Manual Mill Design Help

[GEspo]

Hello all. I have some large setco slides(x/y) 12” travel ea, along with the cast setco right angle 24”x12” (large).. so i could possibly mount a mill head to the angle and then the angle to slides and have a traversing mill head, id make the table/fixture plate stationary, OR i could mount the head on the right angle stationary and have a traversing table(like a standard knee or benchtop mill). So my question is:

What is a better design from dynamics/mechanics and just better for milling? something tells me a traversing head is better since modern machines have adopted.. thx for the help with this

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[peteeng]

Hi GE - Depends on some dimensions of things. A sketch with some dims would help. Peter

[GEspo]

Hi GE - Depends on some dimensions of things. A sketch with some dims would help. Peter

Hello. Heres a few pics, the sides are 24”x12” with a 12”x12” carriage/saddle, the angle is 24”x12”. Ideally the mill head would extend 12” or so but less may be ok too, could resemble a bed mill type head attachment. Thanks

[GEspo]

Also as far as weights go, the slides are about 220# ea and the angle is about 200#.. the mill head will be a bridgeport j head 1hp about 225#

I’d make a bed, probably a steel box design with damping materials

[peteeng]

Hi9 GE - Your travelling mill head is commonly called a travelling column configuration. It has a stationary bed. The knee mill has a moving table. The travelling column is used when the parts are long so the moving table becomes very big and clumsy. The advantage of a moving column is that the job stays still so the machine can be designed optimally as the part weight can be discounted from the calculations. The knee configuration is good for small parts so to answer your question: it all depends on the size of part you mainly do.... I prefer the moving column it also gets the mechanics up out of the muck... Peter

[GEspo]

So to generalize: if the table and workpiece weight exceed the head and column weight a moving head is better, and vice versa for moving table.. (assuming workpiece size is not a consideration)

Ive also read that whatever has more vibration should be kept stationary. If the end goal is a more Rigid complete system, then i suppose id have to figure out which one is more rigid and secure the other in that case.

I guess we could also say that whatever is heavier is going to vibrate less, but that would disagree with where we’re headed with this..

Somehow i feel like im making progress in figuring this out.. reminds me of my college Physics 102 class.. guessing a FBD will probably solve this, ha

I think i need to go back to p = mv and start over..

Thx for the help

[peteeng]

Hi GE - No the workpiece and table weight is not related to the head and column weight. Weight is rarely considered early in a design exercise as it is not important at this point. What is important is the size of the part you want to make (if that part is very heavy then usually you go fixed bed so the foundations take the weight vs the bearings and the part inertia could be an issue. Plus all of these "heavy" and "long" words are relative eg for Hyundai long is 50m for me long is 1200mm etc, the footprint of the machine is an issue vs the space you have available.

So the first order design principles of a machine are the max envelope needed for machining, the machine max footprint if space is an issue, then what material or production processes are to be used for the machine (cast iron, welded steel, mineral cast, beryllium, plywood etc ) then what materials are going to be processed as this dictates the machines rigidity requirements, then max rapid speed if its a production machine as rapids dictate how fast the jobs get out the door this is hand in hand with feed speeds and a target acceleration for the motion. Once you get those things in order you can choose a machine config and start on the structures. Once structures are defined you can move to the motion studies, this sizes the motors and type of motor needed. Then your onto motion control and electronics and you've broken the designs back... Keep at it... Peter

If the part is long the knee configuration becomes too big as the table moves. The moving column config minimses the footprint and makes enclosures and swarf management easier as well. Machines are usually designed for a purpose so its configuration is derived from its purpose. If the machine is a general machine with no particular purpose it becomes difficult to make these sort of decisions. That's why general machines have evolved to certain configurations plus these are optimised on cost considerations as well.

Free body diagrams are sadly lacking around offices these days. We can jump to FE straight off the CAD file and run stuff. But garbage in is garbage out and a correct FBD is useful for sorting the trash....

edit - mass and vibration are not strongly related. A suspension bridge is very heavy yet can get excited with a gentle breeze blowing over it. Same with a machine. Vibration is a big topic make the machine extremely stiff with thick parts and vibration is not an issue.

[GEspo]

This is very helpful. I will look over and begin evaluating. Can you recommend any books for machine design? Thanks again.

