hanermo,
I have to admit, I had to reread your email several times in order to find a clear message in it. I do see lots of interesting facts but was not sure what to do with them. It was only when I did some minor editing did it start to click. I hope I have not corrupted your meaning.
Now I wonder if the accuracy of my RF30 mill/drill could be improved by adding stiffening to the column. It would probably only need to be stiffer front to back. I know that some people have tried filling hollow structural members with concrete but I would prefer something that can be removed if necessary. Maybe something like a 1/2" or even 1" thick steel plate that is welded to another 1/2" or 1" plate on the bottom. This plate would have holes in it to permit the hold down bolts on the column base to pass through. The column casting would then clamp this plate.
The vertical plate would be welded to this bottom plate. Rather than try for a tight fit inside the column, I would prefer to have it not touch the inside until it reached the top of the column. Then there would be some kind of a jacking arrangement that would press on the inside of the column over, say 2" of height.
In this way I would be supporting the column both top and bottom and hopefully reduce column flexure.
What do you think? (I know my RF30 is viewed by many as a "sow's ear" but with care it can do very accurate work. My best test cube was +/- 0.0002". I also just removed my 2 HP import motor and replaced it with a WEG 2 HP 3 phase motor so have plenty of power.)
Rick
Rick.Sparber.org
On Jun 29, 2011, at 3:34 AM, "\"hanermo\" - CNC 6-axis Designs" <gcode.fi@gmail.com> wrote:
> Great!
>
> You note that the quill does not move in xy when pressed on. Everything
> locked. Impulse in xy, in line with x or y ways.
>
> I would expect at least 0.04 mm of movement, from bending the column.
>
> That is about what I get, and that is about what a Bridgeport series 2,
> CNC, a very heavy and stiff mill gets.
> To test, I suggest chucking a max capacity drill rod in the quill, and
> push on that, just like an end mill does.
>
> Approximate forces in milling (machine mass):
>
> 1/2 Hp minimills - MT2/MT3 sockets - 20-50 kg - ( 2-300 kg)
> 3/4 Hp industrial, old Bridgeport M-heads - MT2 sockets - 50-100 kg (800 kg)
> 1 Hp industrial - Bridgeport Series 1, R8 collets - 100 kg ( 1000 kg)
> 2 Hp Hobby R8 collets - 50-100 kg (400-600 kg)
> 2-3 Hp Old Industrial -Bridgeport Series 2 - R8 - 100-200 kg (2000 kg)
> 3+ Hp Modern Industrial - ISO30 - 300-1000 kg (10.000 kg)
>
> Collets rigidity - Spindle rigidity - Machine rigidity - Available Hp -
> Usable Hp - Material removal rate are all inter-related.
>
> These are all in-line ... that is,
> A best known example of an MT2 milling machine, Bridgeport M-head,
> ciraca 1930-1960, had a 3/4 Hp industrial 3-phase motor.
> It used a MT2 socket.
> The socket becomes the limiting factor, and a mill like that cannot
> effectively utilize more than about 1 Hp.
> This head is more acurate, and more rigid, and better made, than any
> modern hobby milling machine head, using an MT2 socket.
>
> In the case of a modern hobby machine, lightweight, even though it uses
> an R8 socket, the machine itself does not have the rigidity required to
> fully utilize either the R8 collets capacity, OR the Hp often supplied
> with it.
> So some small hobby mills, supplied with R8, and say 2HP, cannot by any
> means utilize as effectively large end mills as the same R8 socket on an
> industrial, old, mill like a Bridgeport Series 2.
> In this case, the light modern mills are not meant for industrial
> production, as the old Bridgeport ones were.
>
> The differences between rigidity and Hp are about 3-4:1.
> That is, a typical mill, compared to a best in breed, will have about 4x more
> rigidity in the best vs. the typical example.
>
> Both sockets, and machine mass, are an excellent indicator of machine
> rigidity.
>
> One more thing:
> Machine design affect the rigidity about 3-5:1.
> A knee mill is about 5x less rigid than a portal mill with a moving
> table, of the same size.
> A portal mill is the most rigid design known.
> The moving table one will need to be about 1.5-3x as heavy, with 1.5 being
> necessary and 3x being typical, as it makes it more accurate.
> It also needs twice the size, because a moving table only has half the
> usable area of the table.
>
> My mill is a moving table portal design, in steel.
> Welded, bolted construction, 1600x1600 in size, 2000 kg.
> Head is a Bridgeport M-head, original 3-phase motor, driven by Hitachi VFD.
> Resolution (Accuracy) is about 4x original Bp, about the same as a
> Bridgeport Series 2, or about 0.01 mm.
> Resolution/Accuracy is achieved by using industrial linear guides, THK
> 20 and 25 mm size. Movement is by ballscrew, of low grade rolled type.
> I do not currently have locks, or swarf guards on the ways, which I am
> making.
>
>> This is another in my series for people new to our hobby. It suggests
>> a set
>> of test that will assess the sloppiness of a given mill. Most of these
>> tests
>> can be performed without applying power and might help a person avoid
>> buying
>> a machine beyond help. It might also help a person see the beauty in a
>> machine that just looks beyond help.
>>
>> If you are interested, please see
>>
>> http://rick.sparber.org/mt.pdf
>>
>> I welcome comments and questions. In fact, I'm depending on input from our
>> experts to correct what I have written and supply additional tests
>> that are
>> easy to run. All of us are smarter than any one of us.
>>
>> Rick
>>
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
> ------------------------------------
>
>
>
>
http://groups.yahoo.com/group/gingery_machines/app/peoplemap/view/map
No comments:
Post a Comment