router table template offset calcs

[Alex Stace]

Am I doing the math right here?

using a 1/4 inch bit to copy my plate onto a template...my calculations tell me if I use a 5/8 bushing I can use the same bushing to cut the table slot?

[david brum]

Alex, just to be clear, are you talking about cutting the hole in the table top for your router plate? If so, you don't need a bit and bushing to make a template. You just make a square with it's inside dimensions 1/16" larger all around than the outside of the plate. The 1/16" is the difference between the outside of your router bit and the outside wall of the bushing. You can screw something together with scraps or whatever. Just make sure the corners are 90 degrees.

[Alex Stace]

yes that is what I am doing, think I would find it easier just to outline the plate in MDF and then tranfer it back.

anyway in your example with the 1/16" with a quarter inch bit I would use a 3/8ths template guide bushing right?

[James White]

Alex,

You are correct. But that makes the hole too tight. I think what David is recommending is using a flush trim bit in a 1/16" over sized template. The flush trim bit makes your hole the same size as your template.

[Alex Stace]

ok, so I want a little clearance, so I could make the template with a 5/8 bushing. then transfer back the the table off that with a 1/2 inch bushing and get the same effect of oversizing it by 1/16 all around?

[Larry Browning]

I just did this on my new router table. But I cheated, I purchased the 3/4 mdf template that was the exact same size as the plate. I then used a pattern bit with the bearing on top to cut the hole. It was easy peasy, I didn't have to do any math! I bought the template because I thought it would be difficult to make my own the exact size I needed it to be. My thoughts as to how to create a template was that I would trace the outline of the plate onto the template material, then cut out the hole under sized with a jig saw and use the drum sander to smooth the edges right up to the line. I thought that would be very time consuming and difficult the get exactly right. Now it seems to me that there may be a better way do this using the router to cut the template. I am still a bit confused about the math and what you are using to guide the bushing. Also, how do you get the proper radius on the corners? Or am I not understanding what you are proposing? The more I think about this, the more confused I get. But I am interested in learning how this is done.

[Alex Stace]

the bushing would be guided by the plate to make the template, then the template to make the table top. I had an example in the a book that explained it using a 3/16ths bit, and had two different bushing sizes, which was confusing me because my math was saying to use the same bushing to get the same size hole transfered. I think I understand now that james made the point that you actually want a somewhat larger hole so the plate will have space to fit, using a smaller bushing to get the second cut actually oversizes the hole by a set amount.

second question...

Should I carefully try and get the depth right, or try to use leveling screws in the mdf for the plate?

[david brum]

ok, so I want a little clearance, so I could make the template with a 5/8 bushing. then transfer back the the table off that with a 1/2 inch bushing and get the same effect of oversizing it by 1/16 all around?

Actually, no. You have to take the diameter of the router bit into consideration. If you use your plate as a template with the same bit and bushing, you'll transfer the size of the plate plus the size of the router bit onto your table. Your hole will be too big.

You could do this (borrowed from elsewhere):

In a template material like 1/4” hardboard, use carpet tape to secure the mounting router plate on it.

With a 3/8” OD guide bushing and a 1/4” straight bit route clockwise around the plate and keep the guide bushing tight against the mounting plate. Make several passes plunging a little deeper each time until you are done. Don’t forget to put a scrap piece under the template material to protect your surface.

Now position the template you cut where you want it and secure it with carpet tape. Use a 1-1/4” OD guide bushing and a 5/8” straight bit (you can also use a 1” OD guide bushing with a 3/8” straight bit). Set the plunge depth to match your plate and route around the inner perimeter in a counter clockwise direction. Make again several passes plunging a bit at a time.

Cut the waste with a skill saw.

Now you have a template for your plate which you can use it over and over again

[glenn bradley]

Another simple method is to do like so:

219233

Then use a pattern bit with the same radius as your router plate corners so the fit is correct.

[Jerome Hanby]

Wow, that should have been obvious. Great tip!


Another simple method is to do like so:

219233

Then use a pattern bit with the same radius as your router plate corners so the fit is correct.

[Larry Browning]

Then use a pattern bit with the same radius as your router plate corners so the fit is correct.

But what if you don't have a pattern bit with the proper radius? I think the radius on my plate was about 1/2 - 3/4" The largest pattern bit I have is 3/4" (wouldn't the bit diameter have to be twice the radius?).

[Alex Stace]

so what would the correct math be to use a 1/4 straight bit and two different bushings?

