Combination Tool Cycle Time Calculation | HENTRY

A cycle-time saving is an arithmetic result, not a specification. Two operations become one, and the difference between the two cycles is a number that can be recomputed by anyone holding the same inputs.

Most quotations skip that step and quote a percentage instead, which is why the saving rarely survives the first production run.

This page does the arithmetic in the open: the terms that belong in the cycle, a worked example with every parameter labelled, the annual conversion, and the batch size below which the tool premium is not recovered.

Every time value on this page is an example, chosen to make the arithmetic checkable. Substitute your own machine, holder and gauge values before using any of it in a quotation.

What two operations actually cost

The two cycles are not the sum of two cutting times. They are two sequences of machine events, and the events that are not cutting often decide the result.

For two stations, the sequence is: load and clamp, rapid approach, drill, retract and clear the chip, change to the reamer, rapid approach, ream, retract, gauge, change back.

For one station, the same list loses the change, the second approach and the retract between stages, and keeps everything else.

Write it as a sum and the difference becomes visible:

Four terms are routinely left out of quoted comparisons. The first is the second tool change, which is needed to put the drill back for the next part. The second is the chip-clearing dwell after drilling, which a combination tool removes because the reamer follows the drill without a retract.

The third is the in-process gauge, which in most cells happens at the hole and does not disappear when the operations merge. The fourth is the rapid approach to the second stage, which is a real move even when the tools sit next to each other in the magazine.

What stays the same in both cases matters as much: load and clamp, the gauge, the coolant routine and the part handling. Comparisons that quietly drop one of these are not comparisons.

One term is easy to overstate. If both tools sit in the same magazine, the change is still a change: the spindle still stops, the arm still travels, and the next tool still has to be proven. Magazine position changes the time by a second or two, not by the whole change.

The worked example

The example is a through hole, 12 mm diameter and 25 mm deep, in a steel around 200 HB. The cutting parameters are typical catalogue values, used here as example inputs and not as a recommendation for a specific material.

Drilling at 90 m/min gives 2,387 rpm. At 0.15 mm per revolution that is 358 mm per minute. With a 0.3 x D point allowance the cutting length is 28.6 mm, so the drill cut is 4.8 s.

Reaming at 60 m/min gives 1,592 rpm. At 0.30 mm per revolution that is 477 mm per minute. With 2 mm of approach and overtravel the cutting length is 27 mm, so the ream cut is 3.4 s.

The two totals differ by 15.4 s. To check the arithmetic, add the column: the two-station sequence is 3.0 + 1.0 + 4.8 + 3.0 + 6.0 + 1.0 + 3.4 + 1.0 + 8.0 + 6.0, and the single-station sequence is 3.0 + 1.0 + 4.8 + 0.6 + 3.4 + 1.0 + 8.0.

Two details change the answer more than the cutting parameters do. If the cell gauges only once per batch instead of once per part, the 8.0 s disappears from both columns and the saving falls to 7.4 s. If the machine changes tools in 3 s rather than 6 s, the saving falls by 6 s more.

That is the first useful conclusion: on a bore this short, the comparison is dominated by the non-cutting terms. A shop that quotes the cutting times alone will overstate the saving in one direction and understate it in the other, depending on which terms it forgot.

From seconds per part to hours per year

The annual conversion is multiplication, but the unit matters. Seconds per part become hours per year only after the annual volume is fixed.

At 200,000 parts, 15.4 s per part is 3,080,000 s, or 855.6 hours. Round to 856 hours and state the volume next to it, because the number is meaningless without it.

Compare that with capacity rather than with money first. A cell running two shifts of 7.5 hours over 230 working days has 3,450 hours a year. The saving is 24.8 percent of that capacity, which is why the same arithmetic that looks small in seconds looks large in planning.

In parts per hour the change is 96.8 to 165.1, an increase of 70 percent. That figure is the one to use when the cell is the bottleneck; the hours figure is the one to use when machine time is charged.

If a machine hour is charged at an internal rate, the value of the saving is 856 times that rate. At 45 EUR per hour the example is 38,520 EUR a year, and the rate is the only input that changes between companies.

Apply availability before promising hours. At an OEE of 0.75 the same saving absorbs 1,141 calendar hours, not 856, because the machine is not producing for a quarter of the year.

The batch size where it stops paying

The tool premium is a fixed cost per tool. The time saving is a per-part gain. Break-even is where the two meet, and it can be written as one line:

With a premium of 180 EUR, a saving of 15.4 s and a rate of 45 EUR per hour, the per-part gain is 0.1925 EUR. If the combination tool costs 0.02 EUR more per part in tooling, the net gain is 0.1725 EUR and the break-even batch is 1,043 parts.

