To do or not to do, can it be done? MK4 drill steel.

After spending a few months reading and studying... I have finally advanced to the level of "I know what I don't know"... maybe.

I've watched all the videos of the MK4 and think the dual box design gantry may serve my purpose. Here's the pie in the sky.

A multipurpose machine, mill, drill and plasma. Forget the plasma part for this discussion. I would like to bore mild steel 10 - 15 mm plate. I figure I'd probably have to change out the z axis spindle depending on the operation, a lot of details would need to be looked at there but now my question?

Could a dual box design like routercnc MK4 generate the forces needed to drill a steel plate using an annular bit? Holes would be 10 to 20 mm. I've got a need to drill large plates 1200 x 1200 mm.

Further spec's on the machine would be, milling plastic, aluminum, and wood.

Any help on threads, builders etc... would be appreciated.

BTW kudos on the MK4 that is some impressive work.

With the right spindle on my machine it probably would, but the reason I went with that overall layout was I wanted a large cutting area for sheets of wood, but still be able to mill with it.

If you don't need anything that large then a either a regular mill style (column at the rear XY table in front), or a fixed gantry with moving table would be better.

Either way you would need a spindle with high torque and low rpm, in the hundreds of rpm for the annular drilling.

Fixed Gantry won't give me the versatility. I'm thinking of a spindle motor/gearbox like from a mag drill. The delimma is z axis design. Yours is the closest to what I've imagined but can it be modularized. Something I could swap out for high speed operations. I've even considered electro magnets to lock it all down during drilling. It's probably not necessary with your design although I don't think I would use any aluminum.

Thank you for your thoughts. You've built a great resource here.

Depending on the accuracy you need to hold over time if you swap out a spindle then it might not go back in fully trammed. Maybe you can provide some kind of alignment pins or something, but it needs some thought.

Therefore another option is to have 2 spindles - have the low speed spindle in the main housing. Then on the lowest part of the Z axis have a thick plate hanging out the front with the high speed spindle in. This is cantilevered out, which is not so good, but might be good enough for the high speed lighter work. Some Bridgeport owners do this to give both options. Like this but hanging out the front ahead of the gantry beam:
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If you want all steel construction have a look at the Mori Seki 5000 for ideas:
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https://www.youtube.com/watch?v=XCET5zkA4cU

Yes I've seen the Mori. A feat in machining. On a more realistic feat, at least for the retired hobbyist I got the idea from https://youtu.be/8OTwT0WFBmg Adam Savage Test series. A dual head plasma router. Kind of like you suggested but I don't think it could handle the forces. I'll work on some concept drawings and get some opinions. Thanks again.

What kind of accuracy are you seeing between table extremes?

I'm thinking of something to fulfill not completely dissimilar requirements for my next "big" machine. My current solution is a moving gantry with 2 Z-axes - a high speed cutting spindle on the front of the gantry and a slow speed drilling/tapping spindle on the rear.

Voicecoil, sounds a lot of what I'm thinking. I've considered the front / back idea would be interested in hearing a consensus on this idea. Have you done any drawings?

I've got so little experience with these machines I don't know what to expect. That's why I'm here. To glean whatever I can.

No drawings done yet I'm afraid, it's just an idea in my head at the mo., based on what I've learned from making my little machine. I can tell you that it will have a pretty massive "extrusion" for the gantry though (likely 200 x 130mm), and will use the same dual ballscrew Z-axis arrangement to minimise the offset between the gantry and the spindle - even more important in this arrangement if you want both spindles to be able to cover a decent working area whilst keeping length to a minimum.

I assume you are talking steel gantry. That's heavy. What size ball screws? What are you going to use for a spindle on the drilling axis?

I ran a little test today and it requires between 300 - 500 pounds to make a 25 mm hole with an annular bit. Most mag drills have a +/- 1200 watt motor. Sorry for the conversions, I just took my bathroom scale and centered it under a piece of mild steel on my drill press. I couldn't find any data discussing the physics of annular drilling.

Well no, I was thinking of using aluminium, but it will still be quite heavy! Ballscrews will likely be 1610 or 2010, haven't really though about the drilling spindle as yet. Your test on the drilling force (which is pretty huge) probably means a front/rear approach isn't the best as a couple of quick sums indicate that the twisting moment on the gantry is likely to cause significant errors - a "dual rail" gantry as in routercnc's design will obviously not suffer from that, the main deflection would be an upward bowing which shouldn't impact hole position too much.