(only bit I have that fits my 1/4 shank yardsale special router, which the template bushings work in) -my other router is a big frued 1/2 shank that does not take bushings without an expensive dohickey.

[Larry Browning]

second question...

Should I carefully try and get the depth right, or try to use leveling screws in the mdf for the plate?

The plate I bought had 8 leveling screws accessed from the top. I then cut the mdf deep enough that I could add pieces of laminate to the ledge that would allow the leveling screws rest on the laminate rather than the soft mdf. I cut it deep enough so that the plate without the set screws would be rest below the top about the same thickness as the laminate (1/32 or so, not sure how thick laminate is) . I could then use the leveling screws to raise the plate to the exact height needed.

[Larry Browning]

Now position the template you cut where you want it and secure it with carpet tape. Use a 1-1/4” OD guide bushing and a 5/8” straight bit (you can also use a 1” OD guide bushing with a 3/8” straight bit). Set the plunge depth to match your plate and route around the inner perimeter in a counter clockwise direction. Make again several passes plunging a bit at a time.

Cut the waste with a skill saw.

Are you saying to cut the router plate outside diameter BEFORE cutting out the through hole? I cut the through hole first, leaving about a 1/2" lip to route out.

[Alex Stace]

I will cut the waste out second I would say a I could stuck a wast hunk of mdf on it in the middle to give the router support on both sides ( may just be from my having used a 3 hp Freud, which is a hevy unstable piece of machinery)

[Larry Browning]

I will cut the waste out second I would say a I could stuck a wast hunk of mdf on it in the middle to give the router support on both sides ( may just be from my having used a 3 hp Freud, which is a hevy unstable piece of machinery)
Huh? I never even thought about doing it that way. However, the template has a thickness (in my case it was 3/4") so the router is only supported on the template side anyway, right?

[Lex Boegen]

When making templates, you make the template larger than the desired work piece by the difference in the radius between the guide bushing and the router bit. If the guide bushing is 5/8" and the bit is 1/2", the difference in the radius is 1/16", so you make the template 1/16" larger. Clear as mud?

[James White]

Alex,

I will let others who have already done this recommend the best way to tackle it. However I was able to get the bushing adapter with PC style bushings for about $15 from Harbor Freight. I have the Freud 1700 2-1/4 horse plunge and it fits perfectly. I don't see it on their web site but I found it in my local store. This was last year I think. It looks just like this set only with the adapter included.

http://www.woodcraft.com/Product/2004855/7667/10-Piece-Router-Bushing-Set-With-Case.aspx

James

[Larry Browning]

When making templates, you make the template larger than the desired work piece by the difference in the radius between the guide bushing and the router bit. If the guide bushing is 5/8" and the bit is 1/2", the difference in the radius is 1/16", so you make the template 1/16" larger. Clear as mud?
Lex, The question is more complicated. When making the template, you should use one size bushing when cutting the template and another size when using the template. The question is, how do you calculate the sizes of the bushing used? Put another way if I want my template to be 1/16" larger than the hole, what size bushing do I use to accomplish that for cutting out the template?
BTW: has that been answered, yet?

[Alex Stace]

I think if I did the math right, if I make the template off the original using a 1/4 bit, and 5/8ths template, then make the final piece with a 1/2 inch bushing I should end up with a 1/16th oversized opening, as the difference in sizes is 1/8th in total, half of which would be each side the center of the bit?

or am I reversing the order?

[James White]

See my post 2 posts down from this one.

[Larry Browning]

I think if I did the math right, if I make the template off the original using a 1/4 bit, and 5/8ths template, then make the final piece with a 1/2 inch bushing I should end up with a 1/16th oversized opening, as the difference in sizes is 1/8th in total, half of which would be each side the center of the bit?

or am I reversing the order?
Let me take a shot at this using my silly logic.
When making the template, the template will be larger by the distance from the outside of the bushing to the edge of the bit PLUS the diameter of the bit all the way around. So if you have a 5/8 bushing and a 1/4 bit my math tells me that would be 1/4 plus 1/4 = 1/2. So the template opening is 1" wider and 1" longer than the plate. So now we need a bushing that will produce a gap between the outside of the bushing and the edge of the bit = to the size difference of the plate and template (1/2" in my example) So, my math tells me that would be a 1 1/8" bushing using the same 1/4 bit.
Now if you wanted to make the hole 1/16 larger, then the larger bushing would need to be 1/32 smaller (1 3/32).
So does this seem correct? If not, where did I go wrong? Not sure where you would find a 1 3/32" bushing.

