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Blind Measurement of Casting Thickness for Drilling and Tapping

Gundraw

Aluminum
Joined
Oct 21, 2013
Location
Midwest
I am installing a DRO on my horizontal mill. Unfortunately, there are three locations that require drilling (hopefully) blind holes in castings which hold an oil bath for a gear train (one hole in the knee, two in the column for mounting the display). I don't have good access to the inside of these castings, even if I empty the oil and take off the inspection plates/covers.

I purchased a low-cost ultrasonic tester (offshore GM100) to see if this would work. The device does surprisingly well on the few pieces of steel and aluminum I tested. Cast iron, however, seems to not work with this at all. Even my angle plates with nice flat surfaces give erroneously small thickness readings (ie, reads 4mm, when the angle plate is 23mm thick).

Any suggestions?

ETA: Figured I would add based on the responses so far:

My hope was to use the ultrasonic gauge to find a places where I could drill deep enough to tap without running through into the oil bath cavity. I am NOT concerned with oil leaking from these threads (they are all well above the oil level). My main concern is drilling through then spilling drill/tapping chips in the oil that I will not be able to retrieve.

The one hole in the knee is far more concerning that the column as the scale HAS to mount there. However, the scale uses a 4mm screw. I hoped there would be a place where I could drill 8mm deep, use a bottoming tap, an have plenty of threads of the screw to hold the scale (not much weight).

As for the column, I could likely find another location to mount the display. However, if I had a way to determine thickness, it would be cleanest to mount it there knowing I had the depth on the casting. (these would be drilled for 6mm screws)
 
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You haven't said what size fasteners or how heavy the display is.

My guess is that unless you are very lucky any hole you drill through deep enough to tap for a reasonable size fastener is likely to strike oil.

I don't know of any liquid tight thread inserts so unless you can find some my suggestion is to add external mounting blocks with epoxy. Either that or fabricate a strap-on mount if that is possible.

With mounting blocks, if you know there is a section where you could drill and tap for small screws they could reinforce an adhesive mount. Without more details than you have given all is just a guess.
 
M/C up some blind plugs from brass, threaded od and blind tapped ID leave them a little long past the OD thread and wind them in to a stop (as in the threads jamming when they run out - like a stud) with a known oil proof jointing compound, then trim off and dress up.
 
First see if there is any oil behind the hole
Oillevels are not that high on machinery in general
Drain the oil (or not)
Drill Tap and then a threaded stud with loctite

Peter
 
Put heavy grease on the drill and tap to catch metal chips so they do not fall into gears and precision surfaces. On My Harrison lathe the rubber bellows on the apron is held in by three small screws that go into the oil reservoir. They slowly seep oil. Folks here suggested heavy grease or Teflon paste to help slow the drips. There was another suggestion for something like non-hardening gasket sealer but I forget the name.
Bill D
 
I was not clear enough in my original post (I have since edited). I am not really concerned with oil seepage from these screws, as all of these holes are well above the oil level and I can easily add sealant.

My concern is drilling through the casting and spilling chips into the oil bath cavity. My hope was to use the ultrasonic gauge to find locations I could drill without going through, while still getting sufficient threads using a bottoming tap.
 
in that case I would use a left handed drill so the chips come outward instead of inward.
:confused: Both LH and RH drills are going to bring chips up out of the hole (if you rotate them the appropriate direction). Both are equally likely to drop some chips on the far side as they break through.
 
If the base is anywhere close to being closed, pipe an airline into the empty head space and blow air into the casting while drilling / tapping holes. This should blow most of the chips back out of the hole.
 
As an added precaution you can drop a magnet or two into the sump to permanently hold the errant chips. Drain and filter the oil to collect the floaters.
 
I see a couple of possibilities. First you could try to purchase or rent a better thickness meter. Get assurances of money back if it does not work on CI.

Another possibility would be to clean up a rather large area on the surface of the casting and attach some nut plates with epoxy. You could drill some very shallow holes (1mm?) to provide a better grip.

Drill your holes near the corners of the casting so that they will go into the adjacent side and not into the interior space. You could mount some brackets (1/8" or 2mm thick) to two or more of these holes to extend to where you need the mounting holes.

Mount a full sized panel over the casting, end to end and top to bottom. Fasten it using the methods above or with clamps that go to the sides or even to the back to hold it in place. Mount your scale(s) to that panel. If there is enough space, a piece of channel would be better than a flat panel.
 
In our engine shop, we use a similar Chicom thickness tester and find it to be very accurate on cast iron. In the manual for ours, there are different settings for grey iron, nodular iron and wrought iron.

And yes, IMHO, you're way overthinking a few stray chips might find their way into a machine which is for making chips by the bushel.

jack vines
 
Drain the oil, drill/tap, install a stud with a thread sealant/locker, let dry, refill oil.

N/M ^^^^

Use your shop vac with the fine tip nozzle. I'd pilot the hole in hopes of additional control when using the tap drill. Spiral flute tap, as well.
 
Easy solution. Before drilling and tapping pressurize your oil reservoir slightly using regulated shop air. Any chips will get blown out rather than fall in. Would be a good idea to use a spiral tap too. Those pull the chips back out of the hole rather than push them in.
 
I was not clear enough in my original post (I have since edited). I am not really concerned with oil seepage from these screws, as all of these holes are well above the oil level and I can easily add sealant.

My concern is drilling through the casting and spilling chips into the oil bath cavity. My hope was to use the ultrasonic gauge to find locations I could drill without going through, while still getting sufficient threads using a bottoming tap.

Unless you are using automotive oil, chips in the oil will not be an issue. Machine oil is designed to allow contaminates to settle out into the sump. Automotive oil is designed to keep contaminates in suspension so the oil filter can remove them.
 
even then this seem not to be problem. there are enough guys out there who have drilled their oilpan for a fitting on their car. they probably never heard of a spiral flute tap.
 
You could probably reckon a column to be at least 1/2"thick. Tapping around with a small hammer can be telling. Put some tape on your drill 3/8"from the end-I doubt it's thinner than 3/8"
 
put your finger (even better someone else's) on the inside at the point your drilling, you will feel the heat from the drill when it's close!
 








 
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