Thread: Lathe and BMH

The very first sticky thread at the top of this section of the forum has the information you are looking for. All three stickies have good information you should look at.

Your 4" flex hose is limiting you the most. If you replace it with 6" or better still 6" duct with a minimum of flex you would get an improvement of 2 1/2 to 3 times the airflow at the bell mouth. You'd get far less dust escaping to the floor or air around you. The Oneida can easily handle the 6" pipes and 6" speaker ports are available too.

Not to steal BobL's thunder but hear is a video he did showing his 6" bell mouth. YouTube

Pete

I understand that 6" hose and BMH would be better and an 8" would be even better. The issue for me becomes practically . A 6" inch hose is more expensive and I do not have one. I used what I have available and it works well for what I am doing. Given the amount of lathe work I do, the ROI to setup one with 6" is not worth it for me.

What I have done resulted in a significant improvement in dust collection when turning.

The ultimate/optimum profile for a BHM requires a trombone or horn like structure about 10x the length of the diameter of the duct. For a100mm (4") diameter BMH that translates to about a a 1m (40") long form (and for a 150 or 6" BMH it needs to be 1.5m or 60" long but few people have sufficient space behind machine for such a horn.

For less than a 1% loss of performance compared to a horn structure a simple formula that has the radius of curvature of the lip at half the Diameter of the duct, so for 4" duct the radius of curvature soul be 1". This makes the minimum outer final diameter of a hood on 4"ducting to be 6", for a 6" hood it should be 9". You can use a larger outer diameter but the returns in performance are small (I cannot measure them).

BHM performance is limited by whatever pressure is available at the collection point . If your'e using a small DC, long lengths of 4" ducting and lots of 4" junctions and flexy these will seriously reduce available pressure so a BMH won't help much. Same for 6" ducting/junctions/flexy. The greater the available pressure the more effective the BMH will be.

Interesting point about effectiveness of BMH and available pressure.

Would a properly sized/shaped BMH on a vacuum with high static pressure work well. I am thinking a 2.5" or 1 .25" hose.

Originally Posted by Maple71

Interesting point about effectiveness of BMH and available pressure.

Would a properly sized/shaped BMH on a vacuum with high static pressure work well. I am thinking a 2.5" or 1 .25" hose.

Yes it would. Must do a test in this some time.

One also needs to bear in mind that Vacs are limited by the small size of their impellers which is why they don't draw much more air through a larger diameter hose.