Have I made a mistake buying a MT3 mill?

[s dParticipant @sd79178]

Posted by old mart on 23/06/2023 16:13:54:

Now you have got the MT3 just be careful to not overtighten the drawbar. I expect the same price offer did not include the R8 model, so there should be more funds available for tooling. Many people keep an er32 collet chuck permanently attached and use it for drills as well as milling cutters. You can buy MT3 socket cleaners which will keep the taper pristine and tooling should be wiped with a clean, but very slightly oily tissue or cloth prior to fitting. There may be recommended drawbar torque figures available, and small torque wrenches are not expensive.

Haven't quite got it yet, only odred and paid for it. There's doesn't seem to be an R8 option. Good idea on the torque wrench, and yes ER32 was the plan, other than that I don't yet know what tools I'll be needing.

[Clive FosterParticipant @clivefoster55965]

its remarkably difficult to get authoritative information as to appropriate drawbar torque for draw in collets.

If you search hard enough you will find that Bridgeport advise between 25 and 30 ft lb for the R8 system, which doesn't seem unreasonable. I have, I think, 8 versions of the Bridgeport manual and only one gives a figure, in a rather hidden place too.

ER system torques are relatively findable. In the context of Model Engineer / Home Workshop machines the official torques to guarantee run-out specifications are scary high. Fortunately too much for the usually supplied sheet metal spanner to comfortably reach as the usual spindle lock components really aren't designed for the stresses of an ER25 or 35 system at full torque.

Essentially the requirement for safe cutter holding is to generate hoop stresses in the collet gripping the cutter equivalent to a moderate interference fit. Heat shrinking of cutters into their holders is popular in the CNC world. They temperatures involved suggest the holder contraction is moderate so presumably the forces involved can be calculated.

Because MT tapers are both relatively long and shallow enough to be self holding it can be presumed that the frictional forces between collet and spindle go up very rapidly once contact between collet and cutter has occurred. So the conversion of longitudinal draw force into interference fit equivalent hoop stress in the collet will be inefficient. I suspect that further tightening of the drawbar beyond a fairly moderate torque does just about nothing to increase cutter retention but massively increases the hold between spindle and collet.

In principle all this stuff can be fairly simply, albeit tediously, calculated to a precision adequate for workshop use and verified by comparison with what real world figures can be found for other systems. But its seriously annoying that there are no proper engineering figures out there.

Were I to use an MT3 collet system I'd promptly grind a decent section out of the middle of the MT3 section so only a short part at the front is actually compressed to grip the cutter. Something equivalent to the working section of an R8 should be ample. A fairly short section at the drawbar end should stabilise things OK so the collet actually pulls up straight. R8 has a simple cylinder at the drawbar end but trying to replicate that is way too much bother.

Out in the real world MT shanks with the centre third or more reduced so as to be clear of the spindle walls are not uncommon so clearly the full shank has way more grip than needed for many jobs.

As cutters with flats for sidelock "Weldon" holders are now fairly readily available at affordable prices it's worth considering whether using sidelock holders rather than collets makes economic sense. You don't need many sizes and being able to leave cutters in their holders ready to go has certain advantages.

Much more so nowadays when it's affordable to have a DRO system on the Z axis. Were I starting over and sidelock holders about 1/2 to 2/3rds current prices from the usual ME budget friendly suppliers going all in on sidelock with enough of each size to work in a manner akin to using a QC system on a lathe would be a no brainer.

Given that gripping area of an MT shank in the spindle is hugely greater than that of a collet on a cutter I imagine very modest drawbar torque would suffice to secure the holder against rotation.

Clive

Edited By Clive Foster on 23/06/2023 16:39:55

[old martParticipant @oldmart]

I always advise R8, "IF YOU HAVE A CHOICE", and that does not always happen, rather like the colour of a second hand car, I wanted a red one, but had to have dark grey. I have a plastic duck head on the aerial so I can find it more easily in car parks.

[SillyOldDufferModerator @sillyoldduffer]

Posted by s d on 23/06/2023 16:06:38:

Posted by JasonB on 23/06/2023 15:51:26:

If it is this one then I can't see any mention of brushless which would tend to say it comes with a brushed DC Motor.

…

It is, and you're right its not brushless… all the other ones I looked at were, but not that one. Wonder what else I've overlooked.

…

My advice is not to fret over Brushed vs Brushless. It's true Brushless is 'better', but the extra performance isn't a game changer. Brushed motors may be a notch less desirable than brushless, but they're still a good choice for a machine tool requiring good torque, low vibration and speed control. Main disadvantage is the brushes wear out, which will happen quickly if the machine is pushed hard, but in my experience they last well provided machines aren't pushed too hard. (Hint: hobby machines should never be hammered! )

The main thing I regret about getting into the hobby is the amount of time I wasted dithering about which mini-lathe was 'best' before buying one. With hindsight, I should have just got on with it. I learned more about good and bad by using one than by all my theorising.

True you might do better by shopping around, but the smaller hobby mills are much the same design, with the price reflecting moderate improvements or omissions rather than tool-room quality versus  heaps of junk. Avoid buying cheap direct from abroad though: if by mischance a lemon turns up, UK consumer protection may not apply to a direct purchase or be impossible to enforce.

Though it does what I need, my WM18 has several limitations. It's worst sin is being too small, which has to be forgiven because I don't have space for bigger. I can live with that!

Dave

Edited By SillyOldDuffer on 23/06/2023 18:58:57

[Howard LewisParticipant @howardlewis46836]

I have been using using a WARCO Economy (RF25 ) Mill / Drill with 3MT since about 1990.

The drawbar needs to be sufficiently tight to prevent the taper loosening, but not so tight that it needs more than a sharp tap with a 1.5 lb copper/hide mallet to break the taper.

For End Mills and Slot Drill upto 20 mm diameter ER collets are used in a holder with a 3MT shank. Where headroom is restricted, occasionally MT collets are used direct in the 3MT spindle.

Above that face mills, up to the 63 mm one supplied with the machine are used. Plus flycutters for larger sweeps.

The only thing that has jammed in the taper was an open ended 3/2MT sleeve. (I made an extractor once that had happened ) After that only 3/2MT open ended sleeves with extractor threads / nuts are used!

Howard

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