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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > DOC in hard maple - what is reasonable?
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  1. #1
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    DOC in hard maple - what is reasonable?

    Hello,

    I recently was talking to Ron at precisebits.com, and he indicated to me that for a 1/4 inch end mill, the "correct" DOC is the diameter of the bit - 1/4 inch. It's better for bit life, he said, and also, it makes machining faster. He thought I could get away with about 100 IPM feed rate at that DOC. I had been pretty timid about DOC up until that point, especially in maple. So I tried his recommendation, and immediately discovered that things weren't going well. Now that the dust has cleared (no pun intended) and I've analyzed what is going , it is clear that I've got flex in my system in the direction across the gantry. I suspected that my HPDE gantry side plates were flexing, but that is *not* the issue. The issue is belt flex, caused by three factors - a) the thickness of the belt b) the width of the gantry c) the design of the system, which has the motor on one side an idler on the other, as opposed to a "motor in the middle" design.

    So, here is my question in terms of future expectations and the idea of building a new router: What is reasonable to expect to be able to achieve for DOC in hard maple at 100 IPM feed rate with a 1/4 inch end mill, on a machine that would be somewhat affordable to put together (say $2K or under)? I talking about a slot cut like a contour.

    BTW, my required work area is about 20 inches by 48 inches, an unusual size (I build bass guitars). Another reason for DIY, besides cost and that I feel I understand enough about CNC routers now to pull it off.

  2. #2
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    Re: DOC in hard maple - what is reasonable?

    How thick are your HDPE gantry side plates? Just bolted to the gantry beam? HDPE is a fairly elastic material and could be part of your problem.

    Photos might help understanding the issue better.

  3. #3
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    Re: DOC in hard maple - what is reasonable?

    Nope, it's not the HPDE. I analyzed the problem, as I stated. I'm aware of my current situation. I'm not saying that if the belt issue were to be resolved, that the HPDE wouldn't then become the primary flex issue. You'll just have to trust me that I've evaluated my current situation correctly.

    But that wasn't the question I asked. The question was what performance I should be able to expect if I build a new "budget" router for $2K or less.

  4. #4
    Community Moderator ger21's Avatar
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    Re: DOC in hard maple - what is reasonable?

    Every machine is going to be different. The design, and materials used for construction is far more important than what the machine costs.
    The more rigid the machine is, the deeper and faster you can cut.
    Gerry

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    Re: DOC in hard maple - what is reasonable?

    Hey Gerry,

    I know you. I have your 2017 screenset for UCCNC, it's great! Don't know what I would do without it.

    But isn't your answer sort of obvious? Of course every machine is different. And of course the more rigid the machine, the deeper and faster you can cut. And as far as the design, of course I would want to choose the most rigid design given the materials and parts I can afford to build that design.

    I'm just trying to get a reference point here. On my machine, when I cut into hard maple with a .25 end mill at .25 DOC at 100 IPM, the slot I'm cutting veers off to one side very noticeably. After spending a few days analyzing why and learning more about timing belt systems and specifications than I ever wanted to know, and after watching carefully what my machine was doing, I'm quite confident in my assessment that my primary issue is belt stretch. If I back off to .125 DOC, I don't really have a problem.

    So what I'm trying ask myself is - "is it really going to be worth it to put together a new machine, or should I just deal with what I have"? That is the whole point of the inquiry. So for instance, if I could buy something like an OX kit and beef it up, that would be a very easy thing to do:

    https://www.aliexpress.com/item/32763669730.html

    Or something like that is ball screw based. Or a design I might come up with - I can see potential weaknesses in that design, and CNC parts *seem* pretty cheap. But if cutting through hard maple as described above is generally not done, then the whole question becomes "why bother?"

  6. #6
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    Re: DOC in hard maple - what is reasonable?

    The issue is belt flex, caused by three factors - a) the thickness of the belt b) the width of the gantry c) the design of the system, which has the motor on one side an idler on the other, as opposed to a "motor in the middle" design.
    It reads like your machine has 2 screws on the long axis. Have you considered adding another motor and eliminating the belts ?

    To scratch build a decent 20" x 48" router for $2k could be a challenge, however if the existing machine has usable organs to donate you're not starting from scratch.
    Anyone who says "It only goes together one way" has no imagination.

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    Re: DOC in hard maple - what is reasonable?

    My machine has two belts on the long axis (X axis). The problem I've noticed so far is flex along the other axis - the carriage moving across the gantry (Y axis). It's possible that I also have a flex problem on the X axis, but the nature of the cuts I'm making and the grain direction is such that I haven't noticed that yet. I have thought about converting the Y axis to a ball screw, that is an option. It would be easier than moving to wider belts and/or trying implement a "motor in the middle" design.

  8. #8
    Community Moderator ger21's Avatar
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    Re: DOC in hard maple - what is reasonable?