[peteeng]

Hi GE - Heres some

https://my.mech.utah.edu/~bamberg/re...e%20Design.pdf a bit old but the principles are good. His conclusion is biased I feel if you read the whole thing he seems to step sideways in the conclusion. Its a sponsored work so maybe it was favoured somehow. But principles are correct. Machines have become extremely stiff since he laid this work due to FEA becoming mature and machine elements like linear bearings becoming so good.. a modern VMC is well over 150N/um these days.

enjoy - Peter

[peteeng]

this will help when you get to motors - Peter

[peteeng]

Hi GE - Here are the two machines you are discussing. I'm tending to the LHS image. I want/need to machine gantries which means the machine will be long & thin. Plus the mechanics are up out of the muck and the part is stationary. This means the dynamics do not depend on part weight, which means the machine can be optimised a little better as the part weight is not a variable... Lots to digest welcome to the chase of the Grail... Peter

[GEspo]

Great thank you. I did read the Bamberg MIT paper and found it a good introduction. One thing I couldnt find was: the "top" of his mill looks about 4" thick and I see no good description or specs on what was actually used. They lead us through the design process with a stated 2" thick steel top, however what's on the table doesn't look to be 2". So not sure what happened there or if I just skimmed it, but I"m guessing that top provides a substantial increase in weight and stiffness. Also am curious as to what material is used. As for the pics above, the LHS image, who makes that mill? I'd like to take a more in depth look.

[peteeng]

Hi GE - DMG Mori Seiki for both. Re STG my guess is steel its part of the fabrication. Peter

[GEspo]

The MR-1 from Langmuir looks interesting. The link below describes the base:

https://www.langmuirsystems.com/mr1/base


A standard type concrete is used. I'd be concerned about it shrinking back, is one reason why im looking at epoxy anchoring cement $$ in my application(not that I want to spend the $$ just dont think Id risk it)

[peteeng]

Hi GE - yes interesting but flawed in some ways 1) std concrete will crack over time 2) it provides mass but the load paths are still thru the steel structure. See how they go... Peter

[GEspo]

Jumping back on this. Looking a bit at mid to small vertical cnc designs, I see Haas VF series use an xy table with a spindle traveling in z. I called a Haas parts supplier for a cost on the belt driven spindle but they wouldn't help without a machine serial no. Thinking I can use my BP head, lock the quill and use one of my slides vertically for z. I'll get 12" of travel that way vs 5" if the BP head was fixed/quill travel only. Any idea how much more rigid a BP quill is if its locked and all the way up? I like the idea of 12" of z travel, will make setup easier too vs a fixed table.

Im guessing another big benefit of the slide on z is i'll be able to control z with a lead screw, that alone is a game changer, no?

[handlewanker]

One thing I would say from past experience of using machinery......if it moves it will deflect to some degree.......as it is a manual mill, dovetails will be the way to go, but linear rails and ball screws make it much easier to make due to the accuracy you get and less machining and alignment. etc.

I have a bench top Italian made BERARDI jig borer, that laid in the back of my workshop covered up unused for 35 years, now converted to be a manual vertical mill, and if I was going to make a manual mill I would follow the same design, which is a hollow 300mm wide "C" shaped column with the motor contained inside the column attached to the back of the head and drives the spindle in the head that moves up and down on the face of the column on dovetails.

The rest is conventional X and Y slides..........the X slides are dovetails and the Y are box ways.

As it was originally designed to be a jig borer the Z axis runs up and down on dovetails that only position the head but are not suitable for moving it under feed conditions so it clamps in position and a quill does the work for drilling and depthing etc.

That make it very unstable to move the head up or down without first unclamping it, so.... in the next major refit I intend to remove the dovetails on the face of the column and head and fit linear rails and a ball screw and at the same time a gas strut too.

It also had a 2 Morse taper in the spindle with no possibility of having a drawbar, totally useless for milling, so I cut the Morse taper spindle end off and grafted an ER32 chuck on the spindle end with double row angular contact bearings top and bottom, mainly to cater for lubrication because they come with seals in the double configuration......all of my tooling has 20mm straight shanks like the TTS system.

I lost approx 60mm of Z axis travel by having the ER32 on the spindle end so I made and fitted a 100mm thick solid steel spacer under the column.

So far with a DRO and VFD it works extremely well and accurately.
Ian.