High Speed Machining viable for DIY??

[scavenger]

I have just started a small CNC router project for PCB's and small parts but I have a grandiose idea for a future larger 5 axis machine and I was wondering what everyone thinks of HSM (High Speed Machining) as a strategy for a DIY CNC milling machine. By HSM I mean 8-25K rpm spindle speeds.

From my reading and looking at HS machining center data sheets (also here http://www.mmsonline.com/articles/hsmdm/case1.html) it appears that HSM provides the following benefits;

1. cutting materials from ally to hardened steel,
2. gives same or higher chip removal rates than lower speed cutting (lighter cuts but faster travel)
3. requires smaller range of cutting tools (and hence less tool changing facilitating a simpler collet system rather than big arbours)
4. gives a superior surface finish
5. allows the design of a lighter machine, particularly on the ways (lighter cuts -less torque - cheap belt drive rather than ballscrews?)

I know that high end machining centers often use servos rather than steppers to get the high speeds, but for home use I could put up with a slower job if it came out with a better finish, particularly if the low torque requirements allowed the design of a tilt and turn 4/5th axis table to simplify work holding and increase flexibility. It seems that a HD adjustable speed router mounted atop the Z axis might give enough power and the right rev range.

Does this make sense or have I overlooked some fundementals (other than the constant scream from the router)? This system would need coolant, but I already have that available.

First post so be gentle! Apologies if this has already been thrashed out in the past. Excellent info on this site.

Phil
West London, UK

[randyf1965]

Anything is possible with enough money......

Seriously I would start with a small 3 axis router design. Build it , use it and find its weaknesses. Then improve it or use it to create a new router.

I built my 48"x34" router as a proof of concept... started using it and found all sorts of weaknesses. I am on 3rd revision of machine #2 (bigger stronger and faster... where is Oscar Goldman when I need him!)

[ger21]

I think you're underestimating the forces involved with cutting steel.

[scavenger]

Yeah, I think I did say it was a future fantasy and I am currently on machine No1, a simple small 3 axis acme and delrin style machine.

I was more interested what everyone thinks of high speed machining for diy. Has it just nor caught on yet, too difficult, too many problems or plain don't work? My background is not this type of engineering.

[walter]

This post has a picture of Mikron high speed milling machine (picture#9) http://www.cnczone.com/forums/showpo...21&postcount=1

This is the rigidity that you need for high speed milling. Any lesser frame can't be considered high speed. These frames are between 4-10 tons, not a good choice for hobby machine.

[scavenger]

Originally Posted by walter This post has a picture of Mikron high speed milling machine (picture#9) http://www.cnczone.com/forums/showpo...21&postcount=1 This is the rigidity that you need for high speed milling. Any lesser frame can't be considered high speed. These frames are between 4-10 tons, not a good choice for hobby machine.

Thanks for the input Walter, I'm very interested in the epoxy-granite ideas and had already been looking at those pages. I also have some interests in the use of free heavy precision components such as the blocks of old car engines (as in MultiMachine site but not in the configurations they use (http://groups.yahoo.com/group/multimachine/)). So if I gave the impression in my original post that I was thinking of building something flimsy, that was my fault. I understand that you need a well damped and 'dead' structure that will not resonate easily for any type of serious cutting.

And yet looking at pictures of some modern high speed machining centers with the covers off, although the machine as a whole may be substantial the moving gantry structures appear to be of reasonably lightweight construction, light alloys in some cases. So it seems that High Speed Machining (HSM) must work and require less force at the spindle. After all, my dentist manages to easily cut very hard tooth enamel with a watercooled TC cutter by hand with very little pressure (but the drill does rotate at 400K!)

My background is electro-mechanical rather than machining so I may be talking complete rubbish, but it seemed to me that HSM looked attractive for the reasons I gave in my first post.

Phil

[walter]

Sorry, didn't mean to preach :-)

Someone in Engineering forum started the effort. Should be interesting!

http://www.cnczone.com/forums/showthread.php?t=40225

[scavenger]

Thanks for the link, I hadn't seen that thread.

[stilltheone]

1. cutting materials from ally to hardened steel,
True
2. gives same or higher chip removal rates than lower speed cutting (lighter cuts but faster travel)
True
3. requires smaller range of cutting tools (and hence less tool changing facilitating a simpler collet system rather than big arbours)
May require more tools as length to diameter ratios start to increase requiring different lengths of the same tool as you work down
4. gives a superior surface finish
Only if the machine is rigid enough.
5. allows the design of a lighter machine, particularly on the ways (lighter cuts -less torque - cheap belt drive rather than ballscrews?)
False
heavier structure, stronger ways, higher duty drives
The machine must be rigid (mass) and the response of the drive system fast (expensive). It takes an upper end machine to truely handle HSM.

