So cutter 'A' starts its cut and in so doing causes the cutter to go the the left a few thou but cutter 'B' is free and does not come into operation until cutter 'A' has finished its cut so no problems.
By the time cutter 'A' arrives at the right side it is cutting less therefore less force is acting on the whole cutter now allowing it to resume its original center.
This cutting action gives the required close dimensions required in the slot.
But! please remember if you are cutting a slot out to the sides of the material,slow down.tighten up the Gibbs and keep swarf out of the way.
I am indebted to a gentleman i use to talk to when the annual exhibition of the Model Engineers was held at the Seymore Hall in London during the 50s.and early 60s.
He was always dressed in a white cow gown and making use of a Myford Super 7.
He would chat with any one about engineering problems.
I miss him.
GerryB
