Trepanning Tool

Assuming the blank is < 1/2" thick, idnat why God invented hole saws?

But in a lathe the patient's head would have to be spinning. I think it would be more practical to do this on a Bridgeport or large drill press.

A properly ground bit makes a better one .

I am struggling to find a good way of cutting out 2.5" disks out of

3/16" steel plate. I have not tried trepanning but it is on the list.

Would a hole saw be useful for this application? How many disks would one be able to get out of one saw bit?

BTW if I farmed this out to a local shop for plasma cutting it would cost me $6 per disc. Cutting the same disc with a jeweler's saw costs

1 hour, 2 blades and a sore shoulder.

Michael Koblic, Campbell River, BC

I've not had good results with hole saws - neither cheap nor expensive. They need slow feed and flood coolant. I keep them only for non-metal use now. Likewise, I've cut circles in steel with a (homemade) fly-cutter. It works, but I find it tedious and marginal in terms of speed and finish.

How about rough cutting with a fine oxy torch, rough grind off the "bulbous" dags, clamp together (between centres) and parallel turn to size on the lathe? Auto-feed and auto-switch-off and very fine feed means you can indulge in a peaceful cup of coffee, or read a magazine, while the lathe patiently does the dirty work. Can you drill a central hole? That'd make it much easier to clamp.

Alternatively, hold the rough shape in a 4-jaw and attack it from the face.

(sorry if I've repeated earlier advice - only just jumped in now)

-- Jeff R.

My son could probably stamp out a few thousand per shift once the die is set up. Gerry :-)} London, Canada

Hole saws tend to give really rough finish, and are best used in a vertical position with a drilled hole intersecting the cut line to drain away chips. And, given the number of cutters (teeth), and the radius, it would probably be too much load for your fairly small lathe.

A trepanning bit for a lathe should be ground curved, so its centerline matches the curve of the cut being made. There should be clearance on both the inside and outside of the cut. It sort of should look like this -- end on:

--((--

except that the curve and height of the tool should really stop at the center of the height shown -- as indicated by the "-- --" lines.

So -- you can figure the price of two blades. Do you have a figure of what your time is worth? After a week or so of doing this the arm should become accustomed to this, and the soreness of the shoulder be reduced.

However -- I presume that the $6.00 is for cutting out a single disc. What happens if you stack three or four sheets, so you get multiple discs at a pass? The thicker it gets, the less precise the cut line is I believe, but there should be some tradeoff value to determine where to stop stacking them.

And have you checked what waterjet cutting or laser cutting would cost instead of plasma cutting? Both of those do a cleaner cut when stacked.

Good Luck, DoN.

He says to look for a small stamping shop and ask them for a price on the desired quantity - you may be pleasantly surprised! Gerry :-)} London, Canada

Recently had several hundred 4" diameter X 1/8" thick *galvanized* disks laser cut. Was *way* cheaper than I could have cut them with a hole saw even if I paid myself minimum wage.

That particular shop only had punches to 3 1/2" so I did not get to compare prices of the different techniques.

There are also online retailers that sell plain steel disks in various sizes and thickness. Costs were higher but lacked the minimums and lead time of the local shop. No galvanized and the shipping was outrageous on the few I bought to hold me until the local order was done.

Is the vendor I used, curiously they do not show a 2 1/2" in 3/16".

2 3/8" and 2 7/8" are both offered in 3/16" thick so perhaps it is just missing from the list?

Do you have a (motorized) scroll saw? I can use the same blades on mine as on my jeweler's saw. (But not vice versa -- the jeweler's saw only takes pinless blades, the scroll saw accepts either pinned or pinless 5" blades.) I've cut lots of wood, paper, and plastic on my scroll saw, typically with a 15 or 20-tpi blade, and small amounts of steel with a 40 tpi blade (0.012" x 0.022" cross section). The steel was 1/20" sheet and I advanced it only about 1 IPM with the saw running at about 600 strokes per minute, which gave a milled-looking cut surface. With thicker metal, heavier blade, and higher SPM, you might be able to race along at 2 IPM!

The rough finish would not be a problem as the whole thing gets finished in he lathe anyway. Cost in terms of consumables (blades) is, as well as the duration of the procedure.

I see some people make their own trepanning tools:

Although their skill is admirable I am not sure that I want to invest the time etc. in a single purpose tool.

I looked at what was available commercially and came across this:

What I found interesting was that they seem to use the same cutter over the whole range of radii which is quite wide.

I asked for a quote on 10. $58 including the material. Looking at other vendors on line the drop in price from 10 to 100 is not that significant.

I would be surprised in anyone within 100 km has either of those, but one never knows.

Michael Koblic, Campbell River, BC

Interesting, thanks. It gives me a guideline what to expect. The prices relate well to prices of washers which is my benchmark of expected costs (1" washer which has OD of 2.5" costs about $1.10).

The problem as you mentioned is the shipping which will be a multiple of what you paid and the fact that they ship UPS.

Still, as Gerry suggested one should look for a stamping shop.

Michael Koblic, Campbell River, BC

The short answer is no. I considered buying one and asked around about its performance on steel. The consensus was that for 3/16" plate it was a waste of time, money and a lot of blades.

Michael Koblic, Campbell River, BC

You're around Van Couver, above Washington State? Oh, I see, yer on that big island..... Man, you must be in paradise, over there, altho mebbe not machine-shop wise.

How is industry over there, in general? How bout in places like Van Couver, or Surrey?

I see from Victoria, you could proly row-boat to Seattle, eh??

Greetings Michael, Trepanning the steel would work well if you have a rigid setup and a slow enough speed. Hole saws don't have much room for chip evacuation, so you need to keep pulling the saw out of the hole to clear chips.

3/16 isn't that thick so maybe not too much trouble. A high speed steel trepanning tool would need to be spun pretty slow but if ground properly can take a pretty good chip load. A carbide tool needs more rigidity but will cut much faster. If you can tolerate the noise it's possible to run the carbide fast with a light chip load if you use the right grade of carbide. You might be able to unsolder a tooth from a circular saw blade and use that. Anyway, with a light chip load and spinning fast the carbide will chatter and the chips will come out as flakes. Using spraymist with plenty of air pressure will blow the chips out of the groove so you will be able to make a continuous cut clean through the the steel. Grind the end of any tool you use at an angle so that the tool will break through first at the inside diameter of the groove if it's the discs that you want. Stop feeding the tool when it is just shy of breaking through. Then use a hammer to knock the disc out of the plate. There will be a slight burr that is easily removed with a file or sander. If you cut all the way through there is a good chance that the disc will shift and pinch the tool which will probably break the tool. I must reiterate that using carbide in the above manner will be VERY noisy. Use hearing and eye protection. Cheers, Eric

Perty ingenious.....

Now, iffin only yer politics could follow suit....

Still, ahm gonna move in with you, in a year or so....

Democrats!!!

It's great if you like rain.

There is some. Not a lot. Small machine shops etc.

I suspect there is industry there. Bit out of the way to investigate first hand.

But why would I want to?

Michael Koblic, Campbell River, BC