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Making Circles

Discussion in 'Wood / Woodwork / Carpentry' started by morpheus83uk, 21 Oct 2017.

  1. morpheus83uk

    morpheus83uk

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    Hello,

    I have made a few circle jigs and whenever I use them the circles come out fine however they seem not to have a perfectly flat bottom. I am using a straight bit and where I plunge the bit in it seems to leave a little circle or a little trail for the initial pass but then goes away or I get another one randomly somewhere else.

    Is this normal or am I doing something wrong?

    Thanks

    James
     
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  3. JobAndKnock

    JobAndKnock

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    May seem like an obvious question, but are you using a plunge cutter? They have a small piece of carbide let into the bottom edge
     
  4. morpheus83uk

    morpheus83uk

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  5. JobAndKnock

    JobAndKnock

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    They are plunge bits - in that particular type the carbide extends below the body to achieve the same result as the bottom cutting variety

    Are you making recess or "bowl" cuts? That is are you cutting out recesses in solid timber as opposed to routing either a circular hole or circular pieces? If you are making recess cuts you may want to consider using the largest diameter cutter you can (to reduce the number of passes) and possibly even going to a specialist cutter designed for the task, such as the CMT mortise cutters which have slightly downwards tilted carbide cutting edges specifically formulated to give a smooth bottom finish. Another to consider is whether or not the router is being tilted slightly during the cut - it can sometimes help to have a larger sub base attached to the router to avoid this (basically a piece of 5 or 6mm acrylic plastic, etc)

    CMT down-shear mortise cutter with top guide bearing 001_01.jpg

    This particular cutter has a top guide bearing, but there are also cutters without them, The larger diameter means that they must be used at reduced rpm for safety. TBH much the same effect can often be obtained by using a reasonable quality large (1in or over) diameter straight cutter
     
    Last edited: 22 Oct 2017
  6. morpheus83uk

    morpheus83uk

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    Oh right so I presume that's why I am getting the results below? As it's a plunge cutter?

    These are all test cuts to get the correct diameter and to have a practice but they all come up the same.

    I am currently using a guide bush with the 19mm cutter but I have just found out to get the correct size required I am using the 6.3mm cutter giving me the 39mm diameter I require.

    Would I be able to do the same thing with the mortise cutter?

    Thanks

    James



    DSC_0442.JPG
     
  7. Roger928

    Roger928

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    What are you trying to achieve?
    Why would circles or trails matter?
    Just sand them out if its a problem.
     
  8. morpheus83uk

    morpheus83uk

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    I am trying to make a perfect circle essentially so I can do it over and over again opposed to having to sand each one individually if possible?

    Thanks

    James
     
  9. JohnD

    JohnD

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    is it the disks you want, or the holes?
     
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  11. Roger928

    Roger928

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    A perfect "circle" is achieved using a perfect circle template. Which is possible using a hand held router.

    If the base is not to your liking you have to sand it. Which isn't that difficult.
    You won't achieve perfection regarding the base because of the nature of the tooling.
    If you're finding sanding a little too laborious then perhaps woodworking isn't for you?
     
    Last edited: 22 Oct 2017
  12. JobAndKnock

    JobAndKnock

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    What you are doing is, I think, a combination of three things. The cutter is too small a diameter. Secondly I suspect that your grip on the router may well be rather heavy and last the circular holes indicate that you are halting at at least a couple of points when making the cut.

    I'd recommend that you use a larger diameter cutter for starters - small diameter cutters require a lot more time to be spent in the cut which is more likely to result in the circular marks visible in your photos.

    Another part of the solution is to try to develop what the CNC guys call a "ramped cut". In other words you plunge and keep moving the router at the same time and when you remove the cutter move it into the centre and do the retract smoothly. This is not always easy to achieve so...

    When doing tasks such as this a good approach (even if you can't manage the ramped cut) is to make the cut in two passes. So for example. for a 14mm deep recess that would mean doing a "roughing cut" at about 13mm depth of cut, then following up with a cleaning-up or "finishing cut" to the final depth. This 1mm extra cut is how it is done on CNC routers - and for good reason. It works
     
    Last edited: 22 Oct 2017
  13. morpheus83uk

    morpheus83uk

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    @JohnD It is the holes which I am after.

    @Roger928 I have no issue with sanding its just I thought it would be easier to have the cutter make the bottom smooth to save time and sanding if possible. I am using a circle template made from a hole saw and using a straight cutter with a 24mm guide bush, the circles are coming out as expected however its just the base I am trying to perfect.

    @JobAndKnock I have tried various different sizes of cutters however I have found to get the correct diameter I need I have to use the 6.3mm diameter cutter as I only have the 24mm guide bush which came with the router. I have tried the 19mm diameter cutter however this makes either the hole far too big or too small with the templates I have and the hole saws I have to make the templates.

    I have tried a lighter grip as so I am literally just guiding the router opposed to pressing down (after the initial plunge that is) however I still get the circles or some trails but I think this is because I am stopping. I have given the "ramped cut" a go as well however this didn't seem to help but as you say its difficult to do.

    Ok so if I do the initial cut 1mm shy of what I need and then finish with the last 1mm would I not get a circle following the initial plunge as it would essentially be stopped?

    Thanks

    James
     
  14. Roger928

    Roger928

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  15. JobAndKnock

    JobAndKnock

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    It is because you are going deeper, IMHO, which in turn may or may not be down to stopping. By hogging out most of the material you'll be less prone to that happening, but the problem will also be reduced if you increase the diameter of the cutter - 6.3mm is far too small a diameter for the task you are trying to perform.

    BTW, what diameter of recesses are you trying to produce?
     
  16. morpheus83uk

    morpheus83uk

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    It is a 40mm diameter recess about 6mm deep.

    With what I have its the only cutter I can use to get that given the diameter with the hole saw templates I have and my only one guide bush.

    I get the same issue with the 19mm cutter. So you think it's when I am going deeper and think possibly the mortise cutter will give a smoother bottom?

    Thanks

    James
     
  17. JobAndKnock

    JobAndKnock

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    Is that rough sawn timber you are working with? Have you tried this with planed timber?
     
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