Proper way to cut a Slot

[oskar]

I want to cut the slot shown on the attached and I’m not sure if it’s ok to plunge slowly straight down in several passes and if the end mill I have is the proper one.

The aluminum bar is 2”x5.5”x0.5” thick, the slot is 4.24” long (should go down to 0.5” deep), the end mill is 2 flute, 0.25” OD, 3/8” shank. I have a piece of pressboard under the bar to avoid damage to the vice,

I have drilled the holes shown in the slot to speed up the milling time; they all are with a 7/32” drill except the 2 in the center are with a 17/64” drill. I was thinking to lower the end mill to a 17/64” hole and then move the axis left / right to complete each pass.

Do you have any suggestions?

 

[mikey]

I would use a 3/16" roughing end mill but assuming you don't have one and the slot width is not critical then you can use the 1/4" finishing end mill. I would use a 1/8" depth of cut, speed as fast as the mill will run and feed so you feel a slight resistance to the feed as you turn the hand wheel. Repeat as needed.

For slotting, you can feed in either direction; there is no conventional vs climb milling when slotting. I normally start on one end and feed in a consistent direction but that is just preference.

If you can, use air to clear chips and lots of WD40 for lube.

 

[T Bredehoft]

MIke is right, don't try to take it all at once. The best rule is to baby your cutter, pretend it's glass and will break at any time. 'Cause it might. And keep the chips out of the slot as best you can. The reason to use a 3/16 cutter to make a 1/4 in slot is that a quarter inch cutter will 'bend' and take out wider than .250.

 

[David S]

The other thing to pay attention to is how the work piece is being clamped. Since the vise is camping in the area that you are going to mill straight through, you don't to want it to tighten onto the cutter as you break through or alternately have the workpiece loosen in the vise.

David

 

[Cobra]

If you cut the slot with a 3/16, you can then move over to each side and climb mill the sides of the slot to improve the finish. If it is important to have .25 round ends on the slot, plunge cut the two ends with your .25 and then use the 3/16 to mill the slot and finish the sides. As stated earlier, take it easy with the depth of cut as you cut the initial slot but drop the mill down to cut the entire depth when you climb mill to finish the slot.

 

[oskar]

Thank you all

Sorry David, I don’t understand what you mean. As far as I know the vice holds well and there is no interference with the cutter

Mike I do have a 3/16” shank 4 flute double end endmill with 1/8” OD at each end (total endmill length is 3-1/8”). Perhaps I can use this one? Your tip about the feed direction is excellent because I was wondering about it and now it is all clear.

 

[P. Waller]

Place the work on parallels so that the chips drop through, 1/4" will be more then enough.

 

[Meta Key]

As far as I know the vice holds well and there is no interference with the cutter

I'm not David but I think what he is getting at is that the vice is squeezing the part (with considerable force) just where you are removing material. Thus, as the material is removed the remaining stock may bend slightly inward. So, if you make a finishing pass you may find your slot is a little wider in the middle that at each end due to deflection of the material caused by forces imposed by the vice.

Does that make sense? Not sure I'm explaining it very well..

Anyway, looking at the picture you posted earlier I think it would be better to clamp the stock down. Another alternative would be to use two pieces of steel, same length as your stock, between the stock and each vice jaw. That would help distribute the clamping force across the part, including the ends where stock is not being removed.

Maybe the tolerance for the slot width is such that it wouldn't matter. But, if it does matter, I'd clamp it down at each end with the stock raised up on parallels. That totally avoids the possibility of the vice clamping pressure deforming the stock during machining.

Meta Key

 

[David S]

Thank you all

Sorry David, I don’t understand what you mean. As far as I know the vice holds well and there is no interference with the cutter

Mike I do have a 3/16” shank 4 flute double end endmill with 1/8” OD at each end (total endmill length is 3-1/8”). Perhaps I can use this one? Your tip about the feed direction is excellent because I was wondering about it and now it is all clear.

Hi , What I mean is that your vice jaws are fairly narrow and are gripping the solid work piece in the middle. My concern might be is that as you cut through to the bottom the work piece is no longer solid and may squeeze in a bit, and perhaps come loose.

David

 

[oskar]

Now I get it David and you have a point

The bar is 1/2” thick and I don’t think it will be squeezed by the vise however I will try to see if I can use clamps to hold it down because the parallels under it sound a good idea too. In any case the tolerance of the slot is not critical