Threading drilled holes in PM castings

[kb58]

Just asking to see what others do.

After receiving my PM935 mill, I added various things, which required tapping holes drilled into the housings. Even though I was really careful, and used tapping fluid, I still managed to snap off an 8-32 tap in a casting. I realize the larger the tap, the less likely it'll break, but for those particular holes, 8-32 matched the need. Anyway, reading up on it, I'm reminded that I should probably use rolled-thread taps more often, as they're a lot stronger than the cutting type.

What do you guys do to avoid breaking taps when adding accessories to PM products?

 

[Dan_S]

Did you tap by hand without using a tapping block? Also, what type of tap did you?

 

[macardoso]

I would strongly recommend AGAINST a form tap (roll tap) in cast iron. It relies on plastic deformation of the material where ductile materials like aluminum work best. Cast iron is very brittle and will not take a form tap well.

Short chipping materials like cast iron work best with straight flute taps. Make sure the tap is new and sharp. A lot of the Home Depot brand taps are not ground along the flutes and do not cut well. I found that taps (even the cheap imports) from machine tool companies like Shars, JTS machinery, etc. cut much better than the name brand department store taps. Taps have very finite lifetimes and a buck of old rusty taps will cause you more headaches than they are worth. Mine all ended up in my scrap bin.

Keep the tap straight using a tapping block if needed and go slow. The cast iron should cut with very little pressure but make sure to back the tap out to clear the hole of chips if the threaded hole is deep.
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EDIT: Also make sure you are pre-drilling the hole correctly. An 8-32 tap in cast iron (50% thread) should be pre-drilled with a No. 27 (.1440) drill. The closest fractional approximation is 9/64ths. An 1/8" or 5/32" drill will not work. Also if you do ever use a form tap, remember the pre-drill hole must be larger than a cutting tap. There are manufacturer charts for this.

 

[jbolt]

I generally tap cast iron dry with no cutting fluid and clear the hole more frequently than other metals. Cast iron is carbon heavy and more or less self lubricating. Cutting oils tend to clump the small chips formed from cast iron and will pack up quickly.

I prefer spiral point taps (gun taps) for through holes and starting blind holes and then finish blind holes with a spiral flute tap.

 

[tq60]

Many do not break and clear chips.

We learned way back to simply stop and backup just enough to break the chip.

Yes it adds lots of time but it greatly reduces tap breakage on small taps.

You can feel the break then continue.

Often you turn clockwise only 1/2 a turn at a time and remove completely often on dead or not through holes.

This process results in smaller chips and possibly less strain on the tap as well.

It also has as a benefit better inspection of the threads as you go when you remove tap...can better see where the bottom is and blow chips out.

Sent from my SAMSUNG-SM-G930A using Tapatalk

 

[kb58]

Yes, in the back of my mind, I was wondering how a roll-type tap can work on cast material since it doesn't compress.

When tapping the hole where the tap broke, the tap was brand new, never used (McMaster), and I was turning it maybe half a turn at a time and backing it out a quarter turn to break the chip. I knew I was in trouble when it took a worrisome amount of torque to proceed, but just as much torque to try and back up to break the chip. In fact, that's when it broke, trying to back it out. The depth of the thread at the time it broke was maybe 0.25" or so. About all I can try is to use a slightly larger drill in order to lessen the thread depth.

 

[pacifica]

A double end adjustable wrench gives better feel and it is easier to keep the tap straight and perpendicular to the work.Plus all the points made by tq60 and jbolt.Do all these things and you may never break a tap.

 

[tghsmith]

using a cheap drill press drill stand, a part from micromark , and turned plastic bushing I made a simple tapping fixture for the small stuff.. didn't like the feel of the shaft and the plastic bushing so a brass tubing sleeve was pressed in.. works very nice..

 

[pacifica]

using a cheap drill press drill stand, a part from micromark , and turned plastic bushing I made a simple tapping fixture for the small stuff.. didn't like the feel of the shaft and the plastic bushing so a brass tubing sleeve was pressed in.. works very nice..View attachment 287824View attachment 287825

How would that work on the side of a machine?

 

[macardoso]

Maybe PM uses better cast iron than my ENCO and Grizzly machines! That stuff seems to cut like I was tapping a sand castle.