Jig size for small circles

[Nugene2001]

In inches would be better for an answer but I can work with cm also. I have a thick head so please be kind. I wish to cut a circle of 1 1/2" using a 1" bushing and a 3/8" bit. Now just how large does one made the jig and how did you figure that out? Now this does not apply to you but to me and I must use the KISS rule.

[Dan]

This is not something I do a lot but here's one way of looking at it. If you started out with a hole in your jig of 1" then all you would get is a 3/8 hole (straight plunge). So to get your 3/8 hole to be a 1 1/2" hole you need an extra 1 1/8" (1 1/2" - 3/8). You've already got an imaginary jig with a 1" hole so add another 1 1/8" to give you 2 1/8" hole for your jig.

Or there's another way to look at it. A 3/8 bit and a 1" bush will always cut a circle 5/8" smaller than the hole in the jig (the bush will keep the cutter 5/16" away from the edge of your jig all the way around 2*5/16=5/8). So if you're using the same bit and bush all the time just add 5/8" to arrive at your jig size (1 1/2"+5/8"=2 1/8").

Must go now, head hurts.

[Nugene2001]

This is not something I do a lot but here's one way of looking at it. If you started out with a hole in your jig of 1" then all you would get is a 3/8 hole (straight plunge). So to get your 3/8 hole to be a 1 1/2" hole you need an extra 1 1/8" (1 1/2" - 3/8). You've already got an imaginary jig with a 1" hole so add another 1 1/8" to give you 2 1/8" hole for your jig.

Or there's another way to look at it. A 3/8 bit and a 1" bush will always cut a circle 5/8" smaller than the hole in the jig (the bush will keep the cutter 5/16" away from the edge of your jig all the way around 2*5/16=5/8). So if you're using the same bit and bush all the time just add 5/8" to arrive at your jig size (1 1/2"+5/8"=2 1/8").

Must go now, head hurts.
Please allow me to say "Thank You". This should get my head out of the clouds. The reference to KISS, I'm not sure if you understand it or not and it refers to me alone. "KEEP IT SIMPLE STUPID".

[soundman]

It depends on if you want a hole or a disk & if the template is either a hole or a disk.

But in essence you need to account for half the difference between the bit & the guide.

measure the bit, measure the guide, halve that then add or subtract that to get you result.

If in doubt mark it out on a paper or board & present the router up to it so you can see if you are right.

cheers

[Template Tom]

In inches would be better for an answer but I can work with cm also. I have a thick head so please be kind. I wish to cut a circle of 1 1/2" using a 1" bushing and a 3/8" bit. Now just how large does one made the jig and how did you figure that out? Now this does not apply to you but to me and I must use the KISS rule.
I have always said it is quite impossible to do the calculations in imperial measurement hence the reason why the template guides are seldom used. Convert to metric and the problems are solved easily 1 and 1/2inches is approx 38mm, 1 inch we all know is 25.4mm and 3/8" is approx 12mm Lets round them off to whole numbers 38mm 25mm and 12mm you would require a Jig 51mm diameter
Tom

[Nugene2001]

I "Thank You" for your reply.

[Rocker]

Tom,

I am afraid you have misled Nugene; 3/8" is 9.525 mm, not about 12 mm. Actually 25.4 is a very exact conversion factor, so dividing the decimal inch dimension by that factor will give an accurate result in mm.

Rocker

[Ivan in Oz]

Tom,

I am afraid you have misled Nugene; 3/8" is 9.525 mm, not about 12 mm. Actually 25.4 is a very exact conversion factor, so dividing the decimal inch dimension by that factor will give an accurate result in mm.

Rocker

G'Day Rocker,
What accuracy do you People work to?
OR
How many Decimal points......this I can relate to better :)

I 'Generaly' work in and use Metric,
but occasionaly I revert back to Imperial :confused: :confused:
Probably a Mind slip :eek:

Also some of my tools are Imperial, Verniers stc.

Count

[Rocker]

Ivan,

Because the diameters of my solid carbide spiral router bits (which I use for routing mortices) are fractions of an inch, I find I remember their metric equivalents as 6.34, 7.94, and 12.70 mm, but actually the second decimal place is not really worth bothering with as far as woodworking is concerned. Nevertheless, I think that working with an electronic caliper, which can measure to 0.01 mm is well worthwhile, particularly for those of us whose eyesight makes it hard to measure better than to the nearest millimetre with a ruler.

Rocker

[outback]

I realise I run the risk of getting into trouble for getting off the track.

BUT


I just bought a digital vernier

Goes down as just about the best advice I have received from this BB.

