Maths help required

[Garno]

I need a little help please,

For some reason my brain cells are deserting me at an alarming rate.

If you (me in this case) had a tube that had 1.5mm thick walls, what radius would the inside need to be if I wanted the outside to measure as a 20mm x 20mm square?

I am trying to make a sleeve that will hold round pen kit bushes with varying widths between 8.51 up to 16.52 and lots in-between. Tinkercad thickens the inside of the tube so I would assume that a 10mm Radius would only be 8.5mm on the inside, am I correct?

 

[MikeK]

If I understand your question correctly, a 20mm diameter tube with a wall thickness of 1.5mm would have an inner diameter of 17mm. The inner radius would be 8.5mm. Don't forget to account for the wall thickness on both sides of the diameter.

 

[ajs]

Radius of 8.5mm. 17mm would be the diameter.

 

[Spectric]

I think you are mixing up round and square tubing. You talk about wanting a radius but the outside is a 20 by 20 square!

 

[MikeK]

I think you are mixing up round and square tubing. You talk about wanting a radius but the outside is a 20 by 20 square!

That's why I started my answer with a conditional statement. A 20mm diameter tube would fit in a square opening 20mm on each side.

 

[J-G]

That's why I started my answer with a conditional statement. A 20mm diameter tube would fit in a square opening 20mm on each side.

In an Ideal world with zero tolerence

 

[Garno]

Thanks for the help guys,
I have been all over the place with both radius and diameter and for some reason the old brain just would not kick into gear. I messed up a 90 minute 3D print as the bushes don't fit into the stupid sleeves I made for them. What would not equate for me was the outside radius would be say 10mm and with the thickness that should make the inside wall 8.5mm radius. If I wanted the inside radius to be 10mm does that make the outside radius 11.5mm?

. Don't forget to account for the wall thickness on both sides of the diameter.

That is what I have been forgetting I have only accounted for it on one side.

 

[gcusick]

Another issue with 3D printing circular holes is that they generally print slightly under size because the printer creates the circle as a series of straight-line segments. My experience is that this is generally of the order of 0.3-0.5mm. You can either take care of it in your CAD drawings, or most slicers have a setting for expanding holes by a selected amount.

 

[Sandyn]

I messed up a 90 minute 3D print as the bushes don't fit into the stupid sleeves I made for them.

I nearly always do a sanity print first, just print the first part of the item to check for silly mistakes, which I often make. I also nearly always do a pencil sketch and draw the thing with all the dimensions. It is very easy using any kind of CAD package to muddle radius and diameter. my most popular mistake is dividing a dimension in two in my head, so I ask Alexa. I don't bother about it, which is why I make so many mistakes.

 

[MorrisWoodman12]

Another issue with 3D printing circular holes is that they generally print slightly under size because the printer creates the circle as a series of straight-line segments. My experience is that this is generally of the order of 0.3-0.5mm. You can either take care of it in your CAD drawings, or most slicers have a setting for expanding holes by a selected amount.

You'll probably find a setting in your 3D design programme that defines how many straight-line segments are used to create a circle. At least that's the way I do it using Designspark Mechanical. As long as you've calibrated your printer then it should print accurately to size.
Have fun.
Martin

 

[guineafowl21]

In an Ideal world with zero tolerence

Or in a world with hammers...

 

[Garno]

Another issue with 3D printing circular holes is that they generally print slightly under size because the printer creates the circle as a series of straight-line segments. My experience is that this is generally of the order of 0.3-0.5mm. You can either take care of it in your CAD drawings, or most slicers have a setting for expanding holes by a selected amount.

That is what was happening, all off the tubes/holes were undersized.
I have found the setting in Cura to make the hole bigger and have added +0.5mm.
The print is going to take 1hr 35mins so I will know if I have done it right then.

 

[Garno]

I nearly always do a sanity print first, just print the first part of the item to check for silly mistakes, which I often make

Do you do this on the lowest settings or on the settings that you intend to do your final print?

 

[Sandyn]

Do you do this on the lowest settings or on the settings that you intend to do your final print?

I do it on the final print quality to check for shinkage etc. Sometimes, it's not possible to test, because of the shape, but where it's a bush or something like that. I will stop the print as soon as I can test it. Other times I just print and go, then come back and!! Oh no. I've used the diameter in FreeCad, it should have been the radius!! on the screen, it all looks the same.

 

[Garno]

I've used the diameter in FreeCad, it should have been the radius!! on the screen, it all looks the same.

I have been doing that as well but in Tinkercad

 

[Sandyn]

I have been doing that as well but in Tinkercad

LOL. that moment when you look at the part and think. " that looks bigger than I expected", then the penny drops. D'oh!!!

 

[MorrisWoodman12]

@Garno As a matter of interest which 3D printer do you use?

 

[Gerry]

For what your doing don't use the tube tool in Tinkercad just use a solid shape of the correct size then put an a hole through it of the correct size. Make sure the hole has lots of sides and segments and allow for the plastic spreading as it's laid down.

Gerry

 

[Garno]

@Garno As a matter of interest which 3D printer do you use?

I have a Creality Ender 3 Pro, I got it on eBay brand new for an amazing price, Downside is that it is the 2018 model. I say downside but it seems to be the same as the 2020 model. I paid £127 plus postage so in all £134. So far I am very happy with it.

 

[Garno]

For what your doing don't use the tube tool in Tinkercad just use a solid shape of the correct size then put an a hole through it of the correct size. Make sure the hole has lots of sides and segments and allow for the plastic spreading as it's laid down.

Gerry

Just tried that and it worked an absolute treat, Thank you very much that.