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Guide Bushing Calculations

This is a discussion on Guide Bushing Calculations within the Portable Routing forums, part of the Routers category; Hello, I'm trying to cut out a hole in a table to mount my router ...


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Old 10-05-2005, 06:01 PM   #1 (permalink)
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Hello, I'm trying to cut out a hole in a table to mount my router plate in. It's a woodhaven plate and I don't want to cut a through hole for it. I want to cut a rabbit out for the plate to sit on. I'd like to use guide bushings to make a template from the router plate then transfer that template to the router table. I'm having a bit of trouble determining the proper bit size and bushing sizes to do this. Is there a combination that will allow me to transfer the outside diameter of the router plate to the inside diameter on the template and then back again?

Thanks in advance!
T.

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Old 10-05-2005, 06:36 PM   #2 (permalink)
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Well, T, that depends on a lot of things... the sizes of router bits you have, the size of template guides you have, etc. There are a couple of things you can do.

First, you could start by trying to find a bit with the same radius as your plate. Then you could simply run some straight sticks in a rectangle the same dimension as your baseplate adding the difference in the offset between the bit and the outside edge of the template. Then cut into your tabletop the depth of your baseplate thickness. I would recommend cutting a piece of the same material that you use for your template and placing it in the field of your cut out so the router will not tip away from your template, undercutting your rabbet and possibly making your hole oversize. After this is done, you could use your jigsaw to cut the rest of the hole out, leaving the rabbet, or make another smaller template and cut all the way thru your top with several passes.

I have made a cut out once using the router base for a guide, I just added 3 inches in each direction (the difference between the edge of the router and the cutter. It worked pretty well, but router bases themselves are not as accurate as a properly centered guide bushing.

If you want to make an inlay type pattern, I think it would be more work than you really need to do.
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Old 10-06-2005, 09:49 AM   #3 (permalink)
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To make this as simple as possible use the same size bit as the radius of the plate, if possible, if not use what you have to make a 1/4" BB template. Don't be overly concerned about the off set. Use the router plate ,with two faced tape,a plunging type bit and the guide of your choice to make the cut out in the plywood. Use the plywood template and the same bit and guide you used to make the template to make the cut out. Cut a very little deeper than thickness of router plate,see previous answer about using jig saw, use some means for leveling plate-set screws, woodhaven's levelers ,or something else.
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Old 10-12-2005, 10:43 AM   #4 (permalink)
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Thanks for all the responses. I dug into this a bit more and did some calculations (I can send the diagram if anyone is interested) and determined the following equation:

For the first cut to make the female template from the male baseplate the offset from the plate is:

D = B/2 + O/2

Where B is the diameter of the bit (divided by 2) and O1 is the diameter of the guide bushing (divided by 2) with D being the distance between the baseplate and the (new) inside edge of the template.

When routing the hole in the table using the newly created female template, the offset equation changes to

d = r/2 - b/2

Where b is the diameter of the bit (divided by 2) and r is the diameter of the bushing (divided by 2). Since this is an inlay type operation where the final inside cut needs to match the original baseplate, these two equations need to be set equal to each other to determine the bits that are needed and the guide bushings needed.

So,
D = d
which means

B/2 + O/2 = r/2 - b/2

If you assume that B = b (the same bit used in both operations) then the equation reduces to:

B/2 + O/2 = r/2 - B/2

or (after some algebra)

2B + O = r

Ok, so there's the equation. If you have Excel you can quickly make a table of all router bits (straight cut) and guide bushing coller sizes to determine the values of "O" and "r" that work. There are probably only a few.

For example:
If you have a 1/4in. bit, and a guide bushing "O" of 1/2 in. for the first cut (to make the female cut) then you'd need another bushing "r" with a diameter of 1in.
r = 2*1/4 + 1/2
r = 1/2 + 1/2
r = 1in.

So there's a combination that works (for those that don't want to do the math. There is at least one other with a 5/8 in. bit that works (from the Router book by Spellman) but I don't remember the bushing sizes. It's explained in that book pretty well, but only for that particular size of bushings and bit. If you have a different sizes it doesn't explain how to determine what you need. Hopefully this will help if anyone has different sizes of bushings and bits. BTW, all of the bushing sizes are outside diamters (don't use the radius values if those are given!)

Hopefully this will help anyone else doing this.

Thanks,
T.
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Old 10-12-2005, 11:49 AM   #5 (permalink)
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If one were to go back to the early days of the forum I posted several detailed postings on guide bushing and several others added even more information.... I also posted tables for oak-park style pc style bushing and common sized router bits. You might want to take a look at:
www.routerforums.com/showthread.php?t=163
for starters then do a search on "guide bushings"

I had posted a few tables in the "gallery" but I think that all got trashed. I will dig up the orginals and post them again if anyone is interested. See post:
www.routerforums.com/showpost.php?p=1812&postcount=9

Ed
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Old 10-29-2005, 11:17 PM   #6 (permalink)
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Simply calculate the diameter or Shape of Base
If diameter????? Rout a template in 12mm MDF equal to the dimeter of the base plus the difference between your template guide and cutter that will be used to rout the rebate. As you are not proceeding to rout all the way through simply select a larger template say and then rout all the way through
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Old 02-22-2011, 02:15 PM   #7 (permalink)
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Hello T. I read you post on using Excel to list guide bushings and bits. I wondered if you have done this and if you could email me that Excel file. It would help me set it up and use it.

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Old 02-23-2011, 02:56 AM   #8 (permalink)
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Originally Posted by TomVB View Post
Hello T. I read you post on using Excel to list guide bushings and bits. I wondered if you have done this and if you could email me that Excel file. It would help me set it up and use it.

Tom VB
Tom, whilst charts are useful, like with calculators or electronic cash registers, things can happen to them and so one must be able to use paper and pencil, or better still, calculate in one's head. in a similar fashion, a competent routologist should be capable of calculating off-sets, it really is so simple, template guide diameter minus bit diameter divided by two equals the OFF-SET, or to calculate the size of opening in the template for a given size opening in the finished project: guide dia. minus bit dia. plus finished size of opening equals the size of opening in the template. An example, we wish to rout an opening 9" x 4.5" and we use a 1.5" guide with a 0.25" bit, 1.5" - 0.25" + 9" = 10.25"
1.5" - 0.25" +4.5"= 5.75"
so, the opening in the template has to be 10.25" x 5.75" to end up with an opening in the finished project of 9" x 4.5"
Understanding this means that no project will be held up if the dog chews the chart!
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Old 02-23-2011, 09:38 AM   #9 (permalink)
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Quote:
Originally Posted by TomVB View Post
Hello T. I read you post on using Excel to list guide bushings and bits. I wondered if you have done this and if you could email me that Excel file. It would help me set it up and use it.

Tom VB
While I agree with Harry that having the simple offset formula stored in the cranium is good, I made up a chart for my commonly-used bushings and bits and taped it to the inside of the plastic bushing container. That way, the dog must eat the plastic to get to the chart. And, two coats of cat urine on the container will dissuade all but the most determined dogs.

Oh, and don't forget the utility of bushing collars to add to the offset.
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Old 02-24-2011, 07:46 AM   #10 (permalink)
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While I agree with Harry that having the simple offset formula stored in the cranium is good, I made up a chart for my commonly-used bushings and bits and taped it to the inside of the plastic bushing container. That way, the dog must eat the plastic to get to the chart. And, two coats of cat urine on the container will dissuade all but the most determined dogs.

Oh, and don't forget the utility of bushing collars to add to the offset.
I think this is what you mean Ralph.
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