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Ways, rails and home building

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Dave S14/01/2022 18:08:32
373 forum posts
90 photos

Evening all,

Further progression on my previous xy vs XZ question.

I have engaged in some cardboard engineering to figure out sizes of machine, and come to the conclusion that xy and z is ok for what I want.
I think (very much not sure yet) the the gantry is out and a more traditional column VMC style is in.

So how to arrange the motion?

Linear rails are home brew first choice, partially I suspect because they can be bought and bolted down without any requirements for super precision machining. They also run nicely, so CNC movements are smooth and fast.

I think I’ll be using an old cast iron surface plate as the foundation for my new mill. I could bolt down linear rails to it, but equally I could use it as the basis for a box way. Advantage of that would be much better support for the Y saddle, but it would mean stick slip and possibly slower feed speeds.

The saddle equally could be rails or box ways.

I notice that Denford nova mills and Boxford VMCs seem to use traditional ways, and they seem to be popular.

Thoughts from more experienced people gratefully received.

Dave

mgnbuk14/01/2022 19:19:04
1198 forum posts
72 photos

......bolted down without any requirements for super precision machining

Linear rails require an accurately machined surface to bolt on to if you require an accurate end result - the rails follow the surface they are bolted to.

Rails or traditional slide ways both have their advantages & many machine tool builders used to offer machine with both configurations to suit the users requirements. Rails give low friction for high speed machining, but are poorly damped & relatively fragile - a collison or careless part loading can "brinell" the rails. Turcited box ways are better for heavy machining - better damping, less liable to damage and, as heavier machining is generally done at lower feedrates, the generally lower feedratesfor this arrangement are not much of an issue. On a machine with damaged or worn rails, "just * " replacing the rails gets you back to "as new" performance, where box ways require the guideways to be reqround (so total machine stripdown) , the sliding elements re-Turcited and then re-scraped to get the alignments back.

* We got a Deckel Maho Gildemeister machining centre in for an isurance repair at my last employent that had been crashed during setup. The Y axis linear bearing carriages were smashed by the impact, but the amount of work required to dismantle the machine to get the Y axis off to get to replace the rails & carriages was substantial, plus it had to be removed from site for the work to be carried out . IIRC it took a couple of fitters a fortnight to dismantle, replace the damaged parts & re-assemble the machine - but afterwards the alignments came back to the same values as the original test documentation.

Denford & Boxford probably went with tradiition slideway arrangements for cost reasons + their machines are not that fast WRT to feedrates. My industrial Triac has dovetail ways all round - not the best solution, but just machined into the castings at one hit & "good enough" for the capabilities of the machine. Plus they have the advantage of being compact on a small machine - rails eat up space in comparison.

Nigel B.

HOWARDT14/01/2022 19:55:23
932 forum posts
39 photos

I spent a good deal of my draughting life designing machine tools. Over the period of about twenty years we moved from dovetail, cast iron square ways on hardened rectangular rails, then using Turcite and finally Moglice before using linear rails. If you look at a catalogue for linear rails such as Hiwin or whatever they are these days it will give you a good idea on the selection and mounting design for the rails. While smaller rails may be adequate for your intended use larger ones can work with quite large mounting tolerances. What you have to remember is that you may be using quite a small length of rail during your actual working cutting stroke, so minimising positional errors. Alright you have to achieve a reasonable geometric tolerance on the mountings but with some thought you can achieve it, you just need a machine big enough to do the machining. Most of our mounting faces were only a good milled surface with both rail seats milled in one setting. The likely hood of a damaging crash to the rails is unlikely with the sort of power you will be using on the spindle, serious cnc mills are using high kw integral spindle motors, not a VFD. Over the years the only machines I heard of in need of rail replacement were high power mills and lathes, most of the machines I designed were in the 2 to 20kw range and upto 20 axes. We did occasionally do a refit after a few years because of component change and replaced rails as a matter of course but never found a problem with them, even after running more or less continuously for five years or more.

Good luck, you have a lot to do.

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