Thread: A couple of cracks at a bellmouth form for a dust extractor shute.

Artful,

If you could find one of these non working speaker horns you would be set. They were used behind movie screen, in warehouses, and outdoors.

https://www.google.com/search?q=outdoor+loudspeaker+horns&client=ubuntu&hs=3jB&channel=fs&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiCnpa9sd_gAhXCnOAKHcNkB9oQ_AUIDygC&biw=1440&bih=754

WW2 aircraft locator:

https://2.bp.blogspot.com/-8cbKCMBdD...locator_12.jpg

AB I find using a live centre and a piece of steel or Al plate on the tailstock as a "pusher" a much better way of applying forward pressure.

I use two pieces of plate so that on sits inside the PVC ad this helps centre the PVC on the Timber former.
I also use two difference size pushers - for 100 and 150 mm pipe.
Pusher1.jpgPusher3.jpg Pushers.jpg

The rucking you are seeing is due to too much heat being applied behind the end and not enough at the very end of the pip.
Remember the end of the pipe has to expand by ~50% but the area further back behind only by a few %.
Apply such more heat at the end of the pipe and progressively less further back - it sill needs quiet a but of heat further back just not as much as the front of the pipe.
The PVC needs to be soft enough so that you can easily indent it with a finger nail.

I hold the hot air gun in one hand and a digital IR thermometer in the other and monitor the temperature of the pip around it s end. It needs to be ~110º at the very end , about 105º 25 mm back from the end and ~100º 50 mmm form the end before I start pushing.

I also apply light machine oil or natural wax to two former to help the soft PVC slide over the former.

It looks as though there is too much heat being applied, the pvc is getting glossy. In one picture it’s starting to burn. It shouldn’t need that amount of heat, pull the heat gun away a bit. I found that if I kept a hand on the tailstock and added a bit of pressure, I could feel the point at which the pvc wanted to move, keep adding heat at this point and start winding the tailstock little by little. Gradually take the heat gun a little further away and you should keep the pvc soft enough to finish the bend. It’s certainly a learning curve (pardon the pun) Are you spinning the lathe up or just rotating by hand? The tailstock setup looks like you are just using the lathe to push with. Set it up so the lathe is running slowly, that way you will get an even heat around the circumference of the pipe

Originally Posted by Cal

It looks as though there is too much heat being applied, the pvc is getting glossy. In one picture it’s starting to burn. It shouldn’t need that amount of heat, pull the heat gun away a bit.

It should definitely not be going brown, at that point its starting to decompose(BTW it releases hydrogen chloride gas) which is not good for you.

Actually did not even consider having the lathe running. Was just turning it by hand.
Also had a hell of a time finding the right diameter plastic pipe to fit the 100mm flexible dust tube. The grey pipe I found at a tip shop, Its got an internal i.d of 95mm and o.d of 100mm it is stamped as electrical something or other.
There must be an ideal radius measurement to suit 100mm flexible extraction hose too. My form was just a shot in the dark.

Buy a pvc joiner and see if it fits over the flexible hose, if so use that to join the two bits together. That worked for me on 150mm bits but brands of flexy differ in od and id too.
I followed bobs instructions on this, I also used a little blow heater behind the lathe due to doing it in the middle of winter, here is my setup

Originally Posted by artful bodger

Actually did not even consider having the lathe running. Was just turning it by hand.

You can't heat it evenly enough just turning the PVC by hand - it should be turned slowly about 60 rpm or it will self cool.
This frees up a hand to use a thermometer while ether other holds teh hot air gun.
Some lathes don't turn that slowly - one of the benefits of a VFD.

Once the BMH has been formed you can rack up the the RPMs - I take it to about 250 because this helps it cool down. I also use a wet rag to assist cooling.
I measure the temperature with the IR thermometer. If you remove the BMHs from the former too soon before they have cooled down enough they will shrink - I remove at 50ºC.

Also had a hell of a time finding the right diameter plastic pipe to fit the 100mm flexible dust tube. The grey pipe I found at a tip shop, Its got an internal i.d of 95mm and o.d of 100mm it is stamped as electrical something or other. There must be an ideal radius measurement to suit 100mm flexible extraction hose too. My form was just a shot in the dark.

The electrical stuff is harder than regular PVC so will need higher temperatures - best avoided.

I know its what everyone tries to do it, but it is not a good idea to put flex OVER a pipe - this means the ID of the pipe has to be smaller than the flex which causes a constriction.

100 mm pipe is actually 102 mm and if you drop it to 95 mm that pissy little 7mm will undo a significant amount of the benefit obtained from using a BMH.

The fitting that flex will (with a bit of effort) slide into one side, and a BMH can easily be fitted on the other side, is a female coupler.

Have a look at this post to see one way of doing it.
150mm quick clamp things

That way there's no constriction and 102mm ID (or 154mm for 6" pipe) is maintained all the way.

Most of this and more is in my WIP thread on BMH production - might be time for re-read.
Starts here Improving machine cabinet dust ports but there are a number of posts further along i the thread about the better ways of doing things. Bill Pentz chimes in for a couple of posts unrelated to BMHs so you can skip those.