I think I would have to try it out on scrap before I start cutting on the real thing!

[James White]

Larry,

I didn't check your math. But what Alex proposed would give you a template to make a plate 1/16" larger than the original plate. My fault for not thinking the whole thing out. I will edit my post as to not throw anyone else off.

Alex,

Do you have a flush trim bit?

James

[James White]

Larry,

See Davids post #8. It gives you 1/16" bigger hole.

James

[Larry Browning]

Ok, I got to thinking about this some more on my way to work this morning.
I had 2 thoughts:
1) If you reduced the 5/8 bushing to 3/8 that would allow the larger bushing to be reduced to 1".
2) 1/16 larger seems like too much. You want that plate to fit tight without any "play" in it. How about just adding a few layers of tape to the edge of the plate before cutting the template to give it just a bit of extra size?

[Larry Browning]

Larry,

See Davids post #8. It gives you 1/16" bigger hole.

James
I have 2 problems with David's solution.
1) He is using 2 different size bits and Alex says he only has the one bit (1/4")
2) Best I can tell, his solution adds 1/16 all the way around making the hole actually 1/8" larger. That is WAY too much play.

[John Coloccia]

Wowee....this is all so complicated for a simple task.

Mark the outline of the insert.

Use a Forstner bit of the proper radius to drill the corners.

Cut out the rest with a jig saw....use a straight edge to get it dead straight, if you care about such things.

Glue and screw some cleats to the sides to support the router insert.

Done.

[david brum]

OK guys, I have to agree with John that this is way too complicated. It hurts my brain just thinking of how to explain everything. Plus, I'm sure that I didn't explain very well. The main problem is that there is so much room for math error that you'd be lucky to get it right the first time. I think I can simplify it though:

1) You do NOT want the hole to be bigger than the router plate, well maybe a few thousandths.

2)Trying to use the same size bit and two different bushings to make a template is difficult unless you've got a really, really big bushing to make the actual cut. In your case, you'd need a 1 1/8" bushing if that exists. Even then, the corners would probably end up too round.

3) Glenn perfectly illustrates how to do a fool-proof template out of plywood scraps. Just make it a little bigger on the inside than your router plate. How much bigger? Just measure from the outside edge of your preferred router bit to the outside edge of your preferred bushing. That is the distance that the bushing will hold the bit away from the template, so it needs to be added to the size of the template.

There are other methods which would require that you buy more router bits. This is pretty easy, though.

[Myk Rian]

Another simple method is to do like so:

219233

Then use a pattern bit with the same radius as your router plate corners so the fit is correct.
This is the easiest way.

[Larry Browning]

This is the easiest way.
I agree that this is the easiest way IF you have a router bit of the proper size to do the corners. I measured the radius of my plate and found it to be 3/4. I have never seen a 1 1/2" pattern bit. I do have a 3/4 pattern bit but had to order it from the internet. Or am I missing something on how to do the corners?
As I said earlier, I started thinking about all this a few weeks ago when preparing to do mine. Then I noticed there was a template for my plate available for $15. I jumped on it and it worked out great! I had not revisited the problem until this thread. After thinking about it some more I think I would actually make the template the exact size of the plate by making the 2nd cut with the larger bushing into a 3/4 piece of mdf instead of the table itself. That way it could be placed on the table exactly where the plate went and I could then use a standard pattern bit and not mess with the bushings at all.

No matter how Alex decides to make his template, I have had loads of fun thinking about how to do it. I may just try to make a new template using the 2 bushings, just to see how hard it actually is to do.

[Myk Rian]

I agree that this is the easiest way IF you have a router bit of the proper size to do the corners.
If one is building a router table, there is no better time than now to get a new bit. It will be used in other projects.

[Larry Browning]

If one is building a router table, there is no better time than now to get a new bit. It will be used in other projects. But do they even make a 1 1/2 pattern bit?

[Jerome Hanby]

But do the even make a 1 1/2 pattern bit?
Freud has one but it's pricey at Woodcraft, about $82 (http://www.woodcraft.com/Product/2020476/31591/Freud-50130-Pattern-Router-Bit-Top-Bearing-15D-175CL-12SH.aspx)

[Larry Browning]

Freud has one but it's pricey at Woodcraft, about $82 (http://www.woodcraft.com/Product/2020476/31591/Freud-50130-Pattern-Router-Bit-Top-Bearing-15D-175CL-12SH.aspx)
Well, I gotta tell ya, I love a new tool just as much as the next guy. But that's a "bit" much for what probably is a one time use.