    So for instance, if I could buy something like an OX kit and beef it up, that would be a very easy thing to do:
    The problem with that route is that each time you "beef it up", it moves the flex to the next weakest link, until you've replaced the entire machine. The extrusions are just way too small to be rigid enough.


    What is reasonable to expect to be able to achieve for DOC in hard maple at 100 IPM feed rate with a 1/4 inch end mill
    Imo, an Avid CNC Pro series machine. Nothing in the $2K range will do it with a decent finish. Certainly none of the belt driven hobby machines.

    I'd either save up more for a better machine, or take lighter passes.
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  9. #9
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    Re: DOC in hard maple - what is reasonable?

    I’m not sure I buy your argument that side plate deflection isn’t an issue. I’m not trying to argue the point just that I’ve spent decades in machine repair and build and if I had $10 for every time my first course of action was wrong I’d be retired by now. You could be right of course but it is pretty common to have to reboot ones thinking when it comes to diagnostic work.

    As for your question it won’t be easy to hit the $$ mark you have with a suitably stiff DIY build. The one big advantage you have is a well defined working area. That small area makes it much easier to build a stiff machine at a low cost.

    The first thing to consider is used equipment. A working industrial router will likely be too expensive for the budget but a machine with a dead controller might pop up cheap. Depending on your location you might be able to pick up used machine bases. Often these end up being sold by the pound. The goal here is to start out with a good foundation to build upon.

    Beyond that the only way to control costs is to find parts and materials at rather huge discounts over list. Unless you are extremely lucky hitting $2000 for a DIY machine that can handle these sorts of cuts will be difficult. The other issue is that you need to do some fine machining to build a very rigid machine. What you can accomplish with the tools at your disposal is unknown. This in any event the higher your expectations the greater the likelihood you will need access to a machine shop.

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    Re: DOC in hard maple - what is reasonable?

    Now we are getting somewhere. The last two posts are very useful. Thank you both. Getting a used machine that needs work is an interesting idea. I wonder how I would shop in my area. I'd almost certainly need to see it in person first.

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    Re: DOC in hard maple - what is reasonable?

    Quote Originally Posted by wizard View Post
    I’m not sure I buy your argument that side plate deflection isn’t an issue. I’m not trying to argue the point just that I’ve spent decades in machine repair and build and if I had $10 for every time my first course of action was wrong I’d be retired by now. You could be right of course but it is pretty common to have to reboot ones thinking when it comes to diagnostic work.

    As for your question it won’t be easy to hit the $$ mark you have with a suitably stiff DIY build. The one big advantage you have is a well defined working area. That small area makes it much easier to build a stiff machine at a low cost.

    The first thing to consider is used equipment. A working industrial router will likely be too expensive for the budget but a machine with a dead controller might pop up cheap. Depending on your location you might be able to pick up used machine bases. Often these end up being sold by the pound. The goal here is to start out with a good foundation to build upon.

    Beyond that the only way to control costs is to find parts and materials at rather huge discounts over list. Unless you are extremely lucky hitting $2000 for a DIY machine that can handle these sorts of cuts will be difficult. The other issue is that you need to do some fine machining to build a very rigid machine. What you can accomplish with the tools at your disposal is unknown. This in any event the higher your expectations the greater the likelihood you will need access to a machine shop.
    I have to agree with all of the comments from wizard.I also think that pictures of the current machine would be helpful as it might be possible to suggest improvements.It seems a peculiar approach to consider buying a slightly better machine with a view to doing work to it in order to make it acceptable when the existing machine might be capable of matching the result with less expenditure.To give just one example,we don't have any idea whether the current machine might be susceptible to gantry skewing as we haven't seen it.

    Have you looked at the cost of replacing the timing belts with ballscrews?I would guess about 15% of your budget for the replacement machine-prior to upgrading it.You might also be able to stiffen the current machine by bolting some angle in strategic locations.

  12. #12
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    Re: DOC in hard maple - what is reasonable?

    So many people here want to rediagnose a problem I spent several days diagnosing. I ran my diagnosis by an expert CNC friend who was also dubious and suspected the gantry side plates. So did I, it's the natural assumption. I had a plan to reinforce them, which would have been far easier than dealing with the belt issue. I'm not saying that other sources of flex will not become more apparent after belt issue is dealt with, of course they will. When I contacted the router builder, he told me that his new machines have much wider belts! Unfortunately, the upgrade is not easy.

    But I'm feeling way more comfortable about putting money into this machine now that I've finally heard relevant comments about other budget machines, particularly about the extrusions. Plus, many of them use the same belts as mine, although they do use the motor in the middle design as opposed to the idler design on mine.

