Tapping Question

[tornitore45]

can the part be better designed ?
Dan.

That should always be the first question when faced with a difficult operation.

 

[colby15642]

OK, advice on this problem I'm going to be up against. I want to tap an 8-32 hole that is only 0.150 to 0.165 deep. Hey, it is what I'm up against. I plan to drill the hole depth with a number 29 (0.136). Find zero on my Bridgeport and raise the table, 0.160. Then I have an extra 9/64 end mill that I ground flat on my diamond wheel grinder and use that to flatten the bottom of the hole. I do have a spare 8-32 tap that I ground flat also that I've been using with good results after starting with a taper tap when possible. I have a lot of spare taps that if a taper tap needs some taken off, I can do it. Then a plug tap, then my bottoming tap. I haven't done anything yet as I want to get as much advice before I start this tapping of 16 holes!!!

I have some of the same advice as others have provided.
1. Rack your brain for an alternate design that allows through-tapping and/or more thread engagement.
2. For some reason, I have had poor luck drilling an accurate hole with an end mill. It may have something to do with the shape of the lands on the end mill OD.
3. Thread engagement will be minimal, even if you change to an 8-36 fine-series thread. Subtracting from the theoretical five threads in the hole, you will likely lose at least 1/2 thread at the entrance and another 1/2 thread at the bottom of the hole. Even a standard bottoming tap may not engage a hole that shallow.
4. You haven't said what material the tapped hole will be in. You may experience the best results in free-machining alloy 360 brass. 12L14 free-machining steel may give acceptable results, but it will more aggressively resist the tap at the bottom of that shallow hole.
I will pray for your success!

 

[tornitore45]

End mills are not recommended for drilling because they cut on the side therefore any deflection will oversize the hole. Drill bits do not cut on the sides, the lands are cylindrical ground (with very slight taper) like a reamer.

 

[swood1]

Make yourself a small tapping guide to ensure you keep the tap straight..... Also, you need to realise a tap is cutting steel from the hole so you need to clear the flutes (spaces between the cutting surfaces of the tap), this may require you to remove the tap completely from the hole and then use a small brush to scrape/remove the swarf. This is especially important if your tapping deep holes...

I don't know whether anyone has mentioned but when I was a trainee the old boys used to say cut a 1/4 of a thread at a time and back the tap off each time to allow the swarf to break away.


Steve

 

[davidyat]

The holes are just keeping a compressed air channel in line. I have a 3/16 brass tube, silver soldered into a fixture that will be held in place with these 2 holes that are tapped with a couple of 8-32 screws into a compressed air, rotary valve engine block. At the other end, a brass intake fixture has the other end of the 3/16 brass tube silver soldered into it. With the upper fixture screwed into the head of the cylinder and the lower screwed into the block, I don't see any problem with the compressed air. I'll try to take a picture of what I'm trying to do. I just want to keep the bottom in place of the hole in the block for the intake tube. With the tube silver soldered and screwed into the head, I'm sure the compressed air won't blow the tube out of the block.

 

[davidyat]

Maruo,

Thanks for your advice. I'm not using the "ground flat" end mill to do any drilling, just to flatten the bottom after I drill the hole with a drill bit.

Thanks

 

[BIGTREV]

Something that has not been mentioned is to use break-free for tapping oil when using tiny taps. Not cutting oil per se, but with it, the taps don't get stuck.

H. What s "break-free tapping oil"..Thanks

 

[MachineTom]

A tapping block is the simplest To make, for a 4-40 a 3/16 thick piece of steel is all you need to begin with. I generally add a tap diameter and a body diameter hole as well to the strip. .750 x .187 x 3.
Unless you a doing hundreds of holes no need to harden the piece. I also have a Phase ll tapping fixture but it only begins at #8 size, to big for a #4. Also a #30 tapmatic power tapping head. That will do down to "0" size taps, on a project where I need 40 #4 holes tapped it was great. If you ever come across a Burgmaster Turret drill, buy it. With 6 spindles, each a different speed, a tapmatic tapping head, that a a SCREWING machine.lol

 

[tornitore45]

I rather grind a $1 drill bit to square a hole bottom than ruining a $10 end mill but that is because I am cheap. I have accumulated at least 6 sets of drill bits, have no hesitation to modify each size for squaring bottoms and modify another for drilling in brass.

 

[tornitore45]

A SCREWING machine is off topic here.
A screw-machine is an automatic lathe doing production work, like screws, nuts, valve pieces etc.

 

[tornitore45]

Why #8 screws? The picture appear to be a small engine, two #4 screws may be adequate, even smaller.

 

[davidyat]

Everyone, thanks for all your advice. I took 2 days and a lot of patience and made the 16 tapped holes. They really will work for what I intend them for. Mauro, if you go here:
https://www.homemodelenginemachinis...-rotary-valve-engine-build-2x-scale-up.25083/
you'll see what I've been doing. I've been following Bazmak's build at twice the dimensions. It's possible I could be done in a few days and hope to post a video of the engine running if I don't botch up the drilling of the air ports in the rotary valve.
Grasshopper