[NC Cams]

There is nothing wrong with dreaming and planning but execution ultimately is where even the most optimistic DIY'er gets slapped in the face with reality.

High speed machining involves ground up engineering/re-engineering of the machine, not the least of which is the spindle. Speaking as a former machine tool bearing engineer, high speed spindle bearing engineering was the most closely guarded "secret" of all the machine tool bearings. Why? Because, tolerances became INORDINATELY critical and these were NOT given out - poorly followed/incomplete instructions guaranteed that bearings will fail and that was intolerable for the guys in the M/T bearing group.

The externl forces in/on the bearing are additive to the forces that the spindle speeds themselves are creating. The internal forces are astronomical and MUST be properly addressed. No "close enough/wing its" here. Heat management is absolutely crutial. Grease lubrication is usually forsaken and oil air mist or some other exotic method used - definitely NOT of the DIY ilk.

Finally, there is bearing fittment. It is is definitely NOT of the "close enough/loctite it" grade. IT was not unusual for spindles to be ground and lapped and thermall stabilized for 72 hours to assure absolute proper sizing. Spindle housings are also ground to unbelieveable tolearnces for size and concentricity. Most DIY'ers balk when they see what needs to be held.

Based on the admontions that I've repeatedly gotten on this website to my "industry standard" bearing fit recommendations (IE: "its only DIY, it don't have to be rocket ship grade", and the like), it is pretty safe to generalize that the typical DIY'er can't/won't even be able to meet the minimum requirements to engineer, let alone build/fit a high speed DIY spindle.

Heck, some can't even R&R Bridgeport mill spindle bearings properly and these are hardly ABEC 7's as you MUST use in a H/S spindle.

Granted, there are guys who can/will rise to the occasion. But, most can't/won't.

Yes, with enough money, you should be able to do most anything - like buy the engineering and machining needed to PROPERLY execute the project.

BUT, if you don't know how to engineer it, leave the creation of high speed machining centers to the pro's. You'll spend way less and enjoy yourself more.

[scavenger]

Originally Posted by stilltheone 1. 5. allows the design of a lighter machine, particularly on the ways (lighter cuts -less torque - cheap belt drive rather than ballscrews?) False heavier structure, stronger ways, higher duty drives The machine must be rigid (mass) and the response of the drive system fast (expensive). It takes an upper end machine to truely handle HSM.

Sorry to be dense but I'm afraid I don't really see why cutting with less force as in HSM requires a heavier or stronger structure. More rigid I agree, as vibration is likely to be a bigger issue, particularly at resonant peaks so it needs to be as rigid and well damped as possible but I don't think that necessarily equates to heavier and stronger. It should be possible to address rigidity in the design of the structure and choice of materials. Aircraft manage to achieve very rigid sections in some areas (although other sections are designed to flex) without masses of weight.

I agree that in an ideal world one would want faster drives with HSM but this is just for a home workshop and I'm not looking to turn out any mass products on this machine or try and mill an F1 engine, so if it takes a bit longer to cut something than on a Deckel Maho I'm not too bothered, therefore the lower spec slower drives that I can afford will have to do.

At the moment it is only an idea anyway and is some way down the project list! ...but interesting to think about. But even if I never build it I will NOT be following NC Cams' helpful advice to leave it to the 'pro's' and go and buy an HSM machining centre! Perhaps he could define "INORDINATELY critical tolerances" and "astronomical internal forces" and we could thrash out bearing design in a slightly more constructive manner if he doesn't mind replying to a DIY'er from a different hi-tech industry.

Phil

[turmite]

PHil may I gently urge you to look at NC Cams post count? I can assure you that he does know what he is talking about. The tolerences he is speaking of is .0001" or less and the fit is not something that can be driven on the spindle, nor as he indicated, loctited in to hold it.

I have read all the post here and it seems to me you are under a misconception of what hsm is. If I read your post correctly you indicated it was dictated by spindle speed? Spindle speed is part of it, but it also includes very rapid movements during the machining process and cam software has to be geared to hsm as well so there are as few unwanted moves out of material. I.E. rather than the z axis lifting and moving even a small amount, the cam is designed to keep the bit in the material and cutting if at all possible. Vibration is another factor in hsm and that is the reason for the post about the machines being so massive.

HTH

Mike