    It now seems reasonable to consider going to a ball screw for the Y (carriage across the gantry), and maybe the X (gantry moves along the table). Here's my router:

    https://www.zenbotcnc.com/2448-CNC-Router_p_19.html

    Now I know you are going to say "Oh no, HPDE!". But take a minute to consider the design. There has been a lot of care to use box construction and thick steel rails instead of aluminum construction. The gantry side plates look like the weak spots, which I why I immediately and wrongly assumed that they would be the primary source of flex (they are not), but they would be easy to reinforce.

    This whole thread has been about a three way decision:

    1. Build a new router or
    2. Deal with the limitations of what I have or
    3. Upgrade my machine.

    I'm now feeling - after hearing my suspicions confirmed about the Ox type aluminum extrusion design having flex issues - that option 3 has some merit.

    BTW, as a data point, I found a youtube video about a guy who solved a problem cutting aluminum parts by converting his OX from belts to acme screws. Apparently belt stretch was the primary issue there too, even with the smaller table and the motor in the middle design.

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    Re: DOC in hard maple - what is reasonable?

    Having gone down this same road with a low end AL extrusion machine, ger21 is spot on - each upgrade will reveal the next source of flex. There is no question that upgrading from belts to screws (acme or ball) will be an improvement but how much? I am pretty sure it will be less than you think. I decided to stop upgrading and will be getting a new router that won't require basic upgrading to meet my needs.

    As to used, there may be good deals out there but well built/designed used products fetch a pretty high percentage of their new price and are in demand. I've been looking for about a year locally and the several deals that have come up went fast.

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    Re: DOC in hard maple - what is reasonable?

    Good feedback on the used route. Not what I want to hear but thanks.

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    Re: DOC in hard maple - what is reasonable?

    SBR rails?How far apart are the sliders on the X and Y axes?A little bit of play can be multiplied by quite a bit by the time you get to the tool tip.Ball screws will be an improvement over belts of that length although you do at least have dual belts.I can't see that bolting a section of angle to the side plates would do any harm,even if you are convinced there is no flex.One other consideration would be to amend the cutting routine so that you make an initial cut a bit outside the intended shape and then go for a final trim with a bit more depth of cut and a bit less width of cut.

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    Re: DOC in hard maple - what is reasonable?

    The rails are 2 inch square steel rails. There is an idler on all four sides of the rail. I can't detect any play in the sliders, though it will be easier once the belt flex issue is solved. I can't tell exactly how far the idlers are apart because they are encased in housings. I'll remove one side of the housings to see and report back. I do understand exactly what you are talking about. And yes, finishing cuts are now part of my regimen.

  17. #17
    Community Moderator ger21's Avatar
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    Re: DOC in hard maple - what is reasonable?

    I can't detect any play in the sliders

    Take the bit out of the router/spindle, grab the collet, and push and pull. If there's any movement at all, it'll affect the cut quality.
    All the play, in all parts of the machine are cumulative, and will show at the cutting tool.
    Gerry

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    Re: DOC in hard maple - what is reasonable?

    I already did that. The belt stretch is so dominant, it is difficult to tell what else is going on. That's true in the X direction as well (gantry moving along the rails).

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    Re: DOC in hard maple - what is reasonable?

    I wonder why more people don't supply router kits with ballscrews considering the small proportion of the cost of the machine that they represent.Timing belts would be fine for a 3D printer,but they must always have a question mark over them when the cutting forces applied to a router are involved.I have read one or two accounts of people printing or machining clamps to match the belt profile and that has to be a good,positive solution to reducing flexing at the securing points.I have a hunch that belts operate best when they just move in one direction,such as when driving camshafts in cars and the reversals of force on a router will mean that the initial stretch can occur in both directions and obviously gets worse with increasing length.

    A couple of these,or similar ,on the X axis would be an improvement: https://www.banggood.com/750mm-SFU16...r_warehouse=CN .Admittedly they are 1605 and might struggle with 100 ipm feeds,but other pitches are out there and just about any ballscrew would have a more dependable effect on location.No doubt a few people will jump out of the woodwork and explain that their hobby machines work just fine with belts and for light work that may well be the case.Machines for serious production have ballscrews or rack and pinion drives for a reason.

    Then you can do as ger21 suggested and track down other sources of flexing.A dial indicator on the tool shank would be really useful and you shouldn't rule out the possibility that the spindle clamps can allow movement since some people regard a single clamp at the lower end of the body as sufficient.It would obviously be easier if you had an easy way to mount a dial indicator at various points on the machine while trying to isolate the features that aren't sufficiently rigid.

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    Re: DOC in hard maple - what is reasonable?

    I finally found an easy to use whip calculator and it seems to indicate a 1605 would be fine at 100 ipm with double bearing at one end and single bearing at the other. BUT since the travel per revolution is 5mm or about 1/5 of an inch, to go 100 ipm you'd need to spin the motor at 500 rpm, and a quick google search seems to indicate that a nema 23 motor tops out at 400 ipm. Is this what you mean by struggling?

    It feels more and more as if I just need to reduce the depth of cut and live with what I have. At least belts go fast.

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