I think I have all the necessary bits and bushings to try the 2 bushing method. I may give it go this weekend. Just for fun.

[Alex Stace]

the concensus seems to have been the opening should not actually be much larger then the plate, so I would think you could actually just use one size, as they should be equally offset?
I will be playing with a piece of pine to test it cheaply tonight I think.

[Larry Browning]

Alex,
That absolutely will not work.
My silly logic in post #22 should work though.(I think) But I am very certain that using the same bit and bushing for both cuts will fail. The hole will be way over sized. You will need a larger bushing for the second cut to make the hole smaller.
I believe what will work is: with the 1/4 bit, use a 3/8 bushing to cut the template and then a 1" bushing to cut the table. The gap between the bushing and the edge of the bit for the 2nd cut must be equal to the gap between the bushing and the edge of the bit PLUS the width of the bit used for the 1st cut. I would add a layer or 2 of masking tape around the edge of the plate before making the 1st cut to add just a little extra size to the template. Also, I would think that it would be imperative to use a bit centering tool before doing any of this. If your bit is not perfectly centered inside the bushing you won't have a prayer of getting it to fit well.

[Alan Schaffter]

Wow, sorry I stumbled on this thread so late, I might have saved some people from pulling out hair.

This tip was written by a "friend" :D and published in Woodworker's Journal December 2010. It is kinda like Glenn's method to make a simple template, but (1) this one results in a PERFECT MDF or hardboard template for use with a pattern bit, (2) DOES NOT require careful cutting and fitting of template pieces, instead only needs scraps of random width and length with one square end, and (3) DOES NOT require a pattern bit with the same radius as your router plate. Any patten bit with a radius equal to or less than the corner radius of the plate will do:

Rip four MDF or hardboard strips and square one end of each. Stagger them around your plate and glue the butt ends- that is all you need to hold it together- use clamps to hold everything tight to the plate. Mix up and fill the corner voids with a little auto body filler (Bondo). Let sit until the Bondo is hard (5 min.?). Carefully remove the plate and sand the surface flat at each corner if needed. A little wax applied to the corners makes this easier. What results is a quick template that matches your router plate PERFECTLY!

http://www.ncwoodworker.net/pp/data/500/medium/Router_plate_template.jpg

When I used mine, I didn't cut all the way through the top with the pattern bit. I made the cut just slightly deeper than the thickness of the plate. I rely on leveling screws to bring it up flush. I also made the routed recess wider by freehand cutting an extra 1/2" - 3/4" towards the center. That leaves a nice rabbet after you cut out the middle with a jig saw.

[Larry Browning]

Wow, sorry I stumbled on this thread so late, I might have saved some people from pulling out hair.

This tip was written by a "friend" :D and published in Woodworker's Journal December 2010. It is kinda like Glenn's method to make a simple template, but (1) this one results in a PERFECT MDF or hardboard template for use with a pattern bit, (2) DOES NOT require careful cutting and fitting of template pieces, instead only needs scraps of random width and length with one square end, and (3) DOES NOT require a pattern bit with the same radius as your router plate. Any patten bit with a radius equal to or less than the corner radius of the plate will do:

Rip four MDF or hardboard strips and square one end of each. Stagger them around your plate and glue the butt ends- that is all you need to hold it together- use clamps to hold everything tight to the plate. Mix up and fill the corner voids with a little auto body filler (Bondo). Let sit until the Bondo is hard (5 min.?). Carefully remove the plate and sand the surface flat at each corner if needed. A little wax applied to the corners makes this easier. What results is a quick template that matches your router plate PERFECTLY!

http://www.ncwoodworker.net/pp/data/500/medium/Router_plate_template.jpg

When I used mine, I didn't cut all the way through the top with the pattern bit. I made the cut just slightly deeper than the thickness of the plate. I rely on leveling screws to bring it up flush. I also made the routed recess wider by freehand cutting an extra 1/2" - 3/4" towards the center. That leaves a nice rabbet after you cut out the middle with a jig saw.

That is awesome! The bondo idea is great! That is the piece that was missing from Glenn's solution. You would probably need to remake the template every time you created a new top, but hey! how often do you create a new router table top?

I am still curious to see how hard it would be to make the 2 bushing template and to see if my silly logic is correct.

[Alan Schaffter]

You would probably need to remake the template every time you created a new top, but hey! how often do you create a new router table top?

I don't know, but I cut my third top with the same template a few years ago!