What are reasonable expectation for a desktop mill?

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    Member mea08mw's Avatar
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    Default What are reasonable expectation for a desktop mill?

    Hi, I'm new to machining but have a lot of experiance in design, I cofounded a start up and we are looking at making some prototypes of our product, and later some demo decises to give to focus groups etc., we wouild probably outsource serial production.

    We have had a think about the finances and it would be worth us getting a desktop mill (tormach 440 etc.) if it could do everything we need, we only need to machine small parts in aluminum and many of them say they are capable of this.

    The main part that I think will be difficult has an outer radius that needs a decent tollerance ~ 30 microns on 45 mm diameter and needs to be round. I think this means we need a 4 axis machine. the same part also has a through hole with an internal diameter of 13mm with a tolerance of ~12 microns, again this needs to be round, it is 45mm deep, and perpendicular to the outer diameter. As far as I can tell we would need a boring head to do that, but I can't tell if they fit in the collets that most of the desktop machines seem to use or if the tool would end up being so long we would have no travel left for the actual machining.

    Personally I like the idea of having our own CNC it's somthing I have always wanted to try out, there are also several simple parts that we are 3d printing at the moment, but could machine for learning how to use it and getting a more proffesional look to the product.

    Any other advice/ pitfalls would be great to hear too.
    Thanks
    Mike

    PS we don't have a big workshop at the moment, just standard wall sockets and maybe just enough room for a small machining centre, definately enough for a desktop mill.

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    Default re: What are reasonable expectation for a desktop mill?

    an external od can also be finish machined with a boring head as long as there isn't anything in the design to hinder it . Boring heads are available with 3/4 shank , so fitting a 440 with one won't be a problem .
    The 440's like any of the tormachs are capable machines , but , they aren't as tight as industrial machines . Your may find yourself struggling to uphold some of your tight tolerances . At the same time someone with no experience can struggle with a top of the line machine if they don't know tricks on how to make things work . If you can learn to make it work then it'll work .

    I think that as a designer it is a good exercise in understanding design vs actual machining and how to make the 2 jive for better efficiency and bottom line costs . I'm not referring to you or be offensive by any means , but I've worked with designers who really made our lives difficult when it came to making parts , tight tolerances where none is needed , features that just won't jive etc . It's good to have an understanding from both points of view and how to compromise if and when needed .

    I wish I could give examples of how my 440's hold up to sizes and tolerances , except 95% of my machining is surfacing . The surfaces finish from the mills are fantastic and the parts mesh together well . My purposely undersized alignment pin holes are always spot on position-wise . I finish ream/chase the holes off the machine out of habit . I run to many machines at once and I know occasions will come up where I'd be too consumed mentally and pop in the wrong reamer and fudge a part , finishing off the machine prevents losing an hour or so

    They are good little machines which I'm confident will handle anything I throw at them . You won't find anything better within the size or price range



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    Default re: What are reasonable expectation for a desktop mill?

    Hi Mike - There are two or more sides of the coin here:
    1) You have a small shop and its just not the machine tool you have to consider. Its the associated tooling, material storage and supply, programming & learning time etc etc even cleaning takes time...of being a small manufacturer. This consumes time and effort that your business may not have (especially your time). Stay being a designer and sub contract the work. Develop a good relationship with a machinist (ideally more than one) that has great machines that you'll never be able to afford and that company will fulfil all your growth and machining demands
    2) Its convenient to be able to make something yourself and as you say you will source out production (if you get there). You say you are a start up, resources are thin for a SU and a machine sitting somewhere not making swarf is a waste of resources. Being a SU and I have been through that a few times, there is the risk of trying to do everything and getting spread too thin and things then fall apart. Figure out your critical path and stick to it... A SU has to get from zero to consistent sales before it can be successful. Usually a small VMC in the corner does not get you to that point, especially one that you have to learn about. That can come once you have regular income and you can figure out the $$ benefits of the machine and pay a machinist to run it
    3) Its worthwhile having two or three machine shops on tap. One will do your prototyping, this will be a small even one person shop who is happy to do one offs and small runs. The other will have several big machines and staff etc that can do production work effectively and struggles to do a one off. Setting up these big machines takes more time and cost then the part...
    4) If your 3D printing simple metal parts then I suspect you are over paying for the part or its a part that can't be machined. This says to me you need to learn more about manufacturing. Find more machinists, there maybe one next door that is a perfect fit. In one biz I was walking into the factory one day and started talking to our neighbour. He was a gun smith and had a medium size VMC had no signage and did not advertise. But he became our go to for one offs and odd jobs for several years
    5) Ultimately its up to you and your biz plan. But an expensive mill means the first few parts will cost what the mill costs so be honest to the business and make a solid decision. The second business I set up we bought a small cnc mill and 1 year in due to a slow down we had to sell it at half price... broke my heart.... Good luck Peter

