Tap use limits

In general thread forming taps require a larger hole than thread cutting taps. There is a formula to calculate the hole size needed. Being lazy over the years I've occasionally tried to find a drill tap chart for thread forming taps rather than digging out the Machinery Handbook every time I use them. So far I've run out of patience wading through all the charts before finding the right one
 
I printed out and laminated the Littlemachineshop tap drill chart (sae one side, metric the other), then wrote the form tap drills next to the printed ones for that size
 
Starrett offers free Poster Size Drill Charts (Inch/Metric/Tap). I have one on the wall in the shop. You do need to "sign up" for an account. (Note: I've never received any spam from them)

https://www.starrett.com/catalogs

Scroll down to "Order Literature"
 
Starr, that is a lot of holes! My calculator tells me a #45 drill will give you 90% thread instead of the standard 77%. It will require more torque to tap those holes. As mentioned previously, use quality HSS taps and cutting fluid for sure.
I broke down a few years ago and purchased a Tapmatic tapping head, used on ebay for around $150. It was a great idea. They (like most tapping heads) are adjustable for torque so you can get just what you need. Another upside is that if you start with a new tap, as the tap gets dull, the clutch starts to slip so you know it is time to replace the tap. These units will work in both through and blind holes, and you can set your drill press or milling machine to control depth. By size, they cover a pretty good range of tap sizes. I have purchased a couple others in larger and smaller sizes since. All on ebay, and for under $150. On such a project, having a tap break on hole 105 would hurt much worse than investing in a tool you will enjoy using for many projects that require tapping of multiple holes. My only caution to you if you start looking at tapping heads would be to make sure your purchase includes the necessary rubber collet for the project or at least know they could set you back another $30ish.
 
For a really good tap, buy OSG EXO taps ( Japanese ) - They are super
and what we used for die work when I was in the shop. They Cut superbly and can do really hard steel as well as Aluminum
We used them to do Rc 52-56 H 11 material for example - and they are strong

A bit old fashion, but if you are concerned, get a 'Starting tap" and pre-start the holes .
in the old days , Taps came in sets of three- Taper (for starting !) -Plug ( normal tap) - and Bottom
A Taper (starting tap) is great for hand tapping as it prevents cocked holes, so a Guide is not necessary.

I have a friend building a 9 Cylinder radial engine and he broke 3 tap in the crankcase.
I burned them out for him using Ben Flemings EDM design which used to be on Yahoo Groups
but I believe they moved to
groups.io

EDM-HomeBuilders groups.io Group

Home built EDM
groups.io groups.io

For info go here

Home built sinking EDM - The Home Machinist!

www.chaski.org
I built mine for about $100, but used shop surplus in doing so

Post edit comment
90 % is a lot of force , be careful !
Rich
 
There is a free iPhone app "OSG Calculator" that will calculate tap drill sizes with different thread percentages/engagement so that you can tailor the thread engagement for the type of material and application. Also includes speed feed calculator. No ads, no in-store purchases needed. Metric, inch, form and pipe taps.
 
You must log in or register to reply here.
Back
Top