Add a proving run. Four hours of machine time and two hours of inspection at the same rate is about 260 EUR, which moves the break-even to 2,551 parts. The proving run is not optional on a first tool, so the second number is the honest one.

Below that volume the tool can still be right, but for reasons that are not arithmetic: fewer fixture changes, one setup to qualify, or a bore that is hard to reach with two tools. Those are legitimate reasons and they should be stated as such instead of hidden inside a percentage.

Two conditions make the equation unsatisfiable at any batch size. If the extra tooling cost per part exceeds the per-part time gain, no volume helps. If the combination tool life is shorter than the sum of the two separate lives by more than the time gain, the same thing happens.

This is also the point where small batches stop paying. The premium and the proving run are fixed; a 300-part order pays both and still has to absorb the risk that one tool stops the cell instead of one of two operations.

The two halves of the decision sit on separate pages by design. This page produces the seconds; single-station versus two-station cost turns them into a capital case, and the combination tool cycle time page holds the process-level view.

Whether the operations should be combined at all is screened earlier, in when to use a combination drill-reamer. The tool family itself is described on the combination drill-reamer page.

What the model does not include

The arithmetic above is a cycle-time model. Four effects sit outside it and each can move the answer by more than rounding does.

The first is tool life. A combination tool carries two cutting edges that wear at different rates, and the tool is reground when either reaches its limit. The effective life is therefore set by the shorter stage, not by the average, which is covered in regrinding limits for combination tools.

The second is the failure domain. With two tools, a broken drill stops one operation. With one tool, a chipped reaming edge stops the station. The expected downtime per failure may be similar, but the number of parts affected per event is not.

The third is inspection. If the two operations currently require two gauges and two records, merging them can remove a handling step that the cycle model never sees. That effect favours the combination tool and is not in the 15.4 s.

The fourth is scrap and rework. A combination tool leaves no opportunity to correct the bore between drilling and reaming, so a drifted pre-hole becomes a scrapped part rather than a reworked one. On a stable process this is small; on a process still being dialled in it is not.

Programming and tool data are a fifth, smaller term: one tool offset instead of two, one tool life record instead of two, and one less entry in the setup sheet.

Where the arithmetic usually goes wrong

Six errors account for most of the disagreements we see when two quotations for the same part are compared.

Using the catalogue cutting speed for both stages as if both cut the full length ignores the drill point allowance, which on a 12 mm hole adds 3.6 mm to the drill path and nothing to the ream path.

Counting one tool change instead of two understates the two-station cycle by about 6 s, which on this example is 39 percent of the entire saving.

Dropping the second approach and the inter-stage retract removes another 2 s, and those are real machine moves that the operator sees every cycle.

Comparing a per-part time with a per-pallet time mixes two units. If the fixture holds four parts, the load time divides by four while the tool change does not, and the balance between the two options shifts.

Using the machine's maximum spindle speed instead of its acceleration curve overstates the gain on short cuts, because a 4.8 s cut never reaches the steady-state rapid rate.

Finally, treating the result as value-added time inflates it. The saving is machine time; what it is worth depends on whether the cell was the constraint, which is a separate calculation.

What published sources say about this

These are the pages used for the cutting data, the tool-life context and the measurement points. Each is cited for what that page states, under its own conditions.

• Reamers, machine reamers (toolmaker technical overview): reamer structure, guide part, chamfer lead and flute shapes, the basis for treating the reaming stage as a separate cut with its own parameters.
• Reaming and fine boring (toolmaker technical page): reaming tool designs and holders, including the floating and guide-pad variants that change how the second stage behaves in the cycle.
• Tool life up, costs down (toolmaker application report): a case in which higher parameters and revised geometry doubled tool life, the reason tool life is treated as a separate term from cycle time.
• Overall operating costs (toolmaker application report): a case in which a replaceable-head drill cut machining time by more than half with a stated tool-life increase, and tool-data transparency reduced downtime.
• What are control charts? (NIST/SEMATECH e-Handbook of Statistical Methods): why a characteristic measured over time is plotted in sequence, the same logic as tracking cycle time per shift instead of per quotation.
• What is Process Capability? (NIST/SEMATECH e-Handbook): the comparison of process spread with specification limits, which is what decides whether the saved time is repeatable or borrowed from the tolerance.
• Dimensioning and Tolerancing (ASME Y14.5): the form, orientation, position and runout definitions that any acceptance gauge on the bore has to be read against.

Outbound references are listed because the arithmetic depends on them, not as endorsements. The example parameters on this page are HENTRY's own illustration and are not taken from any of these pages.

Frequently asked questions

Updated 13 November 2026: first publication. Scope: the cycle-time terms in a two-station versus one-station comparison, the worked example, the annual conversion, the break-even batch and what the model leaves out. All times and prices are examples, not HENTRY values.