Milling a T slot - am I doing it right?

Here's a quick one for you. I'm in the initial stages of making a cylindrical grinding attachment for my surface grinder. It involves milling a tee slot along a piece of 70mm x 40mm BMS. I've started by creating an 18mm deep x 12mm wide slot without problems, although I did buy a carbide 10mm roughing end mill to do the major metal removal. To mill the cross "T" part, I've used two of my tee slot cutters in the set up shown below. Progress is painfully slow and at about one third down the slot, the mill actually stalled. My question is: am I using the tee slot cutter correctly or should I be nibbling away at the cut with a smaller diameter cutter? I should add that both cutters have previously only been used on cast iron and looked to be fairly sharp. ( Not now, though! ) I'm using a spindle speed of, I estimate, 5-600 rpm and feeding at about 150mm/min. I've ordered a new cutter from Arc, which ought to arrive tomorrow, but I thought I'd make this enquiry on here, before using it in anger on Wednesday.

John

Does your design allow for cutting the vertical part of the 'T' a little further? Run the 10mm cutter through 0.2mm lower than the last cut. That way your slot cutter won't have to cut the entire width of the horizontal part.

This is a theoretical answer; while I've got a fair selection of T-slot cutters I've never had the need to use one yet. Feeds and speeds seem reasonable, albeit quite a low chip load. I've got two comments. One, I suspect there's a fair amount of flex in the tool spindle; not good as the cutting edge is long and forces will be hgh. Second, as an estimate I reckon the cutter is removing about 1.2 cubic inches of material per minute. I don't know what mill or drive arrangement is being used, but that removal rate will require comfortably over 1hp at the tool.

Another couple of thoughts. If the material is cold drawn BMS has it gone slightly banana shaped due to the initial machining, thus causing the T-slot cutter to jam? It's normal to cut the initial slot slightly deeper than the T-slot so that the T-slot cutter only cuts on the sides and doesn't rub on the bottom. I can't see if that is the case here.

Andrew

Nicholas and Andrew,

Thank you both for your input. I'll try all the suggestions and report back. For further info - the Mill is a Warco VMC - so reasonably substantial with a 1½ HP 3-phase motor/VFD combination fitted by me. No rev counter fitted so I'm just going by years of looking at engines idling at 800rpm to estimate the spindle speed!

John

Hi John

You don't say if you are using cutting fluid or have air jets to blow away the cut chips, both of these would help reduce cutting forces on the tool.

When it stalled part way into the cut I guess the feedrate was too fast so too much chip load for the power available, perhaps another 50 rpm would have prevented the stall ?

Emgee

Emgee,

No, no cutting fluid, just a squirt of cutting oil as it progressed. I don't have compressed air in the garage, either, having given away my compressor when we moved back to the UK. The chips are coming off hot, but not blue, enough to be painful when they land on my hand. I stand back a bit further, now, while wiping chips away with a brush.

John

Edited By John Hinkley on 08/07/2019 12:46:41

That might be the problem. First, you should be able to get the VFD to show the nominal output frequency which would give a fair estimate of motor speed, given the number of poles in the motor. If the speed of the motor has simply been reduced by reducing the frequency, and any belt or gear drive not changed, then you may well have a lot less than 1.5hp at the tool. If anything I'd reduce spindle speed a bit and stay with the feedrate. Provided of course that you're getting full power at the tool.

Andrew

It can help to make the initial slot a touch deeper than necessary.

Neil

Although the picture is not very clear the cutter looks more like a Woodruff cutter than a Tee Slot cutter. Tee slot cutters in this range usually come with staggered teeth and with a lit more of a "Gullet" to help clear the chips.

I would not be running a Tee slot cutter of this size above 300 rpm and there is a real need to clear the chips. Flushing with coolant or an continuous air blast is a must.

Regards

Gray,

I suspect the cutters (including the original 10mm) choked on the chips, adding oil makes them sticky and even less likely to clear.

Even cutting a 3" wide 1/2" deep dovetail slot I found chips getting trapped under the cutter. Your slot is very narrow with nothing to clear / wash the chips out. The further down the cut the worse it may get. Milling half of the slot from the other direction might help + a shop vac to suck the chips out in lieu of compressed air to blow them out.

I would cut each side of the slot separately. Using a smaller cutter first, that way you are more than halving the load on the cutter and allowing the chips to fall away.

Edited By Hollowpoint on 09/07/2019 16:39:24

Graham, Dave and Hollowpoint,

All good, valid points and I will try to incorporate all your suggestions. We've been out all day visiting York and when I got home, found that the new cutter hadn't arrived, so hopefully it will come tomorrow. As for clearing the chips, I'll have to rely on a brush and the Vax vacuum. I think I'll slow the feed rate down a little and also the cutter speed and see if that helps. Time, if not on my side, is not of the essence.

John

A quick update for those interested........

The new cutter arrived yesterday, too late to do any serious work with it, but today I managed to finish off the first cuts. Unfortunately, the shank of the cutter fouls the top of the tee slot when lowered to remove the lower portion of the tee. I'm going to try to grind the shank down to match the section behind the cutter to obtain the required clearance. In the meantime, I've sharpened the old cutters using the setup below:

It works quite well and produces a good, clean cutting face.

John

Great minds... although my version is a tad less elegant!

Nice one, Neil. Elegant or not, if it does the job then that's fine. It's the result that matters.

I managed to reduce the cutter shank as I outlined above and the resulting profile looks like this, the original on the right and modified profile on the left :

I wasn't able to replicate the production tansition slope with the setup I had, though with a bit of fiddling and changing angles, I could probably get it a lot closer.

Graham - you should be able to see from this shot that the teeth are staggered, so it really is a tee slot cutter!

After applying all the advice given, to a greater or lesser extent, I've finished the tee slot and am pleased with the result. Thanks to all who contributed. I have to say, tee slot cutting is not my most favourite procedure and I will try to avoid it in future designs.

John