How can I mill a small slot and reduce tool deflection to keep it square ?

[picclock]

I know that sounds barmy but let me explain the issue. When cutting slots to adjust final positioning during assembly, these always look off. The reason is that the torque from the cutting mill introduces a deflection. So that if cutting a 4mm slot, the tool pulls in opposite directions in each side of the slot. This always leaves the slot with a lopsided appearance. 

This does not happen with larger endmills because the tool deflection is much lower, but sizes from 6mm and under seem to suffer from it.

The picture below shows a 4mm slotted hole in 1/8" (3.1mm) mild steel with 0.3mm deflection occuring resulting in a slanted slot.

I have tried various methods, always starting with a drilled hole. Inserting the mill through the material and doing it as one full depth cut, taking a succession of tiny full depth plunge cuts of say 5 thou etc. On the whole these have had little or no success.

Any help or tips much appreciated.

Best regards

picclock.

 

[philf]

Picclock,

I usually use short carbide tooling to minimise deflection. Even then I do it with several light cuts. If I'm CNCing a slot I will use a smaller cutter and then use a finishing cut to follow the profile.

Phil.

[AdeV]

Also use a 2-flute end mill (or "slot drill" as our USonian cousins refer to them), apparently they're less liable to wander compared to a 4-flute cutter.

Also, why not cut the entire slot in a series of smaller DoC cuts? I've slotted down to 2mm on my manual mill (2mm slot, ~2mm total depth in 4x 0.5mm passes at a fairly conservative feed rate - I didn't want to break the cutter!) and got a lovely straight line. Admittedly, that was in aluminium, but I've cut small (<6mm) slots in steel before and never had that trouble

[picclock]

@ adev
Thanks for the idea, but I have tried 2 flute endmills, and if anything it seemed worse. This may be because the core is thinner or it lacks the support of the extra flutes. If I get a chance I will do a direct comparison of 2 to 4 flute and see how it pans out. This problem always seems worse with small sizes, generally 3-6mm.

@philf
I should have thought of that but I seem to have an aversion to carbide because they break.
The HSS endmills I use normally have a 6mm shank and smaller dia flutes, which I would hope would improve the stiffness. The carbide mills I have generally have the same shank/flute diameter. I will give them a try and lay in a supply if it works out. I seem to have issues with breaking small carbide mills, not normally in use, but just by dropping them or removing them from a tight collets or generally just being clumsy.   Last one I broke was 10mm, 3 flute and managed to snap the flute off. Indispensable for really hard materials, but definitely kid gloves for handling, especially below 6mm.

Many thanks for the ideas and assistance.

Best Regards

picclock

[WeldingRod]

The more flutes, the stiffer the end mill.
I've had good results with those diamond cut carbide burrs, used as mills.  Not a super smooth surface, though.

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[efrench]

You left out the most important thing: What size endmill are you using to cut the 4mm slot?  If you're using a 4mm endmill it's virtually guaranteed to wander.

[appletree]

The correct way is to use a milling cutter smaller than the width of slot you want. That way you have control of the size of cut and the depth of cut too. You can take as many “spring” cuts as required and machine to actual finned size.


Phil

[John Rudd]

what about starting out with a small hole at each end then finishing with the endmill of appropriate size..? this works but if the slot is pretty short then it could be a bit difficult..

[picclock]

Basically sorted it by using Carbide endmills and making sure the cut is close to the top of the tool. Seems to work well.

Many Thanks

Best Regards

picclock