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View Full Version : what do I need to know to machine dovetails?



Black Forest
12-18-2010, 01:30 PM
I want to build a QCTP. I think I will build the one like Sir John built. So I have to make dovetails. I know I should use an end mill and remove as much material as possible before using the dovetail cutter. After that I am clueless!

I assume I take it pretty slow with a very conservative feed?

Dr Stan
12-18-2010, 03:06 PM
Forest,

The answer is yes, rough them out before using the dovetail cutter. Sometimes we would cut the bottom of the slot for female DT .015/.020 too deep on purpose to give some additional clearance making the dovetail easier to move. Note: you would still maintain the correct depth in the actual dovetail contact area.

BTW, you should be able to find the way to measure dove tails with the pin method in Machinery's Handbook.

Stan

danlb
12-18-2010, 03:54 PM
I heard of one technique that uses a t-slot cutter to cut the area that the tips of the dovetail cutter would otherwise cut. The idea is to minimize the wear on the weakest point of the cutter. The tips of the dovetail are not normally used anyway.

Dan

Tony Pratt
12-18-2010, 03:59 PM
Anything that can be done to minimise the actual metal to be cut with the dovetail cutter is good.
Tony

whitis
12-18-2010, 04:15 PM
It is a common practice to undercut the inside corners of the dovetail. This can be done by drilling holes from the side before miling or using an endmill or rounded cylindrical cutter with the head or the work tilted. This serves a number of purposes:

reduces the wear on the corners, which are the weakest part, of the dovetail cutter
Reduces stress risers (rounded cuts are better for this)
provides room for debris clearance
provides clearance for hand scraping operations
Prevents interference with sharper corners on the mating dovetail pieces

However, this tends to involve additional setups and doesn't usually seem to be done on QCTP tool holders.

rohart
12-18-2010, 04:41 PM
Although the downside is all the messing to remove metal prior to getting the dovetail cutter into action, the upside is that the final cutting is not a precision operation.

The final clearance is taken up by the gib strip, which you can size to fit, and you can clean up the edges of the slides after to make it all look neat.

I found that the big decision was how to shape the dimples for the gib adjustment screws. Flat with a gib locating peg, or dimples to do the locating too.

vpt
12-18-2010, 04:47 PM
I did this in my atlas lathe.

http://img227.imageshack.us/img227/6408/millingtoolholder021.jpg

http://img823.imageshack.us/img823/647/millingtoolholder022.jpg

http://img248.imageshack.us/img248/4562/millingtoolholder023.jpg

MotorradMike
12-18-2010, 05:25 PM
VERY nice pictures VPT.
Did the dovetail cutter survive with all points intact?

Rich Carlstedt
12-18-2010, 09:44 PM
Dovetail cutters, love to break their tips off when chips are not cleared.
Use plenty of vacumm or air to keep the chips from coming around and breaking the flute tip.
The problem with too little coolant is it makes the chips stick, whereas flood coolant is great. I run dry, so the chips can be sucked out
Rich

Mcgyver
12-18-2010, 11:19 PM
I heard of one technique that uses a t-slot cutter to cut the area that the tips of the dovetail cutter would otherwise cut. The idea is to minimize the wear on the weakest point of the cutter. The tips of the dovetail are not normally used anyway.
Dan

+1 . The corners of the finished dovetail will benefit from being anything but a sharp corner, the perfect stress raiser. The tips will fail first....so the best strategy is to remove material such that the tips don't cut. key cutter at an angle works; on a recent project i drilled through with a 1/8" hole to provide clearance....I've done the cuts without, but making it so the cutter doesn't cut in the corner is the best way. Like Rich says, flood is good, well, isn't that true for everything? :)

oldtiffie
12-19-2010, 12:24 AM
Dove-tails and a shaper are just made for each other.

RussZHC
12-19-2010, 01:29 AM
What is the reason(ing) behind the use of dovetails for machine slides?

In woodworking the dovetail is used, more or less, as it locks together and there had been some discussion that if the glues of the "old days" had been better, dovetails in wood working may not have been needed (one test I saw proved finger/box joints are stronger using modern glues).

In the my version of Machinery's Handbook the machine slides that are dovetail are "singular" ("bedding strips" have one side dovetail the other gib set in an "L"; "square strips" which have no dovetail; "overhung strips" similar to "bedding"; "special strips" that mix a dovetail gib and a square strip).

In another thread (something to do with "box ways"...as well as Brian's thread about assembling a dovetail from pieces) the reason given was lifting in the direction the forces were makes sense as does another part that if box ways are used it means another gib to control said lifting forces (so more steps in manufacturing).

Anyway, back to the OP's plan to make a QCTP, would "T" slots not fulfill the role of dovetails in this situation? Or, failing that, something similar to the Dickson style

http://www.lathes.co.uk/latheparts/page13.html

that appears to me could be made without a dovetail cutter and its potential difficulties in execution.

Noob, but it seems to me "T" slots have some advantages not the least of which is when using tapered gibs, how does one go about getting those sides that are tapered consistent, in the correct orientation and relative to one another.

jkilroy
12-19-2010, 10:45 AM
Note that unless made to some incredible numbers a T-slot only really constrains movement in one direction. Note the Dickson tool holder has dovetails, just in reverse, that actually provide location, the t-slot is only the force applied to the angled surfaces that locate the holder.

jackary
12-19-2010, 11:11 AM
I have found that when using a dovetail cutter it is good to use the cutter on one face at a time. After end milling to the nearly the depth required the dovetail cutter is used to cut on the sloping surface only by raising the cutter to just clear the floor so to speak. When the sides are finished take a finishing cut with the cutter lowered to the final depth so that it is only cutting on the floor. This avoids chatter which can happen trying to cut on two faces at once.
Alan

BillTodd
12-19-2010, 12:03 PM
Sawed mine with a horizontal mill.

http://billtodd.dyndns.org/test/qctp1.jpg

Bill

RussZHC
12-19-2010, 07:06 PM
I guess my reason for mentioning the Dickson posts was that, to my eyes anyway, one ends up with dovetails but does not have to use a dovetail cutter...and I was thinking of difficulties a noob like me would face and some of the issues others mentioned about the delicate nature of the very tips of dovetail cutters.

vpt
12-19-2010, 07:41 PM
VERY nice pictures VPT.
Did the dovetail cutter survive with all points intact?


Thanks. The dovetail cutter in the pics already had its tips wore down and there were chips in the cutting edge. I had resharpened the one in the pics with a dremmel mounted to the tool post on the lathe. You can tell looking at the pics.

noah katz
12-19-2010, 09:59 PM
Sawed mine with a horizontal mill.Bill

Nice.

I suppose you could do the same thing in a vertical mill with a stub arbor and appropriate cutter.