Cutting a blind keyway

[Pete O]

Being new to this milling caper, I think I need some advice on what appeared to be a simple task but my first attempt has come up no good.
I am in the process of making a shaft to replace one of the missing components of the power feed system on my mill.
The shaft has several features including two 1/4" keyways; one of these is 13" long and goes right to the end of the shaft, it drives a worm gear that moves along the shaft with the Y axis movement. The other keyway is about 1" long and drives a gear that is held in position by a circlip.
I spent about 2 hours getting everything set up, with the shaft held in 2 vices on the mill table, trammed in 3 dimensions. I found centre, locked the Y axis and proceeded to cut the long keyway first, with a 1/4" endmill held in an ER32 collet. This went okay, taking .020" cuts and a .005 finish cut, I'm quite happy with the way this came up and the key slides along the keyway no problem. (cutting a 13" keyway with no power feed is a pain, gotta get this thing working!)

I then did the shorter keyway, this is how I went about it and I'd be pleased to hear what I've done wrong;
I started by plunging the 1/4" endmill to depth at each end of the keyway, then backed out and took .020 cuts each way to the required depth; i.e. cut left to right, lift the knee .020", cut right to left, lift the knee .020", cut left to right etc. I zeroed the dials at each end of the table to give me a finish point at each end of the keyway. I also used plenty of cutting oil. Ran the cutter at approx 1000 RPM.
The finished keyway is a bit of a disaster; one side is straight but the other side has a curve, resulting in a bit of an egg-shaped keyway and a sloppy fit on the key steel. Naturally, this is almost the last operation on this part, all I had left to do was cut the slot for the circlip.
Tomorrow is Monday in my world so I won't get another go at this til late next week, I'd love to get some advice on how to get a satisfactory result before I try this again.

[RobWilson]

Hi Pete


Cutting left to right then right to left nearly always ends up with an over sized slot . 

Just remove metal in one direction of travel and at the end of the slot retract the tool completely  and return to the start position.  And repeat until required depth is acived.

Plunging to depth at each end can cause over size problems if there is play in the machine.

If you want a good fitting key . Set end stops use a smaller cutter to rough out then finish to size with correct sized cutter .

Probably clear as mud lol 


Rob

[Pete O]

Thanks Rob, that is perfectly clear. I thought I was being clever by plunging a hole at either end but obviously not.
So when I get another go at it, I will (1) rough it out with a smaller end mill and (2) cut in one direction, withdrawing the tool each time for the return and (3) plunge at the start of the cut just to the depth for that pass.
I'll have to figure out how to set up end stops on my machine, makes sense rather than relying on the dials.
Any suggestion what would be an appropriate size endmill to rough out with for a 1/4" finished size?

[DaveH]

Pete,
Just to say the way Rob has suggested is the correct way to mill a slot. It is also important to use a slot mill / slot drill and not an end mill. You probably are using a slot mill but you refer to it as an end mill.

This may help a little http://www.metalworkingfun.com/thread-776.html

 
DaveH

[RobWilson]

(09-11-2015, 07:04 AM)Pete O Wrote: Thanks Rob, that is perfectly clear. I thought I was being clever by plunging a hole at either end but obviously not.
So when I get another go at it, I will (1) rough it out with a smaller end mill and (2) cut in one direction, withdrawing the tool each time for the return and (3) plunge at the start of the cut just to the depth for that pass.
I'll have to figure out how to set up end stops on my machine, makes sense rather than relying on the dials.
Any suggestion what would be an appropriate size endmill to rough out with for a 1/4" finished size?

Hi Pete 

yeah you got it , I would go 3/16 , 5mm   to rough your slot out , I have some very short flute TC slot mills that  due to being short and rigid cut bang on 1//4 with no ruffing out .  

Long slots for sliding things I go for a woodruff cutter if I can ,like so . 



bags of coolant , lashings of the stuff 



and the result with a good sharp cutter .




So they are worth keeping an eye out for , or make your own (different tread) 








Rob

[TomG]

I would never trust an end mill to mill a slot the same size, even if was roughed out first with a smaller one. For a 1/4" keyway, I would use a 3/16" end mill to rough it to within .005 of finished depth, take a climb cut on each side an equal distance each side of center, check the width using gauge blocks then finish mill it to width and depth, taking spring passes until the key fits. Climbing is always a good idea when milling tight tolerance slots or grooves because any cutter flex will be to the center of the slot, not to the outside. You want the keyway to be snug enough on the key that it needs to be tapped in place.

Using a key seat cutter as Rob suggested also works well. Unlike an end mill, they will hold a slot to the proper width because they don't cut on the sides.

Tom

[PixMan]

I made my living for 2 years doing nothing but milling 1/4" keyways (closed ends) in shafts for envelope machinery. The shafts were 1045 steel and 1-1/2" to 2" diameter, clamped directly onto the center slot in the Bridgeport table.

The tool given to me to use was a stubby 1/4" 3-flute HSS end mill. It cut right to size every time and held the .2500" to .2505" width. As soon as the .2500" gauge block didn't fit smooth to the bottom and run end-to-end in the 2" to 6" long keyways, it was tossed and a new one took it's place.

Worked for me.

[EdK]

(09-11-2015, 07:57 PM)PixMan Wrote: I made my living for 2 years doing nothing but milling 1/4" keyways (closed ends) in shafts for envelope machinery. The shafts were 1045 steel and 1-1/2" to 2" diameter, clamped directly onto the center slot in the Bridgeport table.

The tool given to me to use was a stubby 1/4" 3-flute HSS end mill. It cut right to size every time and held the .2500" to .2505" width. As soon as the .2500" gauge block didn't fit smooth to the bottom and run end-to-end in the 2" to 6" long keyways, it was tossed and a new one took it's place.

Worked for me.

Hmmm, interesting. Clearly I've got a lot to learn. And that's a good thing.  

Ed

[f350ca]

I'll second Toms method. Wonder if your company was paying extra for sized cutters of some sort Ken.
Wonder if it was chip buildup that made your second keyway dive to one side Pete.

[PixMan]

I still have one of the stubby end mills. They were a standard stock end mill and given the nature of the small job shop couldn't have been expensive. I cannot recall the name brand on it but I'll check it when I go over to my son's home later this morning.

Most times I use Tom's suggested method because it's better to "creep up" on the dimension. The most obvious reason is that by the time you get to putting a keyway or tapped hole in a given part, you've got the bulk of the time and effort into it. No need to take chances. The job I was doing was a constant repeat so the process was well documented and repeatable.

I was just pointing out that I "never say never." Or, very rarely do.