Only had it a week, but just about worn it out, makes setting up and measuring heaps easier and more accurate, and my eyes are fine. :cool:

[Template Tom]

Tom,

I am afraid you have misled Nugene; 3/8" is 9.525 mm, not about 12 mm. Actually 25.4 is a very exact conversion factor, so dividing the decimal inch dimension by that factor will give an accurate result in mm.

Rocker
Rocker you are so correct and I apologize to Nugene for my mistake. With the correct dimension given by Rocker the size of the template cut-out will be 54mm Approx.
It is certainly much easier to work in metric right from the start. If I mention 37/64ths to people here in Australia they will shake their heads, unless they are of my vintage and they also have a good memory.
Again now I know why those working in imperial measurement do not use the guides regularly.
Tom

[John Saxton]

Tom will you be demonstrating your skills once more at the WW Show @ Claremont this year,you were missed last year by many?

Apologies to Nugene for using his thread for a non-related topic!

Cheers :)

[Template Tom]

Tom will you be demonstrating your skills once more at the WW Show @ Claremont this year,you were missed last year by many?

Apologies to Nugene for using his thread for a non-related topic!

Cheers :)
Hi John
I have made a decision not to attend this years show. In fact when I came back from Sydney last year I made the decision not to attend any further shows, that is to say to demonstrate. It is a lot of work in the preparation for such a show. I certainly will be attending the show and I hope we may get together, what day are you intending going. I was looking at Friday, going with a group that meet regularly every Saturday and maybe going a second day on my own. Let me know which day you intend going and maybe we can have a coffee etc.
Tom

[Dan]

I have always said it is quite impossible to do the calculations in imperial measurement hence the reason why the template guides are seldom used.

It is certainly much easier to work in metric right from the start.

Tom
Seeing as how Nugene is an american, wouldn't he find it easier to work in Imperial?
It didn't strike me as impossible to work out even when I'm happier working in metric.

[Template Tom]

Seeing as how Nugene is an american, wouldn't he find it easier to work in Imperial?
It didn't strike me as impossible to work out even when I'm happier working in metric.
Dan
I think you would have to ask Nugene that question. It certainly is not impossible to do the calculation but it is easier to do the calculations in metric
Tom

[Nugene2001]

All of you have been very nice and I appreciate your answers . I have found it nothing but difficult to work in inches since returning to the US. I like the metric system best. Unfortunately it just ain't popular here in the US. May I ask if imperial inche is the same as American inche?

[Template Tom]

All of you have been very nice and I appreciate your answers . I have found it nothing but difficult to work in inches since returning to the US. I like the metric system best. Unfortunately it just ain't popular here in the US. May I ask if imperial inche is the same as American inche?
Nugene
I would be surprised if it was not the same though some measurements I have seen as 6.35inches, (whom I do not know is using what). I was brought up in the imperial measurement when we used 1/8" 5/16' 3/4" 21/32" 19/64" and of course 1/2".
I still prefer the metric.
Tom

[hexbaz]

May I ask if imperial inche is the same as American inche?AFIK, the imperial inch is the same in America or elsewhere.

I think the only imperial measurements that differ in USA relate to the good old pint (quart, gallon). The reason for the difference is that being a unit of weight (not volume, unlike the litre), you have to decide what to weigh, and how much of it... So one fluid ounce is the amount of distilled water which weighs one ounce. So far, so good.... Then us Brits decided that one gallon would weigh 10lbs (pounds), whereas the US decided that one pint would weigh one pound. There are 8 pints in a gallon in either country, so we (Brits) have 20 fluid ounces to one pint, and USA have 16; making their gallon 20% smaller than ours. Q.E.D.!

Incidentally, we are a right strange mixture here in Pom land - we do (mostly) use metric for wood working, buying groceries, milk and even petrol (though most pumps still have a litre-gallon conversion table - manufacturers still quote MPG for economy figures!).

... But we still (and hopefully always will) serve beer in pint (20oz) glasses - the good old British Pint lives on! ... All our road signs are still in miles though, and cars show both MPH (large) and KPH (small) on the speedo. The use of miles may not last as long - but it is of course extremely expensive to change.

[CHJ]

I like the metric system best. Unfortunately it just ain't popular here in the US. I do not know about the Timber business in the US but you may need to check out your local merchants “stock” sizes.
Before I retired I worked for the USAF for a number of years and although all the sheet metals, pipes etc. sizes were quoted as imperial many of them had migrated to metric dimensions in reality. The military had just amended the spread of the specifications to accommodate the difference; you may find Timber sourced from outside the US is similarly “metric” although sold as “imperial”