Davit – Part 1 (Inserts to Attach the Davit to the Swim Platform)

[Don52]

A Davit is a small crane onboard a ship, especially one of a pair that are used for suspending or lowering a lifeboat. My friend and customer Ray has designed a Davit system to lift up his dinghy onto the swim platform of his boat. This post describes the inserts that I made, as well as the drilling and gluing fixture which were used to glue the inserts to the swim platform. The inserts will eventually support the davit. The first three pictures show a pair of Davits from another boat, to give you an idea of the typical design of the Davits.

1. Dinghy on davits 1


2. Davits 2


3. Davits 3


The next picture shows one of the 2” OD Aluminum inserts. The inserts were tapped for a M10 x 1.5 Helicoil. Notice the slight chatter on the grooves in the following picture. I machined this insert using a long piece of stock with steady rest, to minimize the stock wastage.

4. Insert


I left the chatter because it won’t hurt the function of the insert, but I did eliminate the chatter as shown in the following picture by using a more rigid setup.

5. Cutting groove

[Don52]

I used a spiral fluted tap to tap the hole for the M10 x 1.5 Helicoil. This tap is ideal for blind holes because it pulls the chips out of the hole. If you look closely, you can see the chip coming out of the tap.

6. Tapping hole


7. Inserts machined


8. Inserts powder coated


Here is a picture of Ray installing the Helicoils. Ray specified Helicoils that were 2.5 times the diameter or 25 mm long, which made them difficult to install. 1.5 times the diameter, would have been more than adequate and would have been much easier to install.

9. Ray installing Helicoils


I try to be as efficient as possible, so I clamped the two rails to the table of the Bridgeport, at the same time. I used 1-2-3 blocks to space the support rails above the table to avoid drilling into the table.

10. Support rails clamped

[Don52]

In the following picture you can see how I positioned the 1-2-3 blocks so that the drill would miss them.

11. Drilling support rails


In a similar fashion I positioned one block directly under the place, for the milled slot for maximum rigidity.

12. Milling support rails


And here are the support rails all machined.

13. Holes and slots machined


14. Support rails done


The next step was to build a drill fixture that actually had three functions:

1. Drill a ¼” pilot hole for the pilot drill for the 2” hole saw, which would drill the holes in the swim platform for the inserts.
2. Allow Ray to transfer drill holes in a backup bar, which will mount on the bottom of the inserts on the bottom of the swim platform to help maintain spacing of the inserts.
3. Will be used with some custom studs to hold the inserts in position, while waiting for the epoxy to dry.

15. Welding drill fixture

[Don52]

Here are some of my welds on the drill fixture.

16. Weld 1


17. Weld 2


Here is a picture of the custom stud that I made to allow Ray to use the drill fixture to hold the inserts while waiting for the epoxy glue to set, which glued the inserts to the swim platform.

18. Stud with nuts


Below is a cross section of a SolidWorks assembly model showing how the stud will pilot on the drill fixture and hold the insert.

19. Stud asm


The first step in manufacturing the custom studs was to make a spacer with a M10 x 1.5 tapped hole in the middle and a tight fit to the hole in the headstock of the lathe. The purpose of the spacer is to prevent the M10 x 1.5 grade 10.9 threaded rod from vibrating as I machined the other end of the threaded rod.

20. Support for threaded rod

[Don52]

My 15” Clausing Colchester lathe has an English lead screw and I most frequently cut Metric threads, so I have lefthand tooling, which allows me to start threading from the undercut of the thread, run the lathe in reverse and thread to the end of the shaft. I added extra material on the end of the part to allow the threading tool to clear the center as the tool exited the thread. I chased the threads using a center in the threaded shaft to improve the true position of the ¼”-28 thread relative to the M10 x 1.5 thread.

21. Extension for threading


Here are all of the custom studs completed.

22. Studs threaded


I allowed .001 to .003” clearance between the custom stud and the hole in the drilling fixture. As a result, I checked each stud in all of the holes and reamed the holes in the drilling fixture where necessary.

23. Checking stud for fit


Ray used the drill fixture to transfer drill holes in the backup bar, which will support the inserts on the bottom of the swim platform. Here you can see the back up bar and inserts mounted to the drill fixture.

24. Backup bar fit


Here is a picture of one of the plugs cut in the swim platform for the inserts.

25. Plugs cut with hole saw

[Don52]

The final step was to use the drill fixture and the backup bar to hold the inserts in position until the epoxy glue cured. Notice the black spacer, which Ray 3D printed to hold the top of the insert in the correct position.

26. Fixture set for gluing


27. Close up of gluing fixture


Last but not least is a picture of the backup bar holding the bottom of the inserts.

28. Backup bar in place


-Don

[Don52]

The holes were injected with penetrating epoxy to seal the core and bond in the spacers.



You can see the notches Ray filed in the holes to provide a path for the epoxy to flow to the bottom of the hole along side of the spacer

The bottom of the hole was sealed on the underside of the platform using a fairing epoxy.

Ray feels confident that the core is well sealed. It is balsa wood, a lot of this very thin epoxy was absorbed in each hole. Ray noticed and was pleased to see that the cores from the hole saw were dry and firm.

-Don

[ryanjones2150]

Good work and stellar attention to detail.

When you cut the grooves in those aluminum doofloppies, did you cut both sides of that groove at the same time?

Also, stainless helicoils?

I hope Ray was happy with this project, that much work would cost him as much as that dingy for sure!

[Don52]

Good work and stellar attention to detail.

When you cut the grooves in those aluminum doofloppies, did you cut both sides of that groove at the same time?

I did cut both sides of the groove at the same time.

Also, stainless helicoils?

Yes, the Helicoils were stainless.

I hope Ray was happy with this project, that much work would cost him as much as that dingy for sure!

He is. We are in the process of deciding exactly how we want to make the arms of the Davit.
Round, square or rectangular tubing sections.
Welded or bent.

-Don

[ryanjones2150]

That’s where your chatter comes from, cutting both sides of that groove at the same time. But it got the job done!

[Don52]

That’s where your chatter comes from, cutting both sides of that groove at the same time. But it got the job done!

Yeah I know, but I charge by time and material and I was trying to keep the cost down. If I rotated the compound to cut on one side I would also have to re-calibrate my DRO. When making multiple parts it is nice to be able to trust the DRO. With my more rigid setup there was no chatter even cutting on both sides. Of course the chatter actually helps the insert grab the epoxy better, but like you I don’t like to make a part with chatter.

-Don

[ryanjones2150]

I bet that DRO is nice on that lathe. If I ever use one on a lathe I’m sure I’ll be ruined like I was on the Bridgeport!

[Don52]

I bet that DRO is nice on that lathe. If I ever use one on a lathe I’m sure I’ll be ruined like I was on the Bridgeport!

Yes you will be spoiled. I am sure that you will really like a DRO on your lathe, if you ever decide to do it. I can send you drawings of all the parts I made to adapt the DRO to my Clausing Colchester lathe if you wish.

1) Readout


2) X-Axis


3) Cable Carrier


4) Z-Axis Pickup Support Bracket


5.Scale Covers in place


I installed the DRO in 2012 and I have never had any problems.

-Don

[ryanjones2150]

That colchester is the perfect lathe for a DRO too. Looks good!