    afterthought - How are you determining if the part is supplied in spec? and how is the contractor determining this? Its fine to assume the part comes off the machine to spec but this is not a good assumption. Maybe put $$$ into metrology tools is better for you then a VMC...

    Last edited by peteeng; 02-27-2023 at 03:26 PM.


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    Default re: What are reasonable expectation for a desktop mill?

    Hi
    I regularly circularly interpolate holes, external radii etc. I haven't measured exactly how round is but is good enough for a seal surface so I would guess no worse than 15um, otherwise
    seals would not work. My machine is home-built and while I've taken lots of care neither do I claim that it could match well-made commercial machines.

    If you can circularly interpolate not only the outer radius (should be fairly easy) but the internal hole (thats probably about the limit of my machine) then you could make a major saving
    in terms of programming and set-up operations demanding highly skilled operators, boring heads or a fourth axis. Its has got to be worth investigating. Even if you paid rather more than
    you had hoped for something like a Hass Mini-Mill that could get you those tolerances by interpolation rather than the other palava, it would pay for itself handsomely over time.

    May I suggest that you hand a piece of material to a distributor and ask that they machine the external radius and the internal hole and see how good it is. The distributor will no doubt appreciate this
    is a go-no go test.

    Craig



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    Default re: What are reasonable expectation for a desktop mill?

    Hi,
    the more I think about it I would bore the 13mm hole, with a slightly undersized drill and then ream to final size.

    You could use a 12mm endmill and interpolate the hole but the 45mm stickout is going to mean that you have to take it slow to avoid excessive tool deflection, certainly worth experimenting,
    as I said before if you can get away with interpolation it makes the whole process so much simpler.

    I have attached a pic of my machine. The travels are 350mm x 350mm x 350mm. There'd be no concern about running out of Z axis travel even with 100mm stickout.

    Craig

    Attached Thumbnails Attached Thumbnails What are reasonable expectation for a desktop mill?-newmill2-jpg   What are reasonable expectation for a desktop mill?-newmill3-jpg   What are reasonable expectation for a desktop mill?-fourthaxis-jpg  


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    Default re: What are reasonable expectation for a desktop mill?

    I think you are absolutly right, I've had the cold stare from machininsts multiple times, sometimes it's hard to know what is possible vs what is easy/ best, sometimes from the design point of view it's tempting to be lazy and stop at 'works'. Thanks for the advice, beyond these mounts most of the tolerances are nothing special.



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    Default Re: What are reasonable expectation for a desktop mill?

    Quote Originally Posted by joeavaerage View Post
    Hi,
    the more I think about it I would bore the 13mm hole, with a slightly undersized drill and then ream to final size.

    Craig
    I am an idiot, lol, originally the design wanted separate holes boring from each side, but having read around it would be close to impossible to get them concentric, so the bearings in them would likely wear out very quickly. I think I had written off reaming at thtat point and not thought about it again, but you are right it would be perfect with this design. Thinking about it even more, i think you are right again about interpolation, the bearings are not expensive and we are only planning on making demo units or prototypes which will only get used for ~100hrs before getting sent back, it's probably not a big deal if the bearings only last 200hrs.

    Your mill is beautiful BTW.



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    Default Re: What are reasonable expectation for a desktop mill?

    Hi,
    thankyou for the complement, but of course I know all its failings, so my (some) pride is tempered by knowing all its faults.

    This is the second machine of similar description that I have built over ten years and represents the knowledge I've gained largely from making my own mistakes.

    From a business point of view this is definitely 'Avoid like the plague!' machine. It has soaked many more dollars and hundreds of hours more than it should. As a commercial proposition
    its a failure, however this started and remains largely a hobby, and the time and money spent are in line with that hobby. The fact that I'm now using it daily for business purposes is a happy
    outcome.....but was not and still is not the primary motivation.

    peteeng's comments are well placed. Its easy to decide 'Yes, I need this capability' and then invest heavily to do so, only to find a year or two later that you are short $1000 dollars for this months
    bills when you have a machine that you've paid $30,000 for sitting there that has not been used this week.

    Having said that, when you have such a versatile capability like a CNC mill you find an ever increasing number of ways to use it. My own business is a example where I am able to exploit
    my machines capability to make devices and provide services to customers unavailable elsewhere in New Zealand....at least to my knowledge.

    You and you alone can only assess how much use a machine would be and how it would advance your business. Having a friendly machine shop/machinist is the next best alternative to
    in-house capability. In-house capability is costly exercise in itself. You'll need some decent CAD/CAM software and then invest hundreds of hours into using it that the familiarity with it
    allows you to exploit your machine capability time efficiently. You'll also end up with a major collection of tooling, jigs, measuring gear and lots more. If you totalled the cost of all that stuff
    its a major chunk of the cost of the machine.

    I would suggest that you either have to have a known income stream from using the machine...OR....and this is the hard bit......assign a reasonable value to the components that you make in the course
    of developing a product. Lets say for example you made ten widgets, each an improvement on the last until you arrived at a decent design, how much are those widgets worth to your business?.
    If the final product is potentially worth $100,000 in sales a year, then spending $500 on each widget is reasonable, but if the sales are $10,000 per year then $5000 worth of experimental
    widgets is not reasonable.

    Craig

    Last edited by joeavaerage; 02-28-2023 at 05:04 PM.


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    Default Re: What are reasonable expectation for a desktop mill?

    Hi,
    just as an example you will note that there is a large space between the column and the table, in part because of the position of the table at the time the photo was taken,
    but by comparison to other machines of similar sizes the space is large. This allows for larger work parts (desireable) but also compromises the rigidity (undesireable) of the machine for a given size of frame components.
    In particular notice the long extension from the base of the Z axis that supports the spindle. It is made out of 100 x 100 x 9 RHS steel and so is by no means weak....but neither is it a rigid as
    I would like nor as rigid as the machine demands especially given that I have elected to have very generous overhang.

    This part should be cast in iron, and probably SG iron at that, so another $1000NZD give or take. Where do you stop? I have in the wings a much larger spindle motor for low speed/high torque
    applications, ie steel and stainless, and at that time I will have to upgrade the steel part to a cast part whatever it costs. Not this month, nor the next I assure you.

    Craig



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    Default Re: What are reasonable expectation for a desktop mill?

    one big factor is the cost of having a shop machine prototypes . One or 2 pieces can quickly be in the thousands , and if the parts fail in concept then it's money in the toilet . I have a lot of connections and I can't think of anyone I know who would take on that type of work without charging an arm and a leg .
    A good number of needed parts can and will pay off a machine pretty quickly . All my machines paid for themselves in weeks , except I make my own parts on a regular basis vs prototyping . Regardless , machined parts add up dollar-wise , one way or the other , and at the end of the day the tormachs don't lose a lot in resale value if a guy needs to bail



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    Default Re: What are reasonable expectation for a desktop mill?

    Hi Craig - Steel is twice as stiff as CI so your going cast because its damper? But if you make it 8x as stiff dampness does not matter? so steel, bigger section, better design of bolt flanges as they flex heaps and much thicker and your way ahead? Peter

    You seem to have room for a bigger outreach. A 125x125x10 is nearly 4x stiffer a 150x150x10 is 8x stiffer. Or add more bolts to the flange thats the weak link. Flanges deflect a lot. People underestimate that. Use really thick flanges and lots of bolts. The connection is only as stiff as the bolt stiffness... some thoughts

    Attached Thumbnails Attached Thumbnails What are reasonable expectation for a desktop mill?-newmill2-jpg   What are reasonable expectation for a desktop mill?-arm-jpg  
    Last edited by peteeng; 02-28-2023 at 11:23 PM.


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    Default Re: What are reasonable expectation for a desktop mill?

    Hi peteeng,
    damping is nice, but no that's not the reason for going to a cast section. The real reason is that I can make the base of the casting near the full size of the Z axis saddle
    which is 275mm x 275mm. The second reason is that I can make the wall thickness whatever I want, say 20mm or 25mm. 20mm or 25mm of cast iron still beats
    9mm of steel.

    There is a standard steel section used for columns that is 200mm x 200mm x16mm wall. If I could find some, and I only need 400mm of it, that might be an alternative.
    I'd probably get someone else to weld that, 16mm is quite an ask. The job would be to small for Sub-Arc, but maybe 2.4mm or 3.2mm gas shielded flux cored MIG?
    Either way I'd have to send it to Auckland to be stress relieved....so am I saving that much vs getting the exact thing cast?

    It really comes down to what material I can find, 200 x 200mm x 16 is somewhat rarefied around here!

    I'd sort of run out of money at the time that I was considering the Z axis bracket, so I made do with what I could find at the time.....that was about 18 months ago. 'Nothing is so permanent
    than a temporary measure'. It works perfectly well most of the time but if I put on my 1.8kW Allen Bradley servo powered spindle with a 16mm end mill in steel it starts to flex quite a bit.
    It's the sort of detail that you undertake when building your own machine, and the other one is that 'perfect is the mortal enemy of good enough'. So while cast SG iron might be perfect
    would a better piece of steel, or a better engineered steel construction be 'good enough'??

    Craig



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    Default Re: What are reasonable expectation for a desktop mill?

    Add more bolts and upsize the bolts to as big as possible. Also epoxy bed the arm to the saddle. The connection maybe the weak link... Peter
    I deleted the comment about on diamond stiffness. I had a book once with tabulated inertias and it had the diamond stiffness bigger then across the flats. I've always accepted that as true. But I just did the calcs and it was the same so more bolts is best... Peter



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    Default Re: What are reasonable expectation for a desktop mill?

    Then make the 275x275 box out of 16mm steel plate, edge bolt it together and stitch weld it, will be a s good as the SHS... Peter



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    Default Re: What are reasonable expectation for a desktop mill?

    Hi,

    Then make the 275x275 box out of 16mm steel plate,
    Yes, that is a possibility, but I'd still weigh that with having something cast. By the time you add in stress relieving the cost differences narrow.

    Anyway, that's still not my priority. As you may recall my fourth axis is complete with a servo and has been in service for a month or so. I have a 'you bueaty'
    servo reducer with <1 arc min lash on the bench ex the US for 600NZD for a fifth axis, I have the servo for it, another $1000NZD, I have the great big lump of 4140
    already rough turned for the fifth axis platter, another $190NZD, and am waiting on the 32mm med tensile steel plate for the trunnion table to be laser cut. My focus is on
    a trunnion and fifth axis, I can't afford to get distracted onto other projects. I have the best part of $2000NZD already invested in the fifth axis, and that's on top
    of the $2000NZD for the fourth axis.

    My budget forces me to focus my efforts....and I'm getting very close to having a credible fifth axis and still have some time on my Fusion Machining Extensions subscription
    to do five axis some work.

    So don't talk to me about a spindle bracket, tool changing spindles, coolant pumps, a new electrical cabinet, cos I'm going to poke my fingers in my ears and bloody ignore you!

    Craig



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What are reasonable expectation for a desktop mill?

What are reasonable expectation